Bent axle causing hub to loosen, the other way round, or axle gradually bending?

Question

Getting ready for a recent tour I noticed a little play in my back wheel with an intermittent clunking noise as well, so adjusted the cones. The feeling afterwards wasn't perfect, but I put that down to wear and tear, and the noise went away. At the same time I tweaked my shifting.

About 100km into the trip the noise came back, and continued when under effort or especially under heavy braking (disc brakes). The hub was a little loose again, but I had no means of adjusting it and it didn't get worse, though I had some ghost shifting. When I got home I noticed the cassette was wobbling with wheel rotation, though not itself loose, investigated further and the axle was definitely bent. I replaced it and all was well, the ghost shifting had gone. That was at least 1200km on a bent axle and loose cones, mostly laden, but without further incident.

I can't recall an incident that could have bent the axle at around the right time, and surely it would have taken some impact. I'd had trouble getting the cones just right - it would go just too tight when I locked it off, or loosen on a test ride after stripping down the hub and replacing a failed freehub earlier this year. That was without ghost shifting or clunking and I got it stably right in the end. That bike (my Genesis Tour de Fer) takes some knocks but hadn't recently. It uses a QR Deore XT rear hub.

I'm curious, like to learn from these things, and to know what to watch out for. I assume the bent axle caused the loose hub, but could it have been the other way round? Or could the axle have been getting gradually worse over several thousand km?

Answer 1

Bent axles on Shimano cassette hubs always strongly imply dropout alignment problems. Maybe it's possible for it to happen from being overloaded, but the designs (which are different generation to generation, i.e. the standard versus non-standard axles) simply don't put a lot of leverage on the axle. Problems with bent axles on Shimano hubs are rare and I don't know that I've ever seen it and not found a dropout problem to go along with it. (Which doesn't itself prove anything and I won't try to explain the exact mechanism that makes the axle bend over time, but suffice to say you should check the dropout alignment).

Dropout misalignment also flexes the axle in use, which in turn can cause uneven bearing wear, which then in turn can create finicky adjustment and eventually, once the race surfaces are damaged, can cause a noise.

It can be hard to make a hub with asymmetrical wear that's been ridden a lot be really right again. At a certain point the cup wear surfaces will become asymmetrical, and they'll probably have other damage by then too, and since they can't be replaced, a good adjustment will be impossible.