TECH: DIFFS
We take a closer look at the world of differentials...
Firstly, we need to understand what a differential is, what it does, and how it works. Differentials aren’t a new invention, rumour has it that they have been around for over 3000 years with the ancient Chinese first using differential gearing in the South Pointing Chariot.
Modern diffs are a far cry from this first invention, but the principals remain pretty much the same. A diff’s job is to allow the driven wheels to rotate at different speeds and allows three shafts to rotate at different speeds at the same time; the input shaft, and two output shafts (to each wheel of the axle). The input shaft receives power from the transmission, and the differential then transmits this power to the driven wheels via the driveshafts.
So, why do we need a diff then? A diff is needed to allow a car to go around corners. If you draw a radius for the inside wheel of a car going round a corner, and then do the same for the outside wheel, you will see that the outside wheel has to travel further distance than the inside wheel. Therefore, it needs to rotate faster than the inside wheel to keep up. Without this ability the inside wheel would spin, or the outer wheel would drag. So, clearly a differential is needed to allow both wheels to rotate at different speeds when required.
However, allowing the wheels to rotate at different speeds individually does have its own set of problems; mainly when applying power. If the wheels are allowed to rotate independently, the wheel with the least amount of grip has a tendency to break traction and simply spin – the power (more precisely, torque in this instance) will follow the path of least resistance. Clearly, this is not ideal for a performance car, so a limited-slip differential is used to provide the answer; a diff that allows wheels to rotate independently when cornering, but rotate at the same speed when power is applied.
There are several different types of limitedslip diff available, but first let’s familiarise ourselves with the ‘open’ diff technology as found in most standard, non-performance, road cars…