Hub Flange Spacing

Summary

Hub flange spacing is the lateral distance between the spoke flanges of a bicycle hub. Together with each flange’s position relative to the wheel centerline, it determines spoke bracing angles and strongly influences lateral wheel stiffness, spoke tension balance, and wheel dish.

Key Facts

Category: Concept / Wheelbuilding
Defined as: Lateral distance between the hub’s spoke flanges
Measured in: Millimeters (mm)
Applies to: Front and rear hubs
Directly affects: Spoke bracing angle and lateral wheel stiffness
Influences: Spoke tension ratio and wheel dish
Constrained by: Cassette, rotor, bearings, axle, and frame spacing
Closely related to: Wheel Dish, Bracing Angle, Spoke Tension
Important distinction: Total flange spacing alone does not describe flange asymmetry

Overview

Hub flange spacing is a fundamental part of bicycle wheel geometry. The farther the spoke flanges can be positioned from the wheel centerline, the wider the angle at which the spokes support the rim. This wider bracing angle generally increases the wheel’s ability to resist lateral loads.

The challenge is packaging. Rear hubs need room for a cassette or freewheel, pushing the drive-side flange toward the wheel center. Disc-brake hubs require rotor clearance, which similarly constrains flange placement on the rotor side. Bearings, freehub mechanisms, axle interfaces, and frame spacing impose additional limits.

As a result, flange spacing is rarely perfectly symmetrical.

For wheelbuilding purposes, the most useful dimensions are often not simply the distance from one flange to the other, but the individual center-to-flange dimensions. A hub with flanges 60 mm apart could have each flange 30 mm from center, or one flange 20 mm from center and the other 40 mm away. Those two hubs would produce very different dish, bracing angles, and spoke tension ratios despite having identical total flange spacing.

How It Works

Hub Flanges

The flanges are the portions of the hub where the spokes attach. Their geometry is normally described using:

  • Left center-to-flange distance
  • Right center-to-flange distance
  • Flange diameter or spoke-hole pitch-circle diameter
  • Spoke-hole diameter

Center-to-flange distance largely controls lateral spoke bracing geometry. Flange diameter also affects spoke length and, depending on the lacing pattern, the spoke’s tangential relationship to the hub.

Spoke Bracing Angle

Bracing angle is the lateral angle between the spoke and the wheel center plane.

Moving a flange farther outward increases this angle. The spoke then has greater lateral leverage over the rim, improving its ability to resist sideways displacement.

In general:

Wider center-to-flange distance

  • Greater bracing angle
  • Greater lateral support
  • Potentially higher lateral wheel stiffness

Narrower center-to-flange distance

  • Smaller bracing angle
  • Less lateral leverage
  • Greater dependence on the rest of the wheel structure for lateral stiffness

This does not mean that a narrow flange automatically requires higher absolute spoke tension. Appropriate spoke tension is determined by the rim, spokes, hub, and complete wheel design.

Flange Spacing and Wheel Dish

Flange spacing and wheel dish are closely related, but they are not the same measurement.

Dish describes the position of the rim relative to the hub’s installation centerline. Flange spacing describes where the spoke attachment points are located.

On a rear derailleur hub, the cassette usually pushes the drive-side flange inward. To keep the rim centered, the drive-side spokes must normally operate at a higher average tension than the non-drive-side spokes.

A front disc hub commonly has the opposite type of asymmetry: rotor clearance pushes the left flange inward, so the rotor-side spokes may carry the higher average tension.

The important factor is each flange’s distance from the rim center plane.

Increasing total flange width does not automatically improve tension balance. A hub could gain width primarily on one side and become more asymmetric. Wheel geometry therefore needs to be evaluated using both left and right center-to-flange dimensions.

Spoke Tension Ratio

On an asymmetrical wheel, the difference in bracing angle produces a corresponding difference in average spoke tension between the two sides.

The flange closer to the wheel center generally requires higher tension. The farther-out flange can balance that lateral force with lower tension because its spokes operate at a greater bracing angle.

