Visual Examination of a Worn-Out Bearing Thing

This here is an original Shimano bottom bracket spindle off a 1980's Cannondale:

Or rather, that's the diagram, this is the spindle:

This particular spindle is unique because the bearing track on one side is completely worn out while the other is just fine. What does it mean to be worn out? It means that the bearings have created little divots and stress fractures on the "race" or "cone" which is the track the bearings run on. This is caused by friction. Normally the bearings swim in a world of grease, creating a near frictionless state on the highly polished race, but if the grease washes out or of the bearings are adjusted too tightly against the race or a foreign element, like sand, gets in the mix a little wear spot will emerge right at the contact patch. Over time friction and heat caused by the small imperfection will deform the metal, making the race go from this:

 To this:

Those striations at the angle are what I'm talking about. They look small but any tiny pimple is amplified to a boil when bearings are pressed against it.

Here's a different angle. First the good end:

And then the bad end:

In addition to feeling crunchy when pedaling and creating extra drag, a worn out bearing race will eventually chip and cause the bearings to jam.

How do you know your races are kaput? You'll be able to feel it. Bearings should always be buttery smooth. If the bearings are new and feel crunchy they might be adjusted too tightly.

Quality bearing assemblies, like Dura-Ace or Phil Wood, tend to be polished to a high degree and last longer. When your bearings go out (and they always do, everything eventually crumbles to dust) replace them with a high quality brand and have them installed with care and you'll be happier for it.