Bicycle Thru-Axle Sizes and Dimensions by Bike Type

Thru-axles are the modern standard for attaching wheels to bicycle frames and forks. Unlike traditional quick-release skewers, a thru-axle slides through the hub and threads directly into the frame or fork, providing a stiffer, more secure connection. The key dimensions of a thru-axle are its diameter, length, thread pitch, and thread length. These must match your frame, fork, and hub. Road, gravel, mountain, and fat bikes use different standards. This guide explains the common sizes and how to identify them.

How to Read Tire Sidewall Numbers

Before discussing thru-axles, it helps to understand the numbers on your tire sidewall. They tell you the tire size, which affects the overall wheel diameter and the clearance in your frame. A typical marking looks like this: 700 x 25C or 29 x 2.25.

• 700 or 29: This is the approximate outer diameter of the tire in millimeters (700mm) or inches (29 inches). It corresponds to the rim bead seat diameter of 622mm, known as 700C or 29".
• 25C or 2.25: This is the nominal tire width. In the first example, 25mm wide. In the second, 2.25 inches (about 57mm).
• Some tires include an aspect ratio, but bicycle tires rarely do. Width is given directly.
• The rim diameter is fixed by the wheel size (e.g., 622mm for 700C/29").

To calculate the sidewall height, overall diameter, and circumference, use these formulas:

• Sidewall height = tire width (since bicycle tire sidewalls are roughly as tall as the tire is wide, though actual profile varies). For a 25mm tire, sidewall height ≈ 25mm.
• Overall wheel diameter = rim diameter + 2 × sidewall height. For 700 x 25C: 622mm + 2×25mm = 672mm.
• Circumference = π × overall diameter. For 672mm, circumference ≈ 3.1416 × 672 = 2111mm.

These calculated dimensions are theoretical. Actual tire dimensions vary by manufacturer, rim width, and inflation pressure. Always check clearance with your specific setup.

Key Thru-Axle Dimensions

Bicycle Thru Axle Diameter

Thru-axle diameter is the thickness of the axle shaft, measured in millimeters. Common diameters are 12mm, 15mm, and 20mm. The diameter must match the hub's internal bore and the frame/fork dropout holes. A 12mm axle will not fit a 15mm hub. Diameter is often marked on the axle itself (e.g., "12mm").

Axle Length and Thread Pitch

Axle length is the total length from the head to the end of the threaded section, measured in millimeters. The thread pitch is the distance between threads, expressed as millimeters per thread. For example, 12x1.0 means a 12mm diameter with a 1.0mm thread pitch. Thread pitch is critical: using the wrong pitch will damage the threads. Common pitches are 1.0mm (road/gravel front), 1.5mm (most rear axles), and 1.75mm (some front axles). Thread length is the length of the threaded portion that screws into the frame; too short may not engage fully, too long may bottom out.

Hub Spacing and Axle Length by Bike Type

Road Bike Thru Axle Sizes

Road bikes (including endurance, racing, and many gravel bikes) typically use 12mm thru-axles front and rear. Common standards:

• Front hub spacing: 100mm (12x100mm). Axle length: 120–125mm, thread pitch 1.0 or 1.5 depending on manufacturer.
• Rear hub spacing: 142mm (12x142mm). Axle length: 166–172mm, thread pitch 1.0 or 1.5.

Example: A typical road bike front axle might be 12x100mm, length 121mm, thread pitch 1.5. Rear: 12x142mm, length 172mm, thread pitch 1.5. Always check your bike's specifications.

Gravel Bike Thru Axle Size

Gravel bikes use the same axle standards as road bikes: 12x100mm front and 12x142mm rear. Some newer gravel bikes adopt wider mountain bike standards for extra tire clearance, such as 12x110mm (Boost) front and 12x148mm (Boost) rear. Check your frame.

Mountain Bike Thru Axle Sizes

Mountain bikes use larger diameter axles for strength:

• Front: 15x100mm or 15x110mm (Boost) for most modern bikes. Older downhill bikes may use 20x110mm.
• Rear: 12x142mm (non-Boost) or 12x148mm (Boost). Downhill bikes often use 12x150mm or 12x157mm.
• Thread pitch is usually 1.5mm.

Boost spacing (110/148) provides a wider stance for better wheel stiffness and tire clearance.

Fat Bike Thru Axle Size

Fat bikes, designed for wide tires (4–5 inches), use wider hub spacing:

• Front: 15x150mm or 15x110mm (newer).
• Rear: 12x177mm or 12x197mm (for 5-inch tires).
• Axle lengths are longer accordingly, and thread pitch is typically 1.5mm or 1.75mm.

Compatibility: A fat bike fork with 150mm spacing requires a 15x150mm axle and a matching hub.

Comparing Thru-Axle Standards

The table below summarizes common thru-axle sizes by bike type.

Note: Axle length is not the same as hub spacing. The axle must be long enough to pass through the dropout thickness and thread into the frame. Always measure your existing axle or consult the manufacturer's specs.

How to Measure Your Thru-Axle

If you need a replacement, measure your current axle or the frame/fork:

1. Diameter: Use a caliper to measure the shaft diameter. It will be close to 12mm, 15mm, or 20mm.
2. Overall length: Measure from the underside of the head to the end of the threaded portion, in millimeters.
3. Thread pitch: If you have a thread pitch gauge, measure it. Common values are 1.0, 1.5, 1.75. If unsure, consult your bike's specifications.
4. Thread length: Measure the threaded section. It should be long enough to fully engage the frame threads (usually 10–15mm).

When buying a replacement, match all four dimensions. A difference of even 1mm in length can cause fitment issues.

Fork and Frame Compatibility

Your fork and frame dictate the axle standard. The fork dropouts have a specific diameter and thread pitch. The rear frame dropouts also have threads. The hub must match the axle diameter, and the hub width must match the frame spacing. For example, a Boost fork (110mm) requires a Boost hub (110mm) and a Boost axle (15x110mm). Mixing standards is not possible without adapters, which may compromise safety.

Common Thru-Axle Markings

Many axles have markings on the head or shaft. Examples:

• 12x142: 12mm diameter, 142mm hub spacing (not axle length).
• 15x100: 15mm diameter, 100mm hub spacing.
• P1.5: Thread pitch 1.5mm.

Do not confuse hub spacing with axle length. A 12x142 axle may be 172mm long, but the marking only shows diameter and hub spacing.

Final Tips

• Always verify the exact specifications for your bike model. Standards evolve, and some manufacturers use proprietary axles.
• When installing, thread the axle by hand to avoid cross-threading, then tighten to the recommended torque (usually 10–15 Nm).
• If you change wheels, ensure the new hub matches your axle and frame spacing.

Understanding thru-axle dimensions helps you choose the right parts and keep your bike safe and reliable.