RE: [CR]Campy hub bearing adjustment

(Example: Framebuilding:Tubing:Columbus)

From: "Mark Bulgier" <mark@bulgier.net>
To: classicrendezvous@bikelist.org
Subject: RE: [CR]Campy hub bearing adjustment
Date: Sat, 18 Jan 2003 22:17:12 -0800


Now I'm not saying anyone here is wrong, just being devil's advocate because I don't have any hard data. But there is some theoretical backup to Campy's habit of adjusting them "just right" instead of a little loose.

Executive summary: a little preload spreads the load over more balls, since any looseness at all causes the all pressure to fall on one or two balls only.

It's helpful to think of all materials as various hardnesses of rubber. I know most people think of hardened steel balls and races as essentially infinitely hard, but their compression under real loads is enough to matter, especially since their contact, if infinitely hard, would be a mathematical point, with zero contact area. Since stress is force divided by area, any force (rider weight) at all would imply infinite stress. Luckily it is not so; both ball and race compress under load, and so the area of contact becomes non-zero and the stress becomes something less than infinity. Whew!

But when the balls and races compress (mostly the balls, due to the geometry of it, but there's some in the races too), the axle moves a little lower in the hub, making the lower one or two balls carry all the load, and the upper balls are totally loose. Imagine this in your head, exaggerated (rubber). You can see that as a ball passes directly under the cone the cone rises up a bit, and falls until it is right between two balls, then rises again. This "ball drop" is measurable and a well-known problem to bearing designers. It's is minimized with a full complement, that is, as many balls as will fit, as opposed to those bearing retainers that have fewer balls and larger spaces between the balls.

If the bearing has a *little* preload, the balls at the top might still see some load when the static load is applied and the balls at the bottom compress; but the top balls will still be momentarily unloaded and loose when the bearing sees an impact load. So a bearing that sees lots of impact loads might do even better with a bit more preload. To re-iterate: Zero load on the top balls is a good thing since they can't hold the weight of the rider up, but any more looseness than that is a bad thing, so having the upper balls go to *almost* zero load under the static load is probably best. That means they have some amount of preload (perceived as drag when the axle is spun by hand) before the static load (rider weight) is applied.

Whatever the right amount of preload is, it's clear that some is good. A hub that's still loose by any amount, discernable or not, after the QR is tight, is too loose. Maybe the amount of preload applied by the QR is too much, so that some looseness beforehand is OK - I don't know. But it's good to remember that a hub that feels silky-smooth out of the bike may not be the hub with the lowest friction and wear in use, with a real load on it.

ALL the above applies to cup-and-cone adjustable bearings only, not annular bearings like Phil Wood whose preload is set at the factory.

Mark Bulgier
Seattle, Wa
USA