Originally Posted by
terrymorse
That’s not how headset bearings become “indexed”. No amount of pounding is enough to dent a bearing cup or cone.
The culprit is vibration while riding in a straight line, enabled by the looseness. The vibration displaces the bearing lubrication, leaving bare metal against bare metal, which ultimately leads to fretting erosion of the bearing surfaces and the balls.
The "vibration" in the indexing of a headset is the pounding. It's not erosion due to wear but denting due to brinelling. According to
Wikipedia
Brinelling
/ˈbrɪnəlɪŋ/ is the permanent indentation of a hard surface. It is named after the Brinell scale of hardness, in which a small ball is pushed against a hard surface at a preset level of force, and the depth and diameter of the mark indicates the Brinell hardness of the surface. Brinelling is a process of wear in which similar marks are pressed into the surface of a moving part...
Brinelling is a material surface failure caused by Hertz contact stress that exceeds the material limit. It usually occurs in situations where a significant load force is distributed over a relatively small surface area. Brinelling typically results from a heavy or repeated impact load, either while stopped or during rotation, though it can also be caused by just one application of a force greater than the material limit.
Brinelling can be caused by a heavy load resting on a stationary bearing for an extended length of time. The result is a permanent dent or "brinell mark". The brinell marks will often appear in evenly spaced patterns along the bearing races, resembling the primary elements of the bearing, such as rows of indented lines for needle or roller bearings or rounded indentations in ball bearings.
Headset bearings are stationary for most of the time. If the headset is loose, the cup moves away from the bearing on the upward travel of impact and and then moves back down on the bearing in the downward phase. Repeated enough times with enough load and the cup are dented or "brinelled". A brinneled headset will have regular indentations at each of the bearings on the lower cup with deeper ones on the trailing edge where impact is worst.
Fretting, on the other hand is more related to corrosion than to impact. Fretting leads more to pitting and corrosion which are irregular. A fretted headset should fret at just about any bearing and wear should be random. If fretting were the mechanism, you'd expect to see fretting on both the top and bottom cups or races and the fretting would be randomly distributed.
And I don't see why you think hardened steel bearings won't dent unharden steel cups or aluminum cups. Per above, Brinneling is a test used to test metal hardness by pushing a hardened steel ball into a metal surface. A loose headset is just a dynamic version of that test.