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  1. #1
    Senior Member WCroadie's Avatar
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    Chain dropping from 53 to 39 under load

    My bike has 2010 SRAM force with red cranks, when I am in the 53 in the front and 21 or 23 in the back and really putting pressure on the cranks, the chain will drop down to the 39. I know this is cross chaining but sometimes in races I have to ride like that. Took the bike to the LBS friday, the wrench looked at it, adjusted it a little, I took it for a quick spin and it seemed ok. Then in yesterdays race the chaing started dropping again.

    The FD seems to be lined up correctly, any suggestions as to what may be causing this?

    Thanks

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    chainring bolts tight?

  3. #3
    Senior Member mkane77g's Avatar
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    Chain to short?

  4. #4
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    Let's take a close look at exactly what's happening in slo-mo. It could be a simple FD adjustment or chainline/chainring tooth profile issue.

    Use a repair stand, or lean the bike on a wall with about a foot of room to roll forward. Shift to the problem combination, and with the chain tensioned, adjust the trim of the FD so it clears the chain at both the inside back, and outside front at the same time. If you can't do this, it might help to rotate the FD very slightly to bring the back inward, but understand that this might cause trim and shift issues for the outside/outside combinations.

    If the FD can be trimmed to totally clear the chain on both sides of the cage, and so can't be blamed for causing the shift, it's time to look at how the chain engages and chainring.

    Set the bike back up in the problem combination with the FD clear (or removed). Use your left hand to hold the rear wheel back while you load the pedal with your right. Slowly relax the left slightly so the crank can move a bit while looking through the chain from the top as the chainring teeth come up between the inner plates of the chain. The point of each tooth should slide nicely past the outer plate, then move the chain over as the wider part of the tooth engages. If the point of the tooth touches the edge of the outer plate, it'll tend to lift the chain and start a derailment. If