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8f.8 Bottom Bracket Bearing adjustment


This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.

8f.8 Bottom Bracket Bearing adjustment

From: Jobst Brandt <jobst.brandt@stanfordalumni.org>
Date: Wed, 03 Jan 2001 16:50:20 PST

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    36mm   x   24F tpi  right right        tpi (threads per inch)
British    1.370" x   24F tpi  left  right
French     35mm   x   1mm      right right
Swiss      35mm   x   1mm      left  right

Unless there is something wrong with the right hand cup it should not
be removed because it can be wiped clean and greased from the left
side. The type of thread is usually marked on the face of both left
and right cups. Swiss threads are rare, but if you have one, it is
good to know before attempting removal.

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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