[DavidNJ]

As Mike said, geometry.

Also, if you bottom over a bump it is a shock issue not a spring issue. To the best of my knowledge, all race cars in every type of racing (maybe not drag racing) use mono-tube shocks. Some high end Carerra (bought by QA1). However, not the QA1 original line. Bilstein, Penske, Ohlins, Moton, some Konis are mono-tube.