The bottom bracket resides in a circular shell at the base of the frame tubes on your mountain bike. The right and left crank arms attach to each end of a spindle that rotates in the bracket as you pedal. Pedal cranks are specific to the attachment mechanism of the spindles. The type of bottom bracket on your mountain bike depends on the age of the bike and whether it's a single-speed or multi-speed.
Cartridge
Cartridge bottom brackets are on the majority of newer mountain bikes. Bearings inside the bracket are lubricated and sealed during production. Aside from adjusting the bracket when it becomes loose, cartridge brackets are not serviceable. You remove and replace the unit when it's damaged or wears out. Cartridge bottom brackets come in two types: tapered spindles and spline spindles. Tapered spindles have a bolt that attaches the crank arms and spline spindles have a lockring on the sprocket crank. Each type of cartridge requires a specific tool to remove the particular style of cranks and access the bottom bracket.
Adjustable
Adjustable bottom brackets appeared on the majority of mountain bikes prior to the introduction of sealed cartridge brackets. Adjustable brackets come with tapered or spline spindles for attaching either style of cranks. The right side of the bracket has an adjustable cup and a lockring that threads into the bottom bracket shell. Typically, the cup has notches for a pin spanner or flats for a wrench.
The left side of the bracket has a fixed cup with wrench flats and no lockring. Each cup contains a set of ball bearings. You service adjustable bottom brackets by cleaning the unit and greasing or replacing the bearings. You adjust the bottom bracket by loosening or tightening the lockring.
One-Piece
The majority of older single-speed mountain bikes have a one-piece unit that makes up the bottom bracket. A single piece of steel forms the crank arms and an axle. The axle rides on bearings inside the bottom bracket shell that you remove for servicing. You identify one-piece bottom brackets by the large hex-nut at the right side of shell and the absence of bolts or splines used to attach accessory crank arms.
Sizes
With few exceptions, bottom bracket shells are the same width. This is not to say any bottom bracket will work on your mountain bike. The type of bracket, such as cartridge, adjustable or one-piece, must suit your single-speed or multi-speed bike. In addition, the size of the bracket must be correct so the main crank arm does not interfere with the action of the front sprocket or derailleur. Interchangeability charts tell you which size bottom bracket to use when replacing one that's damaged or if you change the pedal cranks on your mountain bike.
Member Comments