Cranksets & Bottom Brackets

The bottom bracket on a bicycle connects the crankset to the bicycle and allows the crankset to rotate freely. It contains a spindle to which the crankset is attached and the bearings that allow the spindle and cranks to rotate. (The chainrings and pedals are attached to the cranks. more...

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) The bottom bracket fits inside the bottom bracket shell, which connects the seat tube, down tube and chain stays as part of the bicycle frame.

\"There is some controversy as to whether \"axle\" or \"spindle\" should be used in particular contexts. The distinction is based on whether the axle/spindle is stationary, as that in a hub, or rotates, as that in a bottom bracket.\" This article uses spindle throughout for consistency.

An old American term for bottom bracket is \"hanger\". This is usually used in connection with Ashtabula cranks, alternately termed \"one-piece\" cranks.

Bottom bracket types

Three-piece

In typical modern bikes, the bottom bracket spindle is separate from the cranks. This is known as a three-piece crank. The cranks attach to the spindle via a common square taper, a cotter or via a variety of splined interfaces.

Loose bearing

Earlier three-piece cranks consist of a spindle incorporating bearing cones (facing out), a fixed cup on the drive side, an adjustable cup on the non-drive side, and loose bearings. Overhauling requires removing at least one cup, cleaning or replacing the bearings, reinstalling the spindle, and adjusting the cups.

Cartridge bearing

Most modern bicycles use what is called a \"cartridge\" bottom bracket instead. Sealed cartridge bottom brackets are normally two pieces, a unit holding the spindle and bearings that screws in to the bottom bracket shell from the drive side and a support cup (often made of light alloy or plastic) that supports the spindle/bearing assembly on the non-drive side. Other designs are three piece, the spindle is separate, but the bearing cups incorporate cheaply replaceable standard cartridge bearing units; Specialites TA's \"Axix\" range is one example. Either arrangement makes servicing the bottom bracket a simple matter of removing the old cartridge from the bottom bracket shell, and installing a new one in its place. Cartridge bottom brackets generally have seals to prevent the ingress of water and dirt.

In general usage, the term 'Three piece' refers to the former design, with sealed bottom brackets being seen as the 'standard'. Designs utilizing separate bearings are still very often found on BMX bikes, due to the means of installation.

Ashtabula (One-Piece)

With an Ashtabula crank and bottom bracket, the spindle and crank arms are a single piece. The bottom bracket shell is large to accommodate removal of this S-shaped crank. Bearing cups are pressed into the bottom bracket shell. The crank holds the cones, facing in; adjustment is made via the left-threaded non-drive-side cone.

Read more at Wikipedia.org