This explains why many rear derailleur wheels have:

  • High drive-side tension
  • Lower non-drive-side tension
  • Correctly centered rims

The durability concern is often the low-tension side rather than simply the high-tension side. If spoke tension becomes too low during riding loads, individual spokes can partially unload. Repeated unloading can contribute to nipple loosening, loss of trueness, and fatigue problems.

Good hub geometry attempts to maintain useful bracing angles without creating an unnecessarily extreme tension ratio.

Front vs. Rear Hubs

Front Rim-Brake Hubs

Traditional rim-brake front hubs can be nearly symmetrical because neither a cassette nor disc rotor occupies space beside the flanges.

These hubs can provide:

  • Similar left/right bracing angles
  • Nearly equal spoke tensions
  • Little or no dish

This represents one of the least compromised conventional wheel layouts.

Front Disc Hubs

A disc rotor requires space between the left flange and dropout. The rotor-side flange is therefore commonly moved inward.

The result is an asymmetrical front wheel with different bracing angles and average tensions on each side.

Rear Hubs

Rear derailleur hubs face the greatest packaging challenge because space is required for both the cassette and, on disc-brake bikes, the brake rotor.

The cassette generally creates the larger constraint, which is why rear drive-side flanges are usually positioned substantially closer to the wheel centerline.

Exact geometry varies considerably between hub models, even when those hubs use the same axle standard.

Hub Spacing Standards

Quick-Release Hubs

Traditional mountain-bike hubs commonly used 100 mm front and 135 mm rear over-locknut dimensions. Road rear hubs commonly used 130 mm spacing.

As cassette widths and gear counts increased, packaging the drivetrain while maintaining favorable flange positions became increasingly difficult.

Thru-Axles

A thru-axle by itself does not necessarily increase flange spacing. A 12 or 15 mm axle can improve the structural connection between wheel and frame, but flange placement still depends on the hub’s overall width and internal architecture.

The major wheel-geometry changes came when wider dropout standards accompanied thru-axles.

Boost

Boost increased common MTB hub spacing to 110 mm front and 148 mm rear. The additional width gave hub designers room to move flanges outward and improve spoke bracing angles.

However, not every Boost hub has identical flange dimensions. Designers still make different choices involving bearings, freehub mechanisms, rotor placement, and hub-shell structure.

Boost therefore creates an opportunity for improved wheel geometry; the 110 or 148 mm number alone does not define it.

Super Boost Plus

Super Boost Plus uses 157 mm rear spacing and provides still more room for outward flange placement. Pivot, one of the companies that promoted the trail/enduro implementation of the standard, specifically identifies wider flange placement as a major design objective and states that its Super Boost Plus configuration can place the flanges substantially farther apart than a 142 mm hub. Pivot’s technical documentation describes increased wheel stiffness as a primary benefit.

As with Boost, however, 157 mm axle spacing does not guarantee a particular flange width or tension ratio. Actual hub dimensions remain what matters to the wheelbuilder.

Flange Spacing and Wheel Stiffness

Increasing spoke bracing angle generally increases lateral wheel stiffness because the spokes have greater leverage against sideways rim movement.

This can be particularly useful for:

  • Large-diameter 29-inch wheels
  • Long-travel mountain bikes
  • E-MTBs
  • Cargo bikes
  • Heavy riders
  • Wheels exposed to high cornering loads

Hub manufacturers have deliberately altered flange geometry to change wheel stiffness. SRAM’s historical Zipp hub documentation, for example, identifies wider and asymmetric flange layouts as tools for increasing lateral stiffness and balancing spoke tension. SRAM/Zipp Hub Technical History

Flange spacing is only one contributor, however. Rim stiffness, spoke count, spoke gauge, lacing pattern, tension, and wheel diameter all influence the finished wheel.

A rider cannot reliably judge hub flange spacing from perceived wheel stiffness alone.

Asymmetric Rims

An asymmetric or offset rim moves the spoke holes away from the rim’s geometric centerline.

This allows the spoke bed to move toward the hub’s more inward flange, reducing the difference between the left and right bracing angles.

