Drivetrains - INFORMATIONAL
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FF LAYOUT
In automobile design, an FF, or front-engine, front wheel drive, layout places both the engine and driven wheels at the front of the vehicle. This layout is typically chosen for its compact packaging - that is, it takes up very little space, allowing the rest of the vehicle to be designed more flexibly. In contrast with the FR layout, the FF layout eliminates the need for a central tunnel or a higher chassis clearance to accommodate a driveshaft providing power to the rear wheels. Like the RR and MR layouts, it places the heavy engine over the drive wheels which aids traction. As the steered wheels are also the driven wheels, FF cars are generally considered superior to FR cars in conditions such as snow, mud, gravel or wet tarmac. However, powerful cars rarely use the FF layout because weight transference under acceleration unloads the front wheels and sharply reduces their grip, effectively putting a cap on the amount of horsepower which could realistically be utilized. Electronic traction control can avoid wheelspin but largely negates the benefit of extra power.
Early cars using the FF layout include the 1948 Citroën 2CV, 1949 Saab 92 and the 1959 Mini. In the 1980s, the traction and packaging advantages of this layout caused many compact and mid-sized vehicles to adopt it. Because the transversely-mounted engine does not require a bevel gear to change the direction of the final drive, coastdown losses are reduced by approximately 2-3% of flywheel power and hence overall efficiency is slightly higher than with a FR design.
There are four quite different particular arrangements for this basic layout, according to the location of the engine, which is the heaviest component of the drivetrain, with respect to the front wheels.
The earliest such arrangement was not technically FF, but rather MF and had the engine mounted longitudinally (fore-and-aft, or north-south) behind the wheels, with the transmission and differential in front. It was designed by Walter Miller, who had the drivetrain double back to put the differential in the middle, with brakes mounted inboard. E. L. Cord took the easier method of putting the differential in front. With the engine so far back, the weight balance of the L-29 Cord was unwieldy; the driven wheels did not have enough weight upon them. His later 810 and 812 cars were similar. The Citroën Traction Avant used the same MF layout, but solved the weight distribution issue with a new, low slung unibody design, resulting in remarkable handling for the era.
The Grégoire Sport, amongst other cars by that firm, had the engine longitudinally in front of the front wheels, with the differential in the middle. This became quite popular, as the German Ford Taunus 12M and the Lancia Flavia used it as well.
Issigonis's Mini and a few successor cars had the engine laterally mounted (east-west), with the transmission in the sump below the crankshaft. This was just about as good as one could do to put the entire weight of the drivetrain on the front wheels.
But the arrangement that really took over was that of Dante Giacosa, who put the transmission on one side of the laterally mounted engine, and doubled back the drivetrain to put the differential just behind it, but offset to one side. Hence the driveshafts to the wheels are longer on one side than the other, something which was avoided in the past. This located the weight just a bit in front of the wheels. This arrangement was first tried out on the Autobianchi Primula, next on the Fiat 128, and finally on the Fiat 127, which became car of the year. It is this system which dominates worldwide at present.
Vehicles with the Giacosa arrangement tend to suffer from torque steer under heavy acceleration since more power is required to overcome the inertia of the longer (and therefore heavier) driveshaft than the shorter, lighter one. The differential then feeds more power to the wheel that's meeting least resistance (ie the one with the shorter driveshaft) and the car pulls to one side under heavy acceleration. For this reason, the Issigonis design (in which the two driveshafts are equal in length) is still preferred by many performance drivers and accounts for much of the Mini's success in rally and short-track circuit racing.
In automobile design, an FF, or front-engine, front wheel drive, layout places both the engine and driven wheels at the front of the vehicle. This layout is typically chosen for its compact packaging - that is, it takes up very little space, allowing the rest of the vehicle to be designed more flexibly. In contrast with the FR layout, the FF layout eliminates the need for a central tunnel or a higher chassis clearance to accommodate a driveshaft providing power to the rear wheels. Like the RR and MR layouts, it places the heavy engine over the drive wheels which aids traction. As the steered wheels are also the driven wheels, FF cars are generally considered superior to FR cars in conditions such as snow, mud, gravel or wet tarmac. However, powerful cars rarely use the FF layout because weight transference under acceleration unloads the front wheels and sharply reduces their grip, effectively putting a cap on the amount of horsepower which could realistically be utilized. Electronic traction control can avoid wheelspin but largely negates the benefit of extra power.
Early cars using the FF layout include the 1948 Citroën 2CV, 1949 Saab 92 and the 1959 Mini. In the 1980s, the traction and packaging advantages of this layout caused many compact and mid-sized vehicles to adopt it. Because the transversely-mounted engine does not require a bevel gear to change the direction of the final drive, coastdown losses are reduced by approximately 2-3% of flywheel power and hence overall efficiency is slightly higher than with a FR design.
There are four quite different particular arrangements for this basic layout, according to the location of the engine, which is the heaviest component of the drivetrain, with respect to the front wheels.
The earliest such arrangement was not technically FF, but rather MF and had the engine mounted longitudinally (fore-and-aft, or north-south) behind the wheels, with the transmission and differential in front. It was designed by Walter Miller, who had the drivetrain double back to put the differential in the middle, with brakes mounted inboard. E. L. Cord took the easier method of putting the differential in front. With the engine so far back, the weight balance of the L-29 Cord was unwieldy; the driven wheels did not have enough weight upon them. His later 810 and 812 cars were similar. The Citroën Traction Avant used the same MF layout, but solved the weight distribution issue with a new, low slung unibody design, resulting in remarkable handling for the era.
The Grégoire Sport, amongst other cars by that firm, had the engine longitudinally in front of the front wheels, with the differential in the middle. This became quite popular, as the German Ford Taunus 12M and the Lancia Flavia used it as well.
Issigonis's Mini and a few successor cars had the engine laterally mounted (east-west), with the transmission in the sump below the crankshaft. This was just about as good as one could do to put the entire weight of the drivetrain on the front wheels.
But the arrangement that really took over was that of Dante Giacosa, who put the transmission on one side of the laterally mounted engine, and doubled back the drivetrain to put the differential just behind it, but offset to one side. Hence the driveshafts to the wheels are longer on one side than the other, something which was avoided in the past. This located the weight just a bit in front of the wheels. This arrangement was first tried out on the Autobianchi Primula, next on the Fiat 128, and finally on the Fiat 127, which became car of the year. It is this system which dominates worldwide at present.
Vehicles with the Giacosa arrangement tend to suffer from torque steer under heavy acceleration since more power is required to overcome the inertia of the longer (and therefore heavier) driveshaft than the shorter, lighter one. The differential then feeds more power to the wheel that's meeting least resistance (ie the one with the shorter driveshaft) and the car pulls to one side under heavy acceleration. For this reason, the Issigonis design (in which the two driveshafts are equal in length) is still preferred by many performance drivers and accounts for much of the Mini's success in rally and short-track circuit racing.
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FR LAYOUT
In automobile design, an FR, or front-engine, rear wheel drive means a layout where the engine is in the front of the vehicle and drive wheels at the rear. This was the traditional automobile layout for most of the 20th century.
Overview
This layout is typically chosen for its simple design and weight distribution. Placing the engine at the front gives the vehicle a traditional long hood (in British English "bonnet") and engine cooling is simple to arrange. Placing the drive wheels at the rear allows ample room for the transmission in the center of the vehicle and avoids the mechanical problems of transmitting drive to steered wheels. The layout is still more suitable than front-wheel drive for engine outputs of more than about 200 bhp, as the weight transference during acceleration loads the rear wheels and increases grip.
The FM layout is based on the FR layout.
History
The first FR car was an 1895 Panhard model, so this layout was known as the "Systeme Panhard" in the early years. Most American cars used the FR layout until the mid 1980s. The Oil crisis of the 1970s and the success of small FF cars like the Mini, Volkswagen Rabbit, and Honda Civic led to the widespread adoption of that layout.
Some manufacturers, such as Porsche (944,924,928) and Chevrolet (C5 and C6 Corvettes), retained this layout but moved the gearbox from behind the engine to between the rear wheels, putting more weight over the driven axle. This configuration is often referred to as a transaxle since the transmission and axle are one unit.
Right after the Arab Oil Embargo of 1973 and the 1979 fuel crises, a majority of American FR vehicles (station wagons, luxury sedans) were being phased out for the FF layout - this trend would spawn the SUV/van conversion market. Chrysler went 100% FF by 1990; only GM and Ford retained FR for their luxury and performance vehicles.
GM phased out its FR luxury cars after the 1996 model year, and its F-car (Chevrolet Camaro/Pontiac Firebird) in 2002. GM reintroduced North American FR luxury cars with the 2003 Cadillac CTS. Currently they produce the Pontiac GTO (imported from Australia), Chevrolet Corvette/Cadillac XLR and the Cadillac CTS/STS. GM Holden continued to produce RWD cars through this period.
Today, most cars are FF, though the limitations of that layout, such as poor traction under acceleration and excessive nose weight, are beginning to become apparent. Many of the newest models have adopted all wheel drive, and some, like the Chrysler 300 are switching back to the FR layout. Most sports cars and luxury cars have always been FR.
In automobile design, an FR, or front-engine, rear wheel drive means a layout where the engine is in the front of the vehicle and drive wheels at the rear. This was the traditional automobile layout for most of the 20th century.
Overview
This layout is typically chosen for its simple design and weight distribution. Placing the engine at the front gives the vehicle a traditional long hood (in British English "bonnet") and engine cooling is simple to arrange. Placing the drive wheels at the rear allows ample room for the transmission in the center of the vehicle and avoids the mechanical problems of transmitting drive to steered wheels. The layout is still more suitable than front-wheel drive for engine outputs of more than about 200 bhp, as the weight transference during acceleration loads the rear wheels and increases grip.
The FM layout is based on the FR layout.
History
The first FR car was an 1895 Panhard model, so this layout was known as the "Systeme Panhard" in the early years. Most American cars used the FR layout until the mid 1980s. The Oil crisis of the 1970s and the success of small FF cars like the Mini, Volkswagen Rabbit, and Honda Civic led to the widespread adoption of that layout.
Some manufacturers, such as Porsche (944,924,928) and Chevrolet (C5 and C6 Corvettes), retained this layout but moved the gearbox from behind the engine to between the rear wheels, putting more weight over the driven axle. This configuration is often referred to as a transaxle since the transmission and axle are one unit.
Right after the Arab Oil Embargo of 1973 and the 1979 fuel crises, a majority of American FR vehicles (station wagons, luxury sedans) were being phased out for the FF layout - this trend would spawn the SUV/van conversion market. Chrysler went 100% FF by 1990; only GM and Ford retained FR for their luxury and performance vehicles.
GM phased out its FR luxury cars after the 1996 model year, and its F-car (Chevrolet Camaro/Pontiac Firebird) in 2002. GM reintroduced North American FR luxury cars with the 2003 Cadillac CTS. Currently they produce the Pontiac GTO (imported from Australia), Chevrolet Corvette/Cadillac XLR and the Cadillac CTS/STS. GM Holden continued to produce RWD cars through this period.
