Old Crow/Wild Turkey writes:
>> Like a few simpler designs in decades past, the entire
>> cartridge can float within its seat.
> How? My cartridge ultegra has shamfered edges on the
> cartridge exterior that are wedged up against the
> shamfered edges of the housing under load. If they did
> float, you'd feel it like any loose bearing.
> The cartridge itself simply contains a single row of
> bearing balls with angular races just like a "normal"
> headset.
It takes a special cartridge bearing to swivel on a
spherical seat.
Subject: 8f.13 Indexed Steering From: Jobst Brandt
<
[email protected]> Date: Wed, 10 Jun 2004
10:08:29 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. 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.
-------------------------
Jobst Brandt
[email protected]
