Hi, I've just bought my first urethane tyres and am having trouble . The wheel surface is flat. Can anyone tell me what is used to crown the wheel. I think something glued/taped????? to the wheel to allow a crown for the tyre to run on. Tyre appears to thin to grind sides . Can't get the blade to run other than on the very back of the wheel :no::no::no::no::no: