Drivetrain Layouts

Front-engine front-drive (FF) cars usually mount the engine transversely with an integrated transaxle and unequal or equal-length half-shafts (an intermediate shaft equalises lengths to limit torque steer). Front weight bias of around 60% gives stable, understeer-biased handling and good traction in snow when cruising, but the front tyres must both steer and drive, so their friction budget is shared. Front-engine rear-drive (FR) cars use a longitudinal engine, gearbox, propshaft and rear axle; weight distribution can approach 50:50, and some place the gearbox at the rear axle (transaxle) for balance. Mid- and rear-engined layouts concentrate mass near the driven axle for traction and low yaw inertia.

Under acceleration a the load on the rear axle rises by ΔF = m a h / L, where h is the centre-of-gravity height and L the wheelbase. For a 1,500 kg car with h = 0.55 m and L = 2.7 m accelerating at 3 m/s², roughly 920 N moves from front to rear. Because tyre grip rises with load, the driven axle that gains load can transmit more force, so RWD and AWD out-launch FWD for a given power.

All-wheel drive needs a way for front and rear axles to turn at slightly different average speeds. Permanent systems use a centre differential (open, limited-slip or torque-sensing) with a fixed nominal split; on-demand systems drive one axle and engage the other through an electronically controlled multi-plate coupling. Traditional part-time 4WD locks front and rear axles together through a transfer case with an optional low range; it should only be used on loose surfaces, because on high-grip tarmac the locked axles fight each other in corners ('wind-up'). Electric vehicles add another option: separate motors on each axle, with no mechanical link between them at all.