Front-engine, front-wheel drive layout
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In automotive design, a 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, 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 RMR layouts, it places the engine over the drive wheels which may aid traction in many applications. As the steered wheels are also the driven wheels, FF cars are generally considered superior to FR cars in conditions where there is low traction such as snow, mud, gravel or wet tarmac. However, powerful cars rarely use the FF layout because weight is transferred to the rear wheels under acceleration, while unloading the front wheels and sharply reducing their grip, effectively putting a cap on the amount of horsepower which could realistically be utilized. Electronic traction control can avoid wheel-spin but largely negates the benefit of extra power.
Early cars using the FF layout include the 1931 DKW F1, 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 an 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 as near as possible to putting the entire weight of the drivetrain on the front wheels.
- Dante Giacosa 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. The shorter drive shaft, being stiffer than the longer drive shaft, transmits the motion to the wheels immediately instead of 'winding' up due to the drive torque[citation needed]. The net result is more tractive force at the wheel with the shorter drive shaft and the car tends to pull to the opposite side. 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.
[edit] References
Sedgwick, Michael Cars of the 50s and 60s. Gotherburg, Sweden: A B Nordbok, 1983. Has pictures of the engine layouts of the Traction Avant and the modern designs as in No. 4 above.
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