Front-wheel drive

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Front-wheel drive is the most common form of engine/transmission layout used in modern passenger cars, where the engine drives the front wheels. Most front-wheel drive vehicles today feature transverse engine mounting, where as in past decades engines were mostly positioned longitudinally instead. Rear-wheel drive was the traditional standard and is still widely used in luxury cars and most sport cars. Four-wheel drive is also sometimes used. See also FF layout.

1 History
2 Records
3 Advantages of front-wheel drive
4 Disadvantages of front-wheel drive
5 See also
6 References
7 External links

[edit] History

Experiments with front-wheel drive cars date to the early days of the automobile. Following the Spanish-American War, J. Walter Christie of the USA was working on designs for a front-wheel drive car, which he promoted and demonstrated by racing at various speedways in the US, and even competed in the Vanderbilt Cup and the French Grand Prix. In 1912 he began manufacturing a line of wheeled fire engine tractors which utilized his front-wheel drive system, but due to lack of sales this venture failed. Alvis Cars of the United Kingdom introduced a front-wheel drive model in 1928, but it was not a success. Neither was the big Cord L-29 of 1929 in the United States.

The first successful models were the 1931 DKW F1 in Germany, the 1934 Traction Avant cars from Citroën in France, the British BSA Three-wheelers and Cars made in Birmingham from 1930 to the outbreak of World War 2 in 1939, and the contemporary 810 from Cord Automobile in the United States. These vehicles featured a mid-engine design — the engine was behind the transmission and ran "backwards", save for the Cord, which drove the transmission from the front of the engine. This layout allows sharp turning circles, but can also make the car quite long. The weight distribution from this layout creates positive handling characteristics due to its low polar inertia and relatively favourable weight distribution (the heaviest component is near the centre of the car, making the main component of its moment of inertia relatively low).

Front-wheel drive was further popularized by the 1948 Citroën 2CV^{[citation needed]}, where the lightweight aluminium flat twin engine was mounted ahead of the front wheels, and 1955 Citroën DS, featuring the mid-engine layout. Also, Panhard in France, DKW in Germany and Saab in Sweden offered only front-wheel drive cars.

The transversely mounted engine combined with front-wheel drive was popularized by the 1959 Mini^{[citation needed]}; there the transmission was built into the sump of the engine, and drive was transferred to it via a set of primary gears. The same transmission concept was also used by Peugeot and Renault on their jointly-developed small car engine of the 1970s where the 4-cylinder block was canted over to reduce the overall height of the engine with the transmission underneath. The tendency of this layout to generate unwanted transmission "whine" has seen it fall out of favour.

The 1965 Triumph 1300 was designed around a longitudinal engine with the transmission underneath. Audi also used a longitudinally mounted engine overhung over the front wheels in its 1970s designs. Also in the 1970s and 1980s, the large Dourvin-series engines used in the larger Renaults (20, 21, 25 and 30) used this longitudinal "forward" layout. The first generation Saab 900, launched in 1978, also used a longitudinal engine with a transmission underneath with helical gears but the thrust bearings involved gave out a characteristic "zing" sound almost like a hairdryer. Audi is one of the few manufacturers which still uses this particular configuration. It allows the use of equal-length half shafts and the easy addition of all-wheel drive, but has the distinct disadvantage that it makes it difficult to achieve 50/50 weight distribution^{[citation needed]}.

The 1966 Oldsmobile Toronado was the first American front-wheel drive car since the Cord 810.^{[citation needed]} It used a longitudinal engine placement for its V8, coupled with an unusual "split" transmission, which turned the engine power 180 degrees. Power then went to a differential mounted to the transmission case, from which half-shafts took it to the wheels. The driveline was set fairly at centre-point of the wheels for better weight distribution, though this raised the engine, requiring lowered intake systems.

The Corporate Average Fuel Economy standard drove a mass changeover of cars in the US to front-wheel drive. The change began in 1978, with the introduction of the first American-built transverse-engined cars, the Plymouth Horizon and Dodge Omni (based on the European designed Simca Horizon)^[1], followed by the 1980 Chevrolet Citation and numerous other vehicles. By reducing drivetrain weight and space needs, vehicles could be made smaller and more efficient without sacrificing acceleration. Some suggest that the introduction of the modern Volkswagen Rabbit in 1975, from a traditional U.S. competitor, served as a wake-up call for the "Big Three" (which already produced front-wheel drive vehicles in their operations outside North America). Front-wheel drive became the norm for mid-sized cars starting with the 1982 Chevrolet Celebrity, 1983 Dodge 600, 1983 Toyota Camry, and the 1986 Ford Taurus. By the mid-1980s, most formerly rear-wheel drive Japanese models were front-wheel drive, and by the mid-1990s, most American brands only sold a handful of rear-wheel drive models.

There were few Japanese or American rear-wheel drive cars by the early 1990s; Chrysler's car line-up was entirely front-wheel drive by 1990.

