Help diagnose a nasty crunch

chatlow
chatlow Posts: 850
edited May 2014 in Workshop
Just went out having replaced the rear wheel, BB and chain on my Scott s20 and the problem remains.. I put some pressure on the pedals and something slipped causing a big crunch and I nearly lose my nuts. Doesn't always happen, but scares the hell out if me when it does.

The hub on this new wheel is good. BB/cranks feels very smooth, Rd is Okay after a clean. Cassette is tight. Skewer is also quite tight..although I did grease it up a bit. Cassette has a fair bit of play and is pretty loose when off the bike, but is fine when locking tight.

Any ideas on what could be causing this? I'm stumped.

Comments

  • me-109
    me-109 Posts: 1,915
    Chain riding off the cassette sprocket?
  • chatlow
    chatlow Posts: 850
    Thought it might gear slip but the chain stays on the same sprocket when it happens. Just changed qr (clutching at straws a bit), went on turbo trainer and all appeared good. Took it up a hill outside and it slipped/clunked again. Seems to only happen when under a lot of pressure and out of the saddle.
  • lpretro1
    lpretro1 Posts: 237
    Worn chainrings
  • brownbosh
    brownbosh Posts: 602
    lpretro1 wrote:
    Worn chainrings
    +1
  • chatlow
    chatlow Posts: 850
    Here are some pics:

    Chain rings:

    rgmt.jpg

    93c7.jpg

    mnze.jpg


    Cassette:

    xnte.jpg

    f9af.jpg
  • sungod
    sungod Posts: 17,351
    you changed the chain but not the cassette

    was the old chain worn 1/8" or more over 12" ? if it was, then the old cassette will have suffered accelerated wear, once this happens a new chain may slip, if that's the case you'll need to fit a new cassette
    my bike - faster than god's and twice as shiny
  • chatlow
    chatlow Posts: 850
    Yes that's right. Forgot that you should always try to get new cassette with new chain, so I've ordered new cassette now. That being said this problem still happened with the old cassette/chain. Worried I'm going to have to replace chainrings now and maybe a new rd (in case jockey wheels are gone).
  • Semantik
    Semantik Posts: 537
    sounds like a freehub problem to me. you did say you'd replaced the wheel...
  • dodgy
    dodgy Posts: 2,890
    Semantik wrote:
    sounds like a freehub problem to me. you did say you'd replaced the wheel...

    I'd put 20 quid on it simply being the cassette. The old cassette had worn in sympathy with the old chain. Old chain replaced, but left existing cassette on the wheel. If you want to make a cassette last more than one or two chains, you have to keep them clean and change them before they're too worn.
  • Bozman
    Bozman Posts: 2,518
    New cassette when you swap the chain? Ar5e.
    Looking back over the last few months on this site you'd put your money on a stiff link or a quarter/half turn on the cable adjuster.
  • homers_double
    homers_double Posts: 8,279
    How have you connected the new chain? Not gone slightly overboard pushing the pin in have you and created a stiffie?
    Advocate of disc brakes.
  • chatlow
    chatlow Posts: 850
    Semantik wrote:
    sounds like a freehub problem to me. you did say you'd replaced the wheel...

    I replaced the wheel with an almost new one so I'd be shocked if the freehub was broken on this new wheel..
    homers double wrote:
    How have you connected the new chain? Not gone slightly overboard pushing the pin in have you and created a stiffie?

    chain was quick link so i think i can rule that out.

    When some pressure if put on the cranks, I hear a tick/clicking. The crunch/slip is not that constant and happens when a good amount of down pressure is applied. I tested two different cassettes with the new chain (both had some usage on them mind) and problem was there with both. How can I confirm chainrings? They look 'ok' to me..barely any fin shaped teeth.. although I have noticed the chain doesn't always sit firmly on the chainring teeth (can press it down slightly with my finger)

    For now, I've put the old chain back on and the slip is happening less often, but ticking still there.

    It couldn't be the bottom bracket could it? I did replace that recently but made sure the cups and crank were tightened properly.

    thanks
  • slowbike
    slowbike Posts: 8,498
    the slip is between the pedal and the wheel - so the bottom bracket won't affect that - but could be the source of the unrelated tick/click ...

    If it was a stiff link then you'd notice when just turning the pedals over and watching the cassette - there would be a noticeable jump - especially when no pressure was being applied.

    If you cross chain it and go for granny cog and small sprocket then apply a lot of pressure then see what gives - if it doesn't happen then either you can't apply enough pressure or it's something to do with that gear combo - ie something less worn.

    If you want to try it on the turbo then put the resistance up to max, gently apply the back brake and pedal .... might be safer than on a hill!
  • chatlow
    chatlow Posts: 850
    thanks. can't see it being a stiff link as it's happened on a brand new chain and old one. will try pedalling hard using middle/big chainring and granny gear to see what that does. Got a turbo in the house so will do it there. If I castrate myself then I'll blame you..
  • slowbike
    slowbike Posts: 8,498
    New/old chain & stiff link - it can happen ... but I don't think it is for you ...

    Please don't castrate yourself - just use the turbo ... .... PUT THAT KNIFE DOWN! ... ;)
  • chatlow
    chatlow Posts: 850
    Never happens on the turbo :roll:

    Just cycled back from work and was fine up all hills.. went through a tunnel on a straight section and as soon as i put a bit (not a lot of pressure) on the right crank, it happened again.Big cracking sound and the crank slips. B@lls are happy as I controlled it this time.

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