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So, this happened. All of this group of bolts but the one you see came out of my crankset:

enter image description here

My question is.. how exactly do all those bolts wriggle themselves loose !? How do I determine what bolt I need?

Let's be honest: I'm not nice to transmissions. I weigh well over 200 lbs, my maximum measured output is about 1000 watts (briefly) and I ride in a fairly big gear all the time.

But what actually happens so that they vibrate out? I didn't think there was enough force to turn against threads offering lateral resistance...

I'm more a commuter than a sportsman, should I suspect sabatoge? (Ie, I leave it for 8+ hours at a time.) To be fair, I hadn't inspected these in errr.... uggh.... well over 500 miles according to my odometer.

I used motorcycle chain lube (it was what the hardware store had.)

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  • I've never seen that before. Cranks come loose and fall off with appalling regularity (even though the symptoms of a loose crank should easily get the rider's attention). I would imagine that a loose ring, though, would be less noticeable until it got near the end, especially if you were on a different ring with different bolts at the time. (My guess would be that improper assembly is at fault.) May 4, 2015 at 2:28
  • You would imagine correctly, failure was sudden. I know this set made it well over 750 miles as it were. Could you clarify your improper assembly guess? May 4, 2015 at 2:51
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    Improper assembly would simply mean that the bolts were not properly torqued (or perhaps the wrong bolts were used) the last time the ring was replaced. It's not really a rocket science operation, but anything can be screwed up, if you try hard enough. May 4, 2015 at 3:02
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    Well, the bolts aren't very long -- maybe 4 threads. And the ring is sort of locked in place by the spider, so even with several bolts gone it wouldn't be obvious. May 4, 2015 at 3:24
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    The correct torque for those is pretty high (depends on chainset I suppose). You could use some Loctite thread lock (correct grade to allow bolt removal eg Blue 242) to avoid them working loose in future.
    – inbike
    May 4, 2015 at 13:53

2 Answers 2

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Chainring bolts do need to be checked/tightened at regular intervals, it's a part of the maintenance. It it quite possible that they loosened in that 500 miles time and worked their way out; it's surprising that it wasn't more noticeable before it came undone. Some of the early indicators of loose chainring bolts include creaking and a rattle.

You can tighten them at home with the proper hex wrench and don't be afraid to strong arm it.

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  • Great answer. Welcome to the site. And I would add use Loctite.
    – paparazzo
    May 4, 2015 at 23:34
  • Come to think of it, I did hear some intermittent squeaking... but it never lasted very long, nor was it present so much as every other ride! I'll step up my attentiveness. May 5, 2015 at 0:17
  • @user1833028 - You should always be attentive of squeaks and clicks coming from your bike. A properly-assembled bike in good working order should be silent, aside from chain noise. And you should feel nothing through the pedals other than smooth motion. May 5, 2015 at 0:25
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I have lost a bolt from my a chain ring and have been completely unaware of it for weeks. I had to have it pointed out to me by a mechanic. In retrospect there was a little more rattle than normal. I guess it is possible one could lose two and not know it either. Once two come off there would be more rattling I would guess. The amount would depend on where the remaining are aligned. The rattle would probably then lead to the third one to come lose more easily at which point it would pivot. I now check them frequently.

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  • I have seen automobile wheels where 4 of the 5 lug bolts have come off, and the only clue was that the lug nuts were rattling around in the hubcap. May 5, 2015 at 0:26

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