Front wheel drive
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.
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.
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 transmission "whine" has seen it fall out of favor, except at Saab. The first-generation Saab 900 had this layout 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. 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.
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 Oldsmobile Toronado, which reintroduced front wheel drive to the U.S. market in 1966, also 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 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 (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.
Acceleration-wise, the Dodge SRT-4 holds the world record for the quickest and fastest front wheel drive production vehicle.
The changeover 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 midsize 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. Some suggest that the introduction of the Volkswagen Rabbit in 1975 led to Detroit's "Big Three" to market the FF layout; Chrysler's vehicle lineup was almost entirely FF by the end of the 1980s. GM followed Chrysler in the mid-1990s when their vehicle lineup was FF (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 (although later then the 1949 Saab 92), used a substantially different arrangement with the transmission in the sump of the engine.
Many American auto manufacturers are shifting expensive models away from front wheel drive. The Chrysler 300 and almost the entirety of the Cadillac lineup are some examples of cars once only offered in front wheel drive form, now coming off showrooms in a rear or all wheel drive layout.
Advantages of front wheel drive
- Packaging efficiency: the powertrain is a single unit contained in the engine compartment of the vehicle, obviating the need to devote interior space for a driveshaft tunnel or rear differential and increasing the volume available for passengers and cargo.
- Cost: Fewer components overall.
- 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.
- 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 about if a wheel is slipping.
Disadvantages of front wheel drive
- The center 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 2/3rd of 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.
- 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. The fact that this does not happen in a front wheel car 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.
- Snap oversteer also known as lift-off oversteer. The virtue of weight over the drive wheels also makes the rear end of the car exceptionally light. In a hard cornering situation, slight weight transfer to the front of the car (ie from lifting off throttle) can easily result in a unrecoverable spin, certain cars such as Peugeot 306 Xsi are well known for this characteristic.Template:Fact
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