Subject: 8f.13  Indexed Steering 
From:  Jobst Brandt <jobst.brandt@stanfordalumni.org>
Date:    Sat, 10 Feb 2007 13:00:12 -0800

> 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 the term "Brinell" 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.  The effect is mainly a perception of
failure from the rattling noise and clunky feel during light braking.
It has such a small effect that it is imperceptible when riding
no-hands, that is, unless head bearing was re-adjusted while aimed
straight ahead with the bearing balls in the dimples.  This will cause
off center steering-bind.

The head bearing problem seems to be twofold in this case, because
properly adjusted steering can only become looser from dimples, and
dimples cannot immobilize steering.  Therefore, adjustment should not
have been made with a damaged bearing.  More to the point, bearing
dimples are not caused by impact, but rather by lubrication failure
that occurs while riding straight ahead, giving the steering a
preferred home position.

Dimpling 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
occur less easily.  Off road bicycles suffer less from this malady
than road bicycles because it is caused primarily by long straight
coasting descents with no perceptible steering motions, ones that
might replenish lubricant.

To show that it does not come from the front tire pushing the bearing
balls into the races, try to causing dimples by hammering on the
underside of the fork crown of a junk bicycle.  Those who hammered
cotters on steel cranks will recall no dimples on spindles, even
though they have a far smaller diameter than the head bearing and
hammer blows were more severe and direct, supported by no more than
one or two balls.

Bearing balls make metal-to-metal contact only under fretting loads
(microscopic oscillations) while the races are not rotating.  Any
perceptible steering motion will replenish lubricant from the oily
meniscus surrounding each ball contact.  The damaging motion can be
observed by peering at the front hub over the bars while coasting down
a road at 20+ mph as 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.  As the steer tube flexes, both top and bottom
head bearings rock in fretting motion, crosswise to their normal plane
of rotation to develop dimples in the forward and rearward quadrant of
both upper and lower bearings.  That dimples form in the upper bearing,
proves that they not directly load related.

Aside from this, there are numerous patented steering mechanisms for
automobiles to prevent indexed steering, a common problem in the early
days of paved highways on which no significant steering motions
occurred for longer periods.  Among these are Eaton, Gemmer, Ross,
Saginaw, Sheppard, and TRW patents that in passenger cars were pushed
aside by the most primitive steering gear, the rack and pinion with
its own way of solving that problem.

Lubrication failure from fretting causes metal to metal contact which
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 where contact pressure is higher than on the outer race.
Various testimonials for the durability of one rigid cup and cone
bearing over another are more likely an indication of lubrication than
its design.  Cup and cone bearings have been used longer than they
should, considering their poor performance.

The question has been raised whether steering to either side would
reveal a second preferred position in which the balls fall into
matching dimples.  Because 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 into one
race and start generating new dimples in the other one.

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 causing
large bearing friction as the rollers skated.

Shimano, Chris King, Cane Creek and others, offer angular contact,
full ball complement, spherically aligned cartridge bearings.  The
Shimano cartridge bearings have contact seals, not exposed to weather,
to retain grease for life of the bearing.  The races are sufficiently
reentrant that they snap permanently together with sufficient preload
to prevent rocking (fretting) motion perpendicular to the rotational
axis.  Spherical steel rings, that move as plain bearings against an
aluminum housing, support the cartridge bearing to absorb, otherwise
damaging, out-of-plane motion while the cartridge bearing does the
steering.