Axle interfaces and rotor type- how do I make the right choice?
With disc brakes being more and more common, mountain bike axle type, rotor type, rotor size and rotor interfaces are becoming an increasingly major question. I would like to try and dispel a few myths here and also try and offer some clarity on the options and what is relevant where. Information on disc brake hubs can be found here.
Disc brakes – IS (international standard 6 bolt) or centre lock?
Ultimately this depends on the hubs you want and the brakes you are planning on running. If you go and buy yourself a centre lock hub, that will allow you to run centre lock rotors. You can also use a converter to run 6 bolt disc rotors. So centre lock probably seems like the more appealing option. However, there are down sides to that. Firstly, not many people make centre lock hubs. White Industries do and DT Swiss do. You can get a centre lock Schmidt hub. Chris King are developing some too.
Why, you may ask, do some manufacturers avoid centre lock? Well, there are three key reasons; first, the centre lock mounting is quite small. That means you have to squeeze a pretty small bearing in, especially if you want to run a large axle diameter which most people do if they want stiff hubs. Second is that the interface is not an open design. You have to be licensed by Shimano to make a centre lock hub. Finally, not that many people want to run centre lock, mostly because only shimano rotors are made in centre lock and most people do not run shimano rotors; so why get a centre lock hub if you have no intention of running centre lock rotors.
Centre lock is a nice interface, you fit the rotor easily with a lockring and fundamentally it is nice and secure and quick and easy to fit/remove. Moreover a lockring is less likely to shear off or work loose, whereas a small rotor bolt can often cause issues.
Which is lighter? Well, it depends. Some of the lightest hubs are 6 bolt, however if you take something like a DT 240s, the centre lock version is lighter. However, it is mainly lighter because the interface allows for smaller flange diameters, so less material is required, however larger flanges make for stiffer wheels, so all being equal, it is about the same.
So if I run shimano rotors, I have to have centre lock? Well, no, Shimano make rotors in 6 bolt as well. While they would prefer you to be on centre lock, they do not want to restrict their sales by only making centre lock.
What about 140mm rotors? Well, there is no shortage of 140mm rotors in centre lock or 6 bolt. However, Shimano do not make 6 bolt disc 140mm rotors. So that means, you have to have centre lock if you want a shimano 140mm rotor. That is hardly a concern if you are taking a MTB to a trail centre, however on road disc, 140mm is the most popular size. Moreover, Shimano is a popular brake choice.
Which should I choose? Well, that is up to you. 6 bolt disc gives you a massive variety of hubs to choose from and still a massive variety of rotors. It does not require a converter and it is an international standard, which is not often something produced in the cycling industry. If you are running road disc and want to run 140mm rotors, you have to think more carefully. However, I would question the wisdom of 140mm rotors anyway – there is no real benefit to them aside from weight. Larger rotors offer more power and better modulation as well as better cooling. You may or may not have a choice on rotor size depending on your frame/fork. If you can go 160mm, you can still run a full Shimano system on any 6 bolt hub.
Quick release and through axles
It is easier to take it front and rear separately. So, starting with Fronts:
9mm quick release
Front traditional quick release forks are nearly always (more than 99.9% of the time) 100mm spaced. You can use a quick release or bolt on (different to bolt through) fit with these forks. So any track or quick release hub will work. The axle itself is longer than 100mm and sits in the fork, a quick release skewer is put through a hole in the axle and clamps it all in place. However, demands in mountain biking and now also on some road/path/cyclocross bikes led to the development of alternatives.
9mm through axle
This was one of the earliest through axle designs and is now among the least popular. It was only adopted for a short period of time and is less stiff than the other through axles. It is not one I would recommend but it does boast increased strength/stiffness over QR.
12 x 100mm through axle
This interface was introduced in 2016, designed primarily for cyclocross and road disc applications. As of 2021 it has become our most popular axle interface due to the popularity of these disciplines. DCR standard disc and centre lock hubs are compatible with this interface.
The reason for the development of through axles was demands for stiffer and stronger systems. In time, demands have also been for this to be lighter, so often thin aluminium axles are now used.
15 x 100mm through axle
This interface is what you will find on some disc brake road bikes, CX (Cyclocross) bikes, a lot of XC (cross country), AM (all mountain) and enduro bikes. Instead of having a hollow axle which goes proud of the internal measurement of the fork, a through axle hub will come up flush with the inside of the fork. An axle is then fed through the fork and hub and then tightened using a variety of systems. While there is some variation in axle types depending on the fork you have, the hub standard (15mm bore, 100mm spacing) remains constant. DCR standard disc hubs are compatible with this interface.
15mm x 110mm BOOST
This is a standard by SRAM introduced in 2015. It is a 15mm axle diameter, however the hub itself is wider, not just the axle. The idea being that the wider hub shell has a bigger bracing angle meaning that the wheels themselves are stiffer and stronger and it also will allow better clearance for wider tyres. This is likely to be a popular standard in the future as it makes a lot of sense.
Careful now: next are two similar standards here but there are differences and it’s important to understand which one you need.
