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MTB Axle Standards Explained

Posted: Thursday 25 May 2017. Category: Cycling

Does the mere mention of Mtb axle standards make you feel tired and slightly confused? Do you find your fingers reaching for your scalp when you think about hub and frame compatibility?

And do you ever wonder why there are so many axle standards in the first place?

If your answer is yes to these questions, then read on, and all will be revealed. 

The 5mm Quick Release

First up in this rather long list is the good old 5mm Quick Release. This system has been around since the beginning of time, (well, about 80 years) and is still used today on nearly all road bikes and most entry-level mountain bikes.

QR skewer

Standard fork and frame 5mm QR skewers with cam-lever locking mechanism 

How are they fitted?

Easy ... 5mm QR wheels are positioned by sliding either the 9mm or 10mm tips of your axle end caps into your dropouts. Once in position, the wheel is then secured by the cam-lever locking mechanism of the 5mm QR skewer.

QR end caps

Modern, push-in fork end caps – the 9mm tips of these front end caps sit in the fork dropouts to align the wheel 

Why 9mm and 10mm?

9mm is the width of a QR fork dropout and the 10mm is the width of a rear QR frame dropout.

The verdict

Fine for most recreational riders. However, if you’re into a more aggressive style of riding or are thinking about starting out in competition, it’s probably a good idea to change to something a bit more substantial.

9mm/10mm QR Thru Axle

Next up is the 9mm and 10mm QR thru axle. The main difference between this system and the 5mm QR is the larger diameter 9mm shaft acts like an actual axle rather than just a securing skewer. 

9mm QR thru axle

A Superstar 9mm fork QR thru axle 

How are they fitted?

With this system, wheels are positioned by dropping the ends of the axle shafts into your open dropouts. The axle is then clamped using the same type of cam-lever as the narrower 5mm QR skewer.

9mm thru end caps

End caps without tips for a 9mm fork QR thru axle. These sit inside the dropouts, not in the dropouts – the ends of the axle sit in the dropouts to align the wheel

Why 9mm and 10mm?

Same as the original QR – 9mm is for the fork and 10mm for the rear dropout.

Compatibility

9mm and 10mm QR thru axles are compatible with all older style QR frame dropouts – and most newer 5mm QR hubs can be easily converted with end cap adapter kits.

The verdict

The jury’s out on these devices with rider opinion differing from a noticeable increase in stiffness, to a slight increase in stiffness, to virtually no difference at all from a standard 5mm QR system. The good news however, it’s a fairly inexpensive experiment with many adapter kits available for under £20.00 and thru axles like the Superstar priced at just £7.99.

Front 15mm and 20mm Bolt Thru Axles

Bolt thru axles are wider, stronger and stiffer than QR systems and have been standard on most high-end performance mountain bikes for about the last ten years. The 20mm is generally used for DH/Enduro and the 15mm for just about everything else.

 

15 bolt thru

 

A Rockshox Maxle – a threaded 15mm thru axle with cam-lever

How are they fitted?

15mm and 20mm bolt thru axles are either screwed directly into a threaded fork dropout and secured with a cam-lever or torque bolt, or are held in place by a twin pinch bolt system.

fork thread in

A Rockshox 15mm Maxle attached to fork – it's threaded on the left and secured on the right with the cam-lever

fork pinch bolted

A Fox bolt thru axle using a twin pinch bolt system 

Compatibility

A lot of newer hubs can be adapted to take either a 15mm or a 20mm thru axle. However, you will need a 15mm or 20mm specific fork for each system.

It's also worth noting that old style QR dropouts are not compatible with either 15mm or 20mm bolt thru axles. However, it is possible to convert some older style forks by changing the lowers to bolt thru compatible – quite a bit of work though.

The verdict

Greater stiffness, translates into all-round better handling. They also don’t succumb to the forces generated by disc brakes, so the chances of them working loose are very remote indeed. A must have for modern styles of aggressive Trail, Enduro and DH.

Boost: 15mm Front Bolt Thru

With its wider hub and flange spacing, Boost is the latest in bolt thru axle standards. It was developed primarily to increase the stiffness of 29inch and 27.5 wheels. 

Boost 110mm

 

A Superstar 110x15mm Boost hub 

How are they fitted?

Boost axles are fitted in the same way as the earlier 15mm/20mm bolt thru axles.

Pike with Boost 15mm

A Rockshox Pike fitted with a threaded Rockshox Boost 15mm Maxle - this particular model is secured with a torque bolt

How do they work?

To increase wheel stiffness, the Boost 15mm uses a wider 110mm hub. This extra width allows for a wider flange spacing, which increases the bracing angle of the spokes.

