Subject: "Indexed Steering"
From: Jobst Brandt
Date: August 23, 2000
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. 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 was re-adjusted in the straight ahead position with the bearing balls in the dimples. Then the bearing will bind off center.
[I disagree: if the cyclist is riding no-hands, "indexed" steering can prevent the bicycle from balancing -- John Allen]
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.
[I disagree: I've seen a headset race damaged by impact. An impact hard enough to bend the fork steerer can sometimes dimple the crown race on the side which takes the impact, generally the rear -- John Allen]
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 cause 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.
[I disagree: I've seen a bottom-bracket cup indented by hammering on a cottered crank, and bottom-bracket bearing races are not subject to fretting -- John Allen]
Bearing balls make metal-to-metal contact only under fretting loads (microscopic oscillations) while the races ares 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 forkends vibrating fore and aft. This motion does not arise at the forkends, but at the fork crown, where it bends the steerer 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 dimples 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 have been, 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 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 which 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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