Summary
Hub spacing is the width of a bicycle hub at the surfaces that contact the frame or fork dropouts. Traditionally called over-locknut dimension (OLD), it is a fundamental compatibility measurement that determines whether a wheel fits a particular frame or fork. Hub spacing also constrains flange placement, drivetrain position, brake-rotor location, and wheel geometry.
Key Facts
Category: Concept / Compatibility
Also known as: OLD, over-locknut dimension
Measured in: Millimeters (mm)
Measured between: Hub locknut or end-cap seating faces
Applies to: Front and rear hubs
Common widths: 100, 110, 130, 135, 142, 148, 157 mm
Directly affects: Frame/fork compatibility and available hub packaging
Influences: Flange spacing, wheel dish, chainline, rotor position
Important distinction: Hub width alone does not define the complete axle standard
Overview
Hub spacing describes the width of the hub where it interfaces with the bicycle. On a traditional quick-release hub, this is measured between the outer locknut faces. On most thru-axle hubs, it is measured between the end-cap faces that seat against the frame or fork.
It should not be confused with axle length. A thru-axle passes through the hub and frame and may extend far beyond the hub’s nominal spacing before threading into the opposite dropout.
Hub spacing has changed repeatedly as bicycle design has evolved. Additional cassette sprockets, disc brakes, larger wheels, wider tires, and increasingly demanding mountain-bike applications have all placed greater packaging demands on hubs and frames.
Wider standards can give hub designers additional room to move spoke flanges outward, improve bracing angles, or provide drivetrain and tire clearance. However, those benefits depend on how the added width is actually used. A wider hub does not automatically have better flange geometry or less wheel dish.
Hub spacing is therefore both a compatibility standard and an engineering constraint.
How Hub Spacing Is Measured
The relevant dimension is the distance between the surfaces of the hub that contact the inside faces of the frame or fork dropouts.
Quick-Release Hubs
On conventional quick-release hubs, this measurement is traditionally called the over-locknut dimension (OLD).
The axle itself normally extends beyond the locknuts and into the dropouts, so measuring the complete axle length gives the wrong dimension.
Thru-Axle Hubs
Modern thru-axle hubs usually use removable end caps. Hub spacing is measured between the end-cap seating faces.
A hub described as 12×148 therefore indicates:
- 12 mm axle diameter
- 148 mm hub/dropout spacing
It does not specify the complete thru-axle length, thread pitch, or threaded length. Those dimensions are determined by the frame.
Hub Spacing Is Not the Entire Standard
Two hubs with the same nominal width are not necessarily interchangeable.
Compatibility can depend on:
| Dimension | What It Defines |
|---|---|
| Hub spacing | Width between dropout seating faces |
| Axle diameter | Diameter of QR or thru-axle interface |
| End-cap design | How the hub locates in the frame or fork |
| Rotor position | Lateral position of the brake rotor |
| Cassette position | Location of the drivetrain relative to the frame |
| Flange position | Spoke geometry within the hub width |
| Thru-axle length/thread | Frame-specific axle requirements |
This distinction becomes especially important with 110 mm front hubs and 157 mm rear hubs, where multiple standards have used the same nominal width.
Common Hub Spacing Standards
| Application | Typical Hub Interface |
|---|---|
| Traditional road front | 100 mm QR |
| Traditional road rear | 130 mm QR |
| Legacy MTB front | 100 mm QR |
| Legacy MTB rear | 135 mm QR |
| Road/gravel thru-axle front | 12×100 mm |
| Road/gravel thru-axle rear | 12×142 mm |
| Legacy MTB thru-axle front | 15×100 mm |
| Boost MTB front | 15×110 mm |
| Boost MTB rear | 12×148 mm |
| Modern DH front | Commonly 20×110 mm Boost |
| DH / Super Boost Plus rear | 12×157 mm |
| Fat-bike rear | Commonly 170–197 mm depending on axle system |
This list is not exhaustive. Numerous transitional, proprietary, tandem, cargo, BMX, track, and fat-bike standards also exist.
