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  1. #1
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    Chain jumping down the cassette when back pedalling

    Ok on my Di2 R8050 system, I switched out a Shimano 11-28 cassette for an 11-30, and now whenever I ever back pedal and the chain is sitting in the big ring and the top 2-3 big cogs... the chain jumps 1 cog down the cassette. Chain will jump back up cassette to original position when I start to pedal forward again though.

    Some more info:
    the 11-28 cassette is a Dura Ace cassette from the previous generation (Di2 9000 series). The 11-30 cassette is from the current generation (Di2 8000). This shouldn't matter right.

    What should I check?

  2. #2
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    Quote Originally Posted by aclinjury View Post
    Ok on my Di2 R8050 system, I switched out a Shimano 11-28 cassette for an 11-30, and now whenever I ever back pedal and the chain is sitting in the big ring and the top 2-3 big cogs... the chain jumps 1 cog down the cassette. Chain will jump back up cassette to original position when I start to pedal forward again though.

    Some more info:
    the 11-28 cassette is a Dura Ace cassette from the previous generation (Di2 9000 series). The 11-30 cassette is from the current generation (Di2 8000). This shouldn't matter right.

    What should I check?
    Nothing. There is no derailleur guiding the chain onto the cog when you're pedaling backwards. The larger big cog you have the worse it will be. You can't do anything to stop other than not pedaling backwards.
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  3. #3
    tlg
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    Only thing I can think of is; do you have drag in your freehub? If your freehub has drag, it could cause your chain to get some slack in it on top. At the extreme chainline, slack on top of your chain could cause it to slip off the cog.
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  4. #4
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    ok after mucking around and following the Shimano's instructions, I found that everything was already set pretty much like it should be. So I put the 28t cassette back on and put the chain in big-big combo and back pedal, and sure enough, chain does jump down the cassette. But this only happens in big-big for the 28t cassette, not in any other combo. Maybe I have never noticed this since I never ride in big-big combo (you're not supposed to cross-chain that much right!).

    So I put back on the 32t cassette, and tried back-pedal in big-big combo, chain jumped agian, as expected. Shift down one cog and back pedal, chain still jumped down. Shift down another cog and back pedal again, this time the chain did try to jump down (I could hear it), but it didn't so it at least held position.

    Hub is smooth, freehub is smooth.

    I guess CX is right! don't pedal backwards when cross-chaining when in big-big combo or near to it. I usually I don't pedal backwards except when sometimes I unclip at a stop and thus will pedal half a revolution backwards to put a foot down.

    I think it also doesn't help that the chainstays of this bike is only 401mm long, making the chain angle really bad when you're cross chaining

  5. #5
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    You should not be using the big-big combination......

    BTW, did you get a chain that was 2 links longer? Your chain is probably too short.
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    patient: doc, it hurts when i do this ...

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  7. #7
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    Quote Originally Posted by No Time Toulouse View Post
    You should not be using the big-big combination......

    BTW, did you get a chain that was 2 links longer? Your chain is probably too short.
    This is complete bullshit, I really wish people that post this would stop. EVERY modern drivetrain is designed to work in big/big.
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  8. #8
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    How fast can you go backpedaling? I think you'll go faster in the little/little, try that and see how fast u can go and get back to us!
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  9. #9
    .je
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    Mine was like that too, at one point it was pretty bad.
    I replaced the chain with a new one, since it was due, but not that bad... I thought...

    2 things were different this time:
    i) the old chain was, I thought, maybe one link too short, so the tension was high. There was only a few degrees movement in the derailleur in the big-big, if I did that. I thought, maybe, that any extra tension might be pulling on it, and it can only be upulled down. The new one is 2 links longer, and it shifts very precisely now. I can also backpedal NP.
    ii) the old chain, even oiled, didn't seem to turn on the pins as smoothly as the new chain, so any binding might pile up, and pop it off the big ring.

    I could have been right, but not sure which might have been the bigger effect. I can say for sure that my backward FTP has gone up almost 20W from before, so you might want to consider.

  10. #10
    'brifter' is a lame word.
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    Quote Originally Posted by .je View Post
    Mine was like that too, at one point it was pretty bad.
    I replaced the chain with a new one, since it was due, but not that bad... I thought...

