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My BB is showing the first signs of being on the way out (a click when I drive either foot down hard; if I remove the chain there's a barely perceptible clicky feeling at the same crank position as the click). It's not convenient to get it fixed in the near future. The last time I wore one out, the ride was horribly notchy by the time I got it done, but I blamed the cassette/chain.

What's likely to happen if I keep riding on it? I can tolerate a rough ride on this bike (I'm not doing much distance on it at the moment). But I don't want to damage anything else (this seems unlikely to me). I also don't want a sudden failure, especially one that causes things to seize up.

  • I was surprised not to find a question on this; there are plenty on how to tell whether it's going, how to find the size, etc. – Chris H Jun 7 '17 at 13:18
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    Happened to me - I cracked the drive-side cup in a circle around the bearing track. All the bearings fell on the road, and I had a very long walk home. Was able to scooter though. – Criggie Jun 7 '17 at 20:12
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    It might need maintenance rather than replacement. Depends if the track has lost metal or not. – Criggie Jun 7 '17 at 20:13
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    @Criggie I assume in this case it's cheap (square taper) and there's not much to maintain assuming there's nothing obviously loose. Unusually for me I'm not inclined to buy the tools - the new bike has octalink which looks like different tools. – Chris H Jun 7 '17 at 21:37
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    @Criggie I rather like it and will keep it but it's no longer my distance bike so a new BB will last a long time. Getting to the bike cooperative is about as awkward as getting to the shop, and I don't mind paying to have it done (I'll get everything else looked at too), it's finding the time that's tricky. – Chris H Jun 8 '17 at 5:34
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Basically it is badly adjusted, damaging things until it fails outright in some manner.

That may happen today, or it might grind on for a decade. munching at the ball bearings and the cups and cones.

  • If it's already spalling, replacement is the best option.
  • If you catch it soon enough, a clean / grease / refit might be all it needs.

Do you intend to throw away the bike when it breaks? Consider a bike-cooperative to supply the tools for a one-time tweak, or just buy the tools. You own the bike and will doubtless use them again.

Bikes can last forever if not crashed, and are properly maintained

and my favourite line

Bikes are not cellphones

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    It turned out I needed to spend less on tools than I thought and they will also fit other bikes I care for. Now I've taken it out I can see that the BB itself is a sealed cartridge type, and in my hand I don't like the feel of it, so I've got a new one ready to fit. Accepted over the other good answers for suggesting that it's worth getting in and having a look -- I hadn't considered that inspection would be worthwhile. – Chris H Jun 21 '17 at 8:26
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Eventually your bearings will disintegrate and your BB will lose its ability to hold the spindle centred withing the BB shell, that will result in the crankset becoming wobbly, your chain will come off, your chainring might even go hit your chainstay and if you're lucky it will happen at low speed.

Here's a BB I personally exploded on my fixed gear commuter

enter image description here

Everything I described above happened and I would not recommend running a clearly faulty BB for too long.

I believe a BB using bearing cartridges might not fail as catastrophically as one with simple cages like the one pictured but I doubt it would end well in the long term.

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"What could happen" depends a lot on the brand and model of bottom bracket and the riding and maintenance conditions it has been subjected. Different quality materials and design may have very different failure modes.

In my view, the failure of a particular BB may not be catastrophic, but may happen on a critical moment of the ride, resulting on an accident, injury or worst.

In the best case, the bearings will only develop some play. If that is the case, it first becomes annoying and ultimately causes problem with the front derailleur or chain guide (if any). In the worst case something may crack. The crack itself may not be terrible, but it may affect the bike frame if used in that condition.

The click you describe may be a crack or dent on one of the races, which may over time cause a bearing ball or roller to break, possibly causing the axle to bind.

This is what happened to me once: I had a Neco brand bottom bracket that developed a noticeable play (On a mountain bike frame with a mountain triple Shimano crankset). The resulting wobble would not cause trouble with shifting and at that time the bike was used for very short urban commuting on flat streets. I neglected for too long until one bearing cage snapped. The bearing balls fell out of place and the axle was bounced all over the shell.

The bike was rendered useless in a second. I could not accelerate or even stand on the pedals while riding a busy street. Luckily it happened right after a red light so no cars where buzzing past me.

The axle wobble in this bottom bracket caused te seals to be less effective, letting water and dirt in, which in turn corroded the bearing cage, accelerating the decay of the whole thing.

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On a Hollowtech II external BB I replaced once, one of the outboard bearings had disintegrated but been successfully ignored by the owner. The metal pieces gouged a big groove into the crank spindle itself (2 piece crank) and the crankset had to be replaced.

Taking care of it sooner would have saved the crankset, but in fairness it didn't seem more knackered than anyone else's creaking BB from the outside. So a little unlucky for the owner.

Sure, this is a fairly extreme example, but ignoring the noises and feelings from a cheap failing part led to the premature destruction of a rather more expensive part.

Not such a risk with cartridge square taper bottom brackets where the whole thing is contained and replaceable together.

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  • The example in my question was also a cheap riveted crankset (Shimano OEM). Strangely this question has suddenly attracted attention almost 3 years later, on the same day I replaced the BB on another of my bikes. – Chris H May 16 at 16:41
  • @ChrisH huh fancy that coinkiydink – Swifty May 16 at 19:10
  • I just thought of another one I'll have to get back to it next week when I can take a photo – Swifty May 16 at 19:15
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Another bike, I was stripping it down for an overhaul but it turned out not to be economical to continue. Nearly everything was worn out pretty hard. There was some play in the bottom bracket. Despite being a cartridge type it disintegrated when I removed it and spilled its balls.

So quite worn out. I then realised this damage on the chain stays is about where the chain rings would be. I think the play was significant enough to allow the chainrings to gouge the chain stay...

enter image description here

enter image description here

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It happened to me once with an old racer and a simple open cup BB. One of the cages holding the bearings decided to involve itself in the action, trapping itself between the race and the bearings.

The thing became harder and harder to pedal, like someone was adding more glue with every rotation. Back-pedalling gave temporary respite, enough to get me limping home, but not enough for another ride.

paraphrase Upshot: as the bearing ate itself, I was unable to keep riding at the same speed, and could have ended up stranded, either walking, or worse having to phone home for a pick-up.

Motto: heed the warning signs and act before it's too late.

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