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Bicycle Spindle Options: 24 MM, GXP, 30 MM, DUB and M30

There is a dizzying array of axle spindle and bottom bracket options that can confuse even the most seasoned rider. As we don’t have enough pages to list all the fun permutations, we’ll stay basic. For this FAQ, we’ll focus on the five most common spindles you will encounter. These are 24 MM, GXP, 30 MM, DUB and M30. All the numbers you see will be axle diameters in millimeters.


24 MM

The 24 mm spindle is a straight standard steel axle used by Shimano and fits 24 mm bottom brackets. These have been the standard for many years so there are and will always be a lot of bottom bracket manufacturer options. However, since these are steel spindles, they are heavier than the others, have smaller bearing contact patches and are more prone to corrosion. The 24 mm spindle is most often used by 4iiii, power2max, ROTOR, Shimano, Stages and SRM.


The GXP is a stepped down 24 – 22 millimeter steel axle. This is a SRAM proprietary product manufactured for many years and along with the 24 mm Shimano is the only option for the Trek BB90 bottom bracket shell standard. Very common, but since steel is heavier and more corrosion prone.

30 MM

The BB30 is a straight 30 mm aluminum spindle that fits 30 mm bottom brackets. The larger diameter equates to increased stiffness as well as lends itself to many different bottom bracket options such as BB30, PF30, 386 EVO and T47, to name a few. In addition, it’s corrosion resistant. One thing to note on the 30 mm spindle is it’s too large for the now discontinued Trek BB90 bottom bracket shell. This spindle is used by many in the cycling industry including Easton, power2max, Race Face, ROTOR and SRM.


DUB is SRAM’s proprietary axle which was introduced in 2019 and uses a 29 mm straight aluminum axle. It fits all current SRAM road, gravel and MTB cranks which used with a SRAM DUB bottom bracket. The large diameter aluminum spindle is light, stiff and corrosion resistant. Since this spindle is proprietary to SRAM, there are limited crank choices and it’s not Trek BB90 compatible.


The M30 is Praxis’ proprietary 30/28 mm stepped aluminum spindle which only fits M30 bottom brackets. Specifically, as you can see in the image below, the axle takes a 30 mm bearing on the drive side and a 28 mm bearing on the non-drive side. In addition to being light weight and corrosion resistant, Praxis says the advantage to this stepped spindle is it pushes the cartridge bearings out as far as possible to create a solid platform for pedaling torque. Please note that this is a proprietary Praxis product so there are limited third party bottom bracket alternatives and it’s not Trek BB90 compatible.

Dave Harris

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