Today, most cars are FF, though the limitations of that layout, such as poor traction under acceleration and excessive nose weight, are beginning to become apparent. Many of the newest models have adopted all wheel drive, and some, like the Chrysler 300 are switching back to the FR layout. Most sports cars and luxury cars have always been FR.
MR LAYOUT
In automobile design, an MR or mid-engine, Rear wheel drive layout is one in which the rear wheels are driven by an engine placed just in front of them, behind the passenger compartment. In contrast to the rear-engined RR layout, the center of mass of the engine is in front of the rear axle. This layout is typically chosen for its low polar inertia and relatively favorable weight distribution (the heaviest component is near the center of the car, making the main component of its moment of inertia relatively low). The layout does suffer from a tendency toward being heavier in the rear than the front, which is not ideal for handling. However, it is generally felt that the lower polar inertia more than makes up for this. The mid-engined layout also uses up central space, making it impractical for any but two-seater sports cars.
In modern racing cars, MR is the usual configuration and is usually synonymous with "rear engine". Due to its favorable weight dynamics, this layout is heavily employed in Formula racing cars (such as F1s). This configuration was also common in very small engined 1950s microcars, in which the engines didn't take up much space. Because of successes in racing, the MR platform has been popular for road going sports cars despite the inherent challenges of design, maintenance and lack of cargo space.
History
The 1923 Benz Tropfenwagen was the first race car with mid-engine, rear-wheel drive layout. It was based on an earlier design named Rumpler Tropfenwagen in 1921 made by Edmund von Rumpler, an Austrian engineer working at Daimler. The Benz tropfenwagen was designed by Ferdinand Porsche along with Willy Walb and Hans Nibel. It raced in 1923 and 1924 and was most successful in Italian Grand Prix in Monza where it stood fourth. Later, Ferdinand Porsche used mid-engine design concept towards the Auto Union Grand Prix cars of the 1930s which became the first winning MR racers. They were decades before their time, although MR Miller Specials raced a few times at Indianapolis between 1939 and 1947. In 1953 Porsche premiered the tiny and altogether new MR 550 spyder and in a year it was notoriously winning in the smaller sports and endurance race car classes against much larger cars- a sign of greater things to come. The 718 followed similarly in 1958. But it was not until the late 1950s that MR reappeared in Grand Prix (today's 'Formula One') races in the form of the Cooper - Climax (1957), soon followed by cars from BRM and Lotus. Ferrari and Porsche soon made Grand Prix MR attempts with less initial success. The mid-engined layout was brought back to Indianapolis in 1961 by the Cooper Car Company with Jack Brabham running as high as third and finishing ninth. Cooper did not return, but from 1963 on British built mid-engined cars from constructors like Brabham, Lotus and Lola competed regularly and in 1965 Lotus won Indy with their Type 38.
Cars Using the MR layout
Pre 1960s
Porsche 550 Spyder (1954), first ever production road car with MR layout, 125 produced (including 90 road versions)
Zündapp Janus (1957), literally mid-engined and nearly symmetrical with passengers on both ends of the engine
1960s to 1970s
De Tomaso Pantera (1971-1996)
De Tomaso Vallelunga (1965-1967)
Ferrari Berlinetta Boxer
Ferrari Dino 206/246 (1968)
Ferrari 308
Ferrari Mondial
Fiat X1/9, (1972-1989) 110,000 produced
Ford GT40 (1966)
Lamborghini Countach (1974-1990)
Lamborghini Miura (1966-1974) 987 produced
Lotus Europa (1966) 9300 produced
Matra Djet (1962-1968) 1,692 produced
Matra M530 (1967-1972) 9,609 produced
Matra Bagheera (1973-1980) 47,802 produced
Maserati Bora (1971-1980)
Maserati Merak (1974-1982)
Porsche 914 (1969) 118,000 produced
1980s to 1990s
Ferrari 328
Ferrari 348
Ferrari F355
Ferrari 360
Honda Beat
Honda NSX (aka Acura NSX in North America)
Lamborghini Diablo
Lotus Esprit
Matra Murena
Mazda Autozam AZ-1
Pontiac Fiero
Porsche Boxster
Toyota MR2
Toyota Previa
2000s
Clio V6 Renault Sport
Ferrari F430
Honda NSX (aka Acura NSX in North America)
Lotus Elise
Porsche Boxster
Porsche Cayman
Toyota MR-S (aka Toyota MR2 Spyder in North America)
Supercars
Bugatti Veyron
Ferrari Enzo
Ferrari 288 GTO
Ferrari F40
Ferrari F50
Ferrari FXX
Ford GT
Jaguar XJ220
Koenigsegg CCR
Lamborghini Gallardo
Lamborghini Murcielago
Maserati MC12
McLaren F1
Pagani Zonda
Porsche Carrera GT
Saleen S7
Spyker C8
In automobile design, an MR or mid-engine, Rear wheel drive layout is one in which the rear wheels are driven by an engine placed just in front of them, behind the passenger compartment. In contrast to the rear-engined RR layout, the center of mass of the engine is in front of the rear axle. This layout is typically chosen for its low polar inertia and relatively favorable weight distribution (the heaviest component is near the center of the car, making the main component of its moment of inertia relatively low). The layout does suffer from a tendency toward being heavier in the rear than the front, which is not ideal for handling. However, it is generally felt that the lower polar inertia more than makes up for this. The mid-engined layout also uses up central space, making it impractical for any but two-seater sports cars.
In modern racing cars, MR is the usual configuration and is usually synonymous with "rear engine". Due to its favorable weight dynamics, this layout is heavily employed in Formula racing cars (such as F1s). This configuration was also common in very small engined 1950s microcars, in which the engines didn't take up much space. Because of successes in racing, the MR platform has been popular for road going sports cars despite the inherent challenges of design, maintenance and lack of cargo space.
History
The 1923 Benz Tropfenwagen was the first race car with mid-engine, rear-wheel drive layout. It was based on an earlier design named Rumpler Tropfenwagen in 1921 made by Edmund von Rumpler, an Austrian engineer working at Daimler. The Benz tropfenwagen was designed by Ferdinand Porsche along with Willy Walb and Hans Nibel. It raced in 1923 and 1924 and was most successful in Italian Grand Prix in Monza where it stood fourth. Later, Ferdinand Porsche used mid-engine design concept towards the Auto Union Grand Prix cars of the 1930s which became the first winning MR racers. They were decades before their time, although MR Miller Specials raced a few times at Indianapolis between 1939 and 1947. In 1953 Porsche premiered the tiny and altogether new MR 550 spyder and in a year it was notoriously winning in the smaller sports and endurance race car classes against much larger cars- a sign of greater things to come. The 718 followed similarly in 1958. But it was not until the late 1950s that MR reappeared in Grand Prix (today's 'Formula One') races in the form of the Cooper - Climax (1957), soon followed by cars from BRM and Lotus. Ferrari and Porsche soon made Grand Prix MR attempts with less initial success. The mid-engined layout was brought back to Indianapolis in 1961 by the Cooper Car Company with Jack Brabham running as high as third and finishing ninth. Cooper did not return, but from 1963 on British built mid-engined cars from constructors like Brabham, Lotus and Lola competed regularly and in 1965 Lotus won Indy with their Type 38.
Cars Using the MR layout
Pre 1960s
Porsche 550 Spyder (1954), first ever production road car with MR layout, 125 produced (including 90 road versions)
Zündapp Janus (1957), literally mid-engined and nearly symmetrical with passengers on both ends of the engine
1960s to 1970s
De Tomaso Pantera (1971-1996)
De Tomaso Vallelunga (1965-1967)
Ferrari Berlinetta Boxer
Ferrari Dino 206/246 (1968)
Ferrari 308
Ferrari Mondial
Fiat X1/9, (1972-1989) 110,000 produced
Ford GT40 (1966)
Lamborghini Countach (1974-1990)
Lamborghini Miura (1966-1974) 987 produced
Lotus Europa (1966) 9300 produced
Matra Djet (1962-1968) 1,692 produced
Matra M530 (1967-1972) 9,609 produced
Matra Bagheera (1973-1980) 47,802 produced
Maserati Bora (1971-1980)
Maserati Merak (1974-1982)
Porsche 914 (1969) 118,000 produced
1980s to 1990s
Ferrari 328
Ferrari 348
Ferrari F355
Ferrari 360
Honda Beat
Honda NSX (aka Acura NSX in North America)
Lamborghini Diablo
Lotus Esprit
Matra Murena
Mazda Autozam AZ-1
Pontiac Fiero
Porsche Boxster
Toyota MR2
Toyota Previa
2000s
Clio V6 Renault Sport
Ferrari F430
Honda NSX (aka Acura NSX in North America)
Lotus Elise
Porsche Boxster
Porsche Cayman
Toyota MR-S (aka Toyota MR2 Spyder in North America)
Supercars
Bugatti Veyron
Ferrari Enzo
Ferrari 288 GTO
Ferrari F40
Ferrari F50
Ferrari FXX
Ford GT
Jaguar XJ220
Koenigsegg CCR
Lamborghini Gallardo
Lamborghini Murcielago
Maserati MC12
McLaren F1
Pagani Zonda
Porsche Carrera GT
Saleen S7
Spyker C8
RR LAYOUT
In Automobile design, an RR, or Rear-engine, Rear wheel drive, layout places both the engine and drive wheels at the rear of the vehicle. In contrast to the MR layout, however, the center of gravity of the engine itself is actually past the rear axle. This is not to be confused with the center of gravity of the whole vehicle, as an imbalance of such proportions would make it impossible to keep the front wheels on the ground.
This layout is typically chosen for a combination of several reasons. For optimal handling and to eliminate the phenomenon known as torque steer, the wheels which propel the car should not be the same ones that steer it. For optimum traction, the engine should be nearest to the driven wheels since the engine is typically the densest/heaviest component of the car. Thus, in a car which steers with the front wheels, it is better for the engine to be located in the rear of the car - either a RR or MR design.
The disadvantage of the RR configuration is that placing the engine outside the wheelbase creates significant problems for handling as, when the car begins to slide on a corner, the end of the car will tend to want to swing wide and overtake the front - especially under braking. This tendency is referred to as oversteer and creates potential safety issues in racing applications as well as for ordinary drivers on wet or icy roads, although such behavior is desirable in drifting, a motorsport based on intentional oversteer.
In addition, even though the rear wheels benefit from the additional traction the added weight of the engine gives, the front wheels still need traction in order to steer the car effectively. For this reason, a RR layout car can also be prone to understeer. Most manufacturers have abandoned the RR layout apart from Porsche who has gradually developed their design with improvements to the suspension as well as electronic aids to reduce the shortcomings of the layout to acceptable levels.
Another manufacturer to implement the RR configuration was the De Lorean Motor Company with its DMC-12 sports car. To compensate for the uneven (35/65) weight distribution caused by the rear-mounted engine, De Lorean used rear wheels with a diameter slightly greater than the front wheels. The last mass produced RR configured car, at least in Europe, was the Škoda 130/135/136 that was produced until 1990.
A range of sports road cars and racing cars with the RR layout were produced by the French company Alpine (badged as Renaults in some countries, including the UK, to avoid trade mark conflicts). These had bodies made of composite materials and used mechanical components made by Renault.
