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Rec.Bicycles Frequently Asked Questions Posting Part 4/5
Section - 8f.14 Roller Head Bearings

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See reader questions & answers on this topic! - Help others by sharing your knowledge
Date: Fri, 13 Feb 2004 12:07:59 -0800
 
Although roller bearing headsets never worked well, they introduced a
positive feature, not directly connected with rollers.  The main
advantage of some rollers was that they had two bearings, the rollers
and a plain bearing back plate that was needed because rollers cannot
run well with even the slightest misalignment of inner and outer race,
something that conventional ball bearings do easily.  The importance
was that this feature separated rotary from swiveling motions.
 
A head bearing serves mainly as the axis about which the fork steers,
but it also carries fore and aft swiveling motion as the fork flexes.
Swiveling motions are the ones that damage head bearings.  As the
bicycle is ridden, the fork absorbs shock by flexing, primarily at the
fork crown, where it rotates fore and aft in the plane of the bicycle
frame, a motion that can be seen by watching the front hub while
sighting over the handle bars while rocking the bicycle fore and aft
with the front brake locked.
 
Although the wheel visibly moves, the angle through which the fork
crown swivels is small and is not in itself damaging because it is
readily absorbed by cup and cone ball bearings.  However, occurring
repeatedly in the absence of steering motions, bearing balls fret in
place and displace lubrication that normally separates them from their
races.  Without lubricant, bearing balls weld to their races and tear
out tiny particles, causing dimples having a matte finish.  This
phenomenon primarily affects road bicycles while coast down hills fast
enough to make practically no steering motions that would move bearing
balls from their straight ahead position to replenish lubrication.
 
Because rollers cannot absorb swiveling motions, some were equipped
with spherical backing plates that could.  This design feature was
then incorporated into ball head bearings that, in contrast to
rollers, stay aligned to their races and cannot bind as rollers do by
sliding off center, an effect that made them hardly useful for this
application.  The combination of ball and plain bearings has replaced
rollers for this job.

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Top Document: Rec.Bicycles Frequently Asked Questions Posting Part 4/5
Previous Document: 8f.13 Indexed Steering
Next Document: 8f.15 Brakes from Skid Pads to V-brakes

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