Bottom bracket - symptoms and diagnosis of impending failure

[wafter]

I've never done sufficient mileage on one bike to experience a BB failure, however..

I've become aware of an oscillation / rumble felt through the pedals on each "power" stroke, typically at higher loads / lower cadence. There's no tangible noise associated with this, while the crank feels smooth to rotate by hand unloaded (as much as one can tell with a chain fitted) and there's no detectable end-float, rock or other movement that I can feel.

The BB is a Shimano MT500 on about 10k miles in my ownership and an indeterminate amount prior to this; from tyre wear I'd reckon maybe another 5k so it owes me nothing.

I'm thinking the acid test would be to remove the crank and manually rotate each bearing to check for roughness and play; which I'll do once I've recovered the necessary tools. In the meantime I'm interested to hear any other thoughts on diagnosis, lifespan and potential failure modes as I need to use the bike but I don't want to end up stranded..

Cheers

 

[chriswoody]

Personally when I start to feel the symptoms you describe, I just unship the chain from the chainrings and loop it round the bottom bracket shell or just take the chain off completely. Then I'll just rotate the cranks using my fingers on the crank arms to make sure I'm not feeling the pedal bearings. That's normally enough to tell if the bearings are running rough.

As you say, the acid test is to take the cranks off completely and feel each bearing with your fingers, but I've been able to feel it through the crank arms as well.

 

[wafter]

Personally when I start to feel the symptoms you describe, I just unship the chain from the chainrings and loop it round the bottom bracket shell or just take the chain off completely. Then I'll just rotate the cranks using my fingers on the crank arms to make sure I'm not feeling the pedal bearings. That's normally enough to tell if the bearings are running rough.

As you say, the acid test is to take the cranks off completely and feel each bearing with your fingers, but I've been able to feel it through the crank arms as well.

Thanks - your post has goaded me into pulling my finger out and whipping the chain off - which was nice and clean thanks to the wax (would have been a good opportunity to check the BB when swapping the chains earlier in the week).

Rotating by hand there's the slightest bit of roughness detectable, especially when applying some force in the direction of load during pedalling, so I suspect that's fairly conclusive.

I have a used apparently-good one that can go on as soon as I've recovered it and the tooling from the homestead at the weekend