The automotive differential has been with us for more than 100 years. Here’s why it’s important.
Remember the famous car-chase scene from the 1968 movie Bullitt? Frank Bullitt (Steve McQueen) chases the bad guys in their Dodge Charger R/T in a 1968 Ford Mustang Fastback GT. During the chase, Frank overshoots a corner in the Mustang and has to reverse in haste. Selecting first gear and dumping the clutch results in V8 muscle melting only one rear tyre in a plume of smoke and a single black line painted up the road. Why does only one wheel spin? An open differential is to blame, but we will get to that.
Why a diff is important
When a vehicle rounds a bend, the outer wheels need to turn faster than the inner wheels because more distance is covered because of a greater radius. This is easy for the non-driven wheels of a vehicle because they aren’t connected and can free-wheel at any speed. The problem occurs on the driven axle because the engine’s torque is delivered through the gearbox and final-drive ratio (the differential) to the two wheels on the axle. The differential allows an even torque split to both wheels while the rotational speeds may differ. Without a differential, it is almost impossible for a vehicle to turn on high-friction surfaces. The mechanical stresses in the driveshafts owing to wind-up can lead to damage or complete failure if no differential is fitted.
A basic diff
A typical automotive differential is a special application of an epicyclical gear set. Engine torque is delivered to the input shaft 1 (see the graphic above) of the diff. On a front-engined, rear-wheel-drive vehicle, the input shaft meets the ring gear (2; connected to the carrier) of the differential where the drive torque is turned through 90 degrees and a reduction ratio occurs (also known as the final drive ratio). The carrier supports the planet gears 3 that are connected to the two driven sun gears 4, which in turn are connected to the driveshafts 5 of the wheels.
When a vehicle drives in a straight line and the two driven wheels rotate at the same speed, the planet gear(s) are stationary and turn the two sun gears (connected to the driveshafts and wheels) at the same speed (see figure above).
When the vehicle rounds a bend, there is a speed differential between the inner and outer wheels, which results in the planet gear turning to allow for the speed difference.
In extreme cases where the input shaft is kept from rotating and one of the driven wheels is rotated, the other driven wheel rotates in the opposite direction at the same speed as the planet gear connected to both the sun gears to reverse the rotation.
This term refers to a conventional unit where there is no limit on the relative speed difference between the two driven wheels. However, the average of the wheel speeds on the driven axle is always equal to the speed of the ring gear (diff rule one). This means that, in an extreme situation where a driven wheel is stationary, the other wheel spins at twice the speed of the ring gear. The Bullit Mustang’s one rear wheel breaks traction and its speed increases dramatically, resulting in the other wheel almost staying stationary.
Diff rule two states that the torque delivered to both individual driveshafts is always equal in an open differential. The maximum torque that can be transmitted to the driven wheels is limited by the minimum traction torque (resistance to turning) at either of the driven wheels, multiplied by a factor of two (both wheels transmit close to the same torque value). Therefore, the spinning wheel of the Mustang can’t transmit much torque to the road because of the loss of traction and low sliding friction coefficient. That low torque figure is then sent to the other wheel with traction and results in the Mustang slowly moving forward while a cloud of rubber smoke fills the air. What Frank Bullit needed in this scenario was a limited-slip differential to improve the traction and resultant acceleration.
Limited-slip diff (LSD)
This type of differential limits the relative rotational-speed difference between the two wheels on a driven axle by providing an unequal torque split. For example, if a rear-wheel-drive car is parked with one rear wheel on ice and the other on tar, the wheel on ice is prone to traction loss and might spin on pull-away. An LSD minimises the speed difference between the two wheels by transmitting more torque to the stationary wheel with traction until it also starts to turn.
There are several mechanical devices that can achieve the above result, but the idea is to limit the speed differential of the sun gears (connected to the driveshafts) to the carrier that is connected to the ring gear. This indirectly limits the rotational-speed difference between the wheels. Popular methods include clutch packs with springs or coned friction surfaces between the sun gears and the carrier. These slightly hamper normal open-differential behaviour, but provide better traction in slippery conditions.
A more advanced solution is an active LSD where the slip between the sun gears and the carrier is actively controlled. This version uses clutch packs and the actuator can be electric or hydraulic. An advanced example is Audi’s Sport diff that has clutch packs and auxiliary transmissions on each differential-output shaft. This allows engineers to employ torque vectoring, where the torque split between the driven wheels can be individually controlled. By supplying more torque to the outside driven wheel(s) of a vehicle rounding a bend, understeer is kerbed by the torque vector.
Another way to mimic the behaviour of an LSD when only an open diff is fitted is to use the ESC system of the vehicle. The ABS setup brakes the spinning wheel on the driven axle to increase the torque that can be supplied to both wheels (refer to diff rule two). This method is not as effective as the action of a true LSD, but better than what can be achieved with a open diff.
This term is often used by the off-road community and describes a mechanism where the open differential is essentially locked to prevent a relative speed difference between the output shafts connected to the wheels on the driven axle. Although a diff-lock improves traction, it is important to employ it only on slippery surfaces, as axle wind-up may occur and damage the drivetrain if no slip between the wheels and surface is possible – especially during cornering. It is also more difficult to turn a vehicle with a diff-lock engaged, as it won’t allow the outer wheel on the drive axle to rotate faster than the inside wheel.
A four-wheel-drive vehicle normally has three differentials, one on each driven axle and the third differential between the output shafts of the transfer case that sends torque to the front and rear axles. A Mercedes-Benz G-Class is an example of a vehicle where all three diffs can be locked to force all four wheels to turn at the same rotational speed.
The arrival of electric powertrains has made it possible to eliminate the differential altogether. If there’s an electric motor on each driven wheel, the speed and torque of each of those wheels can be separately controlled and there is no need for a physical connection between the driven wheels – therefore, no differential is needed. Series production of this setup is still some way off because most electric vehicles use a single electric motor to save money, improve packaging and keep complexity low. In this case, a mechanical differential is still necessary.
- Dragsters do not have diffs because they need maximum traction, and race in a straight line.
- Nascar mandates a solid diff (spool) that degrades handling in large-radius turns, which necessitates more driver input.
- Go-karts have a solid rear axle. Slip between the tyres and road makes cornering possible (but unstable).
- Vehicles with driven wheels close together don’t have differentials, like on BMW’s Isetta.
- Lastly, electric vehicles with an electric motor per driven wheel don’t need diffs. The most prevalent example is Tesla’s Model S.