Bicycle Rear Hub Spacing Chart and Standards

Bicycle rear hub spacing, also known as over-locknut dimension (OLD), is the distance between the outer faces of the locknuts on the hub axle. It determines whether a hub will fit into a frame's rear dropouts. Common rear OLD standards include 120 mm, 126 mm, 130 mm, 135 mm, 142 mm, 148 mm (Boost), and 157 mm (Super Boost). This guide explains the rear hub spacing chart, how to measure OLD, and how standards like Boost and Super Boost affect frame compatibility.

What Is Bicycle Rear Hub Spacing?

Bicycle rear hub spacing is the width of the hub measured across its axle ends, specifically from the outside face of one locknut to the outside face of the other. This dimension is called the over-locknut dimension, or OLD. The frame's rear dropout spacing must match the hub OLD for proper installation. Hub spacing standards have evolved from narrow single-speed hubs to wide thru-axle hubs for disc brakes and larger tires.

Bicycle Rear Hub Spacing Chart

The following table lists common rear hub OLD standards, their typical use, and axle type.

What Does OLD Mean on a Bicycle Hub?

OLD stands for over-locknut dimension. It is the distance between the outer faces of the two locknuts on the hub axle. These locknuts secure the axle in the dropouts. To measure OLD, use a caliper or ruler to measure across the axle ends, excluding any protruding axle threads or quick-release mechanism. The OLD must match the frame's rear dropout spacing.

How Is Rear Hub Spacing Measured?

To measure rear hub spacing:

1. Remove the wheel from the bike.
2. Place the hub on a flat surface with the axle vertical.
3. Use a caliper or a precise ruler to measure the distance between the outer faces of the locknuts.
4. Ignore the axle ends that protrude beyond the locknuts.
5. Record the measurement in millimeters; this is the hub OLD.

For thru-axle hubs, the OLD is measured between the outer faces of the end caps or the hub shell faces that contact the frame's dropouts.

Quick-Release vs. Thru-Axle Hub Spacing

Quick-release hubs have a hollow axle with a skewer that runs through it. The OLD is determined by the axle length and locknut positions. Common quick-release rear OLDs are 130 mm (road) and 135 mm (mountain). Thru-axle hubs have a larger diameter axle that threads directly into the frame. The OLD is effectively the hub width between the end caps. Common thru-axle rear OLDs are 142 mm, 148 mm, and 157 mm, all with 12 mm axle diameter.

Boost Spacing: 148 mm Rear Hub Standard

Boost spacing is a wider hub standard introduced to improve wheel stiffness and tire clearance. The rear Boost OLD is 148 mm, which is 6 mm wider than the previous 142 mm standard. The axle is still 12 mm thru-axle. Boost hubs move the flanges farther apart, allowing a better spoke bracing angle and a stronger wheel. Boost frames have rear dropout spacing of 148 mm. Boost hubs are not compatible with non-Boost frames without adapters.

Super Boost Spacing: 157 mm Rear Hub Standard

Super Boost is an even wider standard with a rear OLD of 157 mm. It uses a 12 mm thru-axle. The extra width allows for even wider flange spacing and is used in some downhill and enduro mountain bikes, as well as fat bikes. Super Boost frames require Super Boost hubs; they cannot be mixed with Boost or standard hubs.

Why Do Rear Hub Spacing Standards Matter?

Rear hub spacing affects frame compatibility, wheel stiffness, tire clearance, and drivetrain alignment. A hub with the wrong OLD may not fit the frame, or may require bending the frame (not recommended for aluminum or carbon) or using adapters. Wider hubs generally offer stronger wheels and more tire clearance but require a matching frame. When upgrading a wheel or frame, always check the hub spacing.

Can You Mix Hub Spacing and Frame Spacing?

Generally, the hub OLD must match the frame's rear dropout spacing. For steel frames, it may be possible to cold-set (bend) the rear triangle to change spacing by a few millimeters, but this is not advisable for aluminum or carbon frames. Some hubs offer convertible end caps to change OLD (e.g., from 142 mm to 148 mm) if the hub shell supports it. Quick-release hubs can sometimes be used in thru-axle frames with special adapters, but this is uncommon for rear hubs.

Rear Hub Spacing and Drivetrain Compatibility

Rear hub spacing also affects the freehub body width and cassette compatibility. Road hubs with 130 mm OLD typically accept road cassettes up to 11 speeds. MTB hubs with 135 mm or 142 mm OLD accept MTB cassettes. Boost 148 hubs often use the same freehub body but with a wider axle; some may require a specific freehub for certain cassettes. Always verify cassette compatibility when changing hub standards.

Conclusion

Bicycle rear hub spacing is a critical dimension for frame and wheel compatibility. The OLD measurement ranges from 110 mm to 157 mm, with specific standards for road, mountain, and fat bikes. Boost (148 mm) and Super Boost (157 mm) are modern thru-axle standards that improve wheel performance but require matching frames. When selecting a hub or frame, always confirm the rear hub spacing in millimeters.