My son is on a 2001 RM125 and is an OK novice (mid-pack at regional level). He weighs about 130 and the stock shock spring is rated for a 165 lb rider. The local suspension shop says he needs to go to a 4.0 (from 4.8 stock). Isn't that a drastic jump? We are not revalving at this time (maybe when he is a little faster).