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By the early 2000s, downhill and freeride bike frames were strong enough to withstand massive jumps that would bend quick release skewers and pull wheels out of dropouts. The thru-axle was invented to combat this problem. It’s essentially a large bolt that screws into the fork or frame, creating a rock solid attachment point for the wheel.
In the past, all bicycle wheels attached to U-shaped dropouts on the forks and frame. Nuts on the axle clamp down on the dropouts and the hub, keeping the wheel in place. Quick release skewers add a cam to this system, so the axle nuts can be tightened down loosely by hand, then clamped onto the dropouts using the cam lever. This system is simple and cheap, but it has some disadvantages.
When used properly, a quick release is a secure way to attach wheels to a bicycle. However, it’s easy to tighten the skewer either too much or too little, and there’s always a chance that the cam lever will catch against something and open up. When that happens, there’s nothing to keep the wheel from separating from the bike. This is so common that more lawsuits have been filed against quick-release skewers than all other bicycle components combined. To combat this, manufacturers added retention lips, often called “lawyer lips,” to stop wheels from falling off. These are washers with small tabs that slide into the fork or around the chainstay to keep the wheel from falling off if the axle is loose.
By the early 2000s, downhill and freeride bike frames were strong enough to withstand massive jumps that would bend quick release skewers and pull wheels out of dropouts. The thru-axle was invented to combat this problem. It’s essentially a large bolt that screws into the fork or frame, creating a rock solid attachment point for the wheel.
Thru-axles spread to other bike categories as disc brakes were introduced. A thru-axle places the wheel in the same spot every time, and it spreads load forces from the brakes across the wheel. This improves braking performance and consistency. It also fixes the quick release skewer’s wheel separation problem. Even if a thru-axle isn’t tightened down correctly, the wheel can’t fall out unless the axle is unscrewed from the bike frame.
Thru-axles are significantly thicker than quick release skewers and are almost always hollow. They always have one threaded end that secures the axle into the frame and/or fork.
Quick release skewers are 5mm thin and always have a lever that locks the skewer in place into the dropout or frame.
Some thru-axles have quick release levers. This type of axle is sometimes called a thru-axle skewer. At a glance, it can be hard to tell the difference between these axles and quick release skewers. Here’s how you can tell which attachment system your bike uses.
Quick release skewers -They are significantly thinner than thru-axles, with a 5mm shaft and a small spring at each end. One side of the skewer always has a lever, while the other side has a cap nut. The skewer does not attach to the frame or fork -- ie. they do not have a threaded end.
Thru-axles - Thru axles always have a threaded end that screws into the frame or fork. The thinnest thru-axle has a diameter of 10 mm, so it’s clearly beefier than a skewer. Thru-axles don’t use springs. Bikes with disc brakes almost always use a thru-axle skewer.
What is an annoyance for local riding can become a logistical nightmare if you’re touring and bikepacking. When debating between thru-axle vs quick release skewers, most long distance riders will go with skewers.
Quick release skewers clamp down on traditional dropouts. These are U-shaped, so the axle can slide into the frame before closing the skewer. Fork dropouts and recent multi-speed bikes have the opening of the dropouts pointing down. Most fixed gear and single speed bikes have rear track dropouts with the opening at the end of the frame, while cruisers and old multi-speed bikes have the openings pointed toward the crank.
A thru-axle must be used with a frame and fork with hole-shaped dropouts. The axle screws into dropouts, so the wheel always ends up in the same position. The axle must be removed from the chainstays and wheel before the wheel can be removed from the bike.
Here an example of how a thru-axle in a rear wheel is attached to the frame. This particular thru axle has a lever that locks in place, however not all thru-axles have this lever.
While thru-axles may seem like a superior choice, they aren’t perfect. There are still many reasons why you may prefer a quick-release skewer, just as there are several reasons you may want to consider thru-axle hubs for your next bike.