    2 things were different this time:
    i) the old chain was, I thought, maybe one link too short, so the tension was high. There was only a few degrees movement in the derailleur in the big-big, if I did that. I thought, maybe, that any extra tension might be pulling on it, and it can only be upulled down. The new one is 2 links longer, and it shifts very precisely now. I can also backpedal NP.
    ii) the old chain, even oiled, didn't seem to turn on the pins as smoothly as the new chain, so any binding might pile up, and pop it off the big ring.

    I could have been right, but not sure which might have been the bigger effect. I can say for sure that my backward FTP has gone up almost 20W from before, so you might want to consider.
    Doesn't matter if the chain is moving backwards. The derailleur has NO effect on the chain at that point. The ONLY reason the chain comes off when you backpedal is because there is no pulley guiding it on to the cog. The tension on the chain is the same.
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  11. #11
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    Quote Originally Posted by cxwrench View Post
    Doesn't matter if the chain is moving backwards. The derailleur has NO effect on the chain at that point. The ONLY reason the chain comes off when you backpedal is because there is no pulley guiding it on to the cog. The tension on the chain is the same.
    ok, the tension on the top part of the chain is the same. But what about the tension on the bottom part of the chain? It's not the same right? I'd think that with a shorter chain, then the tension on the bottom part of the chain (when in big-big combo) would be higher than if the chain was longer right? Well then, could the higher tension on the bottom part of the chain may be acting like a resistance force for the chain to move backwards? and thus causing the chain to jump? I'd think a looser chain would allow the chain to move backwards with less resistance and thus may alleviate the jumping (maybe not totally prevent it).

    I suppose I can prove whether this is true or false by breaking the chain loose (quick link) and then roll it backwards while it's loose not rolling thru the rear derailleur (except for the top pulley) and see if it'll jump. If it doesn't jump, then maybe derailleur tension and thus chain length maybe be a mitigating factor?
    Last edited by aclinjury; 03-19-2018 at 11:54 PM.

  12. #12
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    All are capable, but I thought only SRAM was designed for it. Shimano still says don't do it right?

  13. #13
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    Quote Originally Posted by evan326 View Post
    All are capable, but I thought only SRAM was designed for it. Shimano still says don't do it right?
    SRAM was designed for it? how? Like CX said, there is no guide (eg, a pulley guide) to keep the chain rolling straight when its moving backwards like there is a lower pulley to guide it straight when moving forward. I would think that chainline is the main determinant if a chain will jump when backpedaling, and chainline will depend on bike geometry (eg, chainstay length)

  14. #14
    tlg
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    Quote Originally Posted by No Time Toulouse View Post
    You should not be using the big-big combination......
    Meh. Says you. I live in a hilly area. I use big big all the time. And get well over 5,000mi out of a chain.

    Quote Originally Posted by evan326 View Post
    All are capable, but I thought only SRAM was designed for it. Shimano still says don't do it right?
    Yes, all are capable. Shimano doesn't say don't do it. They recommend against it for two reasons:
    a) it's less efficient
    b) increases wear

    So as long as you don't mind losing a couple watts or a few miles on the life of your chain, there's no reason not to do it.

    This is what the big 4 mfg's say about it.
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  15. #15
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    Does your bike have really short chain stays?

    My experience with two identical groups on different bikes indicates short chain stays is a factor.
    I don't think it's anything to 'fix' per se. It just happens and really isn't a problem.

  16. #16
    tlg
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    Quote Originally Posted by Jay Strongbow View Post
    Does your bike have really short chain stays?
    Which bike? I have a bunch.

    Mine are all, or have been, 54-56cm. And my wife rides 50-52. Never had an issue on any bike.
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    Quote Originally Posted by tlg View Post
    Which bike? I have a bunch.

    Mine are all, or have been, 54-56cm. And my wife rides 50-52. Never had an issue on any bike.
    I was responding to the OP. And asked him about chain stay length not bike size.

  18. #18
    tlg
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    Quote Originally Posted by Jay Strongbow View Post
    I was responding to the OP. And asked him about chain stay length not bike size.
    Oh. Well you responded to my post.