Early cars using the RR layout include the Tucker, the Volkswagen Beetle, the Porsche 911 and the innovative Chevrolet Corvair.
In Automobile design, an RR, or Rear-engine, Rear wheel drive, layout places both the engine and drive wheels at the rear of the vehicle. In contrast to the MR layout, however, the center of gravity of the engine itself is actually past the rear axle. This is not to be confused with the center of gravity of the whole vehicle, as an imbalance of such proportions would make it impossible to keep the front wheels on the ground.
This layout is typically chosen for a combination of several reasons. For optimal handling and to eliminate the phenomenon known as torque steer, the wheels which propel the car should not be the same ones that steer it. For optimum traction, the engine should be nearest to the driven wheels since the engine is typically the densest/heaviest component of the car. Thus, in a car which steers with the front wheels, it is better for the engine to be located in the rear of the car - either a RR or MR design.
The disadvantage of the RR configuration is that placing the engine outside the wheelbase creates significant problems for handling as, when the car begins to slide on a corner, the end of the car will tend to want to swing wide and overtake the front - especially under braking. This tendency is referred to as oversteer and creates potential safety issues in racing applications as well as for ordinary drivers on wet or icy roads, although such behavior is desirable in drifting, a motorsport based on intentional oversteer.
In addition, even though the rear wheels benefit from the additional traction the added weight of the engine gives, the front wheels still need traction in order to steer the car effectively. For this reason, a RR layout car can also be prone to understeer. Most manufacturers have abandoned the RR layout apart from Porsche who has gradually developed their design with improvements to the suspension as well as electronic aids to reduce the shortcomings of the layout to acceptable levels.
Another manufacturer to implement the RR configuration was the De Lorean Motor Company with its DMC-12 sports car. To compensate for the uneven (35/65) weight distribution caused by the rear-mounted engine, De Lorean used rear wheels with a diameter slightly greater than the front wheels. The last mass produced RR configured car, at least in Europe, was the Škoda 130/135/136 that was produced until 1990.
A range of sports road cars and racing cars with the RR layout were produced by the French company Alpine (badged as Renaults in some countries, including the UK, to avoid trade mark conflicts). These had bodies made of composite materials and used mechanical components made by Renault.
Early cars using the RR layout include the Tucker, the Volkswagen Beetle, the Porsche 911 and the innovative Chevrolet Corvair.
A mid-mounted engine describes the placement of an automobile engine between the centerline of the rear and front axles. Another term for this is Mid-ship, this term is used mostly by Japanese manufacturers. Traditionally, the term mid-engine layout has been applied to cars that place the engine and transaxle behind the driver but in front of the rear axles. This configuration is known as a mid-engine, rear-drive An engine placed in front of the driver's compartment but fully behind the front axle line also meets the definition of a mid-engine.
Mid-engine designs are usually used in sports or racing cars, as the engine placement provides a low polar moment of inertia by keeping most of the mass close to the center of the vehicle. This aids in quickly changing the direction of the automobile's travel, albeit at the price of reduced straight line stability.
The drawback of mid-engine cars is packaging--frequently, the space the engine takes up could have otherwise been used for passengers or trunk space. In the "behind the passenger" design, engine cooling can also be a potential problem, as in the Porsche 914 or Lotus Esprit, but that problem seems to have been largely solved in newer designs such as the Porsche 986. The Saleen S7 appears to also resolve the problem with the use of large engine-compartment vents on the sides and rear of the bodywork.
Examples
Midship RWD, with engine infront of driver
Mazda RX-7
Mazda RX-8
Nissan 350Z
Midship RWD, with engine rear of driver
Ferrari F355
Porsche Boxter
Toyota MR2
Acura NSX
Midship FWD
Citroën Traction Avant
Citroën DS
Citroën SM
Mid-engine designs are usually used in sports or racing cars, as the engine placement provides a low polar moment of inertia by keeping most of the mass close to the center of the vehicle. This aids in quickly changing the direction of the automobile's travel, albeit at the price of reduced straight line stability.
The drawback of mid-engine cars is packaging--frequently, the space the engine takes up could have otherwise been used for passengers or trunk space. In the "behind the passenger" design, engine cooling can also be a potential problem, as in the Porsche 914 or Lotus Esprit, but that problem seems to have been largely solved in newer designs such as the Porsche 986. The Saleen S7 appears to also resolve the problem with the use of large engine-compartment vents on the sides and rear of the bodywork.
Examples
Midship RWD, with engine infront of driver
Mazda RX-7
Mazda RX-8
Nissan 350Z
Midship RWD, with engine rear of driver
Ferrari F355
Porsche Boxter
Toyota MR2
Acura NSX
Midship FWD
Citroën Traction Avant
Citroën DS
Citroën SM
MF LAYOUT
In automobile design, an MF or Mid-engine, Front wheel drive layout is one in which the front wheels are driven by an engine placed just behind them, in front of the passenger compartment. In contrast to the front-engined FF layout the center of gravity of the engine is behind the front axle. This layout is typically chosen for its better weight distribution (the heaviest component is near the center of the car, lowering its moment of inertia). The mid-engined layout does, however, use up central space, making the resulting vehicle rather long.
Examples of road cars using the MF layout include the Citroën Traction Avant, Citroën DS, and the Citroën SM.
Traditionally, the term mid-engine has been reserved for cars that place the engine and transaxle behind the driver and in front of the rear axles, as in the Lamborghini Countach or Ferrari Testarossa, but an engine placed in front of the driver's compartment but fully behind the front axle line also qualifies as mid-engine.
In automobile design, an MF or Mid-engine, Front wheel drive layout is one in which the front wheels are driven by an engine placed just behind them, in front of the passenger compartment. In contrast to the front-engined FF layout the center of gravity of the engine is behind the front axle. This layout is typically chosen for its better weight distribution (the heaviest component is near the center of the car, lowering its moment of inertia). The mid-engined layout does, however, use up central space, making the resulting vehicle rather long.
Examples of road cars using the MF layout include the Citroën Traction Avant, Citroën DS, and the Citroën SM.
Traditionally, the term mid-engine has been reserved for cars that place the engine and transaxle behind the driver and in front of the rear axles, as in the Lamborghini Countach or Ferrari Testarossa, but an engine placed in front of the driver's compartment but fully behind the front axle line also qualifies as mid-engine.
FM LAYOUT
The FM layout, standing for front-midships, is a layout of an automobile that places the engine in the front, like the FR layout, but pushed back enough that the engine's center of gravity is to the rear of the front axle. This aids in weight distribution and reduces the moment of inertia, helping handling. Technically FM is a smaller sub-catgeory of FR, since the engine is still in the front third of the car, and FM cars still have rear wheel drivetrains.
FM cars are often recognizeable by an extensively long hood and front wheels unusually close to the front bumper of the car. Grand tourers usually have FM alayouts, as a rear engine would not leave much space for the rear seats.
Typical cars with FM layouts are Aston Martin Vanquish and V8 Vantage 2005, Chevrolet Corvette, Dodge Viper, Honda S2000, Ferrari 612 Scaglietti, Maserati Quattroporte, Mazda RX-7 and RX-8, Mercedes-Benz SLR McLaren, BMW Z4, Nissan 350Z, Porsche 928, 944, and 968.
The FM layout, standing for front-midships, is a layout of an automobile that places the engine in the front, like the FR layout, but pushed back enough that the engine's center of gravity is to the rear of the front axle. This aids in weight distribution and reduces the moment of inertia, helping handling. Technically FM is a smaller sub-catgeory of FR, since the engine is still in the front third of the car, and FM cars still have rear wheel drivetrains.
FM cars are often recognizeable by an extensively long hood and front wheels unusually close to the front bumper of the car. Grand tourers usually have FM alayouts, as a rear engine would not leave much space for the rear seats.
Typical cars with FM layouts are Aston Martin Vanquish and V8 Vantage 2005, Chevrolet Corvette, Dodge Viper, Honda S2000, Ferrari 612 Scaglietti, Maserati Quattroporte, Mazda RX-7 and RX-8, Mercedes-Benz SLR McLaren, BMW Z4, Nissan 350Z, Porsche 928, 944, and 968.
FOUR WHEEL DRIVE - 4WD
Four-wheel drive, 4WD, 4x4 ("four by four"), all-wheel drive, and AWD are terms used to describe a four-wheeled vehicle with a drivetrain that allows all four wheels to receive power from the engine simultaneously. While many people associate the term with off-road vehicles, powering all four wheels provides better control on slick ice and is an important part of rally racing on mostly-paved roads.
Four-wheel drive (4WD or 4x4 for short) is the original term and is often used to describe truck-like vehicles that require the driver to manually switch between a two wheel drive mode for streets and a four-wheel drive mode for low traction conditions such as ice, mud, or loose gravel. The "all-wheel drive" (AWD) term is a marketing term used to sell primarily on-road 4WD vehicles. However, in Australia, AWD is generally used for passenger vehicles that drive all four wheels all the time (e.g., a Subaru), whereas 4WD is used for vehicles designed primarily for heavy off-road use, normally with a low range transfer case (e.g., a Toyota Landcruiser). The terms are thus quite vague in modern usage.
It is common for identical drivetrain systems to be marketed under different names for upmarket and downmarket branding, and it is also common for very different drivetrain systems to be marketed under the same name for brand uniformity. For example, quattro, 4matic and 4motion can mean either an automatically engaging system with a Haldex clutch or a continuously operating system with a Torsen differential.
Design
When powering two wheels simultaneously, something must be done to allow the wheels to rotate at different speeds as the vehicle goes around curves. When driving all four wheels, the problem is much worse. A design that fails to account for this will cause the vehicle to handle poorly on turns, fighting the driver as the tires slip and skid from the mismatched speeds.
A differential allows one input shaft to drive two output shafts with different speeds. The differential distributes torque (angular force) evenly, while distributing angular velocity (turning speed) such that the average for the two output shafts is equal to that of the input shaft. Each powered axle requires a differential to distribute power between the left and right sides. If all four wheels are to be driven, a third differential can be used to distribute power between the front and rear axles.
Such a design would handle very well. It distributes power evenly and smoothly, making it very unlikely to start slipping. Once it does slip though, recovery will be difficult. Suppose that the left front wheel (of a design that drives all four wheels) slips. Because of the way a differential works, the slipping wheel will spin twice as fast as desired while the wheel on the other side stops moving. (the average speed remains unchanged, and neither wheel gets any torque) Since this example is a vehicle that drives all four wheels, a similar problem occurs between the front and rear axles via the center differential. The average speed between front and rear will not change, torque will be matched, torque goes to zero, speed at the rear goes to zero, and the speed at the front goes to double what it should be, making the left front wheel actually turn four times as fast as it should be turning. This problem can happen in both 2WD and 4WD vehicles, whenever a driven wheel is placed on a patch of slick ice or raised off the ground. The simplistic design works acceptably well for a 2WD vehicle. Since a 4WD is just as likely to have a driven wheel on an icy patch, the simplistic design is usually considered marginally acceptable.
