Rubber Side Down | Axle Standards

One of the best parts of the bicycle industry is that everything is standardized. A bunch of guys sat down, decided on widths and diameters, and nothing ever changed… NOT! Nothing is standardized and it feels like every year there is some new part that won’t fit on my bike because I don’t have a compatible frame, hub, fork or bar. While I’m not here to change the course of the industry, I can explain the different axle variations to simplify things for you. Keep in mind that the hub measurements refer to over locknut dimension—OLD—the distance between the outboard faces of the axles locknuts or endcaps.


A 100mm road hub with a QR in a hollow axle

If you are a road cyclist, things are significantly easier for you. There is an extremely high likelihood that you have a 100mm wide front hub and a 130mm rear hub with quick releases. This has remained relatively unchanged since 1927, when Tullio Campagnolo invented the quick release for race use. The Italian had become increasingly frustrated with his cold and numb hands being unable to loosen the wingnuts that were then used to secure wheels in the frame.

This same basic system was adopted by the mountain bike industry as well, with the exception of a 135mm wide rear hub instead of 130mm as with road bikes. As the mountain industry has advanced, it has become apparent that QR’s (quick releases) are not up to task as the drops get bigger and the speeds get faster.

Most mountain forks now use either a 15 or 20mm diameter through axle, with the common open dropout replaced by a closed design with a larger diameter hole through which the through axle slides. The opposite dropout is threaded, and the axle secures into it, facilitating tightening of the assembly. 15mm through axles are seen on most cross country and all-mountain bikes. Longer travel all-mountain and downhill bikes utilize the 20mm axle size.


A 15mm through axle on a mountain front fork

As the 15mm and 20mm fork axle sizes caught on, most mountain designs still used a 135mm rear hub with a QR. Some manufactures experimented with a 12mm through axle with 135mm hubs with the same design as the front fork. This never really caught on, as that axle design didn’t self-center the wheel in the dropouts, which necessitated three hands for installation, or two hands mixed with copious swearing.

Soon after, the 142×12 axle standard emerged. This is a 142mm width rear hub with a 12mm thru axle. It’s important part to understand that the distance between the two dropouts on the frame is still 135 millimeters, with an extra 3.5 millimeters of inset on each side. This 3.5 millimeters is used to guide the hub into the frame and self-center the wheel, making through axle installation much easier. The 135mm dropout distance remains the same to avoid issues with altering the chain line. If a manufacturer changes that, they would then have to consider hub widths and crank arm spacing—opening a can of worms. Because the 142×12 hubs fit inside the same width dropouts, most hub manufactures offer replaceable hub end caps to convert between 135 QR and 142×12. Unless, however, your current bicycle has replaceable dropouts, you won’t be able to switch between standards, so make sure to buy a hub that is compatible with your current frame.


142x12mm through axle rear hub standard—notice how the dropout has a 3.5mm deep channel to guide the hub into the correct position.

Industry Nine Torch hubs offer several end cap options, allowing ultimate flexibility for different setups. They even offer 10×135 rear end caps to pair with DT Swiss RWS Thru Axles. This unique 10mm through axle is compatible with quick release frames, but replaces the hollow axle and QR skewer with a 10mm through axle design providing more stiffness and strength. It’s an upgrade for older frames, which are unable to utilize the threaded 142×12 standard.


Standard QR compared to a DT Swiss 10mm through axle—compatible with 135mm QR style dropouts

And just when you thought we were all done; as road bikes begin to adopt disc brakes, we increasingly see the frame and fork designs include through axles. Though I think the road bike adoption of through axles will occur more slowly than the mountain industry, the design is superior to the 87-year-old QR design in terms of stiffness and strength. However, through axle wheels cannot be swapped as quickly, so some will resist the change strictly for the seconds they could possibly lose in a race.

Included below is a list and chart that show hubs available today and their main application. There are a lot of them, some more common then others. It should give you an idea of how many different options there are and the lack of standardization that exists. I hope this sheds some light on the situation and helps you to understand the “standards” to choose from when selecting your next frame, hub, or fork.

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Rubber Side Down is a weekly column dedicated to the fledgling cyclist in all of us. Art’s Cyclery Web Content Editor, Brett Murphy is not a professional cyclist, and doesn’t try to masquerade as one either, but he does love to ride bikes. Whether you are clipping in for the first time or counting down the days until your first race, read on, learn from his mistakes, and keep the rubber side down.