As the name implies, quick-release skewers are easy to release. Just flip the lever, and you can slide the wheel out of the bike. There are thru-axles with levers, but they don’t work like a skewer. You still have to unthread the axle from the bike. This takes slightly longer to remove. While this isn’t an issue for most riders, it could cost you a few seconds in a race. There are a few unique solutions around this problem, but you aren’t likely to see them unless you have a high end thru-axle for road bikes built for racing.
Track dropouts are still common on fixed gear and single speed road bikes. Moving the wheel back and forth in the dropouts makes it easy to tension the chain. However, most new single speed mountain bikes use thru-axle hubs for added strength.
The nuts on axle washers, axle bolts and quick release skewers have a toothed or textured surface. This bites into the dropout to keep the wheel in place. These fasteners don’t bite down in the same place every time, which can lead to alignment issues. A misaligned front wheel can make the rotor rub against the brake pads. A misaligned back wheel places the cogs at an angle, which makes shifting harsh and unpredictable. There’s only one position for thru-axle wheels, which eliminates these problems. When it comes to thru-axle vs skewers, thru-axles are the clear winner if you’re using modern drivetrain and brake components.
Thru-axles add strength to the bottom of forks, improving torsional rigidity. This helps the bike track through corners. It also reduces twisting forces on suspension forks that can lead to rotor misalignment and broken axles.
On average, a thru-axle weighs about 20 grams more than a quick release of similar quality. The added material used for the dropouts increases weight, but less material is needed for the forks, seatstays and chainstays. That’s because thru-axles act as a structural components. On average, a thru-axle bike weighs around 100 grams more than a quick release bike.
Most internal gear hubs and electric motor hubs won’t work with thru-axle frames. The axle is the anchor point for the sun gear, so it’s under torsional stress. To keep the axle from spinning, keyed washers push against the dropouts to support the axle. There are a couple internal gear hubs from Kindernay and Rohloff that will work with thru-axles. These use adapters that clamp onto the chain and seat stays to keep the axle from spinning. These adapters only work with a few frames.
Quick release axles have been around for almost 90 years, so they’re well established and easy to find. You should have no trouble picking up replacements, no matter where you are in the world. Due to their shorter availability and endless range of sizes, you will probably have to order replacement thru-axles. What is an annoyance for local riding can become a logistical nightmare if you’re touring and bikepacking. When debating between thru-axle vs quick release skewers, most long distance riders will go with skewers.
As the name implies, quick-release skewers are easy to release. Just flip the lever, and you can slide the wheel out of the bike. However, unfortunately this ease of use also makes them extremely vulnerable to theft - See how Hexlox can help you secure almost any part on your bike
There are 5 key measurements you need to know to get the right thru-axle:
Diameter – This is how thick the axle is, excluding the head.
Total length -- This is the length of the axle excluding the head.
Thread length – This is the length of the threaded area on the axle.
Thread pitch -- Thru-axle threads are measured in millimeters between each thread.
Bolt head -- These are either flat or conical (X-12 cone).
Most axles now have this information printed on them.
The most common axle diameters on mountain bikes are 15mm in the front and 12 mm in the rear. Front hubs use thicker axles to increase the fork’s torsional strength. Downhill and early thru-axle forks use 20mm front axles for added strength. The first road bikes to adapt thru-axles use 15 mm mountain bike axles before switching to 12 mm axles to save weight.
In the past, axles and skewers were measured by over lock nut dimension (OLD). OLD is the distance between the inside faces of the dropouts, also called “fork spacing” or “frame spacing.” Hubs are still sold in a few standard sizes, but dropout thickness varies from bike to bike. For this reason, thru-axles should be measured by overall length. Depending on the thickness of the dropouts, total length is usually 20-30mm longer than OLD. The most common front thru axle OLD dimensions are 100 mm, 110 mm, and 150 mm for front axles and 130mm, 135mm, 142mm, 148mm, and 197mm for rear axles. To accommodate both OLD sizes and dropout sizes, there are over 40 different thru-axle lengths on the market today.
