The sizes come from Shimano’s spec sheets (hubs, wheels and E-THRU axles, checked in October 2026) and are cross-checked against Sheldon Brown’s spacing chart. Width is measured between the outer faces of the hub’s locknuts or end caps, known as O.L.D. (over locknut dimension), and matches the inside spacing of the frame or fork dropouts. Axles are measured in millimeters in the US as well.
Bike axle sizes
| Standard | Position | Axle | O.L.D. width | Typical use |
|---|---|---|---|---|
| 100 mm quick release | Front | QR | 100 mm | Older road and mountain bikes, hybrids |
| 130 mm quick release | Rear | QR | 130 mm | Rim brake road bikes |
| 135 mm quick release | Rear | QR | 135 mm | Mountain bikes without thru-axles and older disc road bikes |
| 12×100 | Front | 12 mm | 100 mm | Disc brake road and gravel |
| 15×100 | Front | 15 mm | 100 mm | Mountain bikes before Boost |
| 15×110 Boost | Front | 15 mm | 110 mm | Current mountain bikes: XC, trail, enduro |
| 20×110 | Front | 20 mm | 110 mm | Downhill |
| 12×142 | Rear | 12 mm | 142 mm | Disc brake road and gravel; mountain bikes before Boost |
| 12×148 Boost | Rear | 12 mm | 148 mm | Current mountain bikes: XC, trail, enduro |
| 12×157 | Rear | 12 mm | 157 mm | Downhill and some enduro frames (Super Boost) |
Shimano currently makes hubs in all of these sizes except 20×110, which corresponds to its Saint HB-M820 downhill hub (110 mm with a 20 mm thru-axle). Boost doesn’t change the axle diameter: it spreads the hub flanges to make the wheel stiffer, which is why it needs its own fork, frame and cranks.
Shimano thru-axle lengths
When you buy a replacement thru-axle, width isn’t enough: total length and thread matter too.
| Shimano axle | For hub | Total length | Thread |
|---|---|---|---|
| SM-AX720-100x12 | 12×100 front | 121 mm | M12×1.5 |
| SM-AX720-142x12 | 12×142 rear | 163 mm | M12×1.5 |
| AX-MT700-142x12 | 12×142 rear | 166 mm | M12×1.5 |
| AX-MT700-148x12 | 12×148 rear | 172 mm | M12×1.5 |
| AX-MT700-157x12 | 12×157 rear | 181 mm | M12×1.5 |
Other brands use other lengths and threads: even Shimano’s own Saint downhill axle threads in at M12×1.75. Look for the specs etched on the axle or in your frame manual before ordering a new one.
How to tell which axle your bike has
- Check the axle or the dropout. Many axles are etched with “12×142”, the length and the thread. If not, measure the axle diameter with calipers and the distance between the inner faces of the dropouts.
- Quick release or thru-axle. A quick release is a thin skewer that passes through a hollow axle and clamps with a lever; a thru-axle is a thick tube that threads into the dropout. If your fork has open, U-shaped dropouts, it’s a quick release.
- You recognize Boost by the width, not the look: if there are 148 mm between the rear dropouts, it’s Boost; if there are 142 mm, it isn’t.
- Convertible hubs. Many mountain bike hubs take end caps to go from 15 to 12 mm or from quick release to thru-axle, but only within the same width.
The rest of the wheel’s measurements (rim diameter, tire width, ETRTO) are in the tire size calculator, and the names of every part in six languages are in the frame and geometry glossary and the wheels and tires glossary. If you’re building a wheel with a new hub, the locknut width goes into the spoke length calculator. If you’re switching hubs, also check the freehub body standards and the disc brake rotor sizes.
Frequently asked questions
What does 12x148 Boost mean?
A 12 mm thru-axle and a 148 mm wide rear hub, 6 mm wider than the older 142. Nearly every current mountain bike uses it; the matching front standard is 15×110 Boost.
Can I put a Boost wheel in a non-Boost fork?
No. A 110 mm hub won't fit a 100 mm fork, and a 148 mm hub won't fit a 142 mm frame. Hub end caps can change the axle diameter (from 15 to 12 mm, for example), but never the width.
What axle does a disc brake road bike use?
Today it's normally 12×100 mm front and 12×142 mm rear. Older disc road bikes used a 100 mm quick release up front and 135 mm at the rear.
Will any 12x142 thru-axle fit any frame?
Not always. Besides the width, total length and thread vary: Shimano's are 163-166 mm long with an M12×1.5 thread, but some axles use other threads, like Shimano's Saint (M12×1.75). Check the thread in your frame manual or on the axle itself.