So called cones are the the inner races in angular contact bearings in hubs. The outer races, usually simply called races, are usually integral to the hub and cannot be replaced.
Cones however are easy to replace, often one would simply get a new axle with cones and counter nuts included.
Apparently, it is quite common for cones to become pitted. Where the surface of a race is covered by a number of small depressions and in worse cases becomes rough and This may happen in a process like Brinelling where the hardened steel balls leave a small concave impression on the race or spalling where after an initial crack flakes from the surface are shed.*
Pitting is often encountered only on the cones while the outer races remain in good shape. Evidence for the frequency of this failure mode is that replacement cones are readily available, which also indicates demand.
Why is that so?
Is this perhaps coincidental due to load geometry? (Concave vs convex ball race interfaces.)
Or is this by design, for example by different hardness outer races > balls > cones?
*In case of insufficient lubrication and/or corrosion fretting may also lead to pitting.