Traction control was invented to solve this problem for 2WD vehicles. When one wheel spins out of control, the brake can be automatically applied to that wheel. The torque will then be matched, causing power to be divided between the pavement (for the non-slipping wheel) and the brake. This is effective, though it does cause brake wear and a sudden jolt that can make handling less predictable. By extending traction control to act on all four wheels, the simple 4WD vehicle design based on three differentials can now prevent wheel spin up. One nice feature of this design is that it does not work against traction control - it is traction control. This design is commonly seen on luxury crossover SUVs.
Another way to solve the problem is to temporarily lock together the differential's output shafts, usually just for the center differential that distributes power between front and rear. Recall that a drivetrain without differentials will fight the driver, causing tire wear and handling problems. This is of little concern when the wheels are already slipping. One very common design joins the output shafts together via a multi-plate clutch under computer control. This design causes a small jolt when it activates, which can disturb the driver or cause more wheels to lose traction. Another common design uses a viscous coupling unit. A dilatant fluid inside the viscous coupling unit acts like a solid when under shear stress caused by high shaft speed differences, causing the two shafts to become connected. This design suffers from fluid degradation with age and exponential locking (joining) behavior. It can also can waste fuel, because one possible optimization reduces latency via a small rotational difference (via gearing) to hasten torque transfer. Older designs used manually operated locking devices.
Yet another way to prevent this problem is via a Torsen differential. When a normal differential is replaced with a Torsen differential, it is possible to drive the output shafts with different amounts of torque. While this is useless in a zero-torque situation, it will help greatly when the traction difference is not so extreme. A typical Torsen II differential can deliver up to twice as much torque to the high traction side before traction is exceeded at the lower tractive side. Most Audi quattro cars, notably excluding the A3 and TT, use a center Torsen differential. For a time, the Volkswagen Passat 4motion shared this design. The HMMWV uses front and rear Torsen differentials, but only has a normal lockable differential in the center. Torsen differentials generally work very well, though they can be expensive and heavy.
Many lower-cost vehicles entirely eliminate the center differential. These vehicles behave as 2WD vehicles under normal conditions. When the drive wheels begin to slip, one of the locking mechanisms discussed above will join the front and rear axles. Such systems distribute power unevenly under normal conditions, and thus do not help prevent loss of traction; they only enable recovery once traction has been lost. Most minivan 4WD/AWD systems are of this type, usually with the front wheels powered during normal driving conditions and the rear wheels served via a viscous coupling unit. Such systems may be described as having a 95%/5% or 90%/10% power split. Light trucks and SUVs tend to use multi-plate clutches under computer control, often with 100% of the power going to the rear axle under normal conditions. Sports cars using this type of system usually drive only the rear under normal conditions. For example, Lamborghini uses a viscous coupling unit to drive the front, and the Nissan Skyline GT-R uses a clutch. The Audi TT normally powers the front, and has a multi-plate clutch to power the rear.
History
The first-ever four-wheel drive car (as well as hill-climb racer), the so-called Spyker 60 HP, was built in 1903 by Dutch brothers Jacobus and Hendrik-Jan Spijker of Amsterdam. Designs for four-wheel drive in the US, came from the Twyford company of Brookville, PA in 1905, six were made there around 1906; one still exists and is displayed annually[citation needed]. The second US four-wheel drive vehicle was built in 1911 by the Four-Wheel Drive auto company (FWD) of Wisconsin. FWD would later produce over 20,000 of its four-wheel drive Model B trucks for the British and American armies during World War I. It was not until "go-anywhere" vehicles were needed for the military that four-wheel drive found its place. The Jeep, originally developed by American Bantam but mass-produced by Willys and Ford, became the best-known four-wheel drive vehicle in the world during World War II. Willys (since 1950 owner of the Jeep name) introduced the CJ-2A in 1945 as the first full-production four-wheel drive passenger vehicle. Possibly beaten by the 1941 GAZ-61.
It was in 1948 that the vehicle whose name is synonymous with Four Wheel Drive in many countries was introduced. The Land Rover appeared at the Amsterdam Motor Show, originally conceived as a stop-gap product for the struggling Rover car company, and despite chronic under-investment succeeded far better than the passenger cars. Land Rover also had a luxury 4WD with the Range Rover in the 1970s, which unlike most subsequent offerings from other manufacturers, was capable of serious off-road use.
In 1963, Kaiser Jeep introduced a 4WD wagon called the Wagoneer. It was revolutionary at the time, not only because of its technical innovations such as an independent front suspension and the first automatic transmission with 4WD, but also because it was equipped and finished as a regular passenger automobile. The Super Wagoneer (1966 to 1969) was powered by Rambler or Buick V8s. Its high level of equipment made it the first "luxury" SUV. American Motors (AMC) acquired Kaiser's Jeep Division in 1970 and quickly upgraded and expanded the entire line of serious off-road built 4WD vehicles. The top range full-size Wagoneer Limited continued to compete with traditional luxury cars. It was relatively unchanged during its production, even after Chrysler's buyout of AMC, all the way through 1991.
Jensen applied the Formula Ferguson four-wheel drive system to their 1966 Jensen FF marking the first time 4WD was used in a production sports car. However, with a total of 320 build units this did not sell in appreciable numbers. The first manufacturer to develop four-wheel drive for road-going cars was Subaru, who introduced the mass-produced 4WD Leone in 1972. This model eventually became the best-selling 4WD car in the world. Subaru's success in marketing AWD vehicles has led to an AWD-only lineup in almost all of its markets outside of Japan. By 1998, Subaru discontinued all two-wheel drive vehicles in North America, where it remains the only brand to be exclusively AWD.
In 1980, AMC introduced the Eagle. This was the world's first complete line (sedan, coupe, and station wagon) of permanent automatic all-wheel drive passenger models. The new Eagles combined Jeep technology with an existing and proven AMC passenger car platform. They ushered a whole new product category of "sport-utility" or Crossover SUV. AMC's Eagles came with the comfort and high level appointments expected of regular passenger models and used the off-road technology for an extra margin of safety and traction. The Eagles were popular (particularly in the snowbelt) and innovative. During 1981 and 1982 a unique convertible was added to the line. The Eagle's monocoque body was reinforced for the conversion and had a steel targa bar and a removable fiberglass roof. The Eagle station wagon remained in production for one year after Chrysler bought AMC.
Audi also introduced a permanently all-wheel driven road-going car, the Audi Quattro, in 1980. Audi's chassis engineer, Jorg Bensinger, had noticed in winter tests in Scandinavia that a vehicle used by the German Army, the Volkswagen Iltis, could beat any high performance Audi. He proposed developing a four-wheel drive car, soon used for rallying to improve Audi's conservative image, the resulting rally bred Audi Quattro was a famous and historically significant Rally car. This feature was also extended to Audi's production cars and is still available.
Some of the earliest mid-engined four-wheel drive cars were the various road-legal rally cars made for Group B homologation, such as the Ford RS200 made from 1984-1986. In 1989 niche maker Panther Westwinds created a mid-engined four-wheel drive, the Panther Solo 2. Today, sophisticated all wheel drive systems are found in many passenger vehicles and most exotic sports cars and supercars.
4WD in road racing
Bugatti created a total of three four-wheel drive racers, the Type 53, in 1932, but the cars were legendary for having poor handling. Ferguson Research Ltd. built the front-engined P99 Formula One car that actually won a non-WC race with Stirling Moss in 1961. In 1969, Team Lotus raced cars in F1 and the Indy 500 that had both turbine-engines and 4WD, as well as the 4WD-Lotus 63 that had the standard Cosworth engine. Matra also raced a similar MS84, while Team McLaren tested its design only. All these F1 cars were considered inferior to their RWD counterparts and the idea was discontinued, even though Lotus tried repeatedly.
Terminology
Although in the strictest sense, the term "four-wheel drive" refers to a capability that a vehicle may have, it is also used to denote the entire vehicle itself. In Australia, vehicles without significant offroad capabilities are often referred to as All-Wheel Drives (AWD) or SUVs, while those with offroad capabilities are referred to as "four-wheel drives". This term is sometimes also used in North America, somewhat interchangeably for SUVs and pickup trucks and is sometimes erroneously applied to two-wheel-drive variants of these vehicles.
The term 4x4 (read either four by four or four times four) is used to denote the total number of wheels on a vehicle and the number of driven wheels; it is often applied to vehicles equipped with either full-time or part-time four-wheel-drive. The term 4x4 is common in North America and is generally used when marketing a new or used vehicle, and is sometimes applied as badging on a vehicle equipped with four-wheel drive. Similarly, a 4x2 would be appropriate for most two-wheel-drive vehicles, although this is rarely used in the USA in practice. In Australia the term is often used to describe a pickup truck that sits very high on its suspension. This is to avoid the confusion that the vehicle might be a 4x4 because it appears to be otherwise suited to off-road applications. A 2×4, however, is unambiguously a piece of lumber.
Large American trucks with dual tires on the rear axles (also called duallys or duallies) and two driven axles are officially badged as 4x4s, despite having six driven wheels because the 'dual' wheels behave as a single wheel for traction purposes and are not individually powered. True 6x6 vehicles with three powered axles such as the famous "Deuce and a Half" truck used by the U.S. Army has three axles (two rear, one front), all of them driven. This vehicle is a true 6x6, as is the Pinzgauer, which is popular with defense forces around the globe.
Another related term is 4-wheeler (or four-wheeler). This generally refers to all-terrain vehicles with four wheels and does not indicate the number of driven wheels; a "four wheeler" may have two or four-wheel drive.
In the UK, the derogatory nickname "Chelsea tractor"[1] is sometimes used to describe large privately owned four-wheel drive vehicles that rarely are used off-road. The term originally applies mostly to Range Rovers but may also be applied to any similar large four-wheel-drive vehicle or SUV.
Unusual four-wheel drive systems
Prompted by a perceived need for a simple, inexpensive all-terrain vehicle for oil exploration in North Africa, the French motor manufacturer Citroën developed the 2CV Sahara. Unlike other 4x4 vehicles which use a conventional transfer case to drive the front and rear axle, the Sahara had two engines, each independently driving a separate axle, with the rear engine facing backwards. The two throttles, clutches and gearchange mechanisms could be linked, so both 12 bhp 425 cc engines could run together, or they could be split and the car driven solely by either engine. Combined with twin fuel tanks and twin batteries (which could be set up to run either or both engines), the redundancy of two separate drive trains meant that they could make it back to civilization even after major mechanical failures. Only around 700 of these cars were built, and there are no clear records of how many still exist. Enthusiasts have built their own "new" Saharas, by rebuilding a 2CV and fitting the modified engine, gearbox and axle onto a new, strengthened chassis.
BMC experimented with a twin-engined Mini Moke in the mid-1960s, but never put it into production.
Suzuki Motors introduced the Suzuki Escudo Pikes Peak Edition in 1996. Though actually numbers were never released, this twin-engined vehicle is believed to weigh less than 2000 pounds and produce nearly 1000bhp. Each engine is a twin-turbo charged 2.0L V6 mated to a sequential 6-speed manual transmission.
Most recently, DaimlerChrysler's Jeep Division debuted the twin engine, 670 hp Jeep Hurricane concept at the 2005 North American International Auto Show in Detroit. This vehicle has a unique "crab crawl" capability, which allows it to rotate in 360 degrees in place. It also has dual Hemi V8s.
