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Sram Force rear derailleur chain jumping off lower jockey

patrickfpatrickf Posts: 536
edited September 2013 in Workshop
This is a bit of an odd one. Built "one of those" Scott CR1 SL lovelies up with Sram Force and everything is great apart from the rear derailleur is playing up.

The issue basically appears that the chain under certain circumstances comes off the lower jockey wheel at the point where it comes from the chainset and the chain rides on the edge of the teeth causing quite a racket. Usually this is fixable by pedalling backwards and continuing on.

The problem is worst in big chain ring with the top two sprockets on the cassette. I know that combination is to be avoided, but it should "work" without the derailleur throwing a wobbly. The problem also occurs mid cassette if I ride over bumps that cause the chain to jump. This is probably the most annoying as I had this happen a lot on my commute the other day.

The indexing is set up properly and gear changes are no problem at all.

The chain was cut as per Sram instructions (I hope) wrapping over big-big excluding the RD and then adding one full link (one inner, one outer) plus the power lock to where the chain meets. This method has certainly worked a few times on my other bike running Sram Rival.

Pics are attached, to make it a bit clearer I hope.

I know the B-adjustment screw looks like it'll need adjusting back in again. I adjusted that before the ride to see if that would improve things - hadn't noticed how far out it looks until I looked at the photos.

Chainset is Sram Force and the BB is a Sram GXP Team PF86 press fit. Chain is a PC1071 and cassette is a PG1071 11-28.

Any ideas?

Posts

  • nicklousenicklouse Posts: 81,520 Lives Here
    patrickf wrote:
    The chain was cut as per Sram instructions (I hope) wrapping over big-big excluding the RD and then adding one full link (one inner, one outer) plus the power lock to where the chain meets.
    you mean one inner and the powerlock (one link) what you say would be 1 1/2 links.

    checked for bent chain.

    stiff links?
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • 47p247p2 Posts: 329
    From the pictures it looks like the pulley cage has a slight twist at the lower idler pulley
  • Monty DogMonty Dog Posts: 20,614
    Check the rear mech hanger alignment
    Make mine an Italian, with Campagnolo on the side..
  • slowbikeslowbike Posts: 8,498
    47p2 wrote:
    From the pictures it looks like the pulley cage has a slight twist at the lower idler pulley

    yup- I'd agree - bottom of the cage is twisted in ...

    were those pics taken with the chain on the big or little ring?
  • nicklouse wrote:
    patrickf wrote:
    The chain was cut as per Sram instructions (I hope) wrapping over big-big excluding the RD and then adding one full link (one inner, one outer) plus the power lock to where the chain meets.
    you mean one inner and the powerlock (one link) what you say would be 1 1/2 links.

    checked for bent chain.

    stiff links?
    Yes added 1.5 links :)

    Double checked the Sram documentation yesterday and that's exactly what they say. Measure to add one link, cut the chain and then add the power lock.

    I hadn't checked for a bent chain - didn't think of that.

    Never noticed any stiff links.
  • Slowbike wrote:
    47p2 wrote:
    From the pictures it looks like the pulley cage has a slight twist at the lower idler pulley

    yup- I'd agree - bottom of the cage is twisted in ...

    were those pics taken with the chain on the big or little ring?
    Should have mentioned this really. The picture showing the chain off the lower jockey wheel is with the chain in big-big (I.e. 50/28) as that's where it happens the worst.

    The other picture is a bit easier to see - that's in big-small (I.e. 50/11).

    The cage on the Force RD is carbon isn't it? Would that mean this is a manufacturing defect?

    I presume the best way to check if the cage is twisted is to lie it on a flat surface?

    It's all new kit and has done this from the start.
  • Monty Dog wrote:
    Check the rear mech hanger alignment
    I've got no way of checking this without going to my LBS. Was thinking about getting a spare hanger anyway.
  • slowbikeslowbike Posts: 8,498
    It might not be the cage - it could be the hanger?
  • peteonepeteone Posts: 152
    Had very similar issue last week with with new Cervello build, big front ring to half the cassette upwards the chain runs off the bottom jockey.
    Turned out to be a bent hanger as mentioned above.
    Cervelo R3
    Giant TCR SL
    Ibis Tranny
  • Cheers guys. I think steps for the weekend are:

    1. Swap the Rival RD from my commuter onto the CR1. That should tell me if the RD is at fault.
    2. Double check the chain. I'm fairly confident it's fine but checking it anyway.
    3. Order a new hanger. If that doesn't fix it then it's still good to have a spare. In fact I may order one now.
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