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  1. #1
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    3 Mechanics stumped- Sram xx issue

    So, I've adjusted the bike, buddy adjusted the bike, and the bike has stumped 3 mechanics at the shop so far (on two visits). So really, that's five people.

    The story: The bike is a 29" titanium hardtail fitted with sram xx 2x10 (11-36). It probably has 400-500 miles on it. It shifts flawlessly. However, while climbing STEEP grades with a lot of power (small ring front & 2nd or 3nd from lowest gear in the rear) the chain literally explodes off the small ring in the front.

    I've crashed twice so far. My left calf has a nice 6"x5" bruise from Sunday and I blew out the ass of my shorts trying to save myself. Going backwards clipped in down steep grades is sketchy. Today, while doing a test and tune, the chain came off while hammering up hill. I went over the bars and have a new gash on left shin.

    Any insight or help is greatly appreciated. I'm trying to not kill myself over this. We have a bikepacking trip planned for the Colorado trip next week.

    New Video:
    Sram XX rear derailleur problem - YouTube
    Last edited by rondigs; 08-19-2013 at 05:15 PM.

  2. #2
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    Ti flexes a fair amount. Hold the bike on the ground, and step on the crank. The bottom bracket will flex, I'm willing to bet. Whether or not that's enough to get the chain to hop off, I can't tell you, but maybe try dropping to a lower gear and not mashing so much?

  3. #3
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    Chainline?

  4. #4
    Cleavage Of The Tetons
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    Yup....sounds like frame flex is essentially acting as a derailleur.
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  5. #5
    Elitest thrill junkie
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    It's not a clutch derailleur is it? My clutch thingie engaged and caused the chain to go slack during a race last weekend and knocked my chain off the front ring (which is exactly what the derailleur is supposed to prevent)
    "It's only when you stand over it, you know, when you physically stand over the bike, that then you say 'hey, I don't have much stand over height', you know"-T. Ellsworth

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  6. #6
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    Chainline is good. No noises or rubbing. Shifts flawlessly.

  7. #7
    Poacher
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    My SIR 9 was flexy (worn out, beat down frame ), and no problem with staying on chainring, but I think that COULD be an issue. It sounds to me (without looking at it), like a worn out drivetrain. 400-500 miles of harder than usual climbing, mixed in with grit and grime from wet, and a flexible frame are conspiring against you. I have no evidence though. Jayem made an interesting point.

  8. #8
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    Chains will climb off a cogset that has a bad plating job. I've done this for a living a badly plated parts will be sent through again. This is OK for door hinges but not for a precision part. The chain is climbing the rear cog this then creates a wave that travels to the front making it climb off as well. Shimano has videotaped gear shifting SRAM should do the same using high speed cameras.
    "Dish is illogical." Spoke of Vulcan.

  9. #9
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    Re: 3 Mechanics stumped- Sram xx issue

    I had the same thing happen years ago - the chain would jump off the small ring under power. It turned out that it was a defective chainring. There would have been no way to see the defect or problem based on inspecting it - the shop was stumped until they called Shimano who said a recall had just gone out. They replaced the chainring and problem solved. That was a decade ago, but still proves the point that a bad thing (or possibly cassette as mentioned above) can cause your problem.

    Can you replicate it in a parking lot if you try hard enough? If so, see if your shop will be willing to test a new ring and/or cassette (one change at a time) to see if it solves the problem.


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  10. #10
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    So, I think the rear derailleur is the problem. When I picked it up from the shop, they didn't really have any solutions to the problem. However, the mechanic showed my by bouncing the rear on the ground it produced a lot of chain slap and movement. He suggested that the lack of tension in the rear derailleur is causing the chain to drop.

    Sure enough, I was able to reproduce it today. While on the lower chain ring on the front and a lower gear setting in the rear, I came to a steep hill carrying a lot of momentum. As I came up the hill, without even pedaling, I hit several consecutive rock ledges and the chain came right off. The same scenario happened on another climb during the ride. No pedaling, hit some rock ledges going up hill, chain drops...

    I think the weak spring tension in the derailleur is causing the chain to excessively slap and walk off the front ring...

    A link to a similar story and potential fix:
    very weak cage tension in XX rear Der - Weight Weenies

  11. #11
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    im no pro by any means, but all the tension is on the top of the ring. I cant see slack on the bottom cause this issue unless there is enough side to side play on the bottom of the chain.

  12. #12
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    Quote Originally Posted by machine4321 View Post
    im no pro by any means, but all the tension is on the top of the ring. I cant see slack on the bottom cause this issue unless there is enough side to side play on the bottom of the chain.
    It doesn't take much for slack on the bottom to effect/affect the top expecially on a climb when he is hitting rocks and roots. I am not at all saying this is for sure the issue, but too much slack on the bottom is definitely going to create issues. Hopefully that solves your problems rondigs.
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  13. #13
    I Tried Them ALL... Moderator
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    I vote defective chainring. Mine did exactly the same thing, when I re-installed the small chainring backwards, after deep-cleaning it. I still have the shin scars to prove it...
    "The mind will quit....well before the body does"

  14. #14
    Cleavage Of The Tetons
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    No clutch derailleur?
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  15. #15
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    Quote Originally Posted by rideit View Post
    No clutch derailleur?
    I don't believe it's a clutch style derailleur

  16. #16
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  17. #17
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    Have you checked and made sure you have no play in the bb or crank, Just a tiny wiggle that you may not even notice can pop that chain off.
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  18. #18
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    sounds like your chain may be to long if the derailleur is allowing it to jump that much.

  19. #19
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    Re: 3 Mechanics stumped- Sram xx issue

    Seems like SRAM rd's just have durability issues

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  20. #20
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    I've had the same issue with a bad chainring as well. Silly question, is your rear wheel on there tight enough?

  21. #21
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    Hopefully the video works:
    Sram XX rear derailleur problem - YouTube

    So, the chain has been shortened, limit screw adjusted and nothing is loose. Hopefully the video help. Also, there is no pedaling in the video, so the frame should not be under load/flexing.
    Last edited by rondigs; 08-19-2013 at 05:12 PM.

  22. #22
    Trail Tire TV on blogger
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    3 things I see (thou hard to tell from vid so it could be Optic illusion)

    1 rings look worn,.. like worn out
    2 Front derailleur is WAY to high
    3 Front Derailleur is to far in
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  23. #23
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    3 Mechanics stumped- Sram xx issue

    Keep your feet on the pedals while riding.

    I would bet on worn out ring and/or bent teeth.
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  24. #24
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    The mechanic today suggested possibly replacing the big ring (due to wear). But he didn't mention the smaller ring.

    "keep your feet on the pedals while riding" Not sure what that means. I ride clipless and rarely come out. Unless the chain drops or near crash instance...

  25. #25
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    Slowing the video down to .03x speed shows the bottom chain action getting worse and worse as you bounce. The cage moves 1-1.5" the first bounce and then like 4-5" the second. The chain from the top of the ring>cassette is below the chainstay and the chain from the cage>ring is almost being whipped downward by the cage action. I think this is causing it to ride off the ring from the bottom upwards.

    How does the derailleur feel with no cable/chain installed? Are you able to compare it to another possibly? Here is a picture from the second bounce with the cage almost fully extended and view of the chain as I describe above. Seems like the derailleur should do a better job keeping tension I would think, clutch or no clutch.
    Attached Thumbnails Attached Thumbnails 3 Mechanics stumped- Sram xx issue-2bounce.jpg  


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