Thread pitch is stated as the space between each thread. The easiest way to estimate the thread pitch is to count the threads over a set length:
The side of the bolt head that presses against the dropout is either flat or conical. The conical standard is also called “X-12 cone.”
Tip! - Download our handy Thru-Axle Fitment Guide here - to quickly check your thru axle measurements:
The Hexlox UniversalThru-Axle takes the hassle out of finding the right size thru axle for your bike. No more measuring! Our thru-axle will automatically adjust to your bike's measurements - for a perfect fit.
Selecting the correct size thru axle for your bike is as you can imagine, unfortunately, rather complicated. Even after having read through this article, do you know the length, diameter, and thread count of your current axle?
We thought so 😉
The Hexlox UniversalThru-Axle takes the hassle out of all this. No more measuring! Our thru-axle will automatically adjust to your bike's measurements - for a perfect fit. After years of research and development we at Hexlox has developed our Perfect Fit System.This patent-pending Perfect Fit System adjusts telescopically to perfectly fit your bike's fork/frame. Aero Road, MTB, E-Bike, Fat Bikes*, Trailers*and even Trainers*.
*Might not fit all... - Contact our super-friendly team at support@hexlox.com and we will sort you out.
TheHexlox Universal Thru-Axle has been mercilessly test-ridden by countless riders - including downhill - so you can confidently go out there and just ride.
The Hexlox Universal Thru-axle is precision engineered in Germany and - as the cherry on top - is Hexlox-Ready. This means that you can easily secure your thru-axle and wheel against theft with the Hexlox anti-theft system. We honestly believe that we have created the world's smartest - and best looking - anti-theft system for bike parts.
Check out how Hexlox can help you secure your bike parts here...
Is a gravel bike a road bike or a mountain bike? It’s hard to say, both from an owner’s and a designer’s standpoint. Usually, these bikes use either 12 or 15 mm thick front axles and small “non-Boost” mountain bike rear axles.
Electric mountain bikes may use mountain or downhill components, depending on what the manufacturer thinks is appropriate for the bike’s added speed and power. Likewise, road electric bikes use either road or lightweight mountain components.
These standards use longer axles than previous designs to address common problems with mountain bikes. Increasing axle length allows more space for drivetrain components and stronger wheels. It also moves the chain line outward, keeping the chain from rubbing against wide tires. Most mid to high-range mountain bikes use Boost axles, while most new downhill bikes use Super Boost axles.
Front Boost and Super Boost thru-axles are 110mm long and either 15 or 20 mm thick. Rear Boost axles are 148 mm long and 12 mm thick, while rear Super Boost thru-axles are 157mm long and 12 mm thick. Super Boost axles are the same size as those used on most downhill bikes, but the Super Boost hubs use a different mounting position for the brake rotor.
A fat size thru-axle for MTBs has to be long enough to accommodate giant tires. These bikes typically use a hub that’s 150 mm wide, 40 mm wider than a standard mountain bike hub. 197 mm is the most common fat size axle, but there are many options to fit a variety of dropouts. Currently, the largest axles in use have a total length of 229 mm.
A thru-axle adapter lets you attach the thru-axle forks on your bike to a bike rack. Some adapters are designed to fit in place of the axle, so you can attach your forks to your rack’s existing hardware. Other adapters clamp over your bike’s axle.
In most cases, all you need to worry about when buying an adapter is matching the width of the fork chainstays. Most adapters have sleeves to fit a variety of axle diameters. Some mountain bike forks with oversized, non-threaded fork legs need an axle that adds outward bracing. On these bikes, you should only use adapters that clamp over your bike’s front axle.
There is no universal kit that lets you convert any bike to use thru-axles, or switch between axle sizes. You’ll need to do some research to find out if there’s a solution that works with your bike.
You can install a thru-axle compatible fork on any bike, and pair it with a thru-axle front wheel. Some forks that have replaceable dropout cups, letting you swap in a pair of thru-axle compatible cups to fit a thru-axle wheel. This option is usually limited to high end carbon fiber road forks. If you have mountain bike forks that already support thru-axles, there may be kits available to switch the cups to fit different axle sizes. These kits are usually set up to use larger Boost axles, giving you more choices for wheel upgrades.
