Crankset and bottom bracket compatibility: length is only one field
Keep crank arm length, spindle and shell, chainring interface, chainline and physical clearance separate. Build an exact evidence record instead of treating a shared label as approval.
Published 9 October 2026 · OpenBikeFit
Crank field guide · not to scale
A length cannot answer every interface.
Crank radius · shell/spindle · ring/chainline are separate questions.
Fictional ring plane: centre plane 0 → ring 45 mm; +2.5 → 47.5 mm.
No actual part match, spacer prescription, drivetrain or clearance approval.
On this page
- 01Start with the exact current and candidate identities
- 02Keep the shell and spindle interface separate
- 03Variant markings are evidence with a scope
- 04Name the chainline centre-plane endpoint
- 05Chainring attachment and drivetrain approval are another question
- 06Lateral width is not the length calculation
- 07Frame and ground clearances use different references
- 08Make the hand-off explicit about unresolved evidence
Understand the model first
Inspect the existing crank comparison
The free tool retains its access, hardware and measured re-check boundaries. Reading creates no bike record or installed claim.
Open toolA replacement crankset can have the desired arm length and still be the wrong assembly for a bicycle. The length used by a comparison calculator describes the pedal orbit. It does not identify the spindle, bottom bracket, chainring mounting, drivetrain combination or permitted frame envelope. Before considering a swap, build a record that separates these questions and preserves the exact evidence for each one. An incomplete match is not a partial installation approval.
This is an identification guide, not a fitting procedure. It supplies no removal steps, spacer recipe, tightening sequence, bearing preload or torque values. Do not dismantle a bicycle simply to fill every field. Record what can be safely identified, retain unknowns and ask the frame or component manufacturer or a qualified mechanic to resolve inaccessible details. A photograph, an apparently successful fit or a shared family name cannot replace the applicable current instructions.
Start with the exact current and candidate identities
Keep manufacturer, full model code, generation, variant, left and right arm identity, declared lengths and the source document together. Record the assembled crankset rather than only the visible drive-side arm. A candidate advertisement may show several sizes or variants under one heading; verify the specific item rather than inheriting every property from the product family. A used part also needs a separate condition assessment which this page cannot perform.
For a comparison sheet, give the current assembly and candidate separate columns. Do not copy unresolved fields from one to the other just because both use the same headline label. If the source names a different variant, leave the question open. The aim is an auditable list of matches, differences and missing evidence, not a percentage score or a winner selected by the browser.
Keep the shell and spindle interface separate
The frame’s BB shell has its own interface and dimensions. The crank spindle and bearing assembly have different identities. A shell width is not a spindle length, and an arm length is neither. SRAM’s current DUB MTB chart distinguishes shell width, inner diameter or thread form, crankset type and part numbers. That example demonstrates why a family label alone is incomplete; it does not provide a conversion for every other manufacturer.
Consult the exact frame specification and applicable component chart together. An installed bottom-bracket label can help identify the bearing product without proving which shell is hidden beneath it. Do not infer a complete interface from the outside cup shape. If measurement would require disassembly, obtain the specification or mechanic’s identification instead. A possible adapter does not become approved merely because a diameter appears numerically close.
Variant markings are evidence with a scope
The SRAM chart identifies its crankset types through spindle inscriptions and treats variants separately. Its DUB MTB Wide article provides an example where a wider spindle is associated with a different chainline and frame requirements. The lesson is to preserve the full variant marking, not to adopt one permanent rule that every Wide, Boost or similarly named product from every generation is interchangeable.
A visible marking resolves only what that marking actually identifies. It does not inspect wear, damage, correct bearing engagement or the rest of the drivetrain. Avoid choosing a spacer combination by analogy to a different row. The applicable chart, exact frame specification and current installation manual must agree before a mechanic can evaluate the assembly. If they appear contradictory, stop and ask the manufacturer rather than inventing a hybrid recipe.
Name the chainline centre-plane endpoint
Chainline describes a lateral chainring reference relative to the bicycle’s centre plane. A single-ring plane and a declared middle-of-multiple-rings reference are not automatically the same endpoint. Park Tool’s Chainline Concepts explains that reference distinction. Retain which ring or ring-set plane the source names. The number is not the distance from the visible edge of an arbitrary down tube or the total width between pedals.
