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Crank spindle sizes: 24mm vs GXP vs DUB vs 30mm

Your crank spindle diameter, combined with the frame shell, decides the bottom bracket, so knowing your spindle is half the compatibility answer.

Shimano Hollowtech II, 24mm

Shimano's Hollowtech II cranks use a 24mm steel spindle of constant diameter. It is the most widely supported interface, and because 24mm is relatively small it fits into almost any shell with the right cups or a reducer.

A steel spindle keeps the diameter modest but strong, which is why Shimano has stuck with 24mm across generations of road and mountain groupsets.

SRAM GXP, 24mm stepping to 22mm

SRAM's older GXP spindle is 24mm along most of its length but steps down to 22mm at the non-drive side. That step lets the crank locate against the drive-side bearing to set bearing preload. A GXP crank therefore needs a GXP-specific bottom bracket rather than a plain 24mm one, because the non-drive bearing is sized for 22mm.

SRAM DUB, 28.99mm

DUB is SRAM's more recent single spindle at 28.99mm, sitting between the 24mm and 30mm camps. The idea was one spindle diameter to fit every shell SRAM supports, from BSA to pressfit, using the appropriate DUB bottom bracket for each. Only a DUB bottom bracket carries the right bearing inner diameter for that 28.99mm spindle.

30mm spindles

A true 30mm aluminium spindle, used by Rotor, Race Face and older SRAM cranks, is the stiffest and lightest of the group but demands a large-bore shell. It fits directly in PF30, BB30 and T47, but will not fit a narrow BSA shell without a specific setup, and a 30mm crank cannot run in cups made for 24mm.

This is why spindle diameter has to be checked against shell bore before ordering: the bearing inner diameter of the bottom bracket must match the spindle to the millimetre.

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Sources: SRAM · Shimano