The short version
Most bike wheels are held on by one of three systems: a quick release (QR), a thru-axle, or bolt-on axle nuts. They differ in how the wheel clamps to the frame or fork, how stiff the connection is, and how fast the wheel comes off. Parts are generally not interchangeable between them, so the type and size matter when buying a wheel.
Quick release
A quick release uses a thin skewer that passes through a hollow hub axle. A lever on one side clamps the hub’s end faces against the frame or fork dropouts. QR wheels fit open dropouts, where the wheel slides in from below or behind.
Common sizes you will see:
- Front: 100 mm hub width
- Rear: 130 mm on many road bikes, and 135 mm on many older or mountain-style bikes
Quick release is tool-free and fast. It is also lighter than most thru-axle systems. On some disc-brake bikes, uneven clamping can shift the wheel slightly, which is one reason thru-axles became popular there.
Thru-axle
A thru-axle is a thicker axle that passes through the hub and screws into the frame or fork on the far side, or is held by a nut. It requires closed dropouts (a hole rather than a slot). It generally gives more precise wheel alignment and a stiffer connection, which helps with disc brakes.
Common sizes include:
- 12 x 100 mm front and 12 x 142 mm rear, common on road and gravel bikes
- 15 x 100 mm front, common on mountain bikes
- 15 x 110 mm front and 12 x 148 mm rear, often called Boost on mountain bikes
The first number is axle diameter and the second is hub width. Thread pitch and axle length also vary by brand, so always check the manufacturer’s specification before buying a replacement axle. Some “quick” thru-axles have a lever, but they are still thru-axles.
Bolt-on axles
Bolt-on wheels have a solid axle with threaded ends. Nuts are tightened onto the axle against the dropouts, so a wrench is needed to remove the wheel. You often see this on single-speed bikes, some BMX bikes, budget bikes and some cargo or e-bike designs. They are secure and simple, but slower to remove.
Common sizes at a glance
| Type | Typical front | Typical rear |
|---|---|---|
| Quick release | 9 x 100 mm skewer | 10 x 130 or 135 mm skewer |
| Thru-axle (road) | 12 x 100 mm | 12 x 142 mm |
| Thru-axle (mountain Boost) | 15 x 110 mm | 12 x 148 mm |
Treat these as typical rather than universal. Specifics vary by bike and brand.
How to tell which axle your bike has
- Look at the wheel end. A lever sticking out one side with a skewer through the center is a quick release.
- Check for a thick axle with no lever. A large hex socket or a handle that screws in is usually a thru-axle.
- Look for nuts on the axle ends. That is bolt-on.
- Look at the dropouts. Open slots suggest QR or bolt-on, and closed holes suggest thru-axle.
- Measure. Check the spacing between the frame or fork ends and compare it with the numbers above, or consult the bike’s manual or manufacturer.
If you cannot tell, a local shop can identify it quickly.
Can you swap one type for another?
Usually not by simply changing the axle. The frame or fork has to be designed for the axle type. Some hubs offer end-cap conversion kits so one hub can fit more than one standard, but this depends on the hub model. Check with the hub manufacturer. For more on hub internals, see our bike hub diagram and parts page.
Is 29 the same as 700c?
They are close but not exactly the same thing. Both use a rim with a bead seat diameter of 622 mm, so the rims are the same diameter. The names come from different traditions, and the tires usually differ in width. A 700c wheel can be a 29er wheel size-wise, but check the rim width, brake type and axle standard too.
Care and safety
Always follow the manufacturer’s instructions for closing a QR lever or tightening a thru-axle, and check that the wheel is secure before every ride. A loose wheel can be dangerous. If you are unsure about the right tightness or the compatibility of a part, ask a qualified mechanic. For the wider picture, see the anatomy of a bike wheel.