XRAY - Model racing cars

I beleive Tony was right, and you can add about 3mm of shim before rubbing on the wheel. Download the hudy setup guide
This guide is awesome it explain very well the roll center theory and everything else on the car.

MR Jolly is correct. I was talking about adding shims under the top links on the outside of the car. So at the front that would be between top link and caster block and at the rear between top link and upright.

Raising the lower suspension arms to a higher position on the bulkheads raises the roll center significantly. Raising the top links on the outside will give a smaller change if you use thin shims. I.e. for big changes go for the lower arms but for fine tuning use shims under the outside mounting of the top links. Both changes also affect the camber change (in both case it is increased).

It`s a good tuning tool really, like your car is nearly there but not quite, & the shims under the ouside top links front or rear would be enough to tip it in the right direction, we are only talking .5 -2mm shims

is there any way using shims to adjust the roll centre would cure traction rolling? my son's car is very quick and he's a good driver, but was struggling with traction rolling on the hairpins on carpet, which meant he was having to take them slower than he'd like...

It depends. Generally a higher roll center will keep the chassis more flat during cornering and will make the car loose traction earlier. BUT if the car doesn't loose traction with the higher roll center it will traction roll all the more. In fact, raising the roll center often result in the car traction rolling when you hit a kerb even if it didn't before.