    OP said previously "the chainstays of this bike is only 401mm long"
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  19. #19
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    Quote Originally Posted by No Time Toulouse View Post
    BTW, did you get a chain that was 2 links longer? Your chain is probably too short.
    If the chain spins in the large/large combo without binding, it is NOT too short.
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  20. #20
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    Quote Originally Posted by aclinjury View Post
    ok, the tension on the top part of the chain is the same. But what about the tension on the bottom part of the chain? It's not the same right? I'd think that with a shorter chain, then the tension on the bottom part of the chain (when in big-big combo) would be higher than if the chain was longer right? Well then, could the higher tension on the bottom part of the chain may be acting like a resistance force for the chain to move backwards? and thus causing the chain to jump? I'd think a looser chain would allow the chain to move backwards with less resistance and thus may alleviate the jumping (maybe not totally prevent it).

    I suppose I can prove whether this is true or false by breaking the chain loose (quick link) and then roll it backwards while it's loose not rolling thru the rear derailleur (except for the top pulley) and see if it'll jump. If it doesn't jump, then maybe derailleur tension and thus chain length maybe be a mitigating factor?
    You're trying to infuse this "problem" with issues that are tangential at best. Re-read what CX said in his first post: when you backpedal, there is no derailleur keeping the chain from trying to align with the front ring so it will naturally try to get the chain on a smaller cog. It makes virtually no difference what your setup is. Get that in your head and stop backpedalling. Not sure why you are backpedalling in the first place.

  21. #21
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    Quote Originally Posted by Lombard View Post
    If the chain spins in the large/large combo without binding, it is NOT too short.
    Not necessarily true. Depends on what your idea of 'too short' is. If the derailleur is pulled all the way forward but still works, I'd personally call that too short. I'm guessing your definition of too short would be ripping the derailleur off the bike. Semantics.
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  22. #22
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    Quote Originally Posted by cxwrench View Post
    Not necessarily true. Depends on what your idea of 'too short' is. If the derailleur is pulled all the way forward but still works, I'd personally call that too short. I'm guessing your definition of too short would be ripping the derailleur off the bike. Semantics.
    Pretty much, yes. If it spins freely in large/large, it's OK in my definition. But remember, I'm the guy who favors small/small over large/large.
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  23. #23
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    Quote Originally Posted by cxwrench View Post
    This is complete bullshit, I really wish people that post this would stop. EVERY modern drivetrain is designed to work in big/big.
    I'm going to reply to this at the risk of a virtual flogging by you, CX.

    My take is, sure, if adjusted correctly, it should work. But unless you are a racer who doesn't want to downshift the front for fear of losing a fraction of a second, is the large/large ever really necessary or practical?

    I favor the smaller ring over the larger ring for a couple reasons:

    1) It is quieter.
    2) If I suddenly do need that smaller ring, I am more likely to drop a chain if I am already in the largest cog.

    Generally, I use neither extreme. It is unnecessary. I go to the large ring if my chain starts chattering against the FD. That is usually not until the smallest cog. When I need easier gearing for climbing or if I am stopping, I will go back to the small ring before going to considerably larger cogs.
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  24. #24
    .je
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    Quote Originally Posted by Kerry Irons View Post
    You're trying to infuse this "problem" with issues that are tangential at best.
    Nice play on words.
    What I read from that post was saying, if I'm trying to make sense of that thinking, is that the derailleur is holding the bottom in the right place, but the top is held on the chainline between the chainring and the large cog, which is sort of pulling it outward, if there's tension.
    Is that what was meant? It really shouldn't be able to have enough to pull off the teeth. Apols if that's completely off (play on words again).

  25. #25
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    Quote Originally Posted by .je View Post
    Nice play on words.
    What I read from that post was saying, if I'm trying to make sense of that thinking, is that the derailleur is holding the bottom in the right place, but the top is held on the chainline between the chainring and the large cog, which is sort of pulling it outward, if there's tension.
    Is that what was meant? It really shouldn't be able to have enough to pull off the teeth. Apols if that's completely off (play on words again).
    yeah that's what I meant, I probably didn't explained it all that well.
    Either way, I've been able to verified to myself that tension thru the derailleur makes no difference. So I broke the chain up (quick link), and proceeded to roll it backwards in the top 2 "big-big" combos, and of course with no chain tension, and sure enough the chain did jump too.

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