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Hubs, Axles, Dimensions – State of the Art

Which thru axle is the right one for your hub? We give you an overview of the common dimensions of hubs and axles on MTB, road bike and gravel bike.

Times used to be simpler: quick-release axles with five-millimetre axle diameters were the standard on both road bikes and MTBs. Front wheel hubs were 100 millimetres wide everywhere, rear wheel hubs measured 130 millimetres on the racer and 135 millimetres on the MTB. However, a lot has happened since then: suspension travel has increased on MTBs, disc brakes can now be found on many racers, and instead of eight gears on the rear wheel, the norm has shifted to twelve. In short: The demands on technology grew and the good old quick-release could no longer keep up. One thing is clear: thru-axles (TA) allow a stiffer connection between the wheel and frame or fork due to their larger diameter and the closed dropout in which they are inserted, which was possible with quick-releases (QR). Most of the axles we use on modern bicycles are not thru-axles in the original sense of the word, but screw-in axles, as they are threaded. But because we always have to put them through the wheel for assembly, the term has become widely accepted. Some thru-axles are mounted with tools, some have a screw lever that is either fixed or can be folded down and secured under slight tension. With the introduction of thru axles, however, the number of axle variants has risen sharply. And There has also been a lot of changes in hub dimensions in recent years. So that you keep track and know which axles and hubs you can combine, we are providing you with an overview of the common dimensions of hubs and axles on mountain bikes, road bikes and gravel bikes.

Times were simpler: thru-axles with 5 mm axle diameter used to be the standard.
Times were simpler: thru-axles with 5 mm axle diameter used to be the standard.

Times were simpler: thru-axles with 5 mm axle diameter used to be the standard.

Brave new world: today, thru-axles form a markedly stiffer connection between the wheel and fork or frame.
Brave new world: today, thru-axles form a markedly stiffer connection between the wheel and fork or frame.

Brave new world: today, thru-axles form a markedly stiffer connection between the wheel and fork or frame.

Axle diameter on the modern mountain bike

MTBs are nowadays almost exclusively outfitted with thru-axles. Axle diameters of 15 millimetres at the front and 12 millimetres at the rear have become the established standards. On older bikes with a lot of suspension travel, 20-millimetre axles were sometimes installed at the front. Today, two axle diameters dominate the market, whether boost or non-Boost. Many hubs from manufacturers such as DT Swiss and Hope can be easily converted from quick-release to thru-axle or back again with the help of different axle end pieces.

What is Boost anyway? All about over locknut dimensions

The term has been used more often: with Boost dimensions, the over locknut dimension of the hub has been increased from 100 to 110 millimetres on the front wheel and from 142 to 148 millimetres on the rear wheel. This also had to do with the growing prevalence of larger 29-inch wheels. This is because the distance between the hub flanges increases by these ten or six millimetres, and the wheel thus becomes laterally more stable (see also: Wheel Building 101). At the rear, however, the entire drive moves outwards by three millimetres. That's why you need a Boost chainring with a Boost hub. The space you gain allows more room for manoeuvring at the narrow point between chainring, chainstay and tyre, which manifests for example in more tyre clearance in the frame while maintaining the same chainstay length. The Superboost Plus hub standard on the rear wheel takes things one step further. The over locknut dimension is 157 mm, which allows for a significantly wider flange distance and thus an even stiffer rear wheel. Superboost is still quite niche. However, the measurement is currently becoming more popular with manufacturers like Evil Bikes because it helps to create short chainstays in conjunction with large 29” wheels. Despite the same over locknut dimension, Superboost Plus is not identical to the "old" downhill measurement of 157 millimetres (157DH).

Axle Types: Mountain Bike (Hub Width) Front Wheel (Axle Diameter/Over Locknut Dimension) Rear Wheel (Axle Diameter/Over Locknut Dimension)
Quick Release 5 / 100 mm 5 / 135 mm
Thru-Axle (classic) 20 / 100 mm 12 / 142 mm
Thru-Axle (Non-Boost) 15 / 100 mm 12 / 142 mm
Thru-Axle (Boost) 15 / 110 mm 12 / 148 mm
Thru-Axle (Superboost Plus) 15 / 110 mm 12 / 157 mm

Compatibilities and their limits

You can't fit Boost wheels in a frame and fork with 142 or 100mm over locknut dimension, they are simply too wide. Conversely, you can use a non-Boost wheelset with adapters that fit the hub, such as the MRP Better Boost or the Problem Solvers Booster Adapter Kit to bring it to Boost dimension. You won't benefit from more stable wheels, but you can continue using existing wheels for the time being. Since you have to consider not only the over locknut dimension, but also the position of the brake discs, this conversion is only recommended for experienced mechanics.
But beware: Some manufacturers exclude a conversion of their hubs to boost. DT Swiss, for example, explicitly points out that converted hubs will void their warranty.

Special case: torque caps

To further increase the stiffness of the front wheel/fork unit, RockShox has developed torque caps: the dropouts and their counterparts on the hub each have a significantly larger surface area. This increases support. You can put a hub without into a fork for torque caps, but not the other way around. The good news: For the most common front wheel hubs, there are simple end pieces for conversion.

