I am looking for a pair of wheels for a MTB build project, but I am lost when I read the specifications for wheel sets.

What I am looking for is a pair of wheels with a 9mm rear axle diameter and a 15mm front axle diameter.

But how do I know if a pair with quick releases will fit when it says nothing about the axle diameter?

Example "Available with 9mm or 15mm front axle" so? What is the rear axle diameter?

I am not asking what kind of wheelsmyou like or which you think I should and shouldn't buy. I need help understanding the specifications from the manufacures.

  • I think it would be useful to post a wheel or two which you are considering to give a concrete example of specs.
    – Batman
    Commented Sep 26, 2014 at 15:04

2 Answers 2


Rear wheels have never had 9mm axles, they have always been 10mm. For all but "downhill" MTB[2] and fatbikes[3], they are either 135mm 10mm or the new 12mm 142mm maxle standard.


Unless it specifically states otherwise I would assume that the rear wheel of an MTB wheelset is a 135mm 10mm axle. Anything that "looks" like a standard QR should be 135mm 10mm. But the vendor should make that pretty obvious, and if they don't it should be easy to ask. Wheelsets are expensive, it should be simple to get the answer to what dimensions are used in the rear hub.

[2]- Downhill MTB has a 150mm thru-axle standard.

[3]- Fatbikes use asymmetric 135mm axles (hub shifted to the driveside, relative to the rim) 170mm symmetric axles and 190mm/197mm thru-axles

  • good link, good site, +1
    – PeteH
    Commented Sep 25, 2014 at 19:42
  • Aaand we have 148mm rear length.
    – Vorac
    Commented Sep 26, 2014 at 9:55
  • AAARGH, why would Trek not just use 150mm???
    – Emyr
    Commented Sep 26, 2014 at 10:24

There are a large number of standards when it comes to just axle diameter. largest to smallest, with usual usage:-

20mm - This is used as a front wheel downhill/freeride standard.

15mm - This is front wheel standard for XC

12mm - This is used in rear wheel thru axle

10mm - This is front or rear bolt-through standard

9mm - This is front or rear quick release standard.

This then gets even more confusing when you look at axle length. However for both of the standards you are after, unless the frame is very old, have standard axle lengths. If you are really worried about getting the right/wrong wheels, some hubs have interchangeable axles. For instance Hope Tech.

  • In the higher profile, mid - high end, 15mm XC is common, but its hardly "standard" - a majority of bikes sold are at the cheaper end and still have 9mm QR's.
    – mattnz
    Commented Sep 25, 2014 at 21:31
  • 10mm QR not used on front
    – DWGKNZ
    Commented Sep 25, 2014 at 22:57
  • Thanks,the inner distance between my rear dropouts is 135mm the frame is from 2006. So I should safely assume rear hub is 9mm unless anything specific is stated Commented Sep 26, 2014 at 5:22
  • Why is this answer downvoted? IMHO this is the best answer to the question so far, the other one recommending to 'assume' things.
    – Vorac
    Commented Sep 26, 2014 at 9:53
  • @Vorac because it is inaccurate and lacks information that would make it a complete answer. Widths and diameters should be answered together, not said it is confusing.
    – DWGKNZ
    Commented Sep 26, 2014 at 11:27

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