Automotive (F4) Front-engine, four-wheel-drive layout
In automotive design, an F4, or Front-engine, Four-wheel-drive layout places the internal combustion engine at the front of the vehicle and drives all four roadwheels. This layout is typically chosen for better control on many surfaces, and is an important part of rally racing as well as off-road driving.
Most four-wheel-drive layouts are front-engined and are derivatives of earlier front-engined, two-wheel-drive designs.
Probably the best known users of this layout are the AMC Eagle (August 1979) and the Audi with its quattro four wheel drive system, which first appeared in the late 1980 Audi Quattro road car.
This layout is also the drive train of choice for off-road pickup trucks and SUVs. It allows these vehicles to get the most traction without sacrificing precious cargo or passenger room. The center differential is often not present in these vehicles, meaning the 4WD system does not allow any difference in front and rear axle speeds. For normal road driving, these vehicles are shifted into 2WD mode, preventing damage to the transfer case; though full-time systems cannot go to 2WD mode.