sknhgy 

Probably need someone from the bearing industry to answer this. In the course of changing bearings I noticed that the balls don't match the curve of the races. It's the same on the worn cones that came out of the wheel and the new cones.
This may very well be how these things are designed. I don't know.
Just thought I'd ask.

sweeks

That's the way they're designed. If the race formed a "cup" around the ball, there would be some sliding contact with additional friction.

An individual ball has a "point" contact on its races. This is why proper bearing adjustment involves a slight amount of preload. With preload, all the balls contacting the loaded half of the race share the load. If the bearing is slightly loose, the entire load may be supported by one ball at a time, which concentrates the load enough to cause premature wear which is usually apparent first on the inner race (cone).
Slight preload does no harm because once a rider's weight is on the bearing, the loading is even higher.
Here are a couple references: Jobst Brandt and Bicycles.
I realize this is controversial; make of it what you will.

FBinNY 

Yes, balls are always smaller than the curve of the races. This is to ensure single point contact.

Consider what would happen if it weren't this way.

A cupped system with broader contact area would make it impossible for the ball to roll freely. Since the ball would be touching at different radii with respect to its roll, some sliding would be necessary, greatly increasing friction and wear.

mpetry912 

exactly right, these have a single point of contact and the bearing track described around the cone is a circle.

There are also roller bearings (Stronglight A9 Headset for example) that has a cylindrical roller and there is some of the scrubbing effect alluded to above

tapered roller bearings like a Timken wheel bearing (or the one below) attempt to get around this problem. But for a headset or other low speed applications it doesn't really matter.

/markp

JohnDThompson 

The races are ground parabolicly to minimize contact and make the bearing more tolerant of slight misalignment of the parts.

sknhgy 

Interesting. Thanks.

etherhuffer 

Hey! That race looked dicey! Is it pitted?

sknhgy 

Yes. It's the one I'm replacing.
An LBS gave me some free ones from their parts bin.

etherhuffer 

I did the same. Most bike parts die from neglect more than overuse. Piles and piles of dry hubs, headsets etc that just need cleaning and lube. I also found that various axle races had various quality differences and the angle of the race’s face varied.

jccaclimber

Bicycle races may be different, but all of the cartridge bearings I have opened and checked on a Talysurf have been constant radius. Steering gear ball screws are gothic arch, though even then constant radius on both sides.

To the OP, Google up “raceway curvature ratio” if you’re curious. A smaller ratio will be 52%, and support high loads relative to the bearing size. A high ratio like 57% will have slightly lower drag at moderate loads, but be damaged more easily. Once damaged it won’t matter what the original design was. Go much under 52% and you’ll scrub enough that system life decreases under high loading and go much above 57% and it gets too easy to damage things (search “Hertzian contact” if curious). There are a few more variables in here, so it’s not appropriate to say that bicycle hubs should always be 52%, or 57%, or whatever.

Andrew R Stewart 

I'll add to jccaclimber's great post and mention that the angle, relative to the axle, that the balls see the load through varies from brand to brand. IIRC back in the day the Euro hubs typically had an angle around 22* and as Shimano increased their market share it was known they chose a steeper angle. So even with matching thread specs and cone diameters they weren't a good interchange.

Also there's the cone's ball track small diameter to be aware of. If the replacement cone has too large a small ball track "starting" diameter the balls can end up riding on the corner of that ball track. When the cone is nice and new this only feels like a slightly worn bearing. But this corner breaks down quickly and the bearing soon feels all rough and sloppy. Andy

oldbobcat

Just remember that it's more important to match the race of the new cone to the original rather than try to improve on it. The cups are designed to work with that taper, and they won't work with anything else.