Hub Flange Spacing

Summary

Hub flange spacing describes the lateral distance between the left and right spoke flanges on a bicycle hub. It is a critical structural parameter that influences spoke bracing angle, wheel stiffness, durability, and the degree of wheel dish required to center the rim within the frame.

Key Facts

  • Category: Concept
  • Defined as: Distance between hub spoke flanges
  • Measured in: Millimeters (mm)
  • Applies to: Front and rear hubs
  • Directly affects: Wheel stiffness, spoke tension balance, dish
  • Influenced by: Hub spacing standards, drivetrain layout, disc brakes
  • Closely related to: Bracing angle
  • Optimized in: Boost and Super Boost Plus hub standards

Overview

Hub flange spacing is one of the most influential — and least visible — variables in wheel design. While riders often focus on rim depth, spoke count, or hub engagement, the spacing between hub flanges plays a foundational role in how strong, stiff, and durable a wheel can be.

At a basic level, hub flange spacing determines the angle at which spokes connect the hub to the rim. Wider spacing allows spokes to leave the hub at a broader angle, improving lateral stiffness and reducing the tension imbalance required to center the rim. Narrow spacing forces spokes into steeper angles, increasing dish and reducing wheel strength, particularly on the rear wheel.

Historically, hub flange spacing was constrained by drivetrain and frame standards. As cassettes grew wider and disc brakes were added, flange spacing became increasingly asymmetric. This forced wheel designers to find ways to maintain strength despite growing structural compromises.

In recent years, changes to hub spacing standards — most notably Boost and Super Boost Plus — have been driven in large part by the desire to increase flange spacing and improve wheel integrity. As a result, flange spacing has become a key metric in modern wheel engineering rather than an afterthought.

How It Works

A hub flange is the circular plate on either side of a hub shell where spokes attach. The distance between these two flanges defines flange spacing.

Bracing Angle

Flange spacing directly affects spoke bracing angle — the angle formed between a spoke and the wheel’s centerline.

  • Wider flange spacing:
    • Larger bracing angle
    • Increased lateral stiffness
    • Improved resistance to side loads
  • Narrower flange spacing:
    • Smaller bracing angle
    • Reduced stiffness
    • Greater reliance on high spoke tension

Because bracing angle determines how effectively spokes resist lateral forces, flange spacing is fundamental to wheel strength.

Relationship to Dish

On most modern wheels, the hub flanges are not symmetrically spaced around the centerline. Rear hubs must accommodate the cassette on the drive side, and disc hubs must make room for the rotor.

To center the rim:

  • The flange closer to center requires higher spoke tension
  • The farther flange uses lower tension

Increasing overall flange spacing — even asymmetrically — helps reduce the difference in tension required between sides.

Front vs Rear Considerations

Front hubs typically allow more symmetric flange spacing than rear hubs, though disc brakes introduce offset on the rotor side. Rear hubs are the most constrained, as they must accommodate both drivetrain and disc hardware.

Influence of Hub and Frame Standards

Traditional QR Standards

Older quick-release hubs (100 mm front, 135 mm rear) had limited flange spacing due to narrow overall width. Rear hubs in particular suffered from severe dish and tension imbalance.

Thru-Axles

The adoption of thru-axles allowed hubs to become stiffer and more precisely aligned, but flange spacing gains were modest without wider hub standards.

Boost Spacing

Boost standards (110 mm front, 148 mm rear) were introduced partly to improve hub flange spacing.

Benefits include:

  • Wider flange placement
  • Reduced dish
  • Improved rear wheel durability
  • Better spoke tension balance

Super Boost Plus

Super Boost Plus (157 mm rear) pushes flange spacing even wider. Originally popular in downhill and enduro racing, it allows:

  • Even broader bracing angles
  • Stronger rear wheels
  • Accommodation of wide tires and short chainstays

While not universal, Super Boost Plus demonstrates the structural advantages of increased flange spacing.

Disc Brakes and Flange Offset

Disc brake hubs require space on the rotor side, forcing flange placement inward. This creates asymmetry even on front hubs.

To manage this:

  • Designers increase total hub width
  • Offset flanges strategically
  • Use asymmetric rims to compensate

Flange spacing and disc compatibility are now inseparable considerations in hub design.

Wheel Strength and Ride Characteristics

Lateral Stiffness

Wider flange spacing improves lateral stiffness, which enhances:

  • Precise handling
  • Cornering stability
  • Resistance to wheel flex under load

This is particularly noticeable in aggressive riding or heavier bikes such as e-MTBs and cargo bikes.

Durability

Improved tension balance reduces fatigue on high-tension spokes, increasing wheel longevity and reducing the likelihood of broken spokes.

Weight Trade-Offs

Increasing flange spacing often requires wider hubs and frames, which can add weight. Designers balance these trade-offs depending on the bike’s intended use.

Interaction with Asymmetric Rims

Asymmetric rims offset spoke holes toward one side, helping balance tension when flange spacing is uneven.

When paired with wide flange spacing:

  • Spoke tension becomes more uniform
  • Wheel durability improves
  • Dish-related compromises are reduced

Asymmetric rims are now common on performance mountain bike wheels.

Design Constraints and Limits

Drivetrain Compatibility

Moving flanges outward is limited by chainline requirements. Excessive outward movement on the drive side can compromise shifting performance.

Frame Clearance

Wider hubs require frames designed to accommodate them. This affects chainstay shape, suspension kinematics, and tire clearance.

Standardization

Multiple hub standards complicate compatibility. While wider spacing improves wheels, it also fragments the market.

Why Hub Flange Spacing Matters

Although rarely listed in geometry charts, hub flange spacing underpins many aspects of wheel performance. It influences:

  • Wheel stiffness
  • Spoke life
  • Dish severity
  • Handling consistency

Riders may never measure flange spacing directly, but they experience its effects every time a wheel is pushed hard.

Notable Implementations

  • Boost hubs: Improved flange spacing for trail and enduro bikes
  • Super Boost Plus hubs: Maximum spacing for downhill and e-MTB use
  • High-end road disc hubs: Optimized spacing within narrow constraints
  • Cargo bike hubs: Wide spacing for durability under heavy loads

Related Terms

References

  • Professional wheel-building manuals
  • Hub manufacturer engineering documentation
  • Park Tool: Wheel Geometry Guides
  • BikeRadar: Hub and Wheel Design Explained
  • Pinkbike: Modern Wheel Engineering
  • Academic research on spoke bracing angles
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