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Crank spindle standards, one hub
A crank fits a frame only through a bottom bracket that matches both the shell and the crank spindle. This hub gathers the spindle standards, modern and old, so you can name your crank's interface.
Modern external-bearing spindles
Modern cranks put an oversized spindle through the frame on external or pressed bearings. The three you meet most are the 24 mm Shimano Hollowtech II spindle, the 24/22 mm SRAM GXP spindle and the 28.99 mm SRAM DUB spindle, plus the true 30 mm spindle used by BB30 and PF30 frames.
Each needs a matching bottom bracket for the shell, which is why the fitment tool asks for both the shell and the spindle.
How the modern spindles differ
The modern crank spindles covered here:
24 mm Hollowtech II: Shimano, 24 mm at both bearings.
SRAM GXP: 24 mm drive, 22 mm non-drive.
SRAM DUB: 28.99 mm, one bearing fits every shell.
30 mm: stiff aluminium spindle, needs a wide shell.
Older cartridge interfaces
Before external bearings, the spindle was part of a sealed cartridge and the crank bolted onto it. Square taper clamps onto a tapered square, Shimano Octalink onto an eight-spline shaft in two incompatible versions, and the open ISIS standard onto a ten-spline shaft. None of these three interchange, so an older bike needs the cartridge bottom bracket that matches its crank.
The square taper, Octalink and ISIS guide explains how to tell them apart before buying a replacement cartridge.
How to name your own spindle
To identify a modern spindle, read the diameter where it passes through the frame: 24 mm is Shimano Hollowtech II, a spindle that steps to 22 mm at one end is SRAM GXP, 28.99 mm is SRAM DUB, and a full 30 mm is the BB30 and PF30 spindle. For an older cartridge, look at the crank interface instead of the spindle diameter, because the spindle is sealed inside the unit.
Once you have named the spindle, the fitment tool pairs it with your shell standard and returns the exact bottom bracket, so the spindle and shell together, never the crank alone, decide the part you buy.
Fitting a crank to your frame?
Open the BB compatibility toolPick your shell and crank spindle to see the exact bottom bracket you need.Find crank bottom bracket on eBayaffiliateRelated
Sources: Park Tool: Bottom Bracket Standards ยท Sheldon Brown bicycle glossary (bottom bracket, cranks)