Four-wheel drive, 4WD, 4x4 ("four by four"), all-wheel drive, and AWD are terms used to describe a four-wheeled vehicle with a drivetrain that allows all four wheels to receive power from the engine simultaneously. While many people associate the term with off-road vehicles, powering all four wheels provides better control on slick ice and is an important part of rally racing on mostly-paved roads.
Four-wheel drive (4WD or 4x4 for short) is the original term and is often used to describe truck-like vehicles that require the driver to manually switch between a two wheel drive mode for streets and a four-wheel drive mode for low traction conditions such as ice, mud, or loose gravel. The "all-wheel drive" (AWD) term is a marketing term used to sell primarily on-road 4WD vehicles. However, in Australia, AWD is generally used for passenger vehicles that drive all four wheels all the time (e.g., a Subaru), whereas 4WD is used for vehicles designed primarily for heavy off-road use, normally with a low range transfer case (e.g., a Toyota Landcruiser). The terms are thus quite vague in modern usage.
It is common for identical drivetrain systems to be marketed under different names for upmarket and downmarket branding, and it is also common for very different drivetrain systems to be marketed under the same name for brand uniformity. For example, quattro, 4matic and 4motion can mean either an automatically engaging system with a Haldex clutch or a continuously operating system with a Torsen differential.
Design
When powering two wheels simultaneously, something must be done to allow the wheels to rotate at different speeds as the vehicle goes around curves. When driving all four wheels, the problem is much worse. A design that fails to account for this will cause the vehicle to handle poorly on turns, fighting the driver as the tires slip and skid from the mismatched speeds.
A differential allows one input shaft to drive two output shafts with different speeds. The differential distributes torque (angular force) evenly, while distributing angular velocity (turning speed) such that the average for the two output shafts is equal to that of the input shaft. Each powered axle requires a differential to distribute power between the left and right sides. If all four wheels are to be driven, a third differential can be used to distribute power between the front and rear axles.
Such a design would handle very well. It distributes power evenly and smoothly, making it very unlikely to start slipping. Once it does slip though, recovery will be difficult. Suppose that the left front wheel (of a design that drives all four wheels) slips. Because of the way a differential works, the slipping wheel will spin twice as fast as desired while the wheel on the other side stops moving. (the average speed remains unchanged, and neither wheel gets any torque) Since this example is a vehicle that drives all four wheels, a similar problem occurs between the front and rear axles via the center differential. The average speed between front and rear will not change, torque will be matched, torque goes to zero, speed at the rear goes to zero, and the speed at the front goes to double what it should be, making the left front wheel actually turn four times as fast as it should be turning. This problem can happen in both 2WD and 4WD vehicles, whenever a driven wheel is placed on a patch of slick ice or raised off the ground. The simplistic design works acceptably well for a 2WD vehicle. Since a 4WD is just as likely to have a driven wheel on an icy patch, the simplistic design is usually considered marginally acceptable.
Traction control was invented to solve this problem for 2WD vehicles. When one wheel spins out of control, the brake can be automatically applied to that wheel. The torque will then be matched, causing power to be divided between the pavement (for the non-slipping wheel) and the brake. This is effective, though it does cause brake wear and a sudden jolt that can make handling less predictable. By extending traction control to act on all four wheels, the simple 4WD vehicle design based on three differentials can now prevent wheel spin up. One nice feature of this design is that it does not work against traction control - it is traction control. This design is commonly seen on luxury crossover SUVs.
Another way to solve the problem is to temporarily lock together the differential's output shafts, usually just for the center differential that distributes power between front and rear. Recall that a drivetrain without differentials will fight the driver, causing tire wear and handling problems. This is of little concern when the wheels are already slipping. One very common design joins the output shafts together via a multi-plate clutch under computer control. This design causes a small jolt when it activates, which can disturb the driver or cause more wheels to lose traction. Another common design uses a viscous coupling unit. A dilatant fluid inside the viscous coupling unit acts like a solid when under shear stress caused by high shaft speed differences, causing the two shafts to become connected. This design suffers from fluid degradation with age and exponential locking (joining) behavior. It can also can waste fuel, because one possible optimization reduces latency via a small rotational difference (via gearing) to hasten torque transfer. Older designs used manually operated locking devices.
Yet another way to prevent this problem is via a Torsen differential. When a normal differential is replaced with a Torsen differential, it is possible to drive the output shafts with different amounts of torque. While this is useless in a zero-torque situation, it will help greatly when the traction difference is not so extreme. A typical Torsen II differential can deliver up to twice as much torque to the high traction side before traction is exceeded at the lower tractive side. Most Audi quattro cars, notably excluding the A3 and TT, use a center Torsen differential. For a time, the Volkswagen Passat 4motion shared this design. The HMMWV uses front and rear Torsen differentials, but only has a normal lockable differential in the center. Torsen differentials generally work very well, though they can be expensive and heavy.
Many lower-cost vehicles entirely eliminate the center differential. These vehicles behave as 2WD vehicles under normal conditions. When the drive wheels begin to slip, one of the locking mechanisms discussed above will join the front and rear axles. Such systems distribute power unevenly under normal conditions, and thus do not help prevent loss of traction; they only enable recovery once traction has been lost. Most minivan 4WD/AWD systems are of this type, usually with the front wheels powered during normal driving conditions and the rear wheels served via a viscous coupling unit. Such systems may be described as having a 95%/5% or 90%/10% power split. Light trucks and SUVs tend to use multi-plate clutches under computer control, often with 100% of the power going to the rear axle under normal conditions. Sports cars using this type of system usually drive only the rear under normal conditions. For example, Lamborghini uses a viscous coupling unit to drive the front, and the Nissan Skyline GT-R uses a clutch. The Audi TT normally powers the front, and has a multi-plate clutch to power the rear.
History
The first-ever four-wheel drive car (as well as hill-climb racer), the so-called Spyker 60 HP, was built in 1903 by Dutch brothers Jacobus and Hendrik-Jan Spijker of Amsterdam. Designs for four-wheel drive in the US, came from the Twyford company of Brookville, PA in 1905, six were made there around 1906; one still exists and is displayed annually[citation needed]. The second US four-wheel drive vehicle was built in 1911 by the Four-Wheel Drive auto company (FWD) of Wisconsin. FWD would later produce over 20,000 of its four-wheel drive Model B trucks for the British and American armies during World War I. It was not until "go-anywhere" vehicles were needed for the military that four-wheel drive found its place. The Jeep, originally developed by American Bantam but mass-produced by Willys and Ford, became the best-known four-wheel drive vehicle in the world during World War II. Willys (since 1950 owner of the Jeep name) introduced the CJ-2A in 1945 as the first full-production four-wheel drive passenger vehicle. Possibly beaten by the 1941 GAZ-61.
It was in 1948 that the vehicle whose name is synonymous with Four Wheel Drive in many countries was introduced. The Land Rover appeared at the Amsterdam Motor Show, originally conceived as a stop-gap product for the struggling Rover car company, and despite chronic under-investment succeeded far better than the passenger cars. Land Rover also had a luxury 4WD with the Range Rover in the 1970s, which unlike most subsequent offerings from other manufacturers, was capable of serious off-road use.
In 1963, Kaiser Jeep introduced a 4WD wagon called the Wagoneer. It was revolutionary at the time, not only because of its technical innovations such as an independent front suspension and the first automatic transmission with 4WD, but also because it was equipped and finished as a regular passenger automobile. The Super Wagoneer (1966 to 1969) was powered by Rambler or Buick V8s. Its high level of equipment made it the first "luxury" SUV. American Motors (AMC) acquired Kaiser's Jeep Division in 1970 and quickly upgraded and expanded the entire line of serious off-road built 4WD vehicles. The top range full-size Wagoneer Limited continued to compete with traditional luxury cars. It was relatively unchanged during its production, even after Chrysler's buyout of AMC, all the way through 1991.
Jensen applied the Formula Ferguson four-wheel drive system to their 1966 Jensen FF marking the first time 4WD was used in a production sports car. However, with a total of 320 build units this did not sell in appreciable numbers. The first manufacturer to develop four-wheel drive for road-going cars was Subaru, who introduced the mass-produced 4WD Leone in 1972. This model eventually became the best-selling 4WD car in the world. Subaru's success in marketing AWD vehicles has led to an AWD-only lineup in almost all of its markets outside of Japan. By 1998, Subaru discontinued all two-wheel drive vehicles in North America, where it remains the only brand to be exclusively AWD.
In 1980, AMC introduced the Eagle. This was the world's first complete line (sedan, coupe, and station wagon) of permanent automatic all-wheel drive passenger models. The new Eagles combined Jeep technology with an existing and proven AMC passenger car platform. They ushered a whole new product category of "sport-utility" or Crossover SUV. AMC's Eagles came with the comfort and high level appointments expected of regular passenger models and used the off-road technology for an extra margin of safety and traction. The Eagles were popular (particularly in the snowbelt) and innovative. During 1981 and 1982 a unique convertible was added to the line. The Eagle's monocoque body was reinforced for the conversion and had a steel targa bar and a removable fiberglass roof. The Eagle station wagon remained in production for one year after Chrysler bought AMC.
Audi also introduced a permanently all-wheel driven road-going car, the Audi Quattro, in 1980. Audi's chassis engineer, Jorg Bensinger, had noticed in winter tests in Scandinavia that a vehicle used by the German Army, the Volkswagen Iltis, could beat any high performance Audi. He proposed developing a four-wheel drive car, soon used for rallying to improve Audi's conservative image, the resulting rally bred Audi Quattro was a famous and historically significant Rally car. This feature was also extended to Audi's production cars and is still available.
Some of the earliest mid-engined four-wheel drive cars were the various road-legal rally cars made for Group B homologation, such as the Ford RS200 made from 1984-1986. In 1989 niche maker Panther Westwinds created a mid-engined four-wheel drive, the Panther Solo 2. Today, sophisticated all wheel drive systems are found in many passenger vehicles and most exotic sports cars and supercars.
4WD in road racing
Bugatti created a total of three four-wheel drive racers, the Type 53, in 1932, but the cars were legendary for having poor handling. Ferguson Research Ltd. built the front-engined P99 Formula One car that actually won a non-WC race with Stirling Moss in 1961. In 1969, Team Lotus raced cars in F1 and the Indy 500 that had both turbine-engines and 4WD, as well as the 4WD-Lotus 63 that had the standard Cosworth engine. Matra also raced a similar MS84, while Team McLaren tested its design only. All these F1 cars were considered inferior to their RWD counterparts and the idea was discontinued, even though Lotus tried repeatedly.
Terminology
Although in the strictest sense, the term "four-wheel drive" refers to a capability that a vehicle may have, it is also used to denote the entire vehicle itself. In Australia, vehicles without significant offroad capabilities are often referred to as All-Wheel Drives (AWD) or SUVs, while those with offroad capabilities are referred to as "four-wheel drives". This term is sometimes also used in North America, somewhat interchangeably for SUVs and pickup trucks and is sometimes erroneously applied to two-wheel-drive variants of these vehicles.
