Everything you need to know about Thru Axles: Tullio Campagnolo invented the quick-release skewer almost 90 years ago, never imagining that his simple idea would become the industry standard.
Fast-track to the early 2000s and onwards. The advent of disc brakes on road, CX and MTBs has necessitated a change in thinking to maximise stiffness and minimise power loss. This has led to the introduction of the the thru-axle, which essentially resembles a large pin that passes through holes in the frame and fork tips to secure the wheels – needing to be pulled completely through the spindle before the wheel can be removed.
A 12mm Thru Axle vs. a Quick Release skewer for a road bike.
Improved Steering and Handling
Thru Axles (TA) originally were designed for downhill riders years ago who were popping their front wheels out of their front axles. Because the TA has to pass through the frame and hub, it stops this problem from occurring. The biggest advantages of a TA system vs. traditional quick-release (QR) system is that by pinning the fork legs together, uneven loading is reduced and the lower legs are stiffened. This results in a stiffer front end with improved steering and handling, for example, better tracking through corners at speed.
Better braking with less Drag
Thru Axles also create a stiffer load path between the brake caliper and rotor. If you think of a QR system, they allow the rotor to shift and contact the brake pads, causing brake drag, something you’ll notice when you’re climbing out of the saddle and throwing your bike side-to-side. The TA is larger and has a more rigid connection between the axle and the drop-out, so minimises this happening. This is the biggest benefit for having a TA Disc brake set-up on a road bike. With the proper set up too the wheel is always aligned correctly.
Hub Width & Thru Axle Diameter
Thru axles are frame specific, not hub specific. You have to choose your wheels based on your frame and fork’s compatibility. There are also different thread pitches e.g. coarse, medium, fine for different companies – so bear this in mind when you are doing your research.
To make things even more complicated, there is no industry standard when it comes to frame, hub or wheel designs – it really is dependent on the bike that you end up purchasing.
The two terms that you’ll hear thrown around most are ‘hub width’ and ‘thru axle diameter’. Hub width is the measurement between the dropouts, not accounting for the slot that the hub’s axle ends sit in – as shown in the diagram below.
Thru axle diameter refers to the diameter of the actual thru axle itself, as shown below in the examples of a 12mm and 15mm thru axle:
The following charts summarise the commonly used hub width and thru axle diameter combinations and what they are most used for.
Disc Brake Rotors
Another factor to consider is disc brake rotor size and type. There are two different ‘common’ standards: six-bolt and CenterLock. The latter uses a splined design, which needs to be tightened with a cassette tool. From a mechanic’s point of view there are advantages to centrelock as it makes fitting rotors much quicker, but in terms of performance advantages there’s no real difference in braking performance or weight when comparing the two.
Most road bikes run 140mm or 160mm disc brake rotors. For MTBs, 140mm are the smallest & bare minimum for the rear wheel. 160mm and 180mm are the next size up options. 180mm is most common for MTB. For ultimate downhill stopping power you want 200mm.
Thru Axles for Road Bikes
For off-road bikes, using the TA system is a bit of a no-brainer. For the road, there is some concern about the extra weight of the TA/disc brake set up, which is why a lighter, smaller 12mm diameter has been adopted, with smaller rotors for the road.
There’s also concern about slower wheel changes during races. But quick-release disc brake set ups already slow this process down considerably… To overcome this, many teams are just using spare bikes.
Another issue is if you loose your TA. It can be a real challenge to find a replacement as they vary from frame to frame. But, you shouldn’t lose it as you can always screw it back into the frame once you have removed the wheel.
At the end of the day…
A TA is safer. This may be the deciding factor over the other minor inconveniences of extra weight, longer wheel change times etc. For the weekend warrior, an extra 20 seconds changing a flat tyre on the side of the road isn’t going to make a huge difference in their life!