In an original fictional top-view construction, the bicycle centre plane is X = 0 and the declared ring plane is X = 45 mm. Its chainline is 45 mm. Moving that ring plane outward by 2.5 mm would make it 47.5 mm. This describes the declared geometry only: it does not approve an offset chainring, spacer, crank variant or derailleur combination. The two numbers cannot determine what parts created them.
Chainring attachment and drivetrain approval are another question
Record the exact chainring or spider model, mounting interface, bolt arrangement where applicable and the compatible drivetrain generation named by the manufacturer. A matching bolt count or an apparently matching mount does not settle offset, retention parts, orientation or the rest of the drivetrain. Keep the component-specific instructions rather than turning this guide into a generic assembly checklist with invented universal values.
A crankset supplied without a ring leaves that identification unresolved. A ring from the current bicycle should not be assumed transferable to the candidate. Where a manufacturer provides a compatibility chart, use the exact parts and row names; where it does not, request clarification. The browser’s crank-length subtraction cannot answer a question about attachment or drivetrain operation because those fields are not inputs to its geometry model.
Lateral width is not the length calculation
Q-factor concerns the crank pedal-attachment span. A pedal’s one-sided offset and the installed shoe position are additional quantities. The existing Q-factor/stance-width guide owns the detailed endpoint comparison. A shorter arm does not logically require a narrower spindle, and a wider spindle does not tell the arm radius. Keep these dimensions separately even when a product page bundles them under the same model name.
If the candidate changes lateral endpoints, the comparison is no longer an isolated arm-length change. Likewise, a changed pedal, cleat or shoe can alter the contact stack. Preserve the old baseline and declare the additional variables rather than attributing every later observation to the crank delta. No lateral width or modelled top gap here is an ideal stance target or an individual comfort result.
Frame and ground clearances use different references
A component/frame clearance question concerns particular moving parts, frame surfaces and the manufacturer’s permitted envelope. A pedal/ground question concerns BB height, crank position, the pedal’s lower envelope, wheel and tyre condition, bicycle lean and any suspension movement. They cannot be merged into one fits label. The existing BB drop/height guide starts with the axle and ground origins; read it before interpreting a clearance claim.
For a fictional rigid upright bicycle, let BB ground height be 280 mm, the downward crank radius 170 mm and the pedal’s lowest extent 12 mm below its axle. The static vertical gap is 280 − 170 − 12 = 98 mm. A 165 mm radius under otherwise unchanged assumptions gives 103 mm. This is not a cornering limit, a strike-free guarantee or a procedure for testing clearance while riding.
Make the hand-off explicit about unresolved evidence
Keep exact model/source dates, shell and bearing identification, spindle variant, arm lengths, chainring interface, declared chainline, lateral endpoints and relevant frame limitations. Mark each unknown with the question still to be answered. The unfilled worksheet can carry the baseline without creating an app record. Do not attach a completed private customer sheet to a general business inquiry or treat an educational diagram as a manufacturer part drawing.
Once identities are resolved, the free crank tool can compare declared lengths under its existing assumptions. Any subsequent bike-linked workflow retains its separate access, hardware, mechanic hand-off and re-check boundaries. This article grants no permission to install parts and certifies no fit, braking, drivetrain function or riding outcome. A completed information record reduces ambiguity; it is not a completed or physically verified swap.
Practical questions
Frequently asked questions
Will any crankset with the same arm length fit?
No. Arm radius does not identify the shell, spindle, bearing parts, chainring interface, chainline or frame requirements.
Does a shared DUB label prove the same spindle variant?
No. Keep the full markings and exact applicable chart, frame and current manual. Do not infer one variant from another family label.
Can I calculate pedal clearance from crank length alone?
No. Even a static upright gap needs BB ground height and the lower pedal envelope. Lean, terrain and suspension add further unresolved conditions.
Does this guide provide spacer or torque instructions?
No. It helps identify missing evidence, not assemble parts. Exact current manufacturer instructions and physical checks remain necessary.