MTB hubs

the spacing in the frame or fork, the axle type and diameter, the number of spoke holes, and the brake rotor mounting method. The rear one also has a freehub body tied to the manufacturer and the number of gears. A finer ratchet engagement means the wheel engages sooner when you pedal, which you will notice on technical climbs. Bearings are consumable parts — you can spot play by rocking the rim sideways, and it pays to sort it out immediately before the whole hub is ruined. Therefore, check the serviceability of the bearings before buying.