Hub diagram (text layout)
Without a picture, it helps to read a hub from the outside in. A simplified side-to-side layout of a rear hub looks like this:
[end cap] [bearing] [flange]--- hub shell ---[flange] [bearing] [freehub body] [end cap]
\__________ axle runs through the middle __________/A front hub is the same minus the freehub body. On disc-brake hubs, one side also has a rotor mount.
Hub shell and flanges
The hub shell (or body) is the main barrel of the hub. At each end is a flange, a wider section with holes where the spokes attach. The distance between the flanges and their diameter affect the wheel’s lateral stiffness and how the spokes are laced. Some hubs have flanges with straight-pull slots, others have holes for bent (J-bend) spokes. See bike wheel lacing patterns for how spokes connect to the flanges.
Axle and end caps
The axle runs through the hub shell and supports it. The end caps (or lock nuts on some hubs) hold the bearings in position and set the hub’s width where it meets the frame or fork. Depending on the system, the axle may be hollow for a quick release skewer, or hold a thru-axle. See bike wheel axle types.
Bearings
Bearings let the hub shell spin around the axle. Two main designs exist:
- Cartridge bearings: sealed units pressed into the hub shell. They are usually replaced as a unit.
- Cup and cone: loose ball bearings run between a cup in the hub shell and an adjustable cone on the axle. They can be serviced and adjusted, but need correct preload.
For sizes and how to identify them, see bike wheel bearing sizes.
Freehub body and pawls
On a rear hub, the freehub body carries the cassette and contains a ratchet mechanism. Small spring-loaded pawls (or another engagement design, depending on the hub) lock in one direction to drive the wheel when you pedal and release when you coast. Freehub bodies often come in different standards for different cassettes, so check compatibility before swapping.
Disc rotor mounts and seals
Disc hubs have a mount for the rotor, either a six-bolt pattern or a center-lock spline. Seals or dust covers sit near the bearings to keep dirt and water out. Worn seals are a common reason hubs wear faster.
Front vs rear hub
| Feature | Front hub | Rear hub |
|---|---|---|
| Freehub body | No | Yes |
| Rotor mount | If disc | If disc |
| Complexity | Lower | Higher |
| Spoke tension | Often more even | Often uneven between sides because of dishing |
What holds the hub? (dropouts)
The part of the frame or fork that holds the hub is the dropout. Rear dropouts are at the rear ends of the frame’s seat stays and chainstays, and front dropouts are at the ends of the fork legs. They can be open slots (for quick release) or closed holes (for thru-axles).
Fitting a new hub
You cannot simply bolt a new hub into an existing wheel. Replacing a hub means the wheel has to be unlaced and rebuilt, or the spokes need to be transferred to the new hub. The new hub must match the rim’s spoke count, and the spoke length may need to change. Our guide on how to build a road bike wheel explains the process. If you are not experienced, a wheelbuilding shop can do it.
Basic hub care
Check for play by holding the wheel and trying to rock the axle side to side. Spin the wheel and listen for roughness. Rough or loose bearings need service, and cup-and-cone hubs need careful adjustment. Follow the hub maker’s instructions, or the guidance in your tool maker’s service manual. For the wider context, see the anatomy of a bike wheel.