The vast majority of front-wheel drive vehicles today use a transversely mounted engine with "end-on" mounted transmission, driving the front wheels via driveshafts linked via constant velocity (CV) joints. This configuration was made popular by the 1969 Fiat 128.^{[citation needed]} The 1959 Mini, while a pioneering transverse front-wheel drive vehicle, used a substantially different arrangement with the transmission in the sump of the engine.

American auto manufacturers are now shifting larger models (such as the Chrysler 300^[2] and most of the Cadillac lineup) back to rear-wheel drive.^[3]

[edit] Records

The Cadillac Eldorado, with front-wheel drive introduced in 1967, holds the record for the largest engine in a front-wheel drive production vehicle, at 8.2 L (500 in³), starting with the 1970 model, lasting until the 1976 model year^{[citation needed]}.
In acceleration, the Dodge SRT-4 holds the world record for the quickest front-wheel drive production vehicle.^[4]
A turbocharged Citroën SM broke the land speed record for its class at Bonneville Salt Flats in Utah in 1987, going 202 mph (325 km/h), a record for a front-wheel drive vehicle.^[5]

[edit] Advantages of front-wheel drive

Interior space: Since the powertrain is a single unit contained in the engine compartment of the vehicle, there is no need to devote interior space for a driveshaft tunnel or rear differential, increasing the volume available for passengers and cargo.
Cost: Fewer components overall
Weight: Fewer components mean lower weight
Improved fuel efficiency due to less weight
Improved drivetrain efficiency: the direct connection between engine and transaxle reduce the mass and mechanical inertia of the drivetrain compared to a rear-wheel drive vehicle with a similar engine and transmission, allowing greater fuel economy.
Assembly efficiency: the powertrain can often be assembled and installed as a unit, which allows more efficient production.
Slippery-surface traction: placing the mass of the drivetrain over the driven wheels improves stand-still traction on wet, snowy, or icy surfaces, although heavy cargo can be beneficial for traction on rear-wheel drive pickup trucks.
Predictable handling characteristics: front-wheel drive cars, with a front weight bias, tend to understeer at the limit, which is commonly believed to be easier for average drivers to correct than terminal oversteer, and less prone to result in fishtailing or a spin.
Better crosswind stability.
Tactile feedback via the steering wheel informing driver if a wheel is slipping.
Front-wheel drive allows the use of left-foot braking as a driving technique. However this mostly applies to rally cars.
While driving on snow or ice, the skilled driver can control the movement of the car even while skidding by steering, throttling and pulling the hand brake (given that the hand brake operates the rear wheels as in most cases, with early Saabs being an exception). It takes practice to master but it vastly increases safety while driving in cold environments, given that the driver understands the risks involved in intentional skidding.

[edit] Disadvantages of front-wheel drive

The centre of gravity of the vehicle is typically farther forward than a comparable rear-wheel drive layout. In front-wheel drive cars, the front axle typically supports around 2/3rd of the weight of the car (quite far off the "ideal" 50/50 weight distribution). This is a contributing factor in the tendency of front wheel drive cars to understeer.
Torque steer can be a problem on front-wheel drive cars with higher torque engines (more than 210 N·m ) and transverse layout. This is the name given to the tendency for some front-wheel drive cars to pull to the left or right under hard acceleration. It is a result of the offset between the point about which the wheel steers (which falls at a point which is aligned with the points at which the wheel is connected to the steering mechanisms) and the centroid of its contact patch. The tractive force acts through the centroid of the contact patch, and the offset of the steering point means that a turning moment about the axis of steering is generated. In an ideal situation, the left and right wheels would generate equal and opposite moments, cancelling each other out, however in reality this is less likely to happen. Torque steer is often incorrectly attributed to differing rates of twist along the lengths of unequal front drive shafts. However, Centre-point steering geometry can be incorporated in the design to avoid torque steer. This is how the powerful Citroën SM avoided the problem.
Lack of weight shifting will limit the acceleration of a front-wheel drive vehicle. In a vehicle, the weight shifts back during acceleration, giving more traction to the rear wheels. This is the main reason why nearly all racing cars are rear-wheel drive. However, since front-wheel drive cars have the weight of the engine over the driving wheels, the problem only applies in extreme conditions.
In some towing situations, front-wheel drive cars can be at a traction disadvantage since there will be less weight on the driving wheels. Because of this, the weight that the vehicle is rated to safely tow is likely to be less than that of a rear-wheel drive or four-wheel drive vehicle of the same size and power.
Due to geometry and packaging constraints, the CV joints (constant-velocity joints) attached to the wheel hub have a tendency to wear out much earlier than the universal joints typically used in their rear-wheel drive counterparts. The significantly shorter drive axles on a front-wheel drive car causes the joint to flex through a much wider degree of motion, compounded by additional stress and angles of steering, while the CV joints of a rear wheel drive car regularly see angles and wear of less than half that of front wheel drive vehicles.
The driveshafts may limit the amount by which the front wheels can turn, thus it may increase the turning circle of a front-wheel drive car compared to a rear-wheel drive one with the same wheelbase.