The term 4x4 (read either four by four or four times four) is used to denote the total number of wheels on a vehicle and the number of driven wheels; it is often applied to vehicles equipped with either full-time or part-time four-wheel-drive. The term 4x4 is common in North America and is generally used when marketing a new or used vehicle, and is sometimes applied as badging on a vehicle equipped with four-wheel drive. Similarly, a 4x2 would be appropriate for most two-wheel-drive vehicles, although this is rarely used in the USA in practice. In Australia the term is often used to describe a pickup truck that sits very high on its suspension. This is to avoid the confusion that the vehicle might be a 4x4 because it appears to be otherwise suited to off-road applications. A 2×4, however, is unambiguously a piece of lumber.
Large American trucks with dual tires on the rear axles (also called duallys or duallies) and two driven axles are officially badged as 4x4s, despite having six driven wheels because the 'dual' wheels behave as a single wheel for traction purposes and are not individually powered. True 6x6 vehicles with three powered axles such as the famous "Deuce and a Half" truck used by the U.S. Army has three axles (two rear, one front), all of them driven. This vehicle is a true 6x6, as is the Pinzgauer, which is popular with defense forces around the globe.
Another related term is 4-wheeler (or four-wheeler). This generally refers to all-terrain vehicles with four wheels and does not indicate the number of driven wheels; a "four wheeler" may have two or four-wheel drive.
In the UK, the derogatory nickname "Chelsea tractor"[1] is sometimes used to describe large privately owned four-wheel drive vehicles that rarely are used off-road. The term originally applies mostly to Range Rovers but may also be applied to any similar large four-wheel-drive vehicle or SUV.
Unusual four-wheel drive systems
Prompted by a perceived need for a simple, inexpensive all-terrain vehicle for oil exploration in North Africa, the French motor manufacturer Citroën developed the 2CV Sahara. Unlike other 4x4 vehicles which use a conventional transfer case to drive the front and rear axle, the Sahara had two engines, each independently driving a separate axle, with the rear engine facing backwards. The two throttles, clutches and gearchange mechanisms could be linked, so both 12 bhp 425 cc engines could run together, or they could be split and the car driven solely by either engine. Combined with twin fuel tanks and twin batteries (which could be set up to run either or both engines), the redundancy of two separate drive trains meant that they could make it back to civilization even after major mechanical failures. Only around 700 of these cars were built, and there are no clear records of how many still exist. Enthusiasts have built their own "new" Saharas, by rebuilding a 2CV and fitting the modified engine, gearbox and axle onto a new, strengthened chassis.
BMC experimented with a twin-engined Mini Moke in the mid-1960s, but never put it into production.
Suzuki Motors introduced the Suzuki Escudo Pikes Peak Edition in 1996. Though actually numbers were never released, this twin-engined vehicle is believed to weigh less than 2000 pounds and produce nearly 1000bhp. Each engine is a twin-turbo charged 2.0L V6 mated to a sequential 6-speed manual transmission.
Most recently, DaimlerChrysler's Jeep Division debuted the twin engine, 670 hp Jeep Hurricane concept at the 2005 North American International Auto Show in Detroit. This vehicle has a unique "crab crawl" capability, which allows it to rotate in 360 degrees in place. It also has dual Hemi V8s.
REAR WHEEL DRIVE
Rear-wheel drive (or RWD for short) was a common engine/transmission layout used in automobiles throughout the 20th century. RWD typically places the engine in the front of the vehicle, but the mid engine and rear engine layouts are also used.
The vast majority of rear wheel drive vehicles use a longitudinally-mounted engine in the front of the vehicle, driving the rear wheels via a driveshaft linked via a differential between the rear axles. Some FR layout vehicles place the transmission at the rear, though most attach it to the engine at the front.
Rear wheel drive has fallen out of favor in passenger cars since the 1980s, due in part to higher manufacturing costs, and a perception by many car buyers that front wheel drive is safer, and that it performs better on slippery roads. However, many prestige automobile brands, including Mercedes-Benz, BMW and Porsche continue to use rear wheel drive platforms.
It still sees heavy use in taxi and police fleets, due to cheaper maintenance, and in the case of police fleets, better performance.
Advantages
Better handling in dry conditions - accelerating force is applied to the rear wheels, on which the down force increases, due to load transfer in acceleration, making the rear tires better able to take simultaneous acceleration and curving than the front tires.
Less costly and easier maintenance - Rear wheel drive is mechanically simpler and typically does not involve packing as many parts into as small a space as does front wheel drive, thus requiring less disassembly or specialized tools in order to replace parts.
No torque steer.
Even weight distribution - The division of weight between the front and rear wheels has a significant impact on a car's handling, and it is much easier to get a 50/50 weight distribution in a rear wheel drive car than in a front wheel drive car, as more of the engine can lie between the front and rear wheels (in the case of a mid engine layout, the entire engine), and the transmission is moved much farther back.
Steering radius - As no complicated drive shaft joints are required at the front wheels, it is possible to turn them further than would be possible using front wheel drive, resulting in a smaller steering radius.
Towing - Rear wheel drive puts the wheels which are pulling the load closer to the point where a trailer articulates, helping steering, especially for large loads.
Weight transfer during acceleration. (During heavy acceleration, the front end rises, and more weight is placed on the rear, or driving wheels).
Drifting is much easier in a rear-wheel drive automobile, as power can be applied to keep the drift going--more power aids the drift. Front wheel drive and four wheel drive cars may also drift, but only with much more difficulty. When front wheel drive cars drift, the driver usually pulls on the emergency brake in order for the back wheels to stop and thus skid.
Disadvantages
More difficult to master - While the handling characteristics of rear-wheel drive may be useful or fun in the hands of some drivers, for others, having the rear wheels move about is unintuitive and dangerous. Rear wheel drive rewards skill, and punishes the lack of it. Other layouts are much more forgiving, but don't offer the same rewards in handling.
Decreased interior space - In a passenger car, rear wheel drive means: Less front leg room (the transmission tunnel takes up a lot of space between the driver and front passenger), less leg room for center rear passengers (due to the tunnel needed for the drive shaft), and sometimes less trunk space (since there is also more hardware that must be placed underneath the trunk). There are some exceptions to this as some luxury vehicles have plenty of room and do not suffer a decreaced interior and rear engine designs do not take away interior space. (See Porsche 911, Volkswagen Beatle, De Lorean DMC-12, Chevrolet Corvair, Tucker Automobile and Alpine (car) for rear engine examples.)
Increased weight - The components of your typical rear wheel drive vehicle's power train may be less complex, but there are more of them. The driveshaft adds weight. The transmission is probably heavier. There is extra sheet metal to form the transmission tunnel. There is a rear axle or rear half-shafts. A rear wheel drive car will weigh slightly more than a comparable front wheel drive car (but less than four wheel drive).
Higher purchase price - Probably due to more complicated assembly (the powertrain is not one compact unit) and added cost of materials, rear wheel drive is typically slightly more expensive to purchase than a comparable front wheel drive vehicle. This might also be explained by production volumes, however.
Oversteer and the related problem of fishtailing.
Depending on the automobile model, traction can be less than that of front-wheel drive; however, heavy cargo can be beneficial for traction.
Rear-wheel drive (or RWD for short) was a common engine/transmission layout used in automobiles throughout the 20th century. RWD typically places the engine in the front of the vehicle, but the mid engine and rear engine layouts are also used.
The vast majority of rear wheel drive vehicles use a longitudinally-mounted engine in the front of the vehicle, driving the rear wheels via a driveshaft linked via a differential between the rear axles. Some FR layout vehicles place the transmission at the rear, though most attach it to the engine at the front.
Rear wheel drive has fallen out of favor in passenger cars since the 1980s, due in part to higher manufacturing costs, and a perception by many car buyers that front wheel drive is safer, and that it performs better on slippery roads. However, many prestige automobile brands, including Mercedes-Benz, BMW and Porsche continue to use rear wheel drive platforms.
It still sees heavy use in taxi and police fleets, due to cheaper maintenance, and in the case of police fleets, better performance.
Advantages
Better handling in dry conditions - accelerating force is applied to the rear wheels, on which the down force increases, due to load transfer in acceleration, making the rear tires better able to take simultaneous acceleration and curving than the front tires.
Less costly and easier maintenance - Rear wheel drive is mechanically simpler and typically does not involve packing as many parts into as small a space as does front wheel drive, thus requiring less disassembly or specialized tools in order to replace parts.
No torque steer.
Even weight distribution - The division of weight between the front and rear wheels has a significant impact on a car's handling, and it is much easier to get a 50/50 weight distribution in a rear wheel drive car than in a front wheel drive car, as more of the engine can lie between the front and rear wheels (in the case of a mid engine layout, the entire engine), and the transmission is moved much farther back.
Steering radius - As no complicated drive shaft joints are required at the front wheels, it is possible to turn them further than would be possible using front wheel drive, resulting in a smaller steering radius.
Towing - Rear wheel drive puts the wheels which are pulling the load closer to the point where a trailer articulates, helping steering, especially for large loads.
Weight transfer during acceleration. (During heavy acceleration, the front end rises, and more weight is placed on the rear, or driving wheels).
Drifting is much easier in a rear-wheel drive automobile, as power can be applied to keep the drift going--more power aids the drift. Front wheel drive and four wheel drive cars may also drift, but only with much more difficulty. When front wheel drive cars drift, the driver usually pulls on the emergency brake in order for the back wheels to stop and thus skid.
Disadvantages
More difficult to master - While the handling characteristics of rear-wheel drive may be useful or fun in the hands of some drivers, for others, having the rear wheels move about is unintuitive and dangerous. Rear wheel drive rewards skill, and punishes the lack of it. Other layouts are much more forgiving, but don't offer the same rewards in handling.
Decreased interior space - In a passenger car, rear wheel drive means: Less front leg room (the transmission tunnel takes up a lot of space between the driver and front passenger), less leg room for center rear passengers (due to the tunnel needed for the drive shaft), and sometimes less trunk space (since there is also more hardware that must be placed underneath the trunk). There are some exceptions to this as some luxury vehicles have plenty of room and do not suffer a decreaced interior and rear engine designs do not take away interior space. (See Porsche 911, Volkswagen Beatle, De Lorean DMC-12, Chevrolet Corvair, Tucker Automobile and Alpine (car) for rear engine examples.)
Increased weight - The components of your typical rear wheel drive vehicle's power train may be less complex, but there are more of them. The driveshaft adds weight. The transmission is probably heavier. There is extra sheet metal to form the transmission tunnel. There is a rear axle or rear half-shafts. A rear wheel drive car will weigh slightly more than a comparable front wheel drive car (but less than four wheel drive).
Higher purchase price - Probably due to more complicated assembly (the powertrain is not one compact unit) and added cost of materials, rear wheel drive is typically slightly more expensive to purchase than a comparable front wheel drive vehicle. This might also be explained by production volumes, however.
Oversteer and the related problem of fishtailing.
Depending on the automobile model, traction can be less than that of front-wheel drive; however, heavy cargo can be beneficial for traction.
Current or recent rear wheel drive cars to 2006
While the popularity of rear wheel drive has declined, it is still relatively prevalent, and has been making something of a resurgence. Here is list of current or recent rear wheel drive vehicles. See also Category:Rear wheel drive vehicles.
Almost all non-4WD trucks and most SUVs are rear wheel drive.
