Crank spindle diameter, length and bearing stations: three different references
Identify the exact spindle variant, name each diameter station and retain axial endpoints and bearing locations independently. Equal diameter and overall length do not establish a complete crank interface.
Published 11 October 2026 · OpenBikeFit
Equal length does not locate the bearings
- Two fictional shafts each have declared end-to-end length 120 mm and nominal diameter 24 mm.
- A: bearing-station centres z = 21 and 79 mm → span 58 mm. B: z = 20 and 83 mm → span 63 mm; difference 5 mm.
- Stations are independently declared, not inferred from shell or seal faces. Equal-scale axial lanes are not part profiles, engagement or a spacer recipe.
Original schematic; not to scale. Not a real part drawing, tolerance, conversion, installation or physical inspection.
On this page
- 01Identify the spindle variant first
- 02Diameter requires a named station
- 03Keep stepped journals separate
- 04Overall length needs its own endpoints
- 05Bearing stations are independent declarations
- 06A subtraction does not allocate engagement
- 07Compare sources without inventing a part
- 08Carry the unknowns to the whole crank record
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 spindle variant first
Record the crank manufacturer, model, generation and exact spindle or axle variant before comparing dimensions. Include the receiving frame and complete bottom-bracket identities as separate references. A familiar family name on a crank arm may not distinguish road, mountain, wide or another product-specific construction. A source statement for a different variant cannot silently replace the missing one.
This is a document-reading guide, not a request to dismantle a crank or measure hidden contact surfaces. Use the exact product documentation and existing identifiable markings. If the part cannot be established without physical work, preserve that uncertainty for the appropriate mechanic or manufacturer. Do not use a trial insertion, bearing movement or apparent rotation to certify an interface.
Diameter requires a named station
A spindle diameter is a transverse reference at a stated location. Bearing journal, shoulder, central tube, spline, thread and seal contact can describe different stations. Two numbers with the same unit are comparable only after the subject and convention are established. A maximum outside diameter or visible middle section is not automatically the diameter at a bearing contact.
Shimano describes a 24 mm axle in its Hollowtech II technology reference and an axle integrated with the right crank arm. That statement defines the named technology in its scope; it does not approve any other 24 mm part or every receiving frame. Do not turn the technology description or its performance marketing into a universal interchangeability or superiority claim.
Keep stepped journals separate
Wheels Manufacturing distinguishes uniform and stepped crank interfaces in its overall-width chart, including a 22/24 mm listing. A stepped declaration contains two named diameters, not an average diameter of 23 mm. Averaging loses the separate receiving contacts and produces a number that describes neither journal. Keep the left and right or source-defined station names exactly as documented.
A shell label also does not supply the journal diameter. The preceding guide separates BSA, T47 and press-fit receiving interfaces from crank dimensions. Likewise, a nominal family label such as DUB belongs beside its exact variant and source, rather than becoming a substitute for a physically observed diameter or a tolerance. SRAM distinguishes the named Eagle 90 crank variant in its own product support.
Overall length needs its own endpoints
For any axial length, name both endpoints and which components are included. An end-to-end spindle dimension, shoulder-to-end length, engaged spline extent and assembled outside span can all be different. The chart heading or manufacturer drawing must define which one is meant. Do not infer a total length from a photograph that hides the opposite end or includes an attached crank arm.
The fictional schematic declares two shafts with the same end-to-end length, 120 mm, and the same nominal diameter, 24 mm. These are invented reference records, not specifications for a real product. Both numbers remain true in the construction even though the independently declared bearing stations differ. Equality of two fields does not establish equality of the whole part.
Bearing stations are independent declarations
Use the shaft-axis coordinate z only as the declared reference in this example: z = 0 at the named first end and z = 120 mm at the other. Option A declares bearing-station centres at z = 21 and z = 79 mm, giving a span of 58 mm. Option B declares centres at z = 20 and z = 83 mm, giving a span of 63 mm. The span difference is 5 mm despite equal total length and nominal diameter.
These centres are explicitly supplied fictional data, not values inferred from seals or shell width. The original schematic uses equal axial scale to make the different spans visible. It is not a drawing of actual bearing seats, contact shapes or retaining parts. In a real source, first establish whether a station means a centre, face, shoulder or another manufacturer-defined datum.
A subtraction does not allocate engagement
Subtracting a bearing span from overall length does not reveal usable engagement, allowable exposed shaft or a spacer stack. It only subtracts two declared lengths. Position on each side, axial stops, contact geometry and retention requirements remain independent. The example does not divide a remainder in half or recommend moving a bearing to obtain a desired number.
An installed over-seal span also cannot be substituted for these centre stations. Wheels Manufacturing names the dust-seal convention in its chart separately from the frame shell width. Retain that distinction instead of deriving bearing centres from a seal measurement. A numerically plausible reconstruction can still describe the wrong surfaces and conceal an unresolved interface.
Compare sources without inventing a part
A checkable declaration retains exact crank variant, spindle construction, every named diameter station, source-defined axial endpoints, relevant bearing stations and all applicable receiving references. Mark each item as published, observed or unknown. Equal nominal fields are an observation about the records, not permission for the components to replace each other. Conflicting sources need resolution, not an average or a preferred value.
No preload, spacer, cutting, machining, bearing-pressing or tightening procedure is supplied here. A crank-length comparison models a pedal-axis radius under explicit assumptions; it does not test the spindle. The existing whole-crank guide keeps lateral references, frame conditions and other hardware evidence separate. Neither tool nor article performs a physical inspection or certifies the installed assembly.
Carry the unknowns to the whole crank record
Continue to the chainring mounting guide before returning to the whole-system checklist. Chainring interface, tooth profile, chain requirements and clearance cannot be deduced from spindle diameter or bearing span. The gearing tool can compare declared tooth numbers, but its arithmetic does not establish a physically mountable combination. Keep those source questions beside the numerical result.
Use supplementary paper questions beside the free blank worksheet when you need a hand-off. It has general reference spaces rather than dedicated spindle fields, and opening it does not save a crank profile or send a record. This path preserves the evidence needed for a discussion; it does not recommend a crank, claim a fit improvement or declare the bicycle ready for a ride.
Practical questions
Frequently asked questions
Can I average a 22/24 mm stepped spindle to 23 mm?
No. Keep the two named journal diameters separate. The average describes neither receiving contact and loses the interface distinction.
Do equal 24 mm and 120 mm declarations identify the same spindle?
No. The fictional example has those equal fields but separately declared bearing spans of 58 and 63 mm. Exact stations, construction, stops and retention remain independent.
Can overall length minus bearing span give required spacers?
No. That subtraction does not allocate the remainder or establish engagement, stops or permitted installation. Obtain the exact assembly requirements.
Does the crank-length calculator check axle compatibility?
No. It compares declared crank-radius geometry under explicit assumptions, not bearing seats, spindle variants or physical assembly permission.