From Sheldon's fabulous web site:
http://www.sheldonbrown.com/
Subject: "Indexed Steering" From: Jobst Brandt Date: August 23, 2000
Question: "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." Jobst Answer: 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. The effect is mostly perception of failure from the rattling noise and clunky feel while braking lightly. It has such a small effect that it is imperceptible when riding no-hands unless the bearing clearance has been adjusted in the straight ahead position. Then the bearing will bind off center.
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 bearing and the 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 is not rotating. 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 indication of lubrication than the design of the bearing. Ball bearings with separate cups and cones have been used as head bearings 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. 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 causing large bearing friction as the rollers skate.
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.
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Chuck Schmidt
South Pasadena, Southern California
United States of America
http://www.velo-retro.com (reprints, t-shirts & timelines)