"Indexed Steering" ("Brinelling")

All of the other ball bearings on a bike spend their lives rotating round and round, but headsets spend most of their time pointing straight ahead. Sometimes you will see that the bearing balls have created a neat row of little dents in the bearing surface of the lower frame cup and/or the crown race. This creates the effect commonly known as "indexed steering" where the balls sort of "snap" into place, and the fork tends to stick in the straight-forward position.

The dents in the races resemble the dents created by the Brinell Hardness Test procedure, and as a result, this type of damage is sometimes called "Brinelling." This is a bit of a misnomer, because the Brinell hardness test involves basically hammering a bearing ball into a metal sample with one good hard whack. Some folks assume that the dents in headset races also result from impact, but actually that is not commonly the case. Instead, it has to do with lubricant breakdown. Jobst Brandt has an article on this site descrbing the process in detail.

Ideally, a "brinelled" headset should be replaced, but you can often cure the problem by replacing the retainer with loose balls. You use the same size balls, but can usually fit more of them into the races if you leave the retainer out. As a result, the greater number of balls, closer together, no longer all line up with the dimples in the races.