All you need to know- Hub axle standards on mountain bike


Choices to make

When going out to upgrade your new wheelset, fork or frame you are greeted with a bunch of standards like QR, Thru Axle, Boost and numbers like 100×15 and 142×12, and you might be thinking to yourself: “I don’t care about all this, I just want a part that fits.” – but once you find out what all these mean, it’s really easy to choose the right product for your bike.

So why are there so many different standards?

One word: Evolution. In the early days of mountain biking, there was only one wheel size, the trusty old 26”, but nowadays it seems like there is a new wheel size and width every year or so. Because of this, bike manufacturers decided that new standards are needed in order to cope with all the different forces and demands a wheel is being put through.

Front wheels

Bolt on – This is the oldest standard, but modern bikes don’t use this anymore, though you may still find them on low-end bikes sold in supermarkets & department stores. The hubs axle is threaded on both ends and you tighten the wheel inside your frame or fork with nuts and a 15mm wrench.

 Cheap

 Heavy
  Slow to remove & install
Dying standard

Quick release – Also known as QR, this axle is made up of a 9mm diameter hole inside your hub and a 5mm axle with a quick release lever on one side, and a threaded cap on the other one. Small springs help you keep the skewer centered in the wheel hub, making installation easier. What is great about this axle is that it is lightweight, and as the name says, it is quick. The disadvantages are that you are prone to under or overtightening it and you cannot guarantee the same wheel position once you removed your wheel, as a result you might have to readjust your brakes because of your discs rubbing. Due to its reduced diameter, it is not the stiffest option you can get today.

  Lightweight
  Quick removal

 Inconsistent positioning  Prone to user error
 Not as stiff as thru axle

Thru Axle – As we began riding faster and on more difficult terrain we found the need for stiffer wheels, this is where this standard comes in. Thru axle comes in a few sizes, 110×20, 100×15, and 110×15. The first one is used on Downhill bikes, and the second on pretty much everything else, and the last is also known as Boost, and it is gaining more popularity with the introduction of plus sized wheels.

The first number represents the width of the hub (100mm), and the second the diameter of the axle itself (15mm).

You may still hear some manufacturers use the name “quick release thru axle”, because the thru axle still has a quick release mechanism, i.e. you don’t need any tools to open this type of axle.

110×20 – Downhill bikes are ridden on more challenging tracks, so a wider hub can provide a stiffer wheel. Think about two triangles, one with a narrow base and one with a wide base, which one will be more stable? The same applies for the hub and spokes. The wider the hub (and bigger the axle diameter), the stiffer the wheel.

100×15 – XC and AM bikes don’t need wheels as strong and heavy as DH bikes, so they can get away with narrower and thinner hubs and axles, saving weight in the process.

110×15 – Also known as Boost is the latest axle standard to hit the mountain bike world. As plus bikes gained popularity, forks required more space to fit the large tires (up to 3 inches) so more space was needed between the fork legs. Another 10mm ensures that even the bulkier plus sized tires will fit, with a stiffer wheel coming as a bonus.

What the through axle has going for it as opposed to the 9mm QR is that, apart for being stiffer (because of the larger diameter axle), you can get the same position of the wheel every time, thanks to the axle which threads into the fork itself.

Thru axle has a few minor disadvantages which are not relevant if you are not racing, namely it is not as light as the QR 9mm version and that it is slower to remove. By slower I mean a couple of seconds, so unless you are not racing XC, this will not affect you at all. Thanks to its advantages and negligible drawbacks, the thru axle is the most popular choice for mounting mountain bike wheels.

Rear wheels

Same as the front ones, there was only one way of securing the rear wheel in the olden days, and that is with the bolt on system. What changed in the rear wheel, aside from the reasons specified before was the constant push of manufacturers for more gears.

We started with…well one gear, but now it is not uncommon to see bikes with more than ten gears on the rear wheel. Cramming so many gears in the same amount of space worked for a while, but recently things started changing.

    High stiffness
    Consistent positioning

 Not as light as 9mm
 Slower removal


135×10 – Was the go to size for a long time, and worked with a QR system, with a 5mm axle just like the front.

135×12 – Appeared for the same reason that 100×15 axles appeared, more stiffness in the back wheel. The standard 9mm QR was swapped for the thru axle. There are still some bikes using this standard, but their numbers are decreasing.

142×12 – It is basically the same as 135×12, but manufacturers noticed that mounting the rear wheel wasn’t as easy as it could be, especially with the chain and derailleur tension, it was difficult to get the wheel in the correct position to insert the axle. This is why a 3.5mm groove was machined on both sides of a frame, making it a lot easier to mount the wheel. As with every diameter increase the wheel also gained rigidity.

148×12 – Also known as Boost is the latest axle standard to hit the mountain bike world. Pioneered by Trek and quickly implemented by SRAM, Boost adds another 3mm of length to each side of your hub and frame. What is the benefit? Well apart from the increased rigidity we got before, hubs with wider flanges allow stronger spoke-bracing angle and more stability to the wheel. This was needed for long travel 29” enduro bikes and plus sized bikes, as they became more popular, ensuring a 29” wheel can be as stiff as a 142×12 27.5” wheel, and allowing more space inside the frame for plus sized tires. Since the frame is wider, and the suspension pivots are further apart, the frame itself is stiffer. This is the only standard which affects your crankset

too, because of the 3mm offset of the cassette, you won’t have the exact same chainline as with a standard 135 or 142 width axle.

150×12 – Is a standard only used for downhill bikes, but as hub width is ever increasing we would not be surprised to see this on long travel enduro bikes in the future. Downhill bikes are demanded like no other, so it’s no surprise these bikes get the widest hub width.

157×12 – Also used exclusively for DH bikes, think of it as the same as jumping from 135 to 142.

Axles have changed a lot over the years, but fortunately, you necessarily don’t need a new wheelset every time you change your fork or frame. Depending on the situation, you can get adapters, for example from a standard 9mm QR to a 100mm thru axle, and you can also get different end caps for the same diameter but larger width hub sizes. For example, when changing from 100 to 110 in the front, or 135 to 142 in the rear.

Whether you ride XC, Enduro or even DH, RHC Components has the wheels and hubs which will fit your high end bike with all the above mentioned modern standards.

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