This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
From: Jobst Brandt <email@example.com>
Date: Wed, 23 Aug 2000 17:08:29 PDT
> In the several years I spent working in a pro shop, I have never
> seen a case of "index steering" (yes, we called it that) that was
> _not_ caused by a "brinelled" headset - one with divots in the
> races. I am 99.999 percent certain that that is your problem. What
> are you going to do if you don't fix it? I suggest that you fix the
> headset even if you sell the bike, as a damaged headset could be
> grounds for a lawsuit if the buyer crashes.
I disagree on two points. First, because you use the term "Brinell"
that conveys a notion as incorrect as the phrase "my chain stretched
from climbing steep hills" and second, because there is no possibility
of injury or damage from "indexed" steering head bearings.
Damage to head bearings seems to be twofold in this case because
properly adjusted steering can only become looser from dimples,
dimples that cannot immobilize steering. Therefore, the head
adjustment was too tight. However, dimpling is not caused by impact,
but rather by lubrication failure that occurs while riding straight
ahead, giving the steering a preferred home position. This occurs
more easily with a correctly adjusted bearing than with a loose one
that rattles and clunks. Rattling replenishes lubricant between balls
and races, something that would otherwise not not occur. Off road
bicycles suffer less from this malady than road bicycles because it
occurs primarily during long straight descents that on which no
steering motions, that might replenish lubricant, are made.
If you believe it comes from hammering the balls into the races, you
might try to cause some dimples by hammering on the underside of the
fork crown of a clunker bike of your choice. Those who hammered
cotters on steel cranks will recall no dimples on the spindle, even
though it has a far smaller diameter than the head and the blows were
more severe and direct, supported by no more than one or two balls.
Ball bearings make metal-to-metal contact only under fretting loads
(microscopic oscillations) while the bearing is not turning. Any
perceptible steering motion will replenish lubricant from the oily
meniscus surrounding each ball contact patch. Peering over the bars
at the front hub while coasting down a road at 20+ mph you will notice
the fork ends vibrating fore and aft. This motion does not arise at
the fork end, but at the fork crown, where it bends the steer tube.
Both head bearings rotate in fretting motion crosswise to the normal
plane of rotation as the steer tube bends. Dimples form in the
forward and rearward quadrant of both upper and lower bearings from
this fretting. That they also form in the upper bearing shows they
are not directly load related.
Lubrication failure from fretting causes metal to metal contact that
forms microscopic welds between balls and races. These welds
repeatedly tear material from the softer of the two causing elliptical
milky dimples in both races. Were these brinelling marks (embossed
through force), they would be shiny and smooth and primarily on the
inner race of the bearing. Various testimonials for the durability of
one bearing over another are more likely an idication of lubrication
than the design of the bearing. Rigidly mounted ball bearings have
been used as head bearings longer than they should, considering their
The question has been raised whether steering to either side would
reveal a second preferred position in which the balls fall into
matching dimples. Since bearing balls move at roughly half the rate
of steering motion, with 20 balls, this requires a steering angle of
36 degrees for dimples in both races to match again with the balls.
However, the balls do not arrive exactly at the spot where dimples are
again opposite because they move at a ratio of (od-bd)/(id+bd)
od: outer race diameter, id: inner race diameter, bd: ball diameter.
This ratio not being 1:1, the balls do not naturally arrive at the
second coincidence of the race dimples although they usually drop in.
Roller bearings of various designs have been tried, and it appears
that they were possibly the ones that finally made obvious that fore
and aft motion was the culprit all along; a motion that roller
bearings were less capable of absorbing than balls. This recognition
lead to using spherical alignment seats under the rollers. Although
this stopped dimpling, these bearings worked poorly because the needle
complement tended to shift off center, skewing the needles and cause
large bearing friction.
Shimano, Chris King, Cane Creek and others, offer angular contact,
full ball complement, spherically aligned cartridge bearings. The
Shimano bearings have races that are sufficiently reentrant that the
races snap permanently together. They have contact seals, not exposed
to weather, to retain grease for life of the bearing. The ball
bearings of the Shimano units are supported on spherical steel rings
that slide as plain bearings against their aluminum housing. The
plain bearing takes up the otherwise damaging out-of-plane motion
while the ball bearing does the steering. The bearing is only durable
as long as the plain bearing remains lubricated.