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  1. #1
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    Chain dropping off right side of cassette on downhills

    Sorry to come on here and post a new thread first up, but been struggling with a solution for the first time ever. I've been maintaining and self building bikes for over 20 years now. Mechanically minded.

    But I got a new 11 speed Shimano ultegra cabled synapse at xmas, and from the off on descents if I hit a bump then the chain seems to get thrown, and then it gets caught between the frame and the cassette at the back so you can't rotate the crank either forward or back, so need to dismount and pop it back on. I've checked the limit screws, and tried moving in, slightly out, but seems to keep coming down to a lack of tension in the chain.

    Zipp Course 30s that spins forever when in the stand, so not freewheel stiffness. I'm running the wide rear cassette 11>32 that came with it, and 52-36 front rings. Note that this is throwing on 52-11. When on the 52-32 cross chained (only in workstand, I don't ride cross chain) then there seems to be plenty more slack in the chain, the jockeys aren't vertical (as suggested for 28 or less) but about 5 o'clock.

    So my take is a lack of chain tension, but having been back to the bike shop twice with it and them saying chain is right length, I'm giving them the benefit of the doubt. And of course it's not a problem that can be replicated in the workshop.

    So any suggestions.

    Many thanks in advance to all that offer suggestions. Never needed to ask for help with wrenching before, always used to be the other way....

  2. #2
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    Doesn't sound like chain tension if it does it in 52-11 and not 36/11. It's a long cage derailleur I assume? If you are in 52/32, how much slack in links can you create by pulling the derailleur forward?
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  3. #3
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    Are you really sure your small cog limit screw is adjusted correctly? Screw it in until the chain is trying to jump to the next larger cog, then back it off until it stops trying to jump. Do this in small quarter turn increments.

    Are you sure your derailleur hanger isn't slightly bent? This could cause problems. By any chance did the bike fall on its drive side? Did someone bump your rear derailleur with their front wheel? I would definitely have your shop check the hanger alignment.

    It does sound like your chain has some excess slack, but this should not cause the problem you are having.
    "With bicycles in particular, you need to separate between what's merely true and what's important."-- DCGriz, RBR.

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  4. #4
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    Take it to another shop. In the big/big your RD should be almost all the way forward, where the RD is in Big/little doesn't say anything.
    Set the B screw so the pully is close to the cog. Adj the chain lenght.
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  5. #5
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    Quote Originally Posted by Lombard View Post
    Are you really sure your small cog limit screw is adjusted correctly? Screw it in until the chain is trying to jump to the next larger cog, then back it off until it stops trying to jump. Do this in small quarter turn increments.

    Are you sure your derailleur hanger isn't slightly bent? This could cause problems. By any chance did the bike fall on its drive side? Did someone bump your rear derailleur with their front wheel? I would definitely have your shop check the hanger alignment.

    It does sound like your chain has some excess slack, but this should not cause the problem you are having.
    99% the limit screw is correct. I say this as I've been doing this so long, and of course this was my starting point. Hey, I'm not perfect, but after lots of fettling in the stand then I'm convinced enough to come on here and ask for advice ;-) I used the same approach you suggested to set it. From the shop I thought it was slightly too far in as it was occasionally ramping up to the second cog and I suspected this was what was throwing it when the freehub was new and possibly a little stiffer. But have now ruled that out.

    Cage hasn't ever been banged whilst I've had it, and in fairness to the shop when it was in for the build / check then they were keeping it in the office and not out on the main workshop area. They mostly deal in MTBs and e-bikes nowadays, not $9,000 road bikes. Which in part makes me also wonder at their wrenching skills. However, since I've not been able to sort it either, people in glass stilettos....

    Quote Originally Posted by duriel View Post
    Take it to another shop. In the big/big your RD should be almost all the way forward, where the RD is in Big/little doesn't say anything.
    Set the B screw so the pully is close to the cog. Adj the chain lenght.
    Hmm, this was my thinking as looking at the Shimano dealer install guide then the length is set using the short cage approach. Cage is way off all the way forward.

