Learning by making rookie mistakes

Not long ago, I published a post about how to overhaul a cup-and-cone bearing hub. While I got most of the salient points right, recently I found out I didn’t quite do such a good job putting that rear hub back together.

After eight months of riding that same rear wheel on the trainer, I noticed my brake rotors were rubbing on their pads in a peculiarly regular way. Upon closer inspection, it appeared the rear hub itself wasn’t spinning straight and had an axial wobble. So, on I went, removing the rear wheel, pulling off the cassette and brake rotor, and taking the hub apart.

I was not quite expecting what I saw when I pulled the rear axle out…

Notice how some of the threads are gone in the middle of the axle. More than that, there is a very peculiar sort of galling wear, even on the parts of the axle without any threads in it.

I didn’t know what to make of it at the time, but what I did know was that correctly tightening the cones against the bearings should get rid of the axle wobble, so that’s what I ended up doing. The 1/4″ ball bearings were just nine months old, but I thought I might as well replace them anyway, as I was already this far along and my fingers were smothered in enough grease already.

For a few days after, I was puzzled about what I saw. Even a search online for “bicycle axle wear” did not lead me to anything helpful that specifically had my issue in mind. It wasn’t until I returned to the Park Tool YouTube channel’s really helpful “Tech Tuesday” segments that I realized what was wrong.

The answer was pretty insidious, but it’s also a rookie mistake that could have happened to anybody. I had somehow left one of the ball bearings in the middle of the rear hub shell, not accounting for it when I pushed the rear axle through. That forgotten ball bearing ended up wearing away at the rear axle in a place it wasn’t supposed to.

It was good that I caught and diagnosed this problem when I did. Had I ridden much more, or ridden this wheel outside, there is a good chance that ball bearing could have put a much bigger divot in the axle, leading to a stress riser that could ultimately bend or even break the axle. Even now, the divots on the axle making it thinner present a greater risk of breakage where the material should have been one more-or-less continuous diameter. Fortunately, I recently ordered replacement axle parts for both the front and rear hubs on my Giant S-X2 wheelset, and while they are on back-order, they should be arriving soon. Once those get shipped to my house, I should be able to replace the axles, cone nuts, and whatever else needs replacement (luckily for me, the bearing cups on the hub shells are still good).

Over the six years I’ve written on this blog, I’ve done mostly mechanical and technical content, and I am flattered that people seem to use this space as a reliable reference. Unfortunately, as with this case, there are instances where I get it wrong and things can go sideways. Preferring to look on the bright side, if it’s a day spent where I can honestly say I learned something new, it’s not a complete disaster.

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