Bicycle Quick-Release Axle Dimensions: Diameter, Length and Hub Spacing

A standard bicycle quick-release (QR) setup uses a hollow hub axle of 9 mm diameter at the front and 10 mm diameter at the rear. A thin skewer, about 5 mm thick, passes through the axle. Front hub spacing is 100 mm. Rear hub spacing is usually 130 mm on road bikes and 135 mm on mountain bikes. The axle itself is a few millimetres longer than the hub spacing, so its ends can locate in the frame or fork dropouts. None of these values appears on the tire. The numbers on the tire sidewall describe the tire and rim, not the axle.

Do the Numbers on the Tire Sidewall Tell You the Axle Size?

No. Tire sidewall markings describe tire width and rim diameter only. They say nothing about axle diameter, axle length or hub spacing. This is a common point of confusion, because riders often look at the tire first when identifying a wheel.

What the sidewall markings mean

• ETRTO / ISO marking, for example 37-622: 37 is the nominal tire width in millimetres. 622 is the rim bead seat diameter in millimetres. This is the most precise marking on a bicycle tire.
• Traditional marking, for example 700x35c or 29x2.25: This is a nominal size. The first number is a wheel size class, and the second is an approximate width. The units are mixed: millimetres for 700c, inches for 29 and 27.5.
• Pressure rating, for example 50–85 psi or 3.5–6 bar: This is a performance limit, not a dimension.

Bicycle tires do not normally carry an aspect ratio the way car tires do. Instead, sidewall height is roughly equal to tire width on a typical bicycle tire.

Simple geometry from the sidewall, and its limits

You can estimate the overall wheel diameter as the bead seat diameter plus two times the tire height. Using tire width as the height gives these approximate figures:

• 37-622: 622 + (2 × 37) = 696 mm diameter. The circumference is about 696 × 3.1416 ≈ 2,187 mm.
• 25-622: 622 + (2 × 25) = 672 mm diameter. The circumference is about 2,111 mm.

These are calculated figures. The actual inflated size varies with rim internal width, tire construction and pressure. More importantly, the two wheels above could use exactly the same 100 mm front QR hub. Tire size affects frame and fork clearance. Axle and hub dimensions affect whether the wheel fits between the dropouts. These are separate compatibility checks.

What Is the Standard Quick-Release Axle Size?

The standard quick-release axle size is a 9 mm hollow axle for the front hub and a 10 mm hollow axle for the rear hub. Both are threaded at the ends to carry cones and locknuts. On many hubs the common threads are M9×1.0 at the front and M10×1.0 at the rear, but older hubs and some brands use other threads.

The axle diameter matches the dropout slot width. Front fork dropouts are sized for a 9 mm axle, and rear frame dropouts for a 10 mm axle. That is why a front QR wheel will not simply slot into a rear dropout, and a rear axle will not enter a front fork slot.

Axle diameter versus skewer diameter

The quick-release skewer is the rod with a cam lever on one end and an adjusting nut on the other. It passes through the bore of the hollow axle. The skewer rod is typically about 5 mm in diameter. When people search for a bike quick release axle diameter, they usually mean the 9 mm or 10 mm hollow axle, not the 5 mm skewer rod.

Quick-release axles versus solid nutted axles

Solid axles held by nuts, as found on many utility, kids' and track bikes, are not designed for a skewer. Their diameters overlap with QR sizes, for example 3/8 in (about 9.5 mm) or 10 mm. However, they have no bore for a skewer, so they are not interchangeable with QR axles without changing the axle.

Quick-Release Hub Spacing: What Width Means

When riders talk about quick release axle width, they usually mean hub spacing, also called over-locknut dimension (OLD). OLD is the distance in millimetres between the outer faces of the hub's two locknuts. It must match the dropout spacing, which is the distance between the inner faces of the frame or fork dropouts.

Application	Front hub OLD	Rear hub OLD	Typical QR axle diameter
Modern road bike, QR	100 mm	130 mm	9 mm front / 10 mm rear
Mountain bike, hybrid, touring, QR	100 mm	135 mm	9 mm front / 10 mm rear
Older road bikes (5–7 speed era)	100 mm	126 mm or 120 mm	9 mm front / 10 mm rear

The front hub on almost all QR bikes, road or mountain, is 100 mm. The difference between road and MTB quick release axle sizes is mainly at the rear: 130 mm versus 135 mm. The extra width on a mountain bike rear hub leaves room for disc brake rotors and a wider spoke bracing angle.