The overwhelming majority of sports cars are rear wheel drive, although some also have four wheel drive options. It would be redundant to list them all here.
BMW - All cars except the MINI, and all wheel drive variants.
Cadillac - CTS, SRX, STS, Catera
Chevrolet - Camaro, Corvette
Dodge Viper, Dodge Charger/Dodge Magnum/Chrysler 300
Ferrari
Ford - Crown Victoria, Falcon (Australia), Mustang, Scorpio, Thunderbird, Capri
Holden - Holden Monaro (Australia) and versions sold overseas: Pontiac GTO (USA) and Vauxhall Monaro (United Kingdom), Holden Commodore (Australia)
Honda - S2000, NSX (Acura NSX in North America)
Infiniti - G35,M45,M35,Q45,M30,J30
Jaguar - All except the X-Type
Lexus - IS, GS, LS, SC
Lincoln - Town Car, Mark VIII and LS
Lotus - All except Elan-M100
Maserati - All models
Mazda - MX-5 Miata, RX-7, RX-8
Mercedes-Benz - All cars except A-Class, B-Class and all wheel drive models
Mercury - Grand Marquis, Marauder
Nissan - 350Z, Z Cars, Skyline (except 4WD models), Silvia (200SX in Europe/Oceania, 240SX in USA), 180SX
Pontiac - Firebird, GTO, Solstice
Porsche - All cars except the AWD Carrera 4, 911 Turbo and Cayenne
Saturn Sky
Tofaş - Kartal Şahin Doğan Serçe
Toyota - Supra, MR2, Altezza (Lexus IS), Crown, Mark X, Toyota Corolla GTS 1984-1987 USA Models AE86
TVR - All models
Vauxhall Carlton/Opel Omega - All models
While the popularity of rear wheel drive has declined, it is still relatively prevalent, and has been making something of a resurgence. Here is list of current or recent rear wheel drive vehicles. See also Category:Rear wheel drive vehicles.
Almost all non-4WD trucks and most SUVs are rear wheel drive.
The overwhelming majority of sports cars are rear wheel drive, although some also have four wheel drive options. It would be redundant to list them all here.
BMW - All cars except the MINI, and all wheel drive variants.
Cadillac - CTS, SRX, STS, Catera
Chevrolet - Camaro, Corvette
Dodge Viper, Dodge Charger/Dodge Magnum/Chrysler 300
Ferrari
Ford - Crown Victoria, Falcon (Australia), Mustang, Scorpio, Thunderbird, Capri
Holden - Holden Monaro (Australia) and versions sold overseas: Pontiac GTO (USA) and Vauxhall Monaro (United Kingdom), Holden Commodore (Australia)
Honda - S2000, NSX (Acura NSX in North America)
Infiniti - G35,M45,M35,Q45,M30,J30
Jaguar - All except the X-Type
Lexus - IS, GS, LS, SC
Lincoln - Town Car, Mark VIII and LS
Lotus - All except Elan-M100
Maserati - All models
Mazda - MX-5 Miata, RX-7, RX-8
Mercedes-Benz - All cars except A-Class, B-Class and all wheel drive models
Mercury - Grand Marquis, Marauder
Nissan - 350Z, Z Cars, Skyline (except 4WD models), Silvia (200SX in Europe/Oceania, 240SX in USA), 180SX
Pontiac - Firebird, GTO, Solstice
Porsche - All cars except the AWD Carrera 4, 911 Turbo and Cayenne
Saturn Sky
Tofaş - Kartal Şahin Doğan Serçe
Toyota - Supra, MR2, Altezza (Lexus IS), Crown, Mark X, Toyota Corolla GTS 1984-1987 USA Models AE86
TVR - All models
Vauxhall Carlton/Opel Omega - All models
LIMITED SLIP DIFFERNTIAL
In automotive applications, a limited slip differential (LSD) is a modified or derived type of differential gear arrangement that allows for some difference in rotational velocity of the output shafts, but does not allow the difference in speed to increase beyond a preset amount. In an automobile, such limited slip differentials are sometimes used in place of a standard differential, where they convey certain dynamic advantages, at the expense of greater complexity.
The main advantage of a limited slip differential is found by considering the case of a standard (or "open") differential where one wheel has no contact with the ground at all. In such a case, the contacting wheel will remain stationary, and the non-contacting wheel will rotate at twice its intended velocity – the torque transmitted will be zero and the vehicle will remain stationary. In everyday use on typical roads, such a situation is very unlikely, and so a normal differential suffices. For more demanding use however, such as driving off-road, or for high performance vehicles, such a state of affairs is undesirable, and the LSD can be employed to deal with it. By limiting the velocity difference between a pair of driven wheels, useful torque can be transmitted as long as there is some friction available on at least one of the wheels.
TypesTwo main types of LSD are commonly used on passenger cars – torque sensitive (geared or clutch-based) and speed sensitive (viscous/pump and clutch pack). The latter is gaining ground especially in modern all-wheel drive vehicles, and generally requires less maintenance than the mechanical type.
Mechanical
The use of the word mechanical implies that the limited slip differential is engaged by interaction between two (or more) mechanical parts. This category includes clutch and helical limited slip differentials. For road racing, many prefer a helical limited slip differential, because it does not lock the two output shafts to spin at the same rate, but rather biases torque to the wheel with more grip by up to 80%.
Clutch Type - Driveshaft Torque Activated
Characteristics
The clutch type LSD responds to driveshaft torque. The more driveshaft input torque present, the harder the clutches are pressed together, and thus the more closely the drive wheels are coupled to each other.
With no / little input torque (trailing throttle / gearbox in neutral / main clutch depressed) the drive wheels are still coupled somewhat as the clutches are always in contact to some degree, producing friction. The amount of preload (hence static coupling) on the clutches is determined by the general condition (wear) of the clutches and by how tightly they are shimmed.
Broadly speaking, there are three input torque states - load, no load, & over run. Under load, as previously stated, the coupling is proportional to the input torque. With no load, the coupling is reduced to the static coupling. The behaviour on over run (particularly sudden throttle release) determines whether the LSD is 1 way, 1.5 way, or 2 way.
If there is no additional coupling on over run, the LSD is 1 way. This is a safer LSD, as soon as the driver lifts the throttle, the LSD unlocks and behaves somewhat like a conventional open diff. This is also the best for FWD cars, as it allows the car to turn in on throttle release, instead of ploughing forward. [1]
If the LSD increases coupling in the same way regardless of whether the input torque is forwards or reverse, it is a 2 way diff. Some drifters prefer this type as the LSD behaves the same regardless of their erratic throttle input, and lets them keep the wheels spinning all the way through a corner. An inexperienced driver can easily spin the car when using a 2 way LSD if they lift the throttle suddenly, expecting the car to settle like a conventional open diff.
If the LSD behaves somewhere in between these two extremes, it is a 1.5 way diff, which is a compromise between sportiness and safety. Generally a 1.5 way creates a stronger lock under acceleration than deceleration.
Clutch LSDs are noisy, clunky and expensive, which makes them unlikely to be installed by the factory on a passenger car these days. However their response speed and coupling strength is the best of the commonly available LSDs. They are also the only commonly available LSD able to stand up to extreme motorsport abuse.
Mechanism
The clutch type has a stack of thin clutch discs, half of which are coupled to one of the drive shafts, the other half of which are coupled to the spider gear carrier. The clutch stacks may be present on both drive shafts, or on only one. If on only one, the remaining drive shaft is linked to the clutched drive shaft through the spider gears. If the clutched drive shaft cannot move relative to the spider carrier, then the other drive shaft also cannot move, thus they are locked.
The spider gears mount on the pinion cross shaft which rests in angled cutouts forming cammed ramps. The cammed ramps are not necessarily symmetrical. If the ramps are symmetrical, the LSD is 2 way. If they are saw toothed (i.e. one side of the ramp is vertical), the LSD is 1 way. If both sides are sloped, but are assymmetric, the LSD is 1.5 way.
As the input torque of the driveshaft tries to turn the diff centre, internal pressure rings (adjoining the clutch stack) are forced sideways by the pinion cross shaft trying to climb the ramp, which compresses the clutch stack. The more the clutch stack is compressed, the more coupled the wheels are. The mating of the vertical ramp (80o-85o in practise to avoid chipping) surfaces in a 1 way LSD on over run produces no cam effect and no corresponding clutch stack compression.
Servicing
The break-in of clutch LSDs is quite critical. Manufacturers give detailed instructions on how to break the diff in. [2] If these are not followed, the LSD may be permanently harmed, in that it may engage and disengage erratically, due to irregularities on and damage to the clutch surfaces. Essentially the LSD must be worked hard to remove manufacturing imperfections, then the now metal-laden oil changed immediately following.
Servicing consists of changing the oil after hard sessions, also to remove metal particles, and eventually replacing the clutches, or the centre. In any case the oil should be changed regularly (as opposed to the open diff, where the oil could be left unchanged for several hundred thousand kilometres).
Torsen Diff
Geared, torque-sensitive mechanical limited slip differentials utilize worm gears to "sense" torque on one shaft. The most famous version is the Torsen differential invented by Vernon Gleasman in 1958, then sold to Gleason Corporation, who started marketing it in 1982. Geared LSDs are less prone to wear than the clutch type, but some have found their torque distribution characteristics to be less than ideal.
Speed
Viscous
The viscous type is generally simpler, and relies on the properties of a dilatant fluid - that is, one which thickens when subject to shear. Silicone-based oils are often used. Here, a chamber (a sort of cylindrical toroid) of fluid filled with a stack of perforated discs rotates with the normal motion of the output shafts. The inside surface of the chamber is coupled to one of the driveshafts, and the outside coupled to the diff carrier. Half of the discs are connected to the inner, the other half to the outer, they alternate inner/outer in the stack. Differential motion forces the interlocked (though untouching) discs to move through the fluid against each other. The greater the relative speed of the discs, the more resistance the fluid will put up to oppose this motion. In contrast to the mechanical type, the limiting action is much softer and more proportional to the slip, so for the average driver is easier to cope with.
Viscous LSDs are less efficient than mechanical types, that is, they "lose" some power. They do not stand up well to abuse, particularly any sustained load which overheats the silicone results in sudden permanent loss of the LSD effect. They do have the virtue of failing gracefully, reverting to semi-open diff behaviour, without the graunching of metal particles / fragmented clutches. Typically a visco-differential that has covered 60,000 miles or more will be functioning largely as an open differential; this is a known weakness of the original Eunos Roadster sports car. The silicone oil is factory sealed in a separate chamber from the gear oil surrounding the rest of the diff. This is not serviceable and when the diff's behaviour deteriorates, the VLSD centre is replaced.
Gerotor Pump
This works by hydraulically compressing a clutch pack. The gerotor pump uses the housing to drive the outer side of the pump and one axle shaft to drive the other. When there is differential wheel rotation, the pump pressurizes its working fluid into the clutch pack area. This provides a clamp load for frictional resistance to transfer torque to the higher traction wheel. The pump based systems have a lower and upper limits on applied pressure, and internal damping to avoid hysteresis. The newest gerotor pump bases system has computer regulated output for more versatility and no oscillation
Other Related Final Drives
Spool
A spool limits differential rotation to exactly zero. A spool consists of a pinion & ring gear only, the centre is solid, the axle is one piece. A mini-spool is similar, replacing the usual diff centre with a solid piece, retaining the factory axles. Technically a spool is not a differential at all, but is used to achieve a similar effect to an LSD on some street & race cars.
