Front wheel drive

From Academic Kids

Front wheel drive is the most common form of engine/transmission layout used in modern automobiles, 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 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 CitroŽn in Europe, and to the contemporary 810 from Cord Automobile in the United States.

Front wheel drive was further advanced by the now-legendary Mini of 1959 (even if Saab 92 predates it) although its mechanical layout differed from modern vehicles, since the transmission was built into the sump of the engine, and drive was transferred to it via a set of primary gears. This concept was also used by Peugeot and Renault on their jointly-developed "Douvrin" engine of the 1970s. The tendency of this layout to generate unwanted gearbox "whine" has seen it fall out of favour.

Audi pioneered the "U-Drive" layout where the engine is mounted longitudinally, in an "overhung" position over the front wheels. 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.

The Oldsmobile Toronado, which reintroduced front-drive to the U.S. market in 1966, also used a longitudinal engine placement for its V-8, 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 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 liters (The 1970 edition). The Cadillac Eldorado continued to be the only front wheel drive offering from Cadillac until the mid 1980s. 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.

The change over of cars in the US to front wheel drive began in earnest in the 1980s, with the introduction of the compact Chevrolet Citation probably marking the real beginning, and the 1986 introduction of the Ford Taurus bringing front wheel drive to prominence in larger 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.

The vast majority of front wheel drive vehicles today use the 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.

Advantages of front wheel drive

  • The powertrain can be packaged entirely in the engine compartment of the vehicle, negating the need for a transmission tunnel in the floorpan, thus freeing up interior space - important in small cars.
  • Because the weight of the engine/transmission assembly is directly over the driven wheels, front wheel drive cars offer better traction capabilities in snow and ice. This is of particular advantage in small, low-powered vehicles. (In larger, more powerful rear wheel drive cars, weight can be added to the rear of the vehicle to get the same effect.)
  • When driving on low grip surfaces (wet road, ice, snow, gravel...) the car is pulled, rather than pushed, thus making handling better. This can also be used for better cornering because the cars pulls itself out of a curve.
  • The lack of a drive shaft (propshaft) and a lighter rear axle assembly means that there is less mechanical inertia in the powertrain, meaning that greater fuel economy is achieveable.

Disadvantages of front wheel drive

  • The centre of gravity of the vehicle is typically further forward than a comparable rear-wheel drive layout. In front wheel drive cars, the front axle typically supports around 2/3 the weight of the car (quite far off the "ideal" 50/50 weight distribution). This is a contributing factor towards the tendency of front wheel drive cars to understeer.
  • Torque steer can be a problem on front wheel drive cars with higher torque motors( > 210 Nm ). 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. An example of a car with strong torque steer would be the 257 hp (191 kW) Volvo S60 T5 FWD.
  • 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 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.

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