Transmission wind-up (which is what I presume you're asking about ) is caused by a difference in rolling radius between two tyres (or axles) connected by a differential, and its effects are worsened when the differential concerned has some sort of limited slip capability.

It happens when, as mentioned, there's inequality in rolling radius between the two sides of the differential, for example using tyres of different make/size, with greatly differing tyre pressures, or tread depths.

On a two wheel drive car it's undesirable, as it can cause premature diff wear, as well as upsetting the handling. It's even more of an issue on a four wheel drive car like the Impreza as the centre differential is (either a viscous or electro-magnetic) LSD, so you've got three diffs to keep happy, instead of just one.

If, for example, you put new tyres on the back of the car while keeping badly worn ones on the front, the rear axle will rotate more slowly than the front for any given road speed. The viscous coupling in the centre diff will try and fight this imbalance, with large amounts of heat and diff wear resulting. In extreme circumstances the handling will be upset due to the effective shift in the intended torque split of the diff.

The moral of the story is that it's important to use four tyres of the same make, and as close to equal tread depth as possible to minimise/eliminate this sort of thing. Also, if your car ever breaks down and the clueless AA man offers you a suspended tow, say no, and insist on a trailer...

[Edited by greasemonkey - 8/1/2003 4:11:25 PM]

 

PROPER BO