Subject: Roller-Bearing Headsets
From: Jobst Brandt
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.
I don't agree with Jobst's assertion that these "never worked well."
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 which 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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