Why a 5 mm difference matters

A 130 mm hub in a 135 mm frame leaves a 5 mm gap. A 135 mm hub in a 130 mm frame requires the stays to be forced apart. Either mismatch changes how the dropouts sit against the axle, can misalign the wheel and brakes, and puts stress on the frame. Frames are designed for one spacing. Treat any other spacing as incompatible unless the frame or hub maker provides an approved solution.

Quick-Release Axle Length Explained

Quick release axle length is the total length of the axle from end to end. It is longer than the hub's over-locknut dimension because the axle ends protrude past the locknuts into the dropouts. As a rough guide, a 100 mm front hub uses an axle a little over 100 mm long, and a 135 mm rear hub uses an axle around 145 mm long. Exact lengths depend on the hub design.

Why protrusion must be limited

On a QR hub, each axle end should protrude slightly less than the thickness of the dropout. This allows the axle ends to locate the wheel in the slots, while the skewer clamps the locknuts firmly against the dropout faces. If the axle sticks out beyond the outer face of the dropout, the skewer clamps against the axle end instead of the dropout. The wheel is then not properly secured. This is why a replacement QR axle must be chosen to match the hub's OLD and the original protrusion, not just the diameter and thread.

Skewer length versus axle length

The skewer is always longer than the axle. It has to span the hub, both dropouts, and still engage the adjusting nut securely. Skewers are therefore sold as front or rear and are matched to a hub spacing, for example 100 mm front or 130/135 mm rear. A front skewer is too short for a rear hub.

Quick-Release Rear Axle Size by Bike Type

The quick-release rear axle size is a 10 mm hollow axle on almost all bike types. Rear hub spacing is 130 mm for road QR bikes and 135 mm for mountain, hybrid and touring QR bikes. The axle diameter is the same across road and MTB QR hubs. Only the length and spacing differ. When replacing a rear axle, you therefore need three values: diameter (10 mm), thread (commonly M10×1.0), and length suited to the hub's OLD.

How to Measure Quick-Release Axle Dimensions

Measure axle and hub dimensions with vernier or digital calipers, with the wheel removed and the skewer taken out.

1. Remove the skewer. Unscrew the adjusting nut and slide the skewer out of the hollow axle.
2. Measure axle diameter. Place the calipers across the threaded part of the axle end. A reading of about 9 mm indicates a front QR axle. About 10 mm indicates a rear QR axle.
3. Measure the over-locknut dimension. Measure between the outer faces of the two locknuts, not the axle ends. Expect about 100 mm for a front hub, and 130 mm or 135 mm for a rear hub.
4. Measure total axle length. Measure from one axle end to the other. Subtract the OLD and divide by two to get the protrusion on each side.
5. Measure dropout spacing on the frame or fork. Measure between the inner faces of the dropouts at the axle slots. This figure should match the hub's OLD.
6. Check dropout thickness. Each axle end's protrusion should be slightly less than the dropout thickness on that side.

Small differences of a fraction of a millimetre are normal measuring variation. A difference of several millimetres, such as 130 mm versus 135 mm, indicates a different standard.

Quick-Release Axles Compared With Thru-Axles

A thru-axle is a different system and is not dimensionally interchangeable with a quick-release axle. Thru-axles are much thicker, commonly 12 mm or 15 mm in diameter. They pass through closed holes in the dropouts rather than open slots, and they use their own hub spacings, such as 100×12 mm and 142×12 mm. A 9 mm or 10 mm QR hub cannot be fitted to a thru-axle frame, or the reverse, unless the hub maker offers compatible end caps that convert the hub to that standard.

Key Quick-Release Bike Axle Sizes at a Glance

• Front QR axle diameter: 9 mm, hollow, commonly threaded M9×1.0.
• Rear QR axle diameter: 10 mm, hollow, commonly threaded M10×1.0.
• Skewer rod diameter: about 5 mm.
• Front hub spacing (OLD): 100 mm for road and mountain bikes.
• Rear hub spacing (OLD): 130 mm road, 135 mm mountain bike, with older road bikes at 126 mm or 120 mm.
• Axle length: slightly longer than the OLD. Protrusion on each side must be less than the dropout thickness.
• Tire sidewall size: independent of all axle and hub dimensions.