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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, though many in prior decades were positioned longitudinally instead. Rear-wheel drive was the traditional standard while four-wheel drive is also sometimes used. See also FF layout.

Contents

History

Experiments with front wheel drive cars date to the early days of the automobile. Alvis Cars of the United Kingdom introduced a front wheel drive model in 1928, but it was not a success.

The first successful models were the 1934 Traction Avant cars from Citroën in Europe, and the contemporary 810 from Cord Automobile in the United States. These vehicles featured a mid-engine front wheel drive design — the engine was behind the transmission and ran "backwards."

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 favorable weight distribution (the heaviest component is near the center of the car, making the main component of its moment of inertia relatively low).

Front wheel drive was further popularised by the 1948 Citroën 2CV, where the lightweight aluminum flat twin engine was mounted ahead of the front wheels, and 1955 Citroën DS, featuring the mid-engine layout.

The transversly mounted engine combined with front wheel drive was popularised by the MINI of 1959; 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 gearbox underneath. The tendency of this layout to generate unwanted transmission "whine" has seen it fall out of favor, except at Saab. The first-generation Saab 900 used a longitudinal engine with gearbox underneath with helical gears but the thrust bearings involved gave out a characteristic "zing" sound almost like a hairdryer.

Audi pioneered the "U-Drive" layout where the engine is mounted longitudinally, in an "overhung" position over the front wheels. Also in the 1970s and 80s the Douvrin large 2.0, 2.2 and 2.6 engines used in the larger Renaults (20, 21, 25 and 30) used this longitudinal "forward" layout. 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 - one reason why Audi vehicles have not managed to match the dynamic prowess of BMW, who have tenaciously stuck with the conventional rear wheel drive layout for all its vehicles.

In the German market, premium vehicles developed advanced independent suspension designs for the rear wheels and thus never switched away from rear wheel drive.

All Subaru front wheel drive and all wheel drive layouts are derived from the standard longitudinal front engine/rear drive transmission configuration, however the differential for the front wheels is between the bellhousing and the transmission. Visually, the front CV axles come out of the sides of the bellhousing, directly behind the clutch/torque converter area. The engine sits entirely in front of the front axle centerline, however, the short fore-aft length of the boxer engine allows this configuration without any weight distribution issues.

The 1966 Oldsmobile Toronado was the first U.S. made front wheel drive car since the Cord. It used a longitudinal engine placement for its V8, coupled with an unusual "split" transmission, which turned the engine power 90 degrees. The driveline was set well behind the centerpoint of the wheels, however, for better weight distribution (consequently, it was sometimes called the "Mafia car", because it was "all hood").

The Toronado's cousin, 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/500in³ (the 1970 model). The Cadillac Eldorado continued to be the only front wheel drive offering from Cadillac until 1980, when the front wheel drive Cadillac Seville was introduced. Reasons cited for the enormous engine are smoothness due to the inertia of the engine soaking up much of the vibration caused by low milling tolerances of the era, resulting in a very smooth ride.

In acceleration, the Dodge SRT-4 holds the world record for the quickest front wheel drive production vehicle. A turbocharged Citroen SM broke the land speed record for its class at Bonneville Salt Flats in Utah in 1987, going 202mph (325km/h), a record for a front wheel drive vehicle.

The Corporate Average Fuel Economy standard drove a mass changeover of cars in the US to front wheel drive beginning in the 1980s, with the introduction of the compact Chevrolet Citation, and the 1986 introduction of the Ford Taurus bringing front wheel drive to prominence in mid-size cars. 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.

Making this change easier was the fact that U.S. manufacturers had invested relatively little in the rather expensive and complex task of developing modern rear-wheel drive independent suspension layouts. Detroit's "Big Three" enjoyed a remarkable run of 75 low-cost years building passenger cars with the live rear axle design first seen on the Ford Model T. Some suggest that the introduction of the modern Volkswagen Rabbit in 1975, from a mainstream U.S. competitor, served as a useful wake-up call for the "Big Three." Chrysler's vehicle lineup was almost entirely front wheel drive by the end of the 1980s. GM followed Chrysler in the mid-1990s, with the exception of the Corvette and F-body lineup.

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 Fiat 128 of 1969. 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 some expensive models back to rear-wheel drive. The Chrysler 300 and most of the Cadillac lineup are examples of cars first offered as rear wheel drive, then front wheel drive, and now rear- or all-wheel drive. Horsepower is increasing and many engineers feel the rear wheel drive layout is better in cases of significant weight transfer associated with high powered engines and rapid acceleration.

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
  • Fuel economy: Lower weight means better gasoline mileage
  • 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 be 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 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.

Disadvantages of front wheel drive

  • The center 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 ( > 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, Center-point steering geometry can be incorporated in the design to avoid torque steer. This is how the powerful Citroen SM front-wheel drive car avoided the problem.
  • Lack of weight shifting will limit the acceleration of a front wheel drive vehicle. In a rear wheel drive car the weight shifts back during acceleration giving more traction to the driving wheels. This is the main reason why nearly all racing cars are rear wheel drive. However, since front wheel 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 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.
  • In low traction conditions (ie: ice or gravel) the front (Drive) Wheels lose traction first making steering ineffective.

See also

External links

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