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Mysterious once-per-pedal-revolution clunk

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Mysterious once-per-pedal-revolution clunk

Old 07-14-13, 12:25 PM
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phulin
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Mysterious once-per-pedal-revolution clunk

I have a 6-month-old commuter bike with a clunk that I feel once per pedal revolution. It happens almost every rotation, in the right pedal's 3 o'clock position - but only when the right pedal has load on it. The bike (and bottom bracket) have only about 700 or 800 miles on them, so I would be really surprised if the bottom bracket were due for replacing, but that seems like the only possible solution I can think of at this point. It's an FSA MegaExo external bearing bracket with a two-piece crank. Does anyone have any other ideas for the culprit? I've tried taking everything apart and regreasing the bottom bracket, with no luck. It's possible I over-preloaded the bearings at one point, if that makes a difference.
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Old 07-14-13, 01:23 PM
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The easiest thing to check is the pedals. Remove the pedals and clean and regrease the threads and mating surfaces before reinstalling them. Try swapping them for another pair if one is available.
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Old 07-14-13, 02:17 PM
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Originally Posted by dsbrantjr View Post
The easiest thing to check is the pedals. Remove the pedals and clean and regrease the threads and mating surfaces before reinstalling them. Try swapping them for another pair if one is available.
+1

While pedals are only involved about 1/3rd of the time, they're far and away the easiest to eliminate, so failing to rule them out before taking anything apart is just plain dumb.

OTOH, given that you say clunk, and not click, I suspect it's the right side bearing, either where the bearing fits the frame, or spindle movement inside the bearing race.

After you've eliminated the pedals, remove the left crank, and slide out the spindle and inspect of evidence of wear on the spindle surface, then check that cups a re secure in the frame. While it's apart, you might as well inspect and service the bearings themselves.

Lastly some carbon cranks develop play where the spindle is press-fit or bonded to the crank. If this is the case, the fix depends on the crank, and some cannot be fixed.
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Old 07-14-13, 10:58 PM
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I forgot to mention that the pedals were the first thing I eliminated - readjusted the (cup and cone) bearings on both, and they both feel great. I have also tried reattaching them with both grease and Loctite.

I don't see any wear on the spindle surface, although on both sides of the spindle the contact with the bearings is shiny, not the usual dull aluminum oxide look. No carbon on my bike except in the fork.

Is it time to give up and replace the BB? It's not like they're that expensive, but it feels silly doing it this early.
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Old 07-15-13, 04:35 AM
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What is the warranty on the bike? If the bottom bracket has issues, I would hope that would be covered. I assume it is a cartridge style.
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Old 07-15-13, 06:27 AM
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Do you have a kickstand on that bike? You might be getting just enough flex when under load to for the left crank to strike the kickstand but not be able to duplicate that on the workstand.
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Old 07-15-13, 08:01 AM
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There's no kickstand. I checked, and I don't think there's anything close enough to the cranks or chainrings to make contact under load. Maybe the front derailleur is too close? I'll check that, but I don't see how flex would make that happen.
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Old 07-15-13, 08:08 AM
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(And on a warranty point of view, I bought the bike from Nashbar. They say that if I send the bottom bracket back, they'll replace it, but I don't want to be without a bike for two weeks, and they won't just send me a new one first.)
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Old 07-15-13, 08:41 AM
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Check chainrings.
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Old 07-15-13, 04:39 PM
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The chainrings don't look likely - I doubt my frame is flexing enough for them (or the chain) to be touching the FD.
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Old 07-15-13, 04:44 PM
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Originally Posted by phulin View Post
The chainrings don't look likely - I doubt my frame is flexing enough for them (or the chain) to be touching the FD.
I don't think that that's what Leebo had in mind.

Modern chainrings have shift gates (cut down teeth) usually in two places. These can allow the chain to move sideways a bit so the next full tooth ticks against the edge of the plate rather than sliding in smoothly.

One way to check is to try to make a note of foot position when it clicks, then look to see if the shift gate is at the engage area of the ring. If you suspect the gate, use a dry marker to color the teeth in the area. Go for a short ride, then come back and note if one has had the color scraped off faster than it's neighbors.
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Old 07-15-13, 04:49 PM
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Hm, that could be it - will check tonight. I'd still be surprised, though, as it's really a "clunk" feeling, which doesn't jibe with the chain just clicking against something. I don't notice any associated sound - just the feeling in my feet.
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Old 07-15-13, 04:56 PM
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Originally Posted by phulin View Post
Hm, that could be it - will check tonight. I'd still be surprised, though, as it's really a "clunk" feeling, which doesn't jibe with the chain just clicking against something. I don't notice any associated sound - just the feeling in my feet.
Yes, the shift gate sound when you hear it is a small tick, not a clunk. If it's a low sound and you can feel it, that argues that it's a large deflection of some kind. With your BB, I'd suspect movement of the entire right bearing in the frame, or a right bearing problem of some kind.
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Old 07-15-13, 05:00 PM
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Is there a good way to check the pedal-frame interface?
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Old 07-15-13, 08:20 PM
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Originally Posted by phulin View Post
Is there a good way to check the pedal-frame interface?
I'm not sure what you mean, but the easiest wat to eliminate the pedals is to simply swap them for another pair. If it solves the issue, it was the pedals, if nothing changes it wasn't.

Sometimes small clicks go away simply by the ace of removing and replacing the same pedals. If this happens the problem was a not quite tight pedal, or dirt on the threads or the face where the pedal fits the crank arm.

So if you replace the pedals and the problem comes back you can be fairly sure that the problem is in the pedal itself.
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Old 07-15-13, 09:13 PM
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Originally Posted by FBinNY View Post
I'm not sure what you mean, but the easiest wat to eliminate the pedals is to simply swap them for another pair. If it solves the issue, it was the pedals, if nothing changes it wasn't.
Oops, I misspoke. I meant the BB-frame interface (since you said that it might be the right bearing moving in the frame).
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Old 07-15-13, 09:29 PM
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Originally Posted by phulin View Post
Oops, I misspoke. I meant the BB-frame interface (since you said that it might be the right bearing moving in the frame).
This is hard to diagnose, short of disassembly and sometimes even with. One trick that sometimes helps is to paint a few dots of brittle paint (nail polish (ask your GF which is the cheap crap that always chips) where the cup and frame meets. Allow to dry and get brittle, then go riding and look for some short steep hills to climb in a big gear.

If when you get back the dots are cracked, the cup is moving. OTOH if you get the clunk but the dots are OK, then it's probably something else. I say probably because the test is prone to false negatives.
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Old 07-22-13, 08:54 AM
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Okay. Replaced the BB (since it's not that expensive anyway) and it fixed the problem.
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