Subject: Bottom Bracket Bearing Adjustment
From: Jobst Brandt
Date: January 3, 2001
This concerns conventional threaded adjustable and fixed cup bottom bracket (BB) bearings, not roller bearing or Ashtabula cranks.
The conventional ball bearing Crank assembly, as has been common on three piece cranks, usually has 1/4" balls held in an 11 ball cage. Some less expensive bearings use only 9 or fewer. The balls are best left in the cage because removing it makes assembly difficult, does not make room for additional balls, and saves insignificant weight.
The four kinds of BB threads in common use today are Italian, British, French, and Swiss, possibly in that order of occurrence.
Diameter Pitch Right Left Cup -------- ----- ----- ----- Italian 36 mm x 24F tpi right right tpi (threads per inch) British 1.370" x 24F tpi left right French 35 mm x 1mm right right Swiss 35 mm x 1mm left right
A left hand thread is preferred on the right hand cup because it has a tendency to unscrew if not rigidly tight. The propensity to rotate is small, and will, depending on pedaling, sometimes unscrew a left hand thread that was not tight so that a left hand thread alone will not prevent loosening. The right hand cup should be made as tight as practical and not be removed during regular maintenance. Because cups seldom fail, right hand cups seldom require removal.
No unusual greases are required for this bearing and a can of automotive wheel bearing grease will go a long way to lubricate this and other parts of the bicycle that require grease. After installing the spindle with greased bearings, the (adjustable) left cup should be advanced until an increase in rotational drag can be felt but where the spindle can still be turned using the tip of the thumb and forefinger. Without preload that causes this drag, the spindle will be riding on a single ball as each ball passes under the load.
Known as "ball drop" this phenomenon can best be visualized on a loosely adjusted bearing where the spindle has appreciable clearance. Because the steel of the spindle, balls and cups is elastic, the load can be distributed over several balls, but only if these parts are already in contact before the load is applied. Ideally the preload should be large enough so that the balls on the top do not develop clearance, but this much preload is impractical for such a heavily loaded bearing.
Because the feel of bearing adjustment is delicate, the spindle should be adjusted without the cranks. In a correctly adjusted bearing, the spindle should not spin freely were it not greased. Practically all industrial applications use axial springs (Belleville washers) to preload bearings typically on motor shafts.
Although the BB bearing can operate without preload, its life is substantially extended with a light preload.
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