Depending on the wheel geometry, this can:

  • Improve the left/right tension ratio
  • Increase tension on the lower-tension side
  • Reduce spoke unloading
  • Improve long-term wheel stability

An asymmetric rim does not move the tire away from the bicycle centerline. Only the spoke bed is offset; the rim and tire still need to be correctly dished for the frame.

Offset rims are particularly useful because they improve spoke geometry without requiring a wider frame or axle standard.

Design Constraints

Cassette and Freehub Clearance

The rear drive-side flange cannot simply be moved outward indefinitely. It must clear the cassette, freehub mechanism, bearings, and axle structure.

Changing freehub standards can therefore influence available flange geometry even when dropout spacing remains unchanged.

Disc-Rotor Clearance

The rotor occupies valuable lateral space on the opposite side of a disc hub. Center Lock and six-bolt interfaces also impose specific packaging requirements around the flange and bearing system.

Bearing Placement

Hub designers must accommodate sufficiently large and well-supported bearings. Moving a flange outward may conflict with bearing placement, end-cap design, or hub-shell structure.

Frame and Drivetrain Requirements

Increasing dropout width has consequences beyond the wheel. Chainline, crank clearance, chainstay geometry, tire clearance, and suspension packaging all interact with rear-hub width.

This is why continually widening hubs is not a free improvement. Wheel geometry has to be balanced against bicycle geometry and component compatibility.

Mechanic’s Perspective

Hub flange spacing becomes especially important when building a wheel or replacing a hub.

Two hubs can both be labeled 12×148 Boost yet have different:

  • Flange diameters
  • Left center-to-flange dimensions
  • Right center-to-flange dimensions
  • Spoke-hole diameters

They are therefore not automatically interchangeable while retaining the same spokes.

When replacing a hub, spoke length should be recalculated using the actual hub dimensions and the rim’s effective rim diameter (ERD). Do not assume that matching axle spacing, spoke count, and lacing pattern means the existing spokes will be correct.

A proper spoke calculation normally requires:

  • Rim ERD
  • Left and right flange diameters
  • Left and right center-to-flange dimensions
  • Spoke count
  • Lacing pattern
  • Relevant spoke-hole dimensions

When troubleshooting a wheel with chronically loose non-drive-side spokes, poor flange geometry may be part of the problem. Before simply adding tension, check:

  • Overall wheel dish
  • Drive-side tension against the rim’s maximum specification
  • Non-drive-side tension
  • Same-side tension uniformity
  • Rim offset
  • Spoke and nipple condition
  • Rim damage

If the high-tension side is already near the rim’s allowable limit, increasing all spoke tension may not provide a safe solution. The wheel may simply have an unfavorable tension ratio for its combination of hub and rim.

An asymmetric replacement rim can sometimes materially improve such a wheel, but its offset must be included in the spoke-length calculation.

Hub measurements should also be verified rather than blindly copied from an online database when accuracy matters. Manufacturer drawings occasionally change between hub generations, freehub configurations, or end-cap versions while the commercial model name remains similar.

Common Misconceptions

“Wider Flanges Always Mean Better Spoke Tension Balance”

Not necessarily. Wider total spacing improves potential bracing geometry, but tension balance depends on how far each individual flange sits from the rim centerline.

“Narrow Flanges Require Higher Spoke Tension”

Not inherently. Narrower bracing angles reduce lateral support, but safe absolute tension is determined by the entire wheel system.

“All Boost Hubs Have the Same Flange Spacing”

False. Boost defines the hub/frame interface dimensions, not identical internal hub geometry.

“Thru-Axles Create Wider Flange Spacing”

Not by themselves. Axle diameter and hub width are separate design variables.

“A Wider Hub Automatically Makes a Stronger Wheel”

A wider hub can provide better bracing angles, but wheel durability still depends on rim design, spokes, tension, lacing, spoke count, and build quality.

“Flange Spacing and Wheel Dish Are the Same Measurement”

They are related but distinct. Flange geometry influences the dish and tension required to position the rim correctly, while dish describes the rim’s final lateral position.

Related Terms

References

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