Through-axles or thru-axles (TA) are becoming the standard axle for most bikes, originally used on downhill mountain bikes and freeride bikes starting in the 2000s. Under the demanding conditions, riders were breaking quick-release axles. Quick-release axles could also dislodge from the dropouts or otherwise malfunction under heavy braking. Now, it's possible to find gravel bikes, road bikes with through-axles, even exercise bikes are being equipped with them. The following 2 pictures are a comparison between a thru-axle used for stationary bikes, a MTB thru-axle and a bolt on axle.
Thru-axles offer triple the thickness and a much simpler design. Thru-axles slot through closed dropouts in the fork/frame through the wheel and screw directly into an integrated nut located on the opposite side of the fork/frame. At 12mm thick thru-axles are 7mm thicker than quick-release axles. The thickness results in a stiffer and more controlled ride, especially under braking with disc brakes.
Various widths and applications:
Thru-axles allow for an increased frame width which encourages wider hub widths. Wide hubs combat lateral load rollover when using 36", 29", and 27.5" tires and wheels common on today's mountain bikes. Another benefit is that above 110mm it's possible to build symmetrical hubs with a better Q-factor, which is beyond the scope of this post.
SRAM and Trek have pushed for a standard 148x12mm axle, but thru-axle width can range from 100-150mm front and 130-197mm rear. Within that range, you will find Boost (148mm) and Superboost (157mm) axles which became popular around 2014. E-bikes and fat bike hubs lie on the wider side of the range.
Hex tools are used to install and remove them, although you can buy thru-axles with removable or foldable levers, which make them nearly as convenient as quick-release axles. Thru-axles can get a bit complicated to purchase as frame width and thread pitch can vary per manufacturer. The Robert axle project was developed to aid in the purchasing of uncommon thru-axles or ones specific to bike trailers and accessories. There is also the Rapilock product that has different attachments to change thread pitch and axle width.
What are quick-release axles?
Quick-release skewers were invented by the legendary Tullio Campagnolo to switch gears during road races. It may seem like forever ago but there was a time when you had to flip your wheel around to change gears. That’s a far cry from the state-of-the-art 30-speed electronic transmissions we have today. You can still get fixie bikes that have gears on both sides of the hub, enabling fixed or freewheel riding. At the time of writing hub brakes are still used in road racing where aerodynamics and weight savings are crucial. Qr axles are 100mm front 135mm rear with a diameter of 5mm.
Quick-release axles became popular as they offer a tool less way to remove your wheels for maintenance, transportation, and storage. Quick-release slot into frame and fork dropouts. Rotating the knob adjusts the tension. When the correct tension is reached you lock the lever.
There is a risk of the quick-release axle becoming loose if not fastened properly and might require adjustment every few rides. Trek has recalled several bicycle models as the quick-release lever could rotate 180 degrees. It was possible that the lever could be lodged in the brake rotor if not properly locked. This would be mostly due to human error though. The lever also adds additional weight for those on the competitive edge.
Bolt-Ons:
If you’ve been browsing through the selection of bikes at your local big box store. You might have seen bolt-on axles. They are rods with threaded ends to accept nuts. They require a wrench to remove the wheel. Used to reduce costs on affordable bikes often inexpensive kids' bikes. Bolt-on axles are fine as many people rarely need to remove their wheels and would rather have an affordable bike.
How are bike axles made?
Swiss lathes are a great way to produce axles or other shafted parts. The simplified top-level process is a raw bar, usually, aluminum 7075 or high carbon steel is placed in a machine hopper. It’s fed into the swiss machine, where it’s worked on, and out pops a part via conveyor belt. Right! We make it seem pretty simple.
You can read more about swiss machines here:
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