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On a pedal cycle, the bottom bracket is the bearing assembly on which the cranks rotate. The element of the frame into which it is mounted is the bottom bracket shell. The traditional form of the BB is a cup and cone bearing - a spindle, two cups, one adjustable with lockring and one fixed, using nine 1/4" balls on each side - but these are increasingly being supplanted by cartridge units. Roller bearings are generally unsuitable because of the asymmetric lateral forces imposed by the action of pedalling.

There are at least four mutually incompatible varieties of threading which have been in mainstream use in the last 20 years: UK/US, Italian and two French varieties. The UK/US and one of the French forms use a left-hand thread for the right-hand (chainring side) fixed cup, the others a right-hand thread. Unit bottom brackets generally thread into the right-hand side with a floating sleeve on the left-hand side to keep them aligned.

Fitting or removing the bottom bracket assembly frequently requires proprietary tools - large flat spanners, peg spanners, splined removers and the ilk. Make sure that the frame threads are clean; in particular, if the frame is new or has just been resprayed, make sure they are not choked with paint or shotblasting grit - ideally in these circumstances you - or more probably someone with framebuilder's tools - should run the appropriate tap through them. Grease the threads lightly if you ever want to be able to get the thing apart again.When refitting a cup and cone bracket:

  • Stick the balls into the races of the cups with a plentiful lump of grease.
  • Screw up the fixed cup (bearing in mind that it may be a left-hand thread). Tighten it up to the frame using the appropriate tools, and bearing in mind that stripping the thread makes for a very expensive frame repair. Using inappropriate tools generally results, inter alia, in carving up the paintwork. Make sure the balls are still in place. Sometimes keeping a finger stuck through the hole in the cup helps.
  • Insert the spindle so as to hold the balls on the fixed cup side in place.
  • Tighten up the adjustable cup with your fingers until you just reach a point where the spindle will no longer rotate freely.
  • Holding the adjustable cup in place with the appropriate tool, tighten up the lockring against the BB shell. This tightening action will slightly reduce the compression of the balls and allow the bracket to rotate more freely. If there is any play whatsoever in the spindle, slacken off the lockring a lttle and tighten the adjustable cup a smidgeon; it is better to err on the tight side than the slack side.
While cup-and-cone bottom bracket assemblies are still commonly found on cheaper bikes and many road bikes, most mountain bikes use a Shimano-style bottom bracket cartridge. This cartridge is a single unit with the spindle and bearings in a sealed metal cylinder. Adjustment and lubrication are neither required nor possible.

Removal of this type of a bottom bracket requires a crank puller to remove the cranks, and a special splined tool to remove the "cups" holding the cartridge in the frame. When a new BB is being inserted, tighten the drive (chainring) side first, followed by the other side. The drive side is threaded to tighten *counterclockwise*, while the left side tightens in the regular way.

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