A guide to mountain bike axle standards | Quick-release, thru-axles, Boost and more explained

Are you baffled by the different axle options available for mountain bikes? Our guide below explains the common types of mountain bike thru-axles, as well as traditional quick-release axles.

Like many things bike, axle standards seem to be subject to constant change, with new ones emerging, gaining traction, then sometimes falling by the wayside as tech trends moves on.

The main reason for this with mountain bike axles is the drive to build more robust wheels. A wider axle means that the flanges on each side of the hub can be set further apart, increasing their bracing angle and, hence, the wheel’s strength.

For the rear wheel, working in the opposite direction is the need to keep a reasonably small Q-Factor, so that the pedals aren’t too far apart, as this lowers pedalling efficiency and ground clearance.

The chainline needs to be engineered to run efficiently too, which means moving the chainset and cassette ever-further outboard as axle width increases. Plus the chainstays need to be splayed out more to fit a wider axle, which increases the risk of heel strike as you turn the pedals.

For mid-range and higher spec XC, trail and enduro bikes, thru-axles with Boost spacing are now the norm.

But you can still find lower-priced MTBs running on quick release hubs or non-Boost thru-axles.

Super Boost is the latest thing and downhill bikes use wider spacing too. Meanwhile Cannondale has its Ai system, which addresses the same problem in a different way.

On top of that, road and gravel bike axle standards are different again.

Let’s have a quick rundown of the options.

Table of common mountain bike axle standards

Quick-release skewers

Traditionally, frames and forks had slotted dropouts, which are open at the ends.

Quick-release axles are hollow tubes that slot into the drop outs. The front wheel is 9mm in diameter and 100mm wide. The rear wheel is 10mm in diameter and 135mm wide.

The axle is held in place by a 5mm diameter quick-release skewer that slides through the hole in the axle. There’s a tightening nut at one end of the skewer and a cammed lever at the other that let you tighten the axle in the frame’s dropouts.

That means you can change a wheel rapidly, but there’s a small risk the wheel will come out of the dropouts, alignment of disc brake rotors in the frame can be imprecise and the system isn’t that rigid.

Thru-axles

Rather than an open-ended dropout, bikes with thru-axles have a hole on each fork blade and at the seatstay-chainstay junctions. The thru-axle slots into the hole on one side and screws tight into threads on the other. Some thru-axles have a lever built into them to tighten or undo them, others you need to tighten with an Allen key.

A thru-axle leads to a much more secure connection between the wheel and the bike than a quick release; there’s no chance of it slipping out under load and it’s more rigid.

But things like dropout design, thread pitch and the overall length of the axle differ between makes, even with the same axle spacing, so thru-axles are not always cross-compatible between brands.

There are also options that include an open dropout on one end, so you don’t need to completely remove the thru-axle to release the wheel from the frame.

Front thru-axle standards

The standard size for thru-axle forks on mountain bikes used to be 15mm x 100mm, with 20mm x 110mm used mainly on downhill and freeride bikes.

For trail, XC and enduro bikes that’s largely changed to the newer Boost thru-axle standard, which increases the front axle length to 110mm. At present, there’s no drive to go wider.

Rear thru-axle standards

At first, 135mm x 12mm was the most common size for thru-axle hubs at the rear, with 150mm used for downhill. But 142mm soon superseded these standards, with the extra 7mm allowing a stiffer interface with the frame.

Boost thru-axles

Boost hubs use 15 x 110mm front and 12 x 148mm rear spacing. This means the hub flanges can be set wider apart to increase the lateral stiffness of the wheel, but it makes Boost incompatible with all the older standards.

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

You’ll find Boost thru-axles on most modern mountain bikes.

Super-Boost thru-axles

Super Boost goes one step wider at the rear, increasing the axle width to 157mm. The aim is again to increase rear wheel bracing angles and rigidity and Super Boost can also allow a frame to be designed with increased tyre clearance.

You can find a spattering of Super Boost wheels from many brands, but there’s a limited number of Super Boost bikes out there. They tend to be from smaller brands still, with Pivot being the first mainstream one to offer the standard. Watch out for Super Boost creeping into larger brands’ bikes soon, though.

157mm spacing is also the standard for the rear wheel on downhill bikes. But the hub geometry is slightly different to Super Boost, with different brake rotor placement, so they’re not the same thing.

Cannondale Ai

Cannondale tackles the drive for increased wheel rigidity and greater tyre clearance differently with its Asymmetric Integration, or Ai, geometry, found on bikes like the Scalpel.

In a standard frame geometry, the chainstays spread out symmetrically from the bottom bracket. To allow space for the cassette, the wheel has to be dished asymmetrically, with the rim not centred over the hub. That means that there’s greater tension on the spokes on the cassette side of the wheel, which lowers rigidity, and is more likely to lead to failure.

With Ai, the driveside chainstay spreads out from the bottom bracket more than the non-driveside by 6mm. This allows the rear wheel to be built symmetrically, offering more rigidity – up to 60 per cent according to Cannondale’s claims.

There’s a matching shift 6mm to the right in the chainrings to keep the drivetrain aligned.

Ai requires a specially dished rear wheel. Like Super Boost, that’s something that’s offered by some wheel brands, or you can get your wheel re-dished by a bike shop.

Cannondale also uses Ai on its SuperX cyclocross bike for its improved tyre clearance.