130 and 135 mm
A 100 mm front hub paired with a 130 mm rear hub became a longstanding road configuration, particularly during the rim-brake era.
Mountain bikes commonly used 100 mm front and 135 mm rear quick-release hubs. The additional rear width provided room for MTB drivetrain and wheel requirements and later supported many early disc-brake systems.
Both remain common when servicing older bicycles.
12×142 mm
The move from 135 mm quick-release to 12×142 mm thru-axle is sometimes described as a simple increase in wheel width, but this is misleading.
The 142 mm system primarily changed how the hub locates in the frame. Extended end interfaces allow the hub to sit securely in recessed thru-axle dropouts while preserving wheel and drivetrain geometry very similar to the preceding 135 mm system.
SRAM’s historical hub specifications illustrate this clearly: comparable 12×135 and 12×142 hubs used essentially the same center-to-flange geometry despite their different nominal widths. SRAM Hub Build Specifications
This is why some older hub designs could be converted between 135 and 142 configurations using manufacturer-approved axle or end-cap kits.
Boost 110 and 148
Boost represented a more substantial change in usable hub width.
The common Boost interfaces are:
- 15×110 mm front
- 12×148 mm rear
Compared with 15×100 and 12×142 systems, Boost provides additional lateral space that hub designers can use to move spoke flanges outward and improve bracing angles.
SRAM’s published hub dimensions show this change directly: its Boost versions moved both front and rear flange positions compared with corresponding non-Boost hubs. SRAM Hub Build Specifications
Boost also works as part of a broader drivetrain system, including corresponding chainline and frame-clearance requirements. It is therefore not simply a wider pair of end caps.
The 110 mm Front-Hub Trap
Not every 110 mm front hub is Boost.
Older downhill bikes commonly used 20×110 mm hubs before Boost existed. Although these hubs share a 110 mm overall width with later Boost systems, their rotor and flange positions can differ.
Modern 20×110 Boost systems repositioned the rotor and flanges to use the width differently. Stan’s documentation specifically notes that conventional 20×110 and 20×110 Boost hubs can share axle width while differing in rotor and flange placement. Stan’s: 20×110 vs. 20×110 Boost
For a mechanic, “110 mm” alone is therefore insufficient information.
148 vs. 157 mm
Boost 148 is widely used on modern mountain bikes, while 157 mm spacing appears primarily on downhill, aggressive trail/enduro, and some e-MTB platforms.
Super Boost Plus uses a 157 mm rear interface but is designed to take greater advantage of the available width for flange placement and drivetrain/frame clearance. Pivot’s technical documentation specifically identifies increased flange spacing as one of the system’s objectives. Pivot Super Boost Plus Technical Information
However, “157 mm” and “Super Boost Plus” should not automatically be treated as interchangeable descriptions. Older downhill 157 mm systems and newer Super Boost implementations can use the available width differently. Component compatibility should be verified from the hub and frame specifications.
Fat-Bike Spacing
Fat bikes require substantially wider hubs to provide clearance for oversized tires and correspondingly wide frame structures.
Common rear standards have included:
- 170 mm quick release
- 177 mm thru-axle
- 190 mm quick release
- 197 mm thru-axle
Front standards also vary considerably.
As with conventional hubs, axle diameter, rotor position, freehub configuration, and frame interface must be checked rather than relying only on the nominal hub width.
Relationship to Flange Spacing
Hub spacing and hub flange spacing are different measurements.
Hub spacing describes the frame interface.
Flange spacing describes the locations where the spokes leave the hub.
A wider hub provides the designer with more potential space for wider flange placement, but does not guarantee that the flanges will use all of it.
Bearings, freehub bodies, cassette clearance, rotor mounts, axle structure, and hub-shell design all compete for space.
Two 12×148 hubs can consequently produce different spoke bracing angles and tension ratios.
Relationship to Wheel Dish
Hub spacing also does not directly determine wheel dish.
