The hub is one of the simplest bearings on a bike. A hub is the centre part of a bicycle wheel, the main components consists of an axle, bearings and a hub shell. The hub shell typically has metal flanges for the attachment of spokes. The functionally rear hub is identical to the front. The middle part of a wheel where the ends of the spokes attach consists of an axle, which attaches to the fork ends; a shell to attach the spokes, and bearings to connect the axle to the shell, permitting the shell to revolve around the axle. An axle has a load applied at its ends though the dropouts, and is supported by its bearings. Where these bearings are placed is critical in order to correctly distribute the load from the bike.

Additionally, in the case of a rear hub, the shell would also have a provision for attaching the rear sprocket. This drive side bearing takes more of the load produced when the rider pedals, nevertheless compared with the forces involved when the force a bicycle hits a pothole with this drive force is relatively insignificant. Some hubs also allow space to incorporate a coaster, drum, disc or roller brake.

Hub maintenance is essential for the smooth running of your hubs and freehub. hubs need attention if the bearings become loose, tight or gritty other wise you will loose performance and/or face the consequence of a catastrophic failure. The best way to fix is to take it apart, clean, add grease and replace the ball bearings if necessary. Special tools are needed called cone spanners, these are thin spanners with various sized heads used to adjust the hub bearings. However, you will need two, one for each side. Don't worry about the expense, these spanners tend to be fairly cheap to add to your bike tools

Common problems with hubs

• Symptom: Axles turn stiffly
• Cause: Hub bearing too tight
• Remedy: Loosen bearings

• Symptom: Axles are loose in the hub or wheel rattles sideways in frame
• Cause: Hub bearings are loose or broken axle
• Remedy: Tighten or replace

• Symptom: Hubs dirty, sticky, sounds gritty
  
• Cause: Hub age
  
• Remedy: Re-grease hubs

• Symptom: Creaky hubs
  
• Cause: Water in the hubs
  
• Remedy: Re-grease

Bicycle hubs may be either the adjustable cup-and-cone type or the cartridge style. The adjustable type bearing surfaces can be adjusted for bearing play. Typically the cartridge types  cannot be adjusted for wear or play. Both types can come in either the freehub type or the thread-on freewheel style.

Both Campagnolo and Shimano use cup and cone bearings in their hubs which are a very reliable and easy to service. The cup and cone bearings are a tried and tested concept from the last century. However, in the last two decades another sort of bearing has started to chip away at the established contenders throne. This is the cartridge or annular bearing commonly found in cars and industrial machines. Even though these bearings have been around for years they have only recently made their way into cycle hubs. The big advantage is to the manufacturer, where the bearings integrated with races from the bearing manufacturer and can then just be pressed into a simple hub shell. This leads to the possibility of small scale production as the manufacturer just has to produce the hub shell then fit industrial standard bearing "off the shelf".