REAR-WHEEL-DRIVE

BMW 118i final drive and electrically-controlled differential.

Clutch in power flow:

• minimal drag losses

• highest actuation dynamics

• optional control accuracy

Overdrive speed difference via internal step gear:

• compact design

• minimal weight

• optimal efficiency

• best availability

Hydraulic actuation:

• optimal control

• highest power density

Compared to the engine and gearbox, the mechanism that transfers torque to the reardriven wheels is relatively overlooked. Despite the traditional layout of a north-south (longitudinal) engine powering a rear-mounted final drive differential assembly not being as common as it once was, the arrangement refuses to die completely – as many Audis and BMWs testify.

Axle layouts

Live rear axles are identified by their driveshafts (half shafts) being enclosed within the casting, meaning that the wheels, axle, final drive/differential and axle casing all rise and fall together with suspension movement. While used on many classic cars, live rear axles today tend to be more common on light commercial vehicles, off-roaders and pick-up trucks.

More sophisticated vehicles, with independent suspension, have the final drive and differential assembly fixed to the body/chassis, usually via a cradle or subframe, so that the driveshafts alone move with the suspension. As the outer driveshafts mimic those from front-driven transmissions, the CV joints are protected by flexible rubber/plastic gaiters that need periodic inspection. However, as the rear driveshafts do not have to contend with sharp steering angles, they tend to last longer than those fitted to the front.

You may also encounter transaxles, where the transmission and axle are mounted together towards the vehicle’s rear and tend to work in tandem with independent suspension. While most contemporary examples of transaxles are fairly unusual, their future lies in various high-voltage hybrid and even electric cars. For this reason, this article will leave such vehicles out but we are sure to visit them in the future, as the new technology becomes more familiar and widespread.

The Final Drive/Differential

The hidden mass of cogs and gears can be baffling, although the principle on which the combined final drive and differential unit operates is straightforward and has remained relatively unchanged for over a century.

The final drive reduces the propshaft speed, which, even when bottom gear is selected, rotates too quickly. The gearbox alone cannot reduce the ratio sufficiently, because achieving a typical first gear ratio reduction (of around 18:1) would require excessively large gears, making the gearbox excessively bulky and heavy. Therefore, the final drive assists the gearbox, by reducing the ratio further to allow a greater output torque for the differential to transmit to the half/driveshafts. Additionally, if the entire gear reduction was performed solely by the gearbox, the propshaft would have to tolerate around four times