    Quote Originally Posted by Srode View Post
    Doesn't sound like chain tension if it does it in 52-11 and not 36/11. It's a long cage derailleur I assume? If you are in 52/32, how much slack in links can you create by pulling the derailleur forward?
    Yes, long cage. I don't descend in 36x11 though. Still plenty of derailleur movement / chain slack in the (never used for real) 52x32. I'd say at least 4 links. Will measure tonight (and try to work out how to post a picture).

  6. #6
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    Quote Originally Posted by duncan74 View Post
    99% the limit screw is correct. I say this as I've been doing this so long, and of course this was my starting point. Hey, I'm not perfect, but after lots of fettling in the stand then I'm convinced enough to come on here and ask for advice ;-)
    Understood. Hey, you never know who will show up here on these forums. We've seen everything here.

    Quote Originally Posted by duncan74 View Post
    Cage hasn't ever been banged whilst I've had it......
    I would at least eyeball it. Look at the derailleur cage from behind the bike. Does it look straight?

    Quote Originally Posted by duncan74 View Post
    Yes, long cage. I don't descend in 36x11 though. Still plenty of derailleur movement / chain slack in the (never used for real) 52x32. I'd say at least 4 links. Will measure tonight (and try to work out how to post a picture).
    Yikes! Yes, I would take at least one pair of links out. If nothing else, it will shift more smoothly.
    "With bicycles in particular, you need to separate between what's merely true and what's important."-- DCGriz, RBR.

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  7. #7
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    Thanks. It all seemed to point to chain length too long to me, but when the shop pushed back and said several times it was right then before getting the chain tool out then I wanted some more opinions. Sadly living in a small town of New Zealand then the options for support are somewhat patchy. Also, I admit things have moved on in the last 10 years (the age of all the groupsets on my road bike, commute bike, cyclocross bike, touring bike and time trial bike.....).

    Having said all this then I'm still counting on the increased chain tension being able to be passed around the cassette to the top of the chain to stop it bouncing off.

    In terms of checking alignment, then I'm assuming it's supposed to be straight vertical? Really daft question but I'm assuming this and for all I know then the boffins at shimano have done something really clever and it now has some aero benefit by having a designed cant or something....

  8. #8
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    Quote Originally Posted by duncan74 View Post
    In terms of checking alignment, then I'm assuming it's supposed to be straight vertical? Really daft question but I'm assuming this and for all I know then the boffins at shimano have done something really clever and it now has some aero benefit by having a designed cant or something....
    Yes. And there is always the possibility the hanger was bent in production, shipping or when put together in the shop. It happens. Remember, the hanger is a sacrificial item meant to save the bike frame and derailleur, so it is the weakest link by design.

    Aero design in a rear derailleur?? I won't touch that one.
    "With bicycles in particular, you need to separate between what's merely true and what's important."-- DCGriz, RBR.

    “Statistics are like bikinis. What they reveal is suggestive, but what they conceal is vital.” -- Aaron Levenstein



  9. #9
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    A derailleur hanger alignment tool is a pretty handy tool to own.

    https://www.parktool.com/blog/repair...nger-alignment
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  10. #10
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    Been watching this thread develop. Chain length is not super critical, I will generally size in small/small, pulling the chain just enough so it doesn't rub on itself where it passes through the pulleys/cage.

    ALWAYS check the hanger. It needs to be as near to perfect as possible. Then...

    Check the limit screws. I ALWAYS set up limit screws w/ the cable NOT attached to the derailleur (rear). Inside edge of the upper pulley lined up w/ the outside edge of the small cog. Upper pulley lined up directly below the large cog. Verify both by pushing on the derailleur by hand, trying to push/pull the chain past the small & large cogs. Make sure the b-tension (end adjustment) is set up correctly. Upper pulley should be pretty close to the large cog (check in small c-ring/large cog).

    There is nothing mysterious here, it's simple mechanical parts. They'll only do what you allow them to do.
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  11. #11
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    Quote Originally Posted by cxwrench View Post
    Been watching this thread develop.

    .......

    There is nothing mysterious here, it's simple mechanical parts. They'll only do what you allow them to do.

    And that's what's frustrating me. In this case then there's nothing I can replicate on the workstand, and nothing that 'looks' as if it could let it happen. It's close to defying the laws of physics, or to be more serious, then static physics aren't in play. So it's down to the shock of the bump causing it.