20 x 110mm through axle
More aggressive off road applications demanded a wider spacing and a larger axle for increased strength and stiffness. So the 20 x 110mm interface was used here. Sometimes the 20mm version of a hub is the same as the 15mm, only with essentially a different cap of bearing, in other instances, a completely different shell is used – DT use a different shell on their 240s for 20mm. DCR standard disc hubs are compatible with this interface.
20 x 110mm BOOST through axle
Although the name makes it sound the same as the ‘other’ 20 x 110mm interface, the important thing here is that it’s a boost design. The same principles apply here as they do on other boost hubs; the flanges and disc mount are further apart meaning the two standards are not interchangeable, as the disc rotor will not align with the brake mount on the fork.
Other front variations:
135mm spacing
Well, on the subject of things getting wider. There is also the newer standards for fatbike hubs. How fat do you want to go, well there is 135mm through axles to get the ball rolling here.
150mm spacing
If you want really fat, you can now take it all the way to 150mm. Each of these wider spacings can also have different axle diameters, just to make everyone’s life easier.
Rear hubs:
A the end of all that, you are probably feeling overwhelmed. Well, it is about to get a lot more complicated, because now we have the weird and wonderful world of rear hub axle interfaces and there is a lot more variety there:
Quick release
So this goes back to the standard quick release. However, even here, there is not a fixed standard anymore. The smallest common size is 120mm which is generally not operated by a quick release skewer however the frame/fork looks the same as a standard quick release frame end, which is used on fixed gear and single speed hubs which will almost certainly be horizontal or forward facing dropouts. Not to be confused with mountain bike single speed which can be any of a variety of options. However, there is also a 110mm standard even on a conventional bike (we aren’t going into folding options here) which is an old Kieran track racing standard.
The next size up is 126mm. 126mm is an old fashioned standard that used to be used on road bikes and was used when people started adding a few gears, normally on a screw on freewheel design, however some freehub bodied options did exist allowing up to 7 gears. There is a DCR 126mm screw on freewheel hub which is compatible with this.
Then we get onto our first major standard, which is 130mm quick release. That is the standard for nearly all modern road bikes. By modern, we are also talking about anything in the last 25 years or so, so if you have a road bike with a quick release skewer, it will almost certainly be 130mm unless it is old.
132.5mm. You won’t find a hub in 132.5mm unless you have made one. However 132.5mm is intended to allow you to run both 130 and 135mm hubs. It only works with certain frame materials and is best suited to a steel frame.
135mm. This is the next major standard. It is the go to for quick release mountain bikes, for trekking bikes, hybrids and modern touring bikes. It is naturally stronger than 130mm and especially than 130mm shimano 11s because the dishing is a lot less severe. I.e. the drive side hub flange can be further from the centre line of the hub.
140mm. A tandem size, however still available in qr.
145mm. Another tandem size. Probably the most common tandem size.
160mm. Yet another tandem size.
Through axle
10 x 135mm – this was popular for a bit however its popularity has waned in favour of wider alternatives with larger diameter axles. It was generally paired with the original 9mm through axle front which we talked about earlier.
12 x 142mm – as of 2021, the most common thru-axle interface, due to the rise in popularity of road disc and gravel bikes. Even 12 x 142mm is not a stand alone standard. The original 12mm x 142mm axle standard was developed by Syntace and that standard still remains. That is now referred to as x12. The Syntace 12mm standard had a 12mm bore inside your hub. That meant that the axle that slid through was smaller than 12mm in order to be able to fit. Some other 12mm axles have a 12mm axle, which means that the bore in your hub has to be larger. This is an unusual situation where you can have different hubs of different standard fit in the same frame. It is this time the axle that you have to get right. The X12 system is probably the most widely adopted and for my money, the standard worth sticking with. Sometimes you will still find that a shimano e-thru will work, however some lower tolerance OEM axles based on the e-thru standard will not fit x12.
12 x 148mm – this is the matching partner to the Boost front 15x110mm interface. The idea being it is a bit like 142mm x 12, only you can have the flanges further apart to make for a stiffer wheel, meeting the demands of larger tyres and bigger wheels better.
12 x 150mm – This is a downhill axle, nice and wide and tough for downhill.
12 x 157mm – This is the newer downhill interface.
Fatbike standard – 170, 177, 190, 197mm. These can be in quick release with a skewer or through axle.
Now we have gone through the standards. How do I know what to choose?
Well, it depends a lot on what you are running. Wider is stiffer and generally heavier. It depends on your frame, what it can take. However, from the perspective of choosing bicycle wheels, the most common option now is to get hubs that are within an interface grouping and then have a convertible hub. That way, if you change your frame or fork, you can keep your hub/wheelset. So, for example, there are front hubs that can be 100x9mm qr, 100x9mm, 12mm, 15mm and 20mm through axle. The DCR standard disc comes in this category. You need to get a different hub if you want a fatbike or a boost hub. Similarly at the rear, conversions between 135mm qr, 135mm x 10mm, 12x135mm and 12x 142mm can often be made. Again, the DCR standard disc can do this. Boost is different. Downhill is normally different and Fatbike is different. The quick release standards are normally fixed, so you normally have to have a hub suitable for the quick release standard your frame is built to.
If you need help getting a set of wheels that suits any of these interfaces, feel free to get in touch.