Chain slip under load downhill

cruff
cruff Posts: 1,518
edited January 2015 in Road general
I've had a couple of occasions lately where I've come to the top of a descent and shifted to the big ring/smallest cog and, within about three turns, the chain has slipped off. I've managed to get it back on eventually on both descents, but there's clearly something wrong. I'm on a compact (50/34) and am pretty much wringing the lot out of it when this happens, so maybe I just need to get a bigger Big Dog - but I would have thought there's no way I could put out enough force for this to be the problem.

Chain has about 500 miles on it - so I guess the first thing is to check it for wear - but it certainly doesn't feel or look slack - and I got 1500 miles (cheapskate) out of my last chain without this (before the chain snapped).

Could anything else be causing this? Derailleur is clean, oiled but not too much, and I have no problem in any other gear
Fat chopper. Some racing. Some testing. Some crashing.
Specialising in Git Daaahns and Cafs. Norvern Munkey/Transplanted Laaandoner.

Comments

  • Bobbinogs
    Bobbinogs Posts: 4,841
    Doesn't sound like anything to do with chain wear. When you say the chain is slipping off, do you mean you are overshipping and dropping the chain from the big ring?
  • ForumNewbie
    ForumNewbie Posts: 1,664
    Sounds like the H (high) limit screw on the front mech needs adjusting if your chain is coming off the big ring. If it is coming off the small cassette ring, then it will be the H limit screw on the rear mech than needs adjusting. These screws limit the movement of the mechs so if set right the chain should definitely not come off.
  • gozzy
    gozzy Posts: 640
    As above, wind your limit screw in a bit.
    You won't have caused any significant wear after 500 miles, and 1500 isn't cheapskating on a chain.
    I seriously doubt you could visually notice chain stretch without either a tool or a tape measure anyway.
  • cruff
    cruff Posts: 1,518
    Cheers. It's the front ring it slips off, so will have a go at the limit screw on the front derailleur. Thanks again!
    Fat chopper. Some racing. Some testing. Some crashing.
    Specialising in Git Daaahns and Cafs. Norvern Munkey/Transplanted Laaandoner.
  • iron-clover
    iron-clover Posts: 737
    As said above, the limit screws will help a lot. If you can it's easiest to disconnect the cable to the front mech before starting, then you can move it by hand to see the limits of movement- however it can sometimes be fiddly to reattach and set up.

    Also, it can be worth just changing up into the big ring and then changing the cassette gears once its settled in the meantime to help prevent the chain shipping. It's not the ideal but might help before you can get the limit screws seen to.
  • desweller
    desweller Posts: 5,175
    It's also worth verifying that the derailleur is parallel to the chainrings.
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  • homers_double
    homers_double Posts: 8,293
    I have a similar problem and set up, if I'm in the bottom 12t gear on the back and shift up to the big ring on the double clanger then it over shifts and drops off.

    Limit screws are set correctly, chain length is OK etc. It's puzzeled me for so long I just work around it now.
    Advocate of disc brakes.
  • frisbee
    frisbee Posts: 691
    Could be chain wear, try lifting the chain off the front of the chainring.
  • johnmiosh
    johnmiosh Posts: 211
    Homers double wrote
    if I'm in the bottom 12t gear on the back and shift up to the big ring on the double clanger then it over shifts and drops off.

    If you are on the small chainring and the 12t on the cassette, the RD will not be giving enough tension to the chain. You should change back to the big ring while still in a larger sprocket instead of working all the way down to the bottom of the cassette.
  • Mad_Malx
    Mad_Malx Posts: 5,183
    zebulebu wrote:

    Chain has about 500 miles on it - so I guess the first thing is to check it for wear - but it certainly doesn't feel or look slack - and I got 1500 miles (cheapskate) out of my last chain without this (before the chain snapped).

    No-one else think that 1500 isn't really very much? I get a lot more than this from 10 speed, in all weathers and lots of changing.
  • mfin
    mfin Posts: 6,729
    All this is to do with is it needs the front mech position, limit screws and perhaps the cable/cable tension all set up or checked properly.
  • keith57
    keith57 Posts: 164
    I recently had a very strange chain slip problem. The chain would slip on the inner ring under pressure at odd moments. It didn't seem to slip off sideways so much as just lose grip on the chain. The culprit was a worn chain-ring. I'd just had a new chain and rear cassette. Big ring was ok, but the inner was a bit past its sell-by date. To be fair to my LBS it did work fine for around 100km after fitting. So we swapped out the inner ring for a new one and all was well again..!

    So, don't forget to check the chain-rings for wear as well as all the usual front mech adjustments.
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  • homers_double
    homers_double Posts: 8,293
    johnmiosh wrote:
    Homers double wrote
    if I'm in the bottom 12t gear on the back and shift up to the big ring on the double clanger then it over shifts and drops off.

    If you are on the small chainring and the 12t on the cassette, the RD will not be giving enough tension to the chain. You should change back to the big ring while still in a larger sprocket instead of working all the way down to the bottom of the cassette.

    That's how I'm working through it. It's never done it on the mtb but I have much higher spec gear on that.

    Thanks for confirming that my spannering skills are actually correct.
    Advocate of disc brakes.

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