When a car travels around a corner, the outside wheels travel further and faster than the inside wheels. This means that if the wheels were directly connected, stresses would build between the wheels until one skipped, or a driveshaft snapped. A differential gets its name as it allows the two wheels on an axle to rotate at different speeds when cornering.
THE OPEN DIFFERENTIAL_
This is the most basic type of diff and what you will find on the vast majority of cars. It allows the two driven wheels to rotate at different speeds whilst splitting drive torque 50:50 either side of the axle. Torque distribution of 50:50 sounds great, doesn’t it? Well, it isn’t quite that simple. It all boils down to the path of least resistance and the amount of useable traction, which changes by modifying variables in this equation (you can skip this if you want!):
When you corner at speed, weight is transferred to the outside of the car through lateral acceleration and this lifts weight off the inside wheels. Reducing the weight on the inside wheels means that you have less mechanical grip and therefore require less tractive effort to make the wheel slip. With an open diff, in this situation the inside wheel will start to slip with only a small amount of torque from the engine, and the diff will distribute the same amount of torque to the opposite wheel, which has more grip. The result is one spinning wheel and another that doesn’t have enough torque to move the car. So what can you do if you want to lay down 11s and not 1s on the tarmac? You turn to LSD.
THE BENEFITS OF LSD_
Compared to an open diff, a limited-slip differential (LSD) can help by splitting the drive torque and biasing one wheel with more torque than the other on split surfaces or when cornering hard. Mechanical LSDs generally come in three main types; Torsen, viscous coupling, and clutch.
Torsen LSDs are torque-sensing diffs that can transfer drive torque from the inside wheel to the outside wheel through a torque bias ratio (TBR). If the inside wheel starts to slip, the outside wheel will receive the torque applied to the inner wheel, multiplied by the TBR. A downfall to the Torsen diff is that if the inside wheel is lifted off the ground, no torque is transferred to the outside wheel (zero torque multiplied by anything is equal to zero), and so the Torsen behaves as an open diff.
Viscous LSDs can be found in cars such as the old Nissan S-platform and the newer 370Z. They operate using two sets of discs corresponding to each driveshaft, which are placed very close to (but not touching) each other. A high-viscosity fluid surrounds the discs and when one wheel slips, the corresponding set of discs spins quickly and drags the fluid around with it. This then applies torque to the opposing side’s discs, which rotates the opposing wheel. Instead of being torque-sensitive, this LSD is speed-sensitive and thus takes time to react to the initial wheelspin.
Clutch-type LSDs are generally the types you will find in aftermarket modified cars and higher-end performance cars. They use clutch packs to lock the wheels into a joined rotation. Under normal operation (in a straight line) the clutches aren’t really doing much, but when high torque is applied to the diff’s internal gears, axial forces push the clutch packs into action. When the clutch packs are pushed and engaged, the wheels are locked in unison. This type of diff is far quicker to react than the viscous type and takes far longer to wear out.
So now you know what will stop your car spinning its inside wheel relentlessly when driven hard. Next time, we’ll look at the e-diffs and brake-based traction systems that we see in the latest supercars.
Stay tuned on 'The Technology Tribe' for part 2..