Detroit Locker/Lokka(tm)
A locker locks both wheels under normal conditions i.e. the default state is locked. If a wheel is externally forced to rotate faster than the diff centre (the outer wheel in a corner) the mechanism unlocks that wheel and allows it to turn freely. Thus the locker has the extremely unusual characteristic of applying drive torque through the inner wheel in corners. If one wheel starts to spin (therefore rotational force is being applied internally) the pinion cross shaft locks the centre more firmly. Often used in off-road 4WD applications. Can be very noisy. The traditional American racing diff is a Detroit Locker. Difficult to control under power in corners as the two actions of the mechanism are contradictory, the car will unpredictably alternate between one wheel and two wheel drive.
Air/Hydraulic Locker
Normally an open diff, can be locked by the driver. Compressed air or hydraulic fluid activates the locking mechanism. Generally used by street cars that also drag race, the car drives to the event open, and locks the diff on the strip.
Factory Names
In the 1950's and 1960's many manufacturers began to apply brand names to their LSD units. The most famous of these was Chevrolet's "Positraction". Since then, Positraction (often shortened to "positrac" or merely "posi") has become a genericized trademark for LSDs.
Other factory names for LSD's include
Pontiac: Safe-T-Track
Ford: Equa-Lock and Trac-Lok
American Motors Corporation: Twin-Grip
Mopar: Sure Grip
Ferrari: E-Diff
Fiat: Viscodrive
In automotive applications, a limited slip differential (LSD) is a modified or derived type of differential gear arrangement that allows for some difference in rotational velocity of the output shafts, but does not allow the difference in speed to increase beyond a preset amount. In an automobile, such limited slip differentials are sometimes used in place of a standard differential, where they convey certain dynamic advantages, at the expense of greater complexity.
The main advantage of a limited slip differential is found by considering the case of a standard (or "open") differential where one wheel has no contact with the ground at all. In such a case, the contacting wheel will remain stationary, and the non-contacting wheel will rotate at twice its intended velocity – the torque transmitted will be zero and the vehicle will remain stationary. In everyday use on typical roads, such a situation is very unlikely, and so a normal differential suffices. For more demanding use however, such as driving off-road, or for high performance vehicles, such a state of affairs is undesirable, and the LSD can be employed to deal with it. By limiting the velocity difference between a pair of driven wheels, useful torque can be transmitted as long as there is some friction available on at least one of the wheels.
TypesTwo main types of LSD are commonly used on passenger cars – torque sensitive (geared or clutch-based) and speed sensitive (viscous/pump and clutch pack). The latter is gaining ground especially in modern all-wheel drive vehicles, and generally requires less maintenance than the mechanical type.
Mechanical
The use of the word mechanical implies that the limited slip differential is engaged by interaction between two (or more) mechanical parts. This category includes clutch and helical limited slip differentials. For road racing, many prefer a helical limited slip differential, because it does not lock the two output shafts to spin at the same rate, but rather biases torque to the wheel with more grip by up to 80%.
Clutch Type - Driveshaft Torque Activated
Characteristics
The clutch type LSD responds to driveshaft torque. The more driveshaft input torque present, the harder the clutches are pressed together, and thus the more closely the drive wheels are coupled to each other.
With no / little input torque (trailing throttle / gearbox in neutral / main clutch depressed) the drive wheels are still coupled somewhat as the clutches are always in contact to some degree, producing friction. The amount of preload (hence static coupling) on the clutches is determined by the general condition (wear) of the clutches and by how tightly they are shimmed.
Broadly speaking, there are three input torque states - load, no load, & over run. Under load, as previously stated, the coupling is proportional to the input torque. With no load, the coupling is reduced to the static coupling. The behaviour on over run (particularly sudden throttle release) determines whether the LSD is 1 way, 1.5 way, or 2 way.
If there is no additional coupling on over run, the LSD is 1 way. This is a safer LSD, as soon as the driver lifts the throttle, the LSD unlocks and behaves somewhat like a conventional open diff. This is also the best for FWD cars, as it allows the car to turn in on throttle release, instead of ploughing forward. [1]
If the LSD increases coupling in the same way regardless of whether the input torque is forwards or reverse, it is a 2 way diff. Some drifters prefer this type as the LSD behaves the same regardless of their erratic throttle input, and lets them keep the wheels spinning all the way through a corner. An inexperienced driver can easily spin the car when using a 2 way LSD if they lift the throttle suddenly, expecting the car to settle like a conventional open diff.
If the LSD behaves somewhere in between these two extremes, it is a 1.5 way diff, which is a compromise between sportiness and safety. Generally a 1.5 way creates a stronger lock under acceleration than deceleration.
Clutch LSDs are noisy, clunky and expensive, which makes them unlikely to be installed by the factory on a passenger car these days. However their response speed and coupling strength is the best of the commonly available LSDs. They are also the only commonly available LSD able to stand up to extreme motorsport abuse.
Mechanism
The clutch type has a stack of thin clutch discs, half of which are coupled to one of the drive shafts, the other half of which are coupled to the spider gear carrier. The clutch stacks may be present on both drive shafts, or on only one. If on only one, the remaining drive shaft is linked to the clutched drive shaft through the spider gears. If the clutched drive shaft cannot move relative to the spider carrier, then the other drive shaft also cannot move, thus they are locked.
The spider gears mount on the pinion cross shaft which rests in angled cutouts forming cammed ramps. The cammed ramps are not necessarily symmetrical. If the ramps are symmetrical, the LSD is 2 way. If they are saw toothed (i.e. one side of the ramp is vertical), the LSD is 1 way. If both sides are sloped, but are assymmetric, the LSD is 1.5 way.
As the input torque of the driveshaft tries to turn the diff centre, internal pressure rings (adjoining the clutch stack) are forced sideways by the pinion cross shaft trying to climb the ramp, which compresses the clutch stack. The more the clutch stack is compressed, the more coupled the wheels are. The mating of the vertical ramp (80o-85o in practise to avoid chipping) surfaces in a 1 way LSD on over run produces no cam effect and no corresponding clutch stack compression.
Servicing
The break-in of clutch LSDs is quite critical. Manufacturers give detailed instructions on how to break the diff in. [2] If these are not followed, the LSD may be permanently harmed, in that it may engage and disengage erratically, due to irregularities on and damage to the clutch surfaces. Essentially the LSD must be worked hard to remove manufacturing imperfections, then the now metal-laden oil changed immediately following.
Servicing consists of changing the oil after hard sessions, also to remove metal particles, and eventually replacing the clutches, or the centre. In any case the oil should be changed regularly (as opposed to the open diff, where the oil could be left unchanged for several hundred thousand kilometres).
Torsen Diff
Geared, torque-sensitive mechanical limited slip differentials utilize worm gears to "sense" torque on one shaft. The most famous version is the Torsen differential invented by Vernon Gleasman in 1958, then sold to Gleason Corporation, who started marketing it in 1982. Geared LSDs are less prone to wear than the clutch type, but some have found their torque distribution characteristics to be less than ideal.
Speed
Viscous
The viscous type is generally simpler, and relies on the properties of a dilatant fluid - that is, one which thickens when subject to shear. Silicone-based oils are often used. Here, a chamber (a sort of cylindrical toroid) of fluid filled with a stack of perforated discs rotates with the normal motion of the output shafts. The inside surface of the chamber is coupled to one of the driveshafts, and the outside coupled to the diff carrier. Half of the discs are connected to the inner, the other half to the outer, they alternate inner/outer in the stack. Differential motion forces the interlocked (though untouching) discs to move through the fluid against each other. The greater the relative speed of the discs, the more resistance the fluid will put up to oppose this motion. In contrast to the mechanical type, the limiting action is much softer and more proportional to the slip, so for the average driver is easier to cope with.
Viscous LSDs are less efficient than mechanical types, that is, they "lose" some power. They do not stand up well to abuse, particularly any sustained load which overheats the silicone results in sudden permanent loss of the LSD effect. They do have the virtue of failing gracefully, reverting to semi-open diff behaviour, without the graunching of metal particles / fragmented clutches. Typically a visco-differential that has covered 60,000 miles or more will be functioning largely as an open differential; this is a known weakness of the original Eunos Roadster sports car. The silicone oil is factory sealed in a separate chamber from the gear oil surrounding the rest of the diff. This is not serviceable and when the diff's behaviour deteriorates, the VLSD centre is replaced.
Gerotor Pump
This works by hydraulically compressing a clutch pack. The gerotor pump uses the housing to drive the outer side of the pump and one axle shaft to drive the other. When there is differential wheel rotation, the pump pressurizes its working fluid into the clutch pack area. This provides a clamp load for frictional resistance to transfer torque to the higher traction wheel. The pump based systems have a lower and upper limits on applied pressure, and internal damping to avoid hysteresis. The newest gerotor pump bases system has computer regulated output for more versatility and no oscillation
Other Related Final Drives
Spool
A spool limits differential rotation to exactly zero. A spool consists of a pinion & ring gear only, the centre is solid, the axle is one piece. A mini-spool is similar, replacing the usual diff centre with a solid piece, retaining the factory axles. Technically a spool is not a differential at all, but is used to achieve a similar effect to an LSD on some street & race cars.
Detroit Locker/Lokka(tm)
A locker locks both wheels under normal conditions i.e. the default state is locked. If a wheel is externally forced to rotate faster than the diff centre (the outer wheel in a corner) the mechanism unlocks that wheel and allows it to turn freely. Thus the locker has the extremely unusual characteristic of applying drive torque through the inner wheel in corners. If one wheel starts to spin (therefore rotational force is being applied internally) the pinion cross shaft locks the centre more firmly. Often used in off-road 4WD applications. Can be very noisy. The traditional American racing diff is a Detroit Locker. Difficult to control under power in corners as the two actions of the mechanism are contradictory, the car will unpredictably alternate between one wheel and two wheel drive.
Air/Hydraulic Locker
Normally an open diff, can be locked by the driver. Compressed air or hydraulic fluid activates the locking mechanism. Generally used by street cars that also drag race, the car drives to the event open, and locks the diff on the strip.
Factory Names
In the 1950's and 1960's many manufacturers began to apply brand names to their LSD units. The most famous of these was Chevrolet's "Positraction". Since then, Positraction (often shortened to "positrac" or merely "posi") has become a genericized trademark for LSDs.
Other factory names for LSD's include
Pontiac: Safe-T-Track
Ford: Equa-Lock and Trac-Lok
American Motors Corporation: Twin-Grip
Mopar: Sure Grip
Ferrari: E-Diff
Fiat: Viscodrive
Van's are usually FR Front Engined and Rear Drive layout, because they carry loads, u can see the differences underneat the vehicle .. FR's usually have long drive shaft connected to rear axle
hardly..sakuraguy said:
mostly only van like the Transit has FR layout. most round round looking van like the older toyota liteace usually MR, well, at least the engine is behind the front wheel and below the driver's seat.
Night Fury
Junior Member
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