Dish depends on where each flange sits relative to the rim centerline. Rear cassette clearance normally pushes the drive-side flange inward, while disc-brake clearance affects the opposite side.
A wider standard may allow more favorable flange positions, but actual dish and spoke tension balance must be calculated from the finished hub geometry.
Frame and Drivetrain Effects
Rear spacing affects more than the wheel.
Moving drivetrain components outward can influence:
- Chainline
- Chainring position
- Crank clearance
- Tire clearance
- Chainstay shape
- Suspension-pivot packaging
This is why Boost and Super Boost systems involve corresponding drivetrain considerations rather than being isolated hub changes.
Continually increasing rear spacing also has disadvantages. Frames, wheels, cranks, and drivetrain components must remain compatible, and excessive width can create unwanted changes in crank stance or frame packaging.
Mechanic’s Perspective
Hub-spacing problems are usually compatibility problems before they are wheelbuilding problems.
When identifying an unfamiliar wheel, do not stop after measuring its width. Confirm:
- Front or rear application
- Quick-release or thru-axle
- Axle diameter
- End-cap type
- Hub spacing
- Rotor position
- Freehub standard
- Cassette position
- Manufacturer conversion options
For thru-axle bikes, also verify the frame’s axle length, thread diameter, thread pitch, and threaded length. These are properties of the frame/axle system and are not determined by a label such as “12×148.”
Do not force a wheel into a frame because its width is only a few millimeters different. Carbon and aluminum frames should never be spread or compressed as a routine compatibility solution. Intentional cold-setting applies only to certain steel-frame situations and requires appropriate frame knowledge.
Hub Conversions
Some hubs are designed around replaceable end caps and can legitimately be converted between certain axle interfaces. Others cannot.
A conversion that makes a hub physically fit between the dropouts does not necessarily correct:
- Rotor location
- Cassette location
- Wheel dish
- Flange geometry
- Chainline
This is particularly relevant to aftermarket 142-to-148 or non-Boost-to-Boost adapter solutions. Some systems use axle spacers, rotor spacers, or wheel redishing to achieve limited compatibility, but they do not transform the hub into a native Boost design with Boost flange geometry.
Use the hub manufacturer’s approved conversion configuration whenever possible.
Replacement Wheels
Before ordering a replacement wheel, record the complete interface rather than simply “front 110” or “rear 157.”
A useful service description would be something like:
Front: 15×110 Boost, Center Lock
or
Rear: 12×148 Boost, Micro Spline
That immediately eliminates several common compatibility errors.
Common Misconceptions
“Hub Spacing Is Axle Length”
False. Hub spacing measures the hub’s dropout interface. A thru-axle usually has a different overall length.
“Any Hub With the Same Width Fits”
False. Axle diameter, end caps, rotor position, cassette position, and other interface dimensions must also match.
“142 mm Was Created to Make Wheels Much Wider Than 135 mm”
Not really. 142 mm primarily established a more secure thru-axle/dropout interface while retaining wheel geometry similar to 135 mm.
“All 110 mm Front Hubs Are Boost”
False. Legacy 20×110 and later Boost-based 110 mm systems can have different rotor and flange locations.
“Wider Hub Spacing Automatically Means Wider Flanges”
False. Wider spacing provides additional design room, but actual flange placement depends on the hub.
“A Spacer Can Convert Any Hub to Boost”
False. Physical dropout width is only one part of the Boost system.
Related Terms
- Hub Flange Spacing
- Wheel Dish
- Boost 148
- Super Boost Plus
- Thru-Axle
- Quick Release
- Chainline
- Bracing Angle
- Freehub Body
- Over-Locknut Dimension (OLD)
References
- SRAM MTB/Road Hub Build Specifications
- SRAM Frame and Hub Fit Specifications
- Stan’s: 20×110 vs. 20×110 Boost
- Pivot Cycles: Super Boost Plus Technical Information
- Hub and frame manufacturer dimensional specifications
- Professional wheelbuilding and bicycle service manuals