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This is a similar question to What exactly happened to my rear derailleur? but with some significant differences.

My bike had recently been serviced (about 3 weeks/500 km previously). All that had been done to it since that was a little tightening of new brake cables and oiling the chain the day before a planned big ride. The RD seemed fine when I oiled the chain. Over the first 100ish km of the ride (link includes pictures of the aftermath) I'd used the full range of gears, front and rear. On a gentle uphill, at least a few seconds after changing down, there was a nasty noise and the back wheel locked up. I barely unclipped in time and got off to find the RD tangled in the spokes and wedged against the seat stay. After two failed attempts to set up and emergency singlespeed conversion (snapped chain) I got the train and a lift home. No spokes broke at the time but several are damaged (two snapped after about another 150 km with a borrowed rear mech).

All parts were present including the B screw and both cage/axle screws (though one was torn out of its thread) and the mounting screw was still firmly in the hanger (which despite being solid steel was quite bent).

I've started putting the bike back together, but what should I be looking out for either as I reassemble it, or before a ride - I've got a tour coming up.

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    I don't think the pictures of the end result will add much but I can easily edit them in. – Chris H Jun 3 '19 at 15:36
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    There's sometimes a hint beforehand - if you hear a "tink-tink-tink" noise when in the lowest gear then the rear cage is tapping on a spoke. Doesn't need much more after that to fully catch a spoke. – Criggie Jun 3 '19 at 18:43
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    @Criggie I've learnt to investigate a spoke tapping noise promptly, as even a luggage strap can cause a sudden stop. So I think I'd have noticed on a quiet road, but I can never be sure – Chris H Jun 3 '19 at 19:11
  • It's worth noting that servicepeople are people too and can make mistakes. I used to only have failures shortly after the bike was serviced, usually the technician made a tiny mistake like a bolt not tightened enough, which took a bit of time or rough ride to develop into a problem. I bought all the equipment and turned maintenance errors into a hobby. – Pavel Jun 4 '19 at 9:03
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    @Pavel indeed, even ones that come recommended. After signifcant work, especially done by me, I ride gently at first (a few km - a test ride), then progressively harder but extra cautiously (tens of km - a few commutes or a fasted training ride) before doing anything big. As you rightly point out, this won't catch things that steadily loosen. I wondered about the derailleur fixing bolt but that was tight afterwards and I'd have expected shifting to suffer anyway. The shop didn't dismantle the cage, but I had 2 weeks earlier when cleaning it; the screws were still present in the broken mech – Chris H Jun 4 '19 at 9:10
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Every time I've seen a correctly adjusted rear derailleur go into the spokes while riding it was because something made the chain jam in the rear derailleur and the force of pedaling tore it off and put it into the spokes.

a few possibilities:

  1. broken chain - if the chain breaks in such a way that it won't go through the pulley cage.
  2. Foreign object in the chain - something gets caught in the chain in such a way that it won't go through the pulley cage.
  3. Incorrect chain installation. If a chain was installed such that the pin wasn't seated correctly the chain can pull apart under stress and get stuck in the pulley cage.
  4. On an abused chain (not like your well oiled chain) links can get tight or rusty and fail to make the sharp bends between the pulleys and get stuck.

Do a careful inspection of your chain when putting your bike back together.

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  • That chain is in the bin, badly damaged and I've got a new one on there. While not the most forceful bit of pedalling of the ride it's possible the chain opened up just then - though it had only recently been fitted by the LBS. Something getting caught is certainly a possibility I can't rule out though I try to avoid debris and don't recall any – Chris H Jun 3 '19 at 15:57
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    @ChrisH Even badly damaged if the bin hasn't been taken away it would be good to get a look at the link on either end of the chain. – David D Jun 3 '19 at 16:06
  • I keep a metal bin in my garage for recycling so I've still got at least most of it. I had to drive a pin out to remove it so it wasn't completely broken even after the RD was, though some had a permanent twist (I removed that bit of course, but took it home - any littering was unintentional). There was no pin sticking out when I attempted to convert to single speed, but there was afterward that failed so I can't tell which bit to inspect. – Chris H Jun 3 '19 at 16:32
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On a gentle uphill, at least a few seconds after changing down, there was a nasty noise and the back wheel locked up.

I've had similar things happen not once, not twice, but three times. Uphill, shift to the smallest gear, hear a grinding noise for a few seconds, then BAM the rear wheel locks up with a broken rear derailleur in the spokes.

What I think happened was the chain dropped inside the largest cog, got stuck, and dragged the derailleur into the spokes.

Now, why would an otherwise well-adjusted bike do that in that situation? I've never had problems with the chain dropping inside the largest cog outside of that situation. At all. Ever. Yet I've had it happen three times on two different bikes - and each time when climbing. I think it's related to load and frame/wheel flex when climbing.

Under climbing loads, I think my chainline was changed and/or the derailleur position moved relative to the cogs, making it more likely for the chain to drop inside the largest cog where it got wedged and wreaked havoc on the entire drive train.

I'm also 200+ lbs and have no problems hitting 1200W+ peaks...

So now I ride with a spoke protector/dork disk to prevent the chain from dropping inside the largest cog.

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  • I had a spoke protector (until that point, I've still got pieces of it), and it's out possible that I changed into the biggest sprocket. Of course behaviour under load is one of the hardest things to test, but I wasn't even breathing very hard, probably putting out no more than about 250W (I'll check strava's estimate) unlike the brief 15%er a few minutes earlier. That's one of the things that surprises me - the timing. I don't trust my lack of reflection of a grinding noise – Chris H Jun 3 '19 at 17:02
  • @ChrisH I don't trust my lack of reflection of a grinding noise First, see if there are any deep gouges in the parts of what's left of the spoke protector that was behind the largest cog. If there are, then the chain probably got in there before things snapped. If not, check the chain then. If one of the plates came loose and caught in the RD, that could explain what happened - and that's the kind of failure I'd expect from a new chain, especially if it's one that doesn't use a quick link. – Andrew Henle Jun 3 '19 at 17:09
  • I'll look at the pieces but it's pretty smashed up – Chris H Jun 3 '19 at 19:08
  • And of course it was supposed to be "recollection" but my train was about to go into a tunnel before I could check – Chris H Jun 3 '19 at 19:15
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    @Andrew Henle " - and that's the kind of failure I'd expect from a new chain, especially if it's one that doesn't use a quick link." I'm curious about the reasoning behind this. (Not doubting you, just respectfully trying to learn more). Why would you expect a new chain to do this? – Jeff Jun 4 '19 at 0:08

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