Hub width comparison

This diagram illustrates the difference in flange spacing between a standard 100mm hub a new 110mm Boost hub.

Note: the difference in flange spacing is also the same when a Boost 110mm hub is compared to a 110mmx20mm DH hub 

Compatibility

Boost’s wider hubs mean that they are non-compatible with standard 100mm forks. And even though Boost is the same width as the DH 110x20, the wider flanges change the rotor positioning, also meaning non-compatibility.

The Verdict

Manufacturers claim the increased bracing angle of the spokes give 29er wheels the stiffness of 27.5s and 27.5s the stiffness of 26ers.  So, if you feel you would benefit from a stiffer wheel, Boost would definitely be the way to go. 

And moving on to rear axles …

135x12 Bolt Thru Axles

Like the early 15mm and 20mm fork axles, the 135x12 rear bolt thru was the first big jump from the older 5mm QR system. Like the 15mm and 20mm fork axles, it was introduced to add strength and stiffness.

135x12 maxle

A Rockshox Maxle 135x12 

How are they fitted?

135x12s are fitted in the same way as most front bolt thru axles – they simply screw into a threaded rear dropout and are tightened by a cam-lever or torque bolt.

Why 12mm?

Simple, 12mm is the largest diameter axle that will fit through a cassette, lock-ring and rotor.

Compatibility

Many newer generation 5mm QR frames and hubs can be converted to 135x12 by using brand specific adapter kits.

The verdict

Whilst the 135x12 is certainly a much stronger and stiffer option than its 5mm QR predecessor, wheel installation can be a bit tricky. Basically, when installing a wheel with a 135x12 axle, you have to guide and hold it in position with one hand and secure it with the other.

Because of this alignment issue, the reign of the 135x12 was fairly short lived and was soon replaced by the newer, self-centering 142x12 bolt thru axle.

142x12 Bolt Thru Axles

As mentioned above, the longer 142x12 bolt thru axle system was introduced to make wheel installation easier. The axle itself is pretty much the same as the earlier 135x12, it's just 7mm longer.

How do they work?

To help with wheel alignment, 142x12 dropouts have 3.5mm recesses that allow the wheel to sit in position (very much like the original QR). Once the wheel is in position, the axle is threaded into the dropout and secured using a cam-lever or torque bolt.

A 142x12 dropout – you can clearly see the 3.5mm recess which allow for wheel self-centering

Compatibility

Most frames and hubs that use the 135x12 system can be easily converted to 142x12. Some newer 135mm QR setups can also be adapted to take on this newer standard.

The verdict

If you currently have a 135x12 and are tired of jiggling around your rear wheel with one hand whilst trying to secure it with the other, the self-centering 142x12 could well be the way to go. However, don’t expect a world of difference in feel and handling from the 135x12.

150x12 Bolt Thru Axle

Designed for DH/Enduro, the 150x12 bolt thru axle is essentially just a longer, (non self-centering) version of the 135x12.

150x12

 

A 150x12 DH axle – this model is secured with a torque bolt 

How are they fitted?

The 150x12 is fitted in the same way as the 135x12. 

Compatibility

As you may have guessed, the 150x12 will only fit wider DH/Enduro frames.

The verdict

Been used in DH for years but suffers from the same wheel alignment issues as the 135x12.

157x12 Bolt Thru Axle DH

The self-centering 157x12 was introduced to replace the older, non self-centering 150x12.

157x12 hub

NS Bikes DH hub – this hub can be used for either self-centering 157x12 or the older, non self-centering 150x12

How are they fitted?

Yes, the self-centering 157x12 is fitted in the same way as the 142x12 – drop into recessed dropouts and secure with cam-lever or torque bolt.

Compatibility

Most 150x12 frames and hubs can be converted to this newer, self-centering system.

The verdict

Again, there’s not a world of difference in feel from the earlier version, it’s just easier to change wheels.

Boost 148x12 Bolt Thru Axle

Like the Boost 110x15 system, the rear 148x12 was introduced primarily to stiffen up larger wheels.

How are they fitted?

For easy wheel alignment, the Boost 148x12 system uses the same type of recessed dropouts as the 142x12. They are also threaded into the drive-side dropout and secured with cam-lever ot torque bolt on the left. 

Boost rear fitting

The wider Boost stiffens larger wheels, increases tyre clearance and has self-centering dropout recesses

Compatibility

Like the Boost 110x15, the Boost 148x12 has widened the hub to allow for an increase in the width of the flanges. This has changed the chain line and the rotor positioning, meaning you need a Boost compatible frame as well as Boost compatible hubs to use this system.

The verdict

A great idea, but to utilize this system you will probably need to buy a new bike.

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Posted by DL


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