BCD (Bolt Circle Diameter)

Summary

BCD, or Bolt Circle Diameter, is the diameter of the circle passing through the centers of a chainring’s bolt holes. It determines which chainrings are compatible with a given crankset and is a fundamental specification when replacing, upgrading, or configuring front drivetrain components.

Key Facts

  • Introduced: Standardized during the 1970s–80s
  • Category: Concept
  • Also Known As: Bolt Pattern, Bolt Spacing
  • Used By / Found On: All chainring–crank interfaces
  • Units: Millimeters (mm)

Common BCD Sizes: 130 mm, 110 mm, 104 mm, 96 mm, 94 mm, 88 mm, asymmetric
Important For: Chainring compatibility and drivetrain upgrades

Overview

BCD—short for Bolt Circle Diameter—is a core measurement in bicycle drivetrain engineering. It defines the spacing of the bolts that secure a chainring to a crank spider and, by extension, which chainrings a crankset can accept. A crank with a given BCD can only use chainrings drilled to match that pattern.

Different cycling disciplines and eras have used a wide range of BCD standards. Larger BCDs, common in older road cranks, accommodate big chainrings suited to high-speed racing. Smaller BCDs, prevalent in modern mountain and gravel drivetrains, allow for compact rings with lower gearing appropriate for climbing. Asymmetric and direct‑mount systems in current-generation cranks add another layer of complexity, reflecting evolving drivetrain trends.

Understanding BCD is essential for riders and mechanics performing drivetrain maintenance, chainring swaps, gearing changes, or 1x conversions. Selecting an incompatible BCD will prevent installation entirely, making it one of the few inherently “hard-spec” dimensions in bike componentry.

How It Works

BCD represents the diameter of an imaginary circle connecting all chainring bolt centers. The method for identifying BCD depends on bolt count and bolt arrangement.

1. Measurement Method

For 5‑bolt spiders:
Measure the distance from the center of one bolt hole to the center of the bolt hole two spaces over (not the adjacent one). This value can be used with a conversion chart or trigonometry to determine true BCD.

For 4‑bolt spiders:
Measure directly between opposite bolt centers. This measurement is typically the BCD itself.

For asymmetric patterns:
Use manufacturer specifications, because the bolt holes are not evenly spaced (e.g., Shimano’s 4‑arm asymmetric road design).

Common tools include calipers, printable templates, or manufacturer diagrams when precision is required.

2. Why BCD Matters

  • Defines Chainring Fit:
    A chainring must match the crank’s BCD; mismatched rings cannot be mounted.
  • Limits Minimum Tooth Count:
    Larger BCDs restrict how small a chainring can be. For example, 130 mm BCD road cranks cannot accommodate compact 30T rings, while 94 mm or 96 mm BCD cranks can.
  • Influences Gearing Options:
    Riders converting to 1x drivetrains often seek smaller BCDs to use small narrow‑wide chainrings.
  • Affects Cross‑Brand Compatibility:
    Cranks—even with similar-looking spiders—may be incompatible due to BCD variations.

3. Direct-Mount Exception

Many modern cranksets eliminate the bolt circle entirely. Instead of traditional spiders, they use:

  • Spline interfaces
  • Lockrings
  • Proprietary direct-mount standards

In these cases, chainring compatibility is determined by the mounting interface rather than BCD, though the underlying concept—matching chainring to crank platform—remains unchanged.

Standards & Compatibility

BCD sizes vary widely across road, mountain, gravel, and older bicycle systems. There is no universal standard.

Road BCDs

  • 130 mm (Standard Road):
    Traditional road size for large rings such as 53/39T.
  • 110 mm (Compact Road):
    Enables smaller rings (50/34T) and became widespread in the 2000s.
  • Shimano Asymmetric 4‑Bolt (e.g., 110/112 mm):
    Used on Dura‑Ace and Ultegra; not a true circle and requires model-specific chainrings.

Mountain BCDs

  • 104 mm:
    Common on 4‑bolt MTB cranks; compatible with many 32–36T narrow‑wide rings.
  • 96 mm:
    Used on Shimano 1x and 2x systems; supports smaller chainrings.
  • 94 mm:
    SRAM’s older 4‑bolt standard for 1x drivetrains.
  • 88 mm / 76 mm:
    Found on some triple cranksets and minimalist drivetrain designs.
  • SRAM X‑Sync Direct Mount:
    Uses 3‑bolt or 8‑bolt interfaces—no traditional BCD.

Gravel & Cyclocross BCDs

A mix of 110 mm, 104 mm, and brand‑specific asymmetric formats.
SRAM and Shimano both offer gravel-oriented 2x cranksets with unique chainring spacing and bolt patterns.

Notable Compatibility Issues

  • Shimano asymmetric 4‑bolt chainrings cannot be used on standard 110 mm cranks.
  • SRAM direct-mount chainrings require the correct 3‑bolt or 8‑bolt interface.
  • Vintage cranksets may use now‑obsolete BCDs, such as Campagnolo’s 144 mm pattern.

Notable Use Cases

  • SRAM Force 1 Crankset (110 mm BCD):
    Frequently adapted for 1x gravel setups using narrow‑wide chainrings.
  • Shimano GRX (110/80 mm BCD):
    Dual‑BCD layout supporting compact 2x chainring configurations.
  • Race Face Aeffect (Direct Mount):
    A modern MTB crankset using spline/lockring mounts rather than bolts.
  • Trek 520 Touring Bike:
    Uses a 130/74 mm BCD combination to enable wide‑range gearing for loaded touring.
  • Wolf Tooth CAMO System:
    Modular spider-and-chainring platform allowing riders to switch between multiple BCD standards.

Related Terms

References

  • Park Tool: BCD Measurement Guide
  • Wolf Tooth Components: BCD & CAMO System Explained
  • Shimano Tech Docs: Road & MTB Cranksets
  • SRAM Crankset Standards White Paper
  • BikeRadar: “Bolt Circle Diameter – Why It Matters”
  • Sheldon Brown: Chainring Compatibility Notes
  • Race Face Crankset Product Specifications
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