Rear mech caught in spokes

El Gordo
El Gordo Posts: 394
edited May 2009 in Workshop
This morning, on my way to work, I go over a level crossing and moments later there's a horrible sound from the back of the bike, the pedals and then the rear wheel lock up and I slither to a halt (somehow getting my foot unclipped in time for a gracefull dismount).

From the tangle of chain and mech at the back it looks like the mech caught in the wheel and wrapped itself, and the chain, around the cassette (the tangle was wedged against the seat stay, above the cassette). By the looks of it only the hanger (thankfully replaceable) is broken but I'll have a more carefull look tonight.

What I can't figure out though is why it happened. The limit screws on mech are properly adjusted besides which I was in a middle cog on the back and wasn't changing gear. It's as though the bumps of the crossing instigated it but I can't really see why.

Any ideas?

Comments

  • markos1963
    markos1963 Posts: 3,724
    It might be the level crossing causing the mech to wobble around. The limit stops only work on the top part of the mech but the lower part could move a fair bit around so catching a spoke, especially if you were in a low gear. I had something similar happen to my old bike a few months ago. Just be sure to check the spokes on the drive side as the chain chewed up 8 spokes without me noticing. I was less lucky than you needing to replace the mech and chain, which needed replacing anyway.
    Norfolk, who nicked all the hills?

    http://farm4.static.flickr.com/3013/243 ... 8d.jpg?v=0
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    http://img216.imageshack.us/img216/3407 ... e001af.jpg
  • will3
    will3 Posts: 2,173
    your mech hanger might have been a bit bent. It doesn't take much for it to cause the mech to contact the spokes in the largest cog.
  • MrChuck
    MrChuck Posts: 1,663
    Maybe a spoke broke first and then caught the mech?
  • Monty Dog
    Monty Dog Posts: 20,614
    Bent mech hanger or poorly adjusted / worn mech - has the bike been crashed or knocked that would bend the mech hanger?
    Make mine an Italian, with Campagnolo on the side..
  • Baytt
    Baytt Posts: 102
    unlucky, it happens , could be from a bent hanger, or a slightly miss alligned mech,

    though a guy from the lbs said it could've been from the chain stretched to much.

    i had a similar thing happen during etape caledonia, 4 miles from the finish teh hanger sheers of the frame and mech ended up near the brakes.

    your lucky the hanger was the only thing that needs replaced, i had to get a new chain as well as a hanger.
  • xcracer
    xcracer Posts: 298
    Happened to me when I bought a second hand steel framed bike. On the maiden voyage the chain started jumping around (too long?) and caught in the spokes and pulled the mech around. As the mech hanger was part of the frame and not separate like the modern types it couldn't be repaired. Bike ruined. Gutted.
  • Baytt
    Baytt Posts: 102
    it is a shame it happens but better a £10-20 hanger than a completely new frame, thats the upside of it anyway :)
  • micken
    micken Posts: 275
    Saw this happen to a rider in the Richmond cycling event as he was passing through Carlton today. Fortunately he was OK and a really nice local lady said her husband could give him a lift in towards home/help. The rear derailleur took out several spokes on the drive side, actually severed them at the point of inpact rather than just bending them.
  • jonmack
    jonmack Posts: 522
    xcracer wrote:
    Happened to me when I bought a second hand steel framed bike. On the maiden voyage the chain started jumping around (too long?) and caught in the spokes and pulled the mech around. As the mech hanger was part of the frame and not separate like the modern types it couldn't be repaired. Bike ruined. Gutted.

    It's safe to bend a steel hanger back, not a ruined bike at all :(
  • Mister W
    Mister W Posts: 791
    My GF had this happen to her and we eventually traced it to a broken side plate on the chain. It must have jammed in the mech and caused it to push into the spokes.
  • El Gordo
    El Gordo Posts: 394
    Having now had a chance to untangle the mess and inspect the damage it seems the mech is a write off aswell. The assembly which adjusts the pitch of the mech seems to now be 180 degrees out of position and I can't see any way of getting it back into place. One of the jockey wheels is broken too although that would be replaceable. It's only a Sora mech so not too pricey.

    The hanger looks like it was stretched to the point of breaking rather than bent sideways. The surface of the break is corroded over 3/4 and bright on 1/4 which makes me wonder if a crack has been developing for some time (that hanger has covered about 40000 miles). The chain looks intact. A couple of spokes are nicked and the wheel out of true but that's easily sorted.

    I'm sure the mech was adjusted OK and everything was running smoothly previously.

    As people have said, it should be cheap to fix. I just need to find a hanger which fits.

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