MTB bottom brackets

How to choose a mountain bike bottom bracket

A bottom bracket is chosen at the intersection of two constraints: the frame on one side and the crankset on the other. Getting either one wrong makes installation impossible. Shimano, SRAM and Race Face cover all common combinations.

Which frame standard?

• BSA (threaded) : 68 or 73 mm wide, the bottom bracket screws in. The easiest to fit and maintain.
• Press Fit : the bottom bracket is press-fitted. The 89.5 and 92 mm widths (BB92) are dominant in mountain biking.
• PF30 and BB30 : larger diameter, found on some frames.
• The standard is engraved on or can be measured from the frame: width and internal diameter.

Which crank spindle?

• 24 mm for a Shimano Hollowtech II crankset.
• 28.99 mm for a SRAM DUB crankset.
• 30 mm for Race Face Cinch and equivalent systems.
• An adapter often allows a given spindle to be fitted in a bottom bracket of another diameter — but not always the other way round.

Bearings: when should they be replaced?

• A tight spot, lateral play at the crank arms or regular creaking indicates bearings nearing the end of their service life.
• On a BSA bottom bracket, the whole assembly can be replaced in one piece using two spanners.
• On a Press Fit, removal requires a dedicated tool and clean reassembly, otherwise it may creak.
• Bearings ceramic reduce friction but require more regular maintenance.

When fitting

Apply plenty of grease to the threads of a BSA bottom bracket, or use assembly grease on a Press Fit. Observe the specified torque and do not forget the spacers: their absence causes most play and creaking after reassembly.

To find out more: How to choose a mountain bike bottom bracket

See also: MTB cranksets and crank arms · Bearings · Drivetrain parts and accessories