There are some mountain bike frames that are designed to be upgradable to thru-axles using a kit. These kits have adapters that fit into the dropouts, adding the threads and axle supports needed for a thru-axle. This type of conversion is dropout specific, so you’ll need to contact the frame or bike manufacturer to see if this is an option.
There are three ways to rotate thru-axles when attaching or removing them from a dropouts:
Any type of axle can be used with any hub, as long as the axle is the correct size.
Since thru-axles have to accommodate both the dropout and the hub width, some bike manufacturers have introduced their own axle sizes to fit their frames and forks. Logically, you would think that if you buy a bike from a company that makes their own axles, you need to buy your axles from that brand, right?
Not really. Most brands that offer bikes that use proprietary axles also sell bikes that use standards like Boost. There are also proprietary axles that work in several brands of bikes, because these companies work with the same component suppliers. For example, some Scott and Santa Cruz bikes use proprietary 168mm length rear axles with a 12 mm thread length and 1 mm thread pitch. However, other bikes in these company’s lineups use SRAM’s Maxle Stealth. Scott and Santa Cruz are not related in any way, they just happen to build bikes that use the same unique axle size. There are aftermarket manufacturers that make axles in proprietary sizes, so you don’t have to buy a replacement from the bike manufacturer. At least you shouldn’t, unless you have a high-end road bike.
How do you get the advantages of a thru-axle with the speed of a quick release skewer? Several manufacturers have tried to answer that question with their own unique axle designs. This includes everything from using retractable pins in place of threads to replacing the non-threaded dropout with an open one, so the wheel falls out as soon as the axle is unscrewed. Since production numbers are low, there isn’t much demand for aftermarket parts that work with these systems.
On axles with cam levers, manufacturers usually recommend tightening down the axle, then backing it off a half turn. Folding the lever takes up the remaining slack. The lever should fold in next to the fork arm, chainstay or seatstay. If the front axle lever points forward, it could be pulled open by branches and other obstacles, and if it folds over the fork, it won’t close all the way. Likewise, it’s easy to accidentally open a rear axle lever if it’s pointing forward.
How do you get the correct axle tightness and get the handle in a safe location? Some manufacturers use a splined axle end with a matching handle. Pull the handle out slightly, and you can spin it around without moving the axle. This way, you position the lever so it folds down where you want it. There are also attachment systems like the QR-15 that use a captive nut that fits into the dropout in several orientations. Rotating this nut changes where the axle bottoms out. Get the nut in the right position, and the axle will always tighten in a place that leaves the lever in a safe position.
Thru-axles are supremely solid and strong, and is becoming the new standard, even for weight-sensitive road riders aka. weight-weenies
When it comes to thru-axle vs skewers, your best option depends on how your bike is equipped, and how you use it. Thru-axles are better for disc brakes, they’re stronger, they increase the rigidity of the frame and forks, and they’re safer overall. Quick release skewers are cheap, easy to replace, and make removing and installing wheels faster. Axles that experience torsion, like those used in hub motors and internal gear hubs, usually aren’t thru-axle compatible.
Finding the dimensions of a thru-axle are more important than trying to find the name or standard, because there are so many variations. That's why, in case you're unsure about your bikes dimensions and measurements, we recommend you to consider the Hexlox Universal Thru-Axle that automatically adjust to your fork and/or frame - for a perfect fit.
Thru-axles for road bikes, mountain bikes, e-bikes and gravel bikes all share components, while the largest axles are reserved for downhill and fat bikes. While OLD is mostly standard, the actual length of axles varies from bike to bike due to different dropout widths. Proprietary axles aren’t necessary limited to one brand.
Only a handful of frames can be upgraded from quick releases to thru-axles. Upgradable wheels and forks are more common.
If you don’t like the position of your thru-axle handle, check the lever end and nut. Some adjustability is built in so you can get the axle tightened down and have the handle in a safe location.
