When it comes to bikes, there are a lot of components to learn about. What makes it tougher is that they are constantly advancing, and standards are changing.

Over the past decade, we have seen a lot of advances in bike design, but one of the biggest was the change in bike axle types. 

Not only do we have completely new standards, but all new sizing. In this article, I will tell you exactly what you need to know about axles. We will discuss each type and sizing and even speak about adaptors you can use. We will discuss:

Bike Axle Types

Let’s get straight to the exciting part! Let’s start discussing the different bike axle types and what bikes you are going to find them on! Let’s go!

Bolt-On Axles

© Robbie Ferri/BikeTips
  • Classic Bikes
  • BMX Bikes
  • Fixed and Single Speed Bikes
  • Track Bikes
  • Utility Bikes
  • Commuter and City Bikes

You will find bolt-on axles on older bikes and unique types such as BMX. They are a solid axle and are tightened onto the bike with a 15 mm hexagonal nut and bolt on either side. The axle sits inside the wheel and is easy to install and uninstall. 

They are great for many reasons, the first of which is that they are incredibly strong. This makes your wheels very difficult to steal, as not many people walk around with a 15 mm spanner in their pocket. 

Bolt-on axles are old technology now, and we don’t see it on many common modern bikes unless it’s a BMX or a unique style of bike.

Common Sizing 

9 mm thickness across the width of the axle. Depending on the type of bike, the front axle will be 100 mm to 120 mm and the rear axle 120 mm to 140 mm. 

Pros

  • Easy To Use
  • Very Strong
  • Tough To Remove Wheels Without A Large Spanner

Cons

  • Have To Carry A Large Spanner
  • Not Very Good Performance
  • Very Little Precision When Installing Compared To Other Axles

Quick Release Axles

Bike Axle Types & Standards Explained 1
© Robbie Ferri/BikeTips
  • Road Bikes
  • Older Mountain Bikes
  • Hybrid Bikes
  • Touring Bikes
  • Gravel Bikes
  • Commuter Bikes
  • Cyclocross Bikes

Quick-release axles are very common, and you will find them on many different bike types, as listed above. They have a small metal axle that goes through the wheel with a nut on one side and a lever on the other.

When the lever is engaged, it grips the dropout, securing the wheel into place. The beauty of a quick release is that it can be undone quickly and is easy to use without any tools required. You can also opt for Allen key versions. 

Quick-release skewers are very cheap to buy and come in various sizes. When using them, it’s important to ensure the wheel is in the correct place and that they are tight enough. Although most are good, some come loose if they are not tight enough.

Common Sizing 

The most common size is 9 mm on the fork axle, but it’s 5 mm on the quick release itself. The front is 100 mm long, and the rear is normally 135 mm on mountain bikes and 130 mm on road bikes. This can differ depending on the brand.

Pros

  • Easy To Use Axle Of Bike
  • No Tools Required
  • Very Lightweight 
  • Standard Sizing
  • Very Cheap To Buy And Replace

Cons

  • Make It Easy For Thieves To Steal Wheels
  • Wheels Can Be Misaligned
  • They Can Get Knocked Open

Thru Axles

The non drive side of a thru axle on a gravel bike.
© Robbie Ferri/BikeTips
  • Road Bikes
  • Mountain Bikes
  • Gravel Bikes
  • Electric Bikes

Thru axles are the most common axle type you will see on most high-quality modern bikes. They use a small hollow tube that goes through the frame or forks and then screws into the other side. They are not only very easy to use but come with great advantages.

Firstly, they are very strong and perfect for off-road bikes, which you expect to take a knock here and there. They are also very lightweight, and when installing them, they always go into the correct place for the disc brake or rim brake. 

Perhaps most importantly, they hold the wheel very securely when used with disc brakes. This is vital, as there have been instances of disc brakes causing wheels secured with quick-release systems to come loose.

They come in many different shapes and sizes but are not all universal. They differ in width and length, and depending on the bike frame you are using, you also get different pitch threads. 

When wheels are released now, they typically come through axles instead of quick-release skewers or bolt-on axles. If you are looking to future-proof a bike, then thru axles are the way forward in many cases. 

Common Sizing 

Regarding thru-axles, length, width, and pitch thread size all differ. The front’s length ranges from 100 mm to 120 mm. The rear range is between 142 mm and 148 mm, with some fat bikes being larger.

For width (diameter), you either get 15 mm, which is common on mountain bikes, or 12 mm, which is common on mountain and road bikes. Pitch thread sizing can be 1 mm, 1.5 mm, or a unique size for a particular manufacturer.

Pros

  • Easy To Use
  • Incredibly Strong
  • Wheel Goes In The Same Place Each Time
  • Modern Standard For Wheels

Cons

  • Bikes With Thru Axles Cost More
  • A Lot Of Different Types And Sizes
  • More Expensive

Modern Bike Thru Axle Standards

As you can see above, there are many different ways to attach wheels to your bike. We often get asked what the current thru-axle standards are for different bikes. Here, we will discuss road bikes, gravel bikes, mountain bikes, and more.

Road Bike Thru Axle Standards

With road bike standards previously came with quick-release wheels years ago, which was the standard, and that’s the case with budget options at the moment. In modern times, we now have thru-axles.

Typically, road bike thru axles are 12 mm wide on both the front and back, with a longer axle on the rear for the cassette.

MTB Thru Axle Standard

With MTB axle standards, you are generally using a 15 mm or 12 mm thru-axle on the front and back. Although it might seem large, they must ensure the wheels stay in place and can take a knock when hit. 

Gravel Bikes

Budget gravel bikes are typically on quick-release skewers, but mid-range to high-end bikes will almost always have thru-axles.

They typically have the same format as road bikes, with 12 mm on either side, but I have seen one or two 15 mm gravel bike axles from some manufacturers. 

Single-Speed And Track Bikes

A big exception to the rule is single-speed and track bikes. These use the bolt-on method. Not only is it required to adjust the chain tension, it makes them harder to steal, and they don’t require room for a large cassette.

A fork with a quick release skewer.
© Robbie Ferri/BikeTips

Bike Axle Adaptors 

We often get asked if it is possible to change the axles on a set of wheels so you can fit it onto a bike or convert a bike to different axles. There are, but there are many limitations when it comes to doing this. Here are the options that you generally have. 

Changing The Wheel Axles

The first option is to change the axle fitment on the hub of the wheels. Only some wheels can do this, and the bigger brands generally offer compatibility. It’s the best way to go, in my opinion, but it can be an expensive change sometimes. 

Swapping The Frame Dropouts

Some bikes offer the capability to swap the frame dropout to work with thru-axles instead of quick-release wheels. This is what I once did on a Lynskey frame, and then I changed the forks at the front to match, too. 

Using Specialist Adapters

Special adapters are also an option. I used adapters to fit a quick-release bike with thru-axle wheels. It worked on the front, but I had to change the dropouts on the rear. These options are few and far between, but sometimes, they will get you out of a mess.

Future Proofing Your Bike

This is something I have had in conversation with many people who are looking to buy a new bike. The last thing you need is to invest thousands into a bike and then realize it won’t use modern equipment.

For axles, I will always recommend going for thru-axles over anything else on standard bike types. Manufacturers such as Giant are stopping quick release because thru axles outperform them in most departments.

Finding a wheel upgrade in the future could be challenging, and you also miss out on all the benefits of thru-axles.

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