Article and Photos by Scott Wilson
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 "cone" sometimes referred to as the “tapered race” 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 cone surface. But if the grease washes out, the bearings are adjusted too tightly against the taper, 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:
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. Even worse, the added friction from all those bumps will cause the ball bearings to deform, and eventually break, causing whatever component it’s a part of to lock up. This can happen to the bottom bracket, the headset, and most frequently, the wheel hubs.
How do you know your races are kaput? You'll be able to feel it in your fingers by spinning the component. In extreme cases you can feel a grinding through the bike itself, as well as a noise kind of like a bunch of very small bowling balls being thrown down an endless alley. Bearings should always be smooth as a dolphin on a water slide and quiet as a box of pillows.
If the bearings are new and feel crunchy they might be adjusted too tightly, or there might be a manufacturer’s defect.
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.
Scott Wilson is an MFA writing student at Columbia College as well as a seasoned professional bike mechanic. Scott’s “wrenching” experience includes bike shops, racing teams, and professional triathletes across the US. The aim of Scott’s technical articles is to explain in detail how bicycles and their individual components work...and in doing so, help you keep your own bikes running better and lasting longer.