The hub is located in the middle of the wheel and houses the axle and bearings which the wheel rotates around.
Hub bearings can be cartridge or cup and cone. Most road bike wheels use cartridge bearings, as they require no maintenance, whilst cup and cone bearings need some servicing. For the smoothest ride, look for ceramic bearings– these are usually on the lighter and more expensive wheels but are most pleasant to ride on.
The rear hub also contains a freehub mechanism – this contains the bearings, and clutch system, which often uses ratcheting teeth and pawls. This might sound a bit technical and boring – but the interesting point is that the a higher number of pawls means that the time taken for the drive chain to start moving as you shift from freewheeling to pedalling is quicker.
Therefore, if you’re racing, and the seconds are as important to you as minutes, you’ll want a higher quality hub with a high pawl engagement. Hope Hoops Pro 2 Evo wheels, for example, have a 40 tooth ratchet and 4 pawl engagement and are known for the satisfying click they make as they turn.
Your hubs need to be compatible with your cassette – so do check that if you’ve got an 11 speed cassette, for example, the hubs on your would-be wheels are compatible.