Formula A: 50% rebounce. Made to be consistent and leave hideously thick thane lines when sliding.Disadvantage is wheels are not icy but grippy. Can not last long time.price is cheapest in 4 formulas. Formula B: 60% rebounce. Formula for amateur. More smooth than Formula A,last longer than it too.This formula occupy most of the market share judging from our distribution records.It is good but not good enough as famous brands.But some clients sell it due to good performance on controllable speed and thaney quality. Formula C: 70% rebounce. Designed for professional players.This is the most smooth formula for our Downhill and grippy wheel.Under same condition it will last longer than many brands on the market. We now have supplied it to many brands in USA EU . Formula D: 80% rebounce. the best choice for sliding wheel. this formula is designed to be the perfect wheel for players to learn how to slide on and do some basic freeriding on.Once broken in wheels leave small thane lines and start sliding more controllable and longer.This formula give you more life out of your wheel.With unparelleled durability it can significantly improves performance and creates one of the longest-lasting wheels on the market.