A guide to mountain bike axle standards

Don't know the difference between Boost and QR? Our guide is here to help…

Are you baffled by the different axle options available for mountain bikes? Our guide below explains the different types.


1. Quick-release skewers

Quick-release skewers are now mostly found on cheaper bikes
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Traditionally, frames and forks had slotted dropouts. The hub axle, which was 9mm in diameter and 100mm (front) or 135mm (rear) wide, rested in these.

A thin (5mm diameter) QR skewer was then slid through the hollow axle to clamp everything in place. Not as stiff or secure as so-called ‘thru-axle’ systems, QRs are now mostly found on cheaper bikes.

2. Rear thru-axles

Rear thru-axles screw into closed dropouts
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Thru-axles are also inserted through a hollow hub axle, but they’re bigger (rear ones are 12mm in diameter) and screw into closed dropouts.

At first, 135mm was the most common rear axle spacing, with 150mm used for DH. But 142mm and 157mm soon superseded these standards, with the extra 7mm allowing a stiffer interface with the frame.

3. Front thru-axles

Some front thru-axles are available with replaceable end-caps
Jonny Ashelford/Immediate Media

Until Boost came along, 15x100mm was the most common thru-axle standard for forks, with 20x110mm used mainly on downhill and freeride bikes.

As with the 135mm and 142mm rear standards, some hubs are available with replaceable end-caps so you can adapt them to fit either size.

4. Boost thru-axles

Boost thru-axles are incompatible with all the older standards
Jonny Ashelford/Immediate Media

Boost hubs use 15x110mm front and 12x148mm rear spacing. This means the hub flanges can be set wider apart to increase the lateral stiffness of the wheel, which makes Boost incompatible with all the older standards.


It also places the chainline 3mm further outboard, which improves clearance between the tyre and chainring.