The worn ones do start to go off a tad when you're on a lot of Carpet, but as myself, Paul and Lloyd found at the last Newbury meet that new ones can be AWFUL on the slippery stuff, they seem to bed in after about 4 mins but we all pretty much wrote that run off as soon as we tried the cars in practise - it was like ice skating on the slippery bits, while good fun it doesn't really get you anywhere
