CALCULATING DIFFERENTIAL WIDTH

SO HOW do we work out that ideal diff width using science and logic instead of guesswork? The diagram on the right shows that by measuring your vehicle, you can work out your optimum diff width.

Start by measuring the inside width between your outer guards to get dimension ‘X’. This is not always easy; sometimes you may need to use spirit levels held vertically on the outer skin of the body, measure across the bootlid and then subtract the inner roll of the guard.

Next, measure the distance between your inner guards to get dimension ‘Y’. This can be measured under the car or by subtracting the two tub widths from dimension ‘X’.

Now it’s time to notionally put the width of your chosen wheel into the tub area. You must have clearance (dimension ‘C’) between your wheel and the inner and outer guards. This clearance is recommended to be 25-30mm or ‘two fingers’, to allow for lateral movement of the diff (and it does move) and bodyroll of the vehicle. Any less and the result is scrubbing, which will make for uncomfortable driving. For a front suspension, the pros allow a ‘C’ of 50-75mm, to allow wheels to still turn inside the guards if the car is sitting low.

Of course, if your wheel doesn’t fit within the guards, it’s either too wide or your tub is too narrow – so more decisions!

Now choose a backspace for your wheel that will give you the look you want while still meeting the track rules we mentioned earlier. This is dimension ‘B’. Keep in mind though that wheel-makers measure backspace to the outside of the wheel, but tyre-makers measure to the internal tyre shoulder. So a 10-inch wheel will actually be 11 inches wide outside to outside when the tyre is taken to account. That’s why we add 12.7mm (half an inch) per side when working out backspace.

If you have done this correctly, you can now work out the optimum width for the differential from wheel flange to wheel flange by using this formula: Y+(2XB)+(2XC).

Now you can order your wheels and your differential.