The evolution of over locknut dimension on the road bike

On the road bike and its cousins, the gravel and cyclocross bike,meanwhile, the disc brake has arrived – and with it the thru-axle. Only extremely lightweight or cheaper new bikes come with rim brakes, quick release skewers and traditional over locknut dimensions. While the first gravel bikes still used the usual MTB axle dimensions (15/100 and 12/142 mm), the twelve-millimetre axle has now become common on the front wheel of road and gravel wheels. The over locknut dimensions largely correspond to the non-Boost dimension, i.e. 100 millimetres at the front and 142 millimetres at the rear. The more stable connection between wheels and fork or frame is not only necessary for the disc brake, but also provides the necessary stiffness for mostly suspension-less off-road riding as well as higher loads on bikes with bikepacking equipment.
The latest development called "Road Boost" should also be seen in this light: Twelve-millimetre front and rear axles are combined with Boost over locknut dimensions from MTBs, i.e.: 12/110 and 12/148 mm. As with mountain bikes, the wheels thus gain lateral stiffness and the frame gains tyre clearance. The narrower front axle is designed to reduce weight and drag. These axles are also found in series production in the fast-growing segment of road E-bikes.

Axle Types: Road Bike (Hub Width) Front Wheel (Axle Diameter/Over Locknut Dimension) Rear Wheel (Axle Diameter/Over Locknut Dimension)
Quick Release 5 / 100 mm 5 / 130 mm
Thru-Axle 12 / 100 mm 12 / 142 mm
Road Boost 12 / 110 mm 12 / 148 mm
The choice is yours: classic style..
The choice is yours: classic style..

The choice is yours: classic style..

...or rather sporty and aerodynamic?
...or rather sporty and aerodynamic?

...or rather sporty and aerodynamic?

How do I find the right axle?

If you need a replacement axle, if you want to mount your bike on a trainer or if you want to alter it in terms of weight or colour, you first have to find the right axle. Most thru-axles are labelled by dimension, which makes searching a lot easier.

  • To determine the sizes yourself, first measure the diameter of your thru-axle – whether it’s 15 or 12 mm can be determined quite easily.
  • Secondly, you now need the over locknut dimension of the corresponding hub.
  • Thirdly, the effective length of the thru-axle is often specified for rear wheel axles, which is of course significantly greater than the over locknut dimension. Measure the length of the axle without the axle head, i.e. from the collar on the outside of the frame and behind the point where the lever or the hex or torx head meets. 
  • Fourth, and this is where it usually gets tricky, there are different threads on thru-axles. This is because the axle must always fit its counterpart, the "nut" in the frame or fork, which is why a thru-axle is always included with the frame or fork. With suspension forks, the decision is relatively simple: you can reach your goal quite quickly by looking at the manufacturer's specifications and Boost or Non-Boost. For rear wheel axles, you may need to determine the thread pitch. M12x1.0, M12x1.5 or M12x1.75 are the most common dimensions – for this purpose it is best to print out a template, such as those available from the Robert Axle Project.

By the way, the same procedure is recommended if you need an adapter for hitching a trailer. For this reason, we have axle adapters from Robert Axle Project in our selection, which allow for every imaginable combination. But make sure that your frame is approved for towing a trailer!

To know which (replacement) axle you need, measure the diameter of your thru-axle.
To know which (replacement) axle you need, measure the diameter of your thru-axle.

To know which (replacement) axle you need, measure the diameter of your thru-axle.

For rear wheel axles you need the effective length of the axle, which of course is greater than the installation width of your hub. Measure the length of your axle without the axle head.
For rear wheel axles you need the effective length of the axle, which of course is greater than the installation width of your hub. Measure the length of your axle without the axle head.

For rear wheel axles you need the effective length of the axle, which of course is greater than the installation width of your hub. Measure the length of your axle without the axle head.

You also need the thread pitch of your thru axle.
You also need the thread pitch of your thru axle.

You also need the thread pitch of your thru axle.

The axle finder from The Robert Axle Project, which you can also download from us, is very helpful.
The axle finder from The Robert Axle Project, which you can also download from us, is very helpful.

The axle finder from The Robert Axle Project, which you can also download from us, is very helpful.

Exotic and speciality shapes

The dimensions mentioned in this text are only the most common variants on mountain bikes, gravel bikes and road bikes. There are plenty of other types of dimensions. On folding bikes, for example, there are extra-narrow front wheel hubs (e.g. Brompton: 74 mm), track bikes have extra-narrow rear wheel hubs with 120 millimetres, tandems are built extra-wide at the rear wheel (145 or 160 mm) and fatbikes can have a rear wheel dimension of 135 millimetres at the front or require over locknut dimensions of 170, 177, 190 or 197 millimetres at the rear. Before you buy the wrong item or damage components when mounting incompatible axles, better measure twice or contact our service team. We are happy to help you find the right parts for your bike!