Thru-Axles

Sifting through all of the new jargon and technology while shopping for an e-bike can be daunting. Whether you are already familiar with old steel bike standards or you are entirely new to looking at bike specifications, modern bikes and especially higher-end bikes can bring a dizzying number of new terms. 

Let’s take a couple minutes and run through one important but seemingly small one: thru-axles.

Thru-axle refers to the system in which a wheel is mounted using a separate, removable axle that is installed from the side rather than the bottom. In contrast to the other kinds of axle systems: quick release (QR) and bolt-on, which also are named based on how the wheel is installed into the bicycle. If you look at your fork or your rear dropouts from the side, looking across the axle horizontally, you can identify a thru-axle as it is surrounded by the fork/frame on all sides. In contrast, a QR or bolt-on system must be dropped out of the bike, so the dropouts are generally some sort of U-shape.

LEFT: thru-axle on the fork of the Urban Arrow Family (MY 22-). RIGHT: Quick Release (not a thru axle!) on the Tern NBD. See the cutout at the bottom-right of the dropout for removing the wheel, whereas the thru axle must be pulled out from the side.

The removable axle itself in the thru-axle system is often referred to as the thru-axle, but you’ll also see thru-axle used as an adjective to describe the hub, wheel, frame, etc, designating it is designed to be used with a thru-axle. 

I can find no actual sources, but the common lore is that thru-axles started to show up in mountain bikes sometime in the early 2000s and have slowly crept into other bicycling disciplines since.

Thru-axles came from motorcycles and dirt bikes, and bring the benefit of adding substantial stiffness without significant added weight. Thru-axles also provide better wheel retention, and ensure the wheel goes in the same way every time, reducing the likelihood of requiring a brake adjustment after reinstalling the wheel. It is fairly common for bikes to have a thru-axle on only one of the wheels, often the front.

These benefits make them popular on cargo and electric bikes, including models from Tern, R&M, Urban Arrow, and a few Gazelles. 

The biggest cost for thru-axles is financial – they are more expensive, and require the wheel and frame/fork be designed for them. It is usually impossible to upgrade a bike to thru-axle, so if you want a thru-axle system, make sure to buy a bike that comes with one. Thru-axles also take more time to handle than a traditional quick release, which may be an issue for pro-racers, but is not significant for city riders.

Another important consideration for thru-axles: trailers. Many trailers mount to bicycles on the rear axle, and so the trailer hitch needs to be compatible with the axle type. Or, some bikes, like the Tern GSD, have an alternate mounting option to mount the trailer to the frame directly rather than the axle.. Generally, the trailer companies put out various adapters and guides (like this one from Burley) to help you figure out if it will work on your bicycle, but we’re also happy to help!

Getting into the weeds a little, there are multiple standards, and the naming conventions for thru-axles are often confusing. Replacing a thru-axle can be surprisingly complicated. Thru-axles are generally described by the diameter and hub width, though there are other parameters to consider as well. Overall, if you have the old thru-axle, that is going to be easiest to use as a guide to source a replacement. But we’re happy to help source thru-axle replacements for the bikes we are familiar with.

A couple different thru-axles showing some of the different lengths, diameter, and attachment styles.

What other terms are you curious about hearing more about?