Soft wheels below 84A need a hub to reinforce the bearing seat in the wheel. Under normal conditions, an 84A wheel will hold the bearings, but not under extreme conditions. Hubs are used on harder wheels only to lower the weight of larger wheels. Usually nylon or aluminum hubs are much lighter than the urethane they replace. Metal hubs are preferred over nylon hubs to produce a much stiffer wheel and provide a better feel to the top end skaters. Nylon hubs tend to flex while skating.