Chainring BCD, asymmetric bolts and direct mount: identify the receiving interface
Separate a bolt-circle diameter from angular spacing and direct-mount families, then keep exact crank generation, offset, tooth profile and whole-system conditions unresolved until supported.
Published 11 October 2026 · OpenBikeFit
One circle, different adjacent chords
- Fictional BCD D = 100 mm in both regular patterns. Adjacent chord d = D × sin(π/n).
- Four equally spaced centres: d ≈ 70.7 mm. Five equally spaced centres: d ≈ 58.8 mm. Both circles use the same drawing scale.
- The formula assumes equal angular spacing; it is not an asymmetric-pattern conversion. BCD is not tooth count, ring outside diameter or direct-mount permission.
Original schematic; not to scale. Not a real part drawing, tolerance, conversion, installation or physical inspection.
On this page
- 01Identify the ring and receiving crank
- 02BCD names a circle through bolt centres
- 03Equal angular spacing gives a conditional chord
- 04Asymmetric spacing changes the question
- 05Direct mount is not one universal interface
- 06Offset does not finish the drivetrain record
- 07Record conflicts without rounding them away
- 08Return to the whole assembly questions
Free blank worksheet
Keep supplementary paper references
General reference and question spaces, not dedicated bottom-bracket or chainring fields or assembly approval.
Open toolIdentify the ring and receiving crank
Start with the exact chainring and receiving crank or spider: manufacturer, model, generation and variant. Preserve the product source and any exclusions before comparing a number. Tooth count and number of visible bolts are not complete interface identities. A covered bolt, unusual arm position or direct-mount attachment can make an assembled photograph an incomplete description.
This guide reads existing documents and identifiable markings; it does not ask you to remove a ring, loosen a bolt, file a component or trial-fit a candidate. If an interface cannot be established without physical work, keep it unknown for the appropriate mechanic or manufacturer. An apparently aligned hole or engaged mounting shape does not prove that the whole assembly is permitted.
BCD names a circle through bolt centres
Bolt-circle diameter, or BCD, is the diameter of the circle passing through the specified bolt centres. Wolf Tooth explains that geometric reference separately from the distance between neighbouring bolts. Keep the centre-based convention explicit. Ring outside diameter, tooth tips, bolt-head outside edges and a spider outline are not the same circle.
A BCD value therefore does not state tooth count or gearing ratio. Two rings can have the same mounting-circle label and different tooth inventories, profiles or product conditions. The gearing comparator works with entered tooth numbers and conditional transmission assumptions; it does not inspect mounting holes. The crank-radius model is another separate construction and also cannot identify the ring interface.
Equal angular spacing gives a conditional chord
For n equally spaced bolt centres on a circle of diameter D, the straight chord between adjacent centres is d = D × sin(π/n). Conversely, D = d / sin(π/n). These are planar geometric relationships under the equal-spacing assumption, not a measurement tolerance or a way to manufacture permission. Name the selected centres, count and assumption before using the formula.
The original example chooses a fictional D = 100 mm. Four equally spaced centres give adjacent chord 100 × sin(π/4) ≈ 70.7 mm; five equally spaced centres give 100 × sin(π/5) ≈ 58.8 mm. Both circles are 100 mm, but their neighbouring distances differ. This independently derived example is not a copied chainring-size table or a real product specification.
Asymmetric spacing changes the question
Do not apply the equal-spacing formula to an asymmetric bolt layout as though every adjacent pair subtended the same angle. A shared BCD and bolt count can still accompany different angular positions or model-specific construction. The fictional four- and five-point drawings show regular spacing only; neither reproduces an asymmetric crank or tells you how to convert one.
Wolf Tooth separates asymmetric measurement from its regular-pattern formula and lists exact applicability conditions on its current asymmetric Shimano product page. The useful principle is that the named crank generation and product exclusions still matter after a 110 BCD or four-bolt label has been read. This guide does not reproduce the manufacturer part matrix or adopt an older page statement as current catalogue availability.
Direct mount is not one universal interface
Direct-mount attachment needs its own exact family and receiving-part evidence rather than a BCD inferred from the visible chainring. Manufacturer terminology can identify distinct mounting arrangements within apparently related products. Number of fasteners, profile, orientation and retaining parts still belong to the named design; do not treat every three-bolt or eight-bolt label as universal interchangeability.
SRAM distinguishes an eight-bolt direct-mount interface for the named Eagle 90 Transmission crank from a three-bolt interface for Eagle 70 Transmission. Its support pages also scope the ring and offset declarations to those products. Those two examples demonstrate why the exact model is needed; they do not approve another ring or a conversion between the families.
Offset does not finish the drivetrain record
A source-defined chainring offset is an axial relationship with a particular datum and construction. It must not be confused with BCD, outside diameter or tooth count. A shared offset label still leaves the mounting family, exact crank, ring profile and receiving system unresolved. Do not infer a whole chainline or preferred riding setup from that one number.
Keep chain requirements, ring profile, any directional or paired-ring conditions, clearance and retaining hardware as separate source questions. The existing whole-crank guide owns the broader chainline and lateral-reference discussion; the cassette guide owns its different driver interface. A complete drivetrain declaration can connect all these records without merging them into a single compatible badge.
Record conflicts without rounding them away
A useful comparison preserves the original product words, source revision and exceptions beside each field. Separate documented nominal values from observations. If a current product excludes a crank generation or the sources describe different layouts, retain the conflict and stop treating the match as complete. Do not round a chord to a familiar standard or silently change bolt count to force the formula.
The arithmetic cannot establish hole tolerance, material, thread condition, torque, engagement, chain retention or installation readiness. No universal bolt, washer, spacer, filing or tightening prescription is provided. This path also gives no rider-fit, comfort, power or braking outcome. Correct circle geometry is one scoped statement about a declaration, not a physical assessment of the bicycle.
Return to the whole assembly questions
Carry the exact ring identity, receiving crank or spider, mounting family, BCD where applicable, angular-layout evidence, source-defined offset and all unresolved drivetrain conditions to the existing whole-crank checklist. Keep shell and spindle records alongside it. A ring that matches one interface still does not resolve the frame, bottom bracket, crank axle or permitted complete configuration.
The free blank worksheet has general references and question spaces, not dedicated chainring fields; use supplementary paper notes without relabelling a fit dimension. Open the free gearing tool only for its declared numerical comparison. Reading a guide, opening a worksheet or producing a conditional ratio neither saves an installation record nor approves a part change, a gear combination or a ride.
Practical questions
Frequently asked questions
Is BCD the chainring outside diameter?
No. It is the diameter of the circle through specified bolt centres. Tooth-tip diameter, tooth count and bolt-head edges are different references.
Can every four-bolt chord be converted with the regular formula?
No. The adjacent-chord formula assumes equal angular spacing. An asymmetric layout needs its exact pattern and product evidence.
Does three-bolt direct mount mean every such ring fits?
No. Resolve the exact mounting family, receiving crank generation, offset, retaining parts and drivetrain conditions. Bolt count alone is incomplete.
Does a gearing result approve a chainring combination?
No. Numerical tooth ratios do not inspect ring mounting, chain requirements, clearance or whole-assembly permission. Keep those source and physical questions separate.