Jump to: navigation, search

FF layout


In automobile design, an 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 - that is, it takes up very little space, allowing the rest of the vehicle to be designed more flexibly. In contrast with the FR layout, the FF layout eliminates the central tunnel needed to accommodate a driveshaft taking power to the rear wheels. Like the RR and MR layouts, it places the heavy engine over the drive wheels which aids traction. As the steered wheels are also the driven wheels, FF cars are generally considered superior to FR cars in conditions such as snow. However, powerful cars rarely use the FF layout because weight transference under acceleration unloads the front wheels and reduces grip. Electronic traction control can avoid wheelspin but largely negates the benefit of extra power.

Early cars using the FF layout include the Citroën Traction Avant, Saab 92 and the 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 a 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.

1.) The earliest such arrangement 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 and Citroën's Traction Avant were similar to Miller's original.

2.) 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.

3.) 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 just about as good as one could do to put the entire weight of the drivetrain on the front wheels.

4.) But the arrangement that really took over was that of Danti Giacosa, who 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.

See also


Automobile layouts
Layouts:
FF | FMR | FR | MF | RMR | RR | F4
Engine positioning:
Front-engine | Mid-engine | Rear-engine
Drive:
Front-wheel drive | Rear-wheel drive | Four-wheel drive | Six-wheel drive