    And just thinking about this, the synapse is designed to flex in the rear triangle to absorb bumps. I'm wondering if that flex is somehow changing something from the static alignment. Also if there is something setting up some harmonic slapping that's ultimately letting it jump sideways.

    Broken 3 mech hangers so far. 1 Kinesis commute when a mech folded into the rear wheel locking the back as I was commuting in Manchester CBD on a tram track. One on a Cannondale MTB and one on 'Lazerus' - the Cannondale R700 road bike I bout second hand that was legendary in the local club for it's ability to come back from seemingly terminal failures and led to 2 bike shops going bust waiting for me to replace it, only earning $30 for a replacement hanger.

    The only plus from this thread so far is I've an excuse to buy another Park Tool......

    Right, next step is to check the mech (and possibly pull it off, make sure that there's no paint/swarf where it is mounted / the hanger is mounted. And of course the chain length with the confirmation that on long mechs then I should not be using the same rules as short hanger setup.

  12. #12
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    Chain dropping off right side of cassette on downhills-20180327_192146.jpgChain dropping off right side of cassette on downhills-20180327_192412.jpg

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    Another photo

    Chain dropping off right side of cassette on downhills-20180327_1922262.jpg

    Hanger is dead vertical. I judged this by taking a rear photo, putting lines on the rear mech plates, extending to the height of the saddle / floor, then offsetting and putting on centre of rear wheel / seatpost. And any twist of the hanger is well within the accuracy of my method, and I'd say of any physical methodology (noting I'm measuring the plates, not the angle of the bolt thread).
    Last edited by duncan74; 03-26-2018 at 11:15 PM.

  14. #14
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    You could certainly take some chain out of that set up. I would check the RDR alignment, it's hard to eyeball those unless they are grossly out of alignment, but probably not your problem - the limit screw set up seems more likely to me.
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  15. #15
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    Quote Originally Posted by duncan74 View Post
    I judged this by taking a rear photo, putting lines on the rear mech plates
    What are these 'mech plates', cause if someone try's to put serving platters on my bike, it ain't happening.
    This is definitely not the way to check the RD hanger alignment.
    Chain is still too long.
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  16. #16
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    Quote Originally Posted by duriel View Post
    What are these 'mech plates', cause if someone try's to put serving platters on my bike, it ain't happening.
    This is definitely not the way to check the RD hanger alignment.
    Chain is still too long.
    No, it's not. OP show us the the chain in small/small.
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  17. #17
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    This similar issue happened to me on one of my bikes. One of my bike had a short cage derailleur, so I went to put on a medium-cage derailleur (RD-5700). Then all of the sudden, if I pedal in the 50x11 or 50x12 combination while going over some small bumps, the chain would jump, causing to pause for a moment and "softpedal" so that the chain would jump back to its original position. Luckily for me, my chain never jumped off the 11 cog and into the frame. For some time, this baffled me. So, like you, I checked my chain length (measured in small-small cog combo), and I checked my high limit screw like CXwrench described. Everything seemed to be right.

    Frustrated, I put back the shortcage derailleur, and problem went away.

    Then put back the medium-cage der, problem again showed up.

    So, I got on ebay and bought me a Shimano long-cage mountain bike derailleur with the CLUTCH (and I turned the clutch ON), and boom the problem went away. So in my case, the clutch helped to control chainslap and thus fixed the problem. And to test my theory, I turned the clutch OFF (allowing the chain to slap freely), and sure enough the problem did come back every once in a while but not nearly to the same extent as it had happened to the medium-cage road derailleur that I had tried earlier. So I only activate the clutch when I'm descending. I don't like using it though, becauase when the clutch is turned ON, it makes the chain really stiff (that's why no chainslap!) and shifting is also suck due to this.

    Eventually I ditched the clutch derailleur because I can't stand how stiff it makes my shifting is, and put the medium-cage road derailleur back on and live with an occasional chain jumping on the cassette (I just have to be mindful not to pedal when going over bumps).

    Here's something you could test out to see if problem is repeatable on the bike stand. With bike on the stand, put it into the highest gear, or next to highest gear, then spin the crank really fast, then as you spin it, bounce the rear derailleur REALLY HARD (like you would by going over very rough cobbles at high speed!), so the chain is going to slap like crazy here. Now observe if the chain jumps on the cassette. In your case, I would put the chain on the 12t cog (big ring up front) and try that first since putting into the 11t cog may cause the chain to jump into the frame itself, which you probably don't want. If you put the chain in the 11t cog, then make sure you're prepared to deal with the chain possibly dropping into the frame. I was able to replicate this problem on the bike while on the stand. Although I couldn't fix it, but at least I could replicate the issue.
    Last edited by aclinjury; 03-27-2018 at 08:14 AM.

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    Quote Originally Posted by duriel View Post
    What are these 'mech plates', cause if someone try's to put serving platters on my bike, it ain't happening.
    This is definitely not the way to check the RD hanger alignment.
    Chain is still too long.
    The mech plates are the bits that go on the side of the jockey wheels. https://www.google.co.nz/search?q=me...w=1368&bih=795 I agree eyeballing isn't accurate enough which is why I overlaid extruded lines onto the image. That's the same approach taken to bike fit, and in effect how almost all buildings, bridges and other structures are built. Could well have been my description that didn't do the approach justice.

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    Quote Originally Posted by cxwrench View Post
    No, it's not. OP show us the the chain in small/small.
    Small - Small? Ok, didn't take a picture of that, but can do tonight. But interested to know what you're looking for here.

    Must admit, looking at the sag when I pushed the mech forward in big-big and saw how much chain sag there was I was thinking that there was plenty of chain to come out. However, when I tried looping it to count links then it was actually a lot less than I thought. Photo didn't show that too well as i was a couple of hands short of being able to do this and take a decent photo.

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    Quote Originally Posted by aclinjury View Post
    This similar issue happened to me on one of my bikes. One of my bike had a short cage derailleur, ....

    Frustrated, I put back the shortcage derailleur, and problem went away.

    Then put back the medium-cage der, problem again showed up.
    I think my solution will be to swap the cassette to a 'normal' road one without the cheat gear (that I don't need / use) and a short cage rear mech. High speed decents, rough roads and long chain combo required to go around the big cog just aren't compatible.

  21. #21
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    Quote Originally Posted by duncan74 View Post
    TThat's the same approach taken to bike fit, and in effect how almost all buildings, bridges and other structures are built. Could well have been my description that didn't do the approach justice.
    Please don't compare what you did (adding 3' of line to a line 1" by eyeballing it) to how engineers build buildings, I know, I am one.

    That is not how you check the RD, PERIOD.
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    Quote Originally Posted by duriel View Post
    Please don't compare what you did (adding 3' of line to a line 1" by eyeballing it) to how engineers build buildings, I know, I am one.
    Umm. Ditto.

    Ok, I appreciate your input, I know you're giving your time to assist. I take on board your point, but I'm happy that the rear mech alignment isn't at fault. Thanks.

  23. #23
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    Quote Originally Posted by duncan74 View Post
    Small - Small? Ok, didn't take a picture of that, but can do tonight. But interested to know what you're looking for here.

    Must admit, looking at the sag when I pushed the mech forward in big-big and saw how much chain sag there was I was thinking that there was plenty of chain to come out. However, when I tried looping it to count links then it was actually a lot less than I thought. Photo didn't show that too well as i was a couple of hands short of being able to do this and take a decent photo.
    I size chains small/small because it gives you the longest possible chain that will work w/ your drivetrain as long as all parts of the drivetrain are within the spec of the derailleur. Since you're using the smallest cog (11t) when sizing the chain that won't ever change anything. This way you can size the chain using a 25, 28, or 32 and still be able to throw a 34 on and it will work. This is what every race mechanic I know and have worked with over the last 15 years has done.
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  24. #24
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    Quote Originally Posted by duncan74 View Post
    The mech plates are the bits that go on the side of the jockey wheels. https://www.google.co.nz/search?q=me...w=1368&bih=795 I agree eyeballing isn't accurate enough which is why I overlaid extruded lines onto the image. That's the same approach taken to bike fit, and in effect how almost all buildings, bridges and other structures are built. Could well have been my description that didn't do the approach justice.
    Derailleur cage, jockey wheel cage
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    Well??????
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