Hub Spacing

Summary

Hub spacing refers to the distance between the outer faces of a bicycle hub’s axle, measured where it interfaces with the frame or fork dropouts. It defines frame compatibility, wheel alignment, and structural constraints for hubs, influencing wheel strength, drivetrain layout, brake placement, and overall bicycle design.

Key Facts

  • Category: Concept
  • Defined as: Distance between hub locknuts or end caps
  • Measured in: Millimeters (mm)
  • Applies to: Front and rear hubs
  • Common standards: 100, 110, 135, 142, 148, 157 mm
  • Directly affects: Wheel dish, flange spacing, frame compatibility
  • Closely related to: Axle type, drivetrain spacing, brake standards
  • Evolved with: Disc brakes, wider tires, suspension designs

Overview

Hub spacing is one of the most fundamental — and consequential — standards in modern bicycle design. It determines how a wheel fits into a frame or fork, but its influence extends far beyond simple compatibility. Hub spacing shapes wheel strength, dictates drivetrain alignment, affects brake placement, and constrains suspension and frame design.

Historically, hub spacing was relatively simple. Early road and mountain bikes used narrow quick-release hubs designed around rim brakes and narrow tires. As bikes evolved to include more gears, disc brakes, wider tires, and suspension systems, those early spacing standards became limiting.

Each expansion of hub spacing has been driven by a specific problem. Rear spacing grew to accommodate wider cassettes and improve wheel strength. Front spacing widened to offset disc brake forces and improve stiffness. In mountain biking, wider spacing also allowed designers to shorten chainstays, increase tire clearance, and improve suspension kinematics.

Today, hub spacing is best understood not as a single measurement, but as part of a broader system of standards that balance wheel performance, drivetrain efficiency, and frame design. Changes to hub spacing ripple through the entire bicycle, affecting nearly every component it touches.

How It Works

Hub spacing is measured as the distance between the outer faces of the hub’s axle — traditionally the locknuts on quick-release hubs, or the end caps on thru-axle hubs. This measurement must match the internal spacing of the frame or fork dropouts exactly.

Interface with the Frame

The frame or fork is built to accept a specific hub spacing. If the spacing does not match, the wheel cannot be installed without modification. This makes hub spacing a hard compatibility limit.

Relationship to Axle Type

Hub spacing is closely tied to axle standards:

  • Quick-release (QR): Uses a hollow axle clamped by a skewer
  • Thru-axle: Uses a solid axle that threads directly into the frame

While axle type affects stiffness and alignment, hub spacing defines the width the frame must accommodate.

Influence on Internal Hub Design

As spacing increases, hub designers gain more room to:

  • Move flanges outward
  • Improve bracing angles
  • Reduce wheel dish

This is why many modern spacing changes were driven as much by wheel strength concerns as by drivetrain needs.

Common Hub Spacing Standards

Road and Rim Brake Standards

  • 100 mm front / 130 mm rear: Traditional road spacing for rim-brake bikes
  • 135 mm rear: Used on touring and early MTB rim-brake bikes

These standards assumed narrow tires, rim brakes, and minimal hub asymmetry.

Disc Brake Road and Gravel

  • 100 mm front / 142 mm rear: Became standard with road and gravel disc brakes
  • Rear spacing increased to accommodate disc hardware and improve wheel strength

This spacing remains common on modern endurance road and gravel bikes.

Mountain Bike Standards

  • 100 mm front / 135 mm rear (QR): Early MTB standard
  • 110 mm front / 148 mm rear (Boost): Modern trail and enduro standard
  • 157 mm rear (Super Boost Plus): Used in downhill, enduro, and some e-MTBs

Each step wider improved wheel stiffness and frame design freedom.

Utility, Cargo, and Specialty

  • 170–197 mm rear: Fat bikes
  • Wider spacing supports very wide tires and extreme loads

Why Hub Spacing Changed

Wheel Strength

Narrow hub spacing forces hub flanges closer together, reducing spoke bracing angles and increasing dish. Wider spacing improves wheel durability, especially for rear wheels under high loads.

Disc Brake Forces

Disc brakes apply braking force at the hub rather than the rim. Wider front spacing improves torsional stiffness and helps forks handle braking loads more predictably.

Tire Clearance

As tire widths increased, especially in mountain biking, frames needed more lateral room. Wider hub spacing allows chainstays to be pushed outward without compromising chainline excessively.

Suspension Design

Full-suspension bikes benefit from additional space around the bottom bracket and rear triangle. Wider hubs give designers more freedom to place pivots and links.

Boost and the Modern Era

Boost spacing marked a turning point in how hub spacing was justified. Rather than being framed as a marketing-driven change, Boost was explicitly tied to measurable benefits:

  • Improved wheel stiffness
  • Reduced spoke tension imbalance
  • Better handling under load

While controversial at launch, Boost is now widely accepted and has influenced nearly every modern MTB platform.

Super Boost Plus extends this logic further, prioritizing wheel and frame performance over backward compatibility.

Trade-Offs and Limitations

Compatibility Fragmentation

Each new spacing standard reduces cross-compatibility. Wheels, frames, and hubs must all match exactly, complicating upgrades and replacements.

Weight and Packaging

Wider hubs and frames can add weight. Designers must balance structural gains against mass and efficiency.

Drivetrain Constraints

Rear hub spacing cannot increase indefinitely without affecting chainline and shifting performance. Each standard represents a compromise between wheel strength and drivetrain efficiency.

Hub Spacing vs Hub Flange Spacing

While related, these terms are not interchangeable.

  • Hub spacing: Overall width of the hub at the frame interface
  • Hub flange spacing: Distance between spoke flanges within that width

Increasing hub spacing often allows wider flange spacing, but the two are distinct measurements.

Notable Implementations

  • Road disc bikes: 100 / 142 mm spacing
  • Trail and enduro MTBs: 110 / 148 mm Boost
  • Downhill bikes: 110 / 157 mm Super Boost Plus
  • E-MTBs: Often Boost or Super Boost for durability
  • Cargo bikes: Extra-wide rear spacing for load capacity

Related Terms

References

  • Bicycle frame and hub standard documentation
  • Professional wheel-building manuals
  • Park Tool: Hub and Axle Standards
  • BikeRadar: Hub Spacing Explained
  • Pinkbike: The Evolution of Boost
  • Academic studies on wheel stiffness and hub geometry
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