BCD (Bolt Circle Diameter)

Summary

BCD, or Bolt Circle Diameter, describes the diameter of the circle passing through the centers of a chainring’s mounting bolts. It is an essential measurement when replacing a chainring on a traditional crank spider. However, BCD alone does not guarantee compatibility: bolt count, symmetric or asymmetric spacing, chainring position, mounting orientation, and drivetrain generation must also match. Many modern cranksets use proprietary direct-mount interfaces instead of a conventional bolt circle.

Quick Facts

  • Category: Drivetrain measurement
  • Full name: Bolt Circle Diameter
  • Units: Millimeters
  • Used on: Bolt-mounted chainrings and crank spiders
  • Common examples: 144, 130, 110, 104, 96, and 94 mm
  • Does not describe: Bolt-hole diameter or chainring tooth count
  • Modern alternative: Direct-mount chainring interfaces

Overview

BCD identifies the size of the imaginary circle formed by a chainring’s mounting-bolt centers. A conventional bolt-on chainring must match the crank spider’s BCD and bolt pattern before it can be installed.

BCD patterns have varied throughout bicycle history. Larger patterns were commonly used with large road and track chainrings, while smaller patterns allowed lower gearing for mountain bikes, touring bikes, and compact road cranks.

Modern compatibility has become more complicated. Two rings may both be described as 110 mm BCD but use different bolt counts, asymmetric spacing, hidden-bolt positions, or chainring profiles. BCD should therefore be treated as one part of the mounting specification rather than a complete compatibility standard.

How BCD Is Measured

BCD is easiest to measure when the crank center or two opposing bolts are accessible.

Symmetric Four-Bolt Patterns

Measure from the center of one bolt to the center of the bolt directly opposite it. Because those bolts lie across the circle’s diameter, the measurement is the BCD.

Symmetric Five-Bolt Patterns

Five-bolt patterns do not have directly opposing bolts. Measure between the centers of two adjacent bolts and multiply by approximately 1.701.

The general formula for an evenly spaced pattern is:

BCD = adjacent bolt spacing ÷ sin(180° ÷ number of bolts)

Another method is to measure from the crank’s rotational center to the center of one mounting bolt and multiply by two.

Asymmetric Patterns

Asymmetric chainrings may still use a nominal BCD, but the bolts are not evenly spaced around the circle. The standard adjacent-bolt formula cannot be used reliably. Identify these patterns through manufacturer documentation, a dedicated gauge, or a verified template.

Wolf Tooth’s BCD guide provides the measurement formula and reference dimensions for common symmetric patterns.

Why BCD Matters

Physical Fit

A chainring with the wrong BCD will not align with the crank’s mounting holes. Even a small difference prevents proper installation.

Minimum Chainring Size

BCD limits how close the chainring teeth can be placed to the mounting bolts. A large BCD therefore restricts the smallest practical chainring size.

Some aftermarket rings use threaded bosses or shaped mounting tabs to fit a smaller tooth count than a conventional flat ring. This does not mean every chainring of that size will clear the same spider.

Available Gearing

Changing chainring size changes the bicycle’s gearing. Smaller rings reduce the gear ratio for climbing, while larger rings provide more development per crank revolution. BCD determines which sizes are physically available for a particular crank.

BCD itself does not change ride feel. Riders notice the resulting gear ratio, cadence, chainline, or shifting behavior.

BCD Is Not the Complete Bolt Pattern

Before ordering a chainring, verify:

  • BCD
  • Number of mounting bolts
  • Symmetric or asymmetric spacing
  • Hidden-bolt or crank-arm clocking
  • Inner, middle, or outer chainring position
  • Chainring-bolt and nut requirements
  • Chainline or chainring offset
  • Chain width and drivetrain speed
  • Tooth profile for 1× use
  • Shift-ramp and pin compatibility for 2× or 3× systems

Shimano, for example, uses 110 mm four-arm road patterns, while GRX double cranksets may use separate 110 and 80 mm mounting dimensions for the outer and inner rings. Those chainrings cannot be selected solely by the general description “110 BCD.” Shimano GRX FC-RX820-2 Specifications

On a front-shifting drivetrain, a replacement ring may bolt onto the crank but still shift poorly if its ramps, pins, tooth difference, or orientation were designed for another chainring combination.

Direct-Mount Chainrings

Direct-mount systems attach the chainring or spider directly to the crank using a proprietary spline, threaded interface, or mounting pattern. These systems are normally identified by interface family and offset rather than conventional BCD.

Examples include SRAM three-bolt and eight-bolt systems. Matching the bolt count is still not enough: SRAM states that its eight-bolt road rings and eight-bolt Eagle Transmission cranks are incompatible because they produce different chainlines. SRAM 8-Bolt Compatibility Guidance

Direct-mount compatibility may depend on:

  • Crank generation
  • Road, gravel, MTB, or e-bike application
  • Chainring offset
  • Rear hub spacing
  • Chainline
  • Chain type
  • Standard or oval orientation

Mechanic’s Perspective

The most reliable identification method is the crank model number, followed by the chainring markings and manufacturer compatibility chart. Measuring BCD is useful when markings are missing, but appearance alone is unreliable.

Common service mistakes include:

  • Ordering a symmetric ring for an asymmetric spider
  • Confusing 130 mm road and 135 mm Campagnolo patterns
  • Using single-ring bolts on a double crank
  • Installing an outer ring backward or in the wrong clocking position
  • Ignoring a hidden bolt behind the crank arm
  • Selecting the correct mount with the wrong chainring offset
  • Mixing chainrings that were not designed to shift as a pair

When installing a ring, follow the manufacturer’s torque specification and tightening sequence. Check that orientation tabs, ramps, pins, and printed markings face the correct direction. Loose bolts can creak or damage the ring and spider; overtightening can strip aluminum hardware.

Buying Considerations

Do not purchase a replacement based only on tooth count and BCD. Search using the crank model whenever possible.

For a 1× conversion, also verify narrow-wide tooth compatibility, chainline, frame clearance, and available chainring sizes. A smaller ring may fit the bolt pattern yet contact the chainstay.

For older cranks, uncommon BCDs can limit replacement selection. Before buying an expensive chainring, compare the cost and availability of maintaining the original crank with replacing the crankset and possibly the bottom bracket.

Advantages of Bolt-On BCD Systems

  • Chainrings can often be replaced independently
  • Multiple tooth counts may be available
  • Some patterns have broad aftermarket support
  • Damaged or worn rings do not always require crank replacement

Trade-Offs

  • Numerous patterns create compatibility confusion
  • BCD alone does not fully describe the interface
  • Large patterns restrict minimum chainring size
  • Optimized multi-ring shifting may require matched chainring sets
  • Uncommon and discontinued patterns can have limited parts availability
  • Direct-mount systems may provide better packaging but are often more proprietary

Common Questions

Can any chainring with the same BCD be installed?

No. Bolt count, spacing, clocking, chainring position, hardware, and drivetrain compatibility must also match.

Can BCD be measured across the bolt-hole edges?

Center-to-center measurement is the correct method. Calipers, a BCD gauge, or a verified template reduce measurement errors.

Does a smaller BCD make a drivetrain lower-geared?

Not by itself. It permits smaller chainrings, but the installed tooth count determines gearing.

Do direct-mount chainrings have a BCD?

Not in the traditional crank-spider sense. They are identified by their proprietary mounting interface, offset, chainline, and crank compatibility.

Related Topics

References

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