Originally Posted by
ksisler
Exactly right. Also willing to bet one would be hard pressed to find any of the cups or races on a typical bike that were machined to better than a few thousanths, so the balls aren't even close to the limiting factor... and given the low rpm, it is not worth worrying about much /K
I'll take that bet. Even the cheapest cups and cones are held to within .001" or better for roundness. Ground cups and cones such as used on better components are round to within 10ths (.0001") Roundness is fairly easy to achieve, and the beauty of the bike world's angular contact systems is that it's very forgiving of actual dimensions (within reason) as long as roundness is kept to spec.
Likewise for bike purposes, the actual size of the balls doesn't matter, and even roundness isn't as critical (both within reason) but the matching of the balls within a set is very critical. Matched balls ensure that each ball takes it's share of the load and the contact pressure is distributed evenly. With unmatched balls, only a few, as few as the three largest, support the bearing with the others doing nothing. The hardened steel of bearing races cannot accommodate the variation, so while poorly made balls won't cause the vibration and heat buildup of high speed bearings, it does cause increased wear through fretting and shorter life.
If you look at the extremely long life of better hubs like Campag, or Dura Ace, it's the precision of the ground races and the use of matched balls that makes it possible.
Whatever quality level ball you choose to buy, good practices in keeping sets from single lots, like good cleanliness and lubrication ensures you'll the service life you paid for. Conversely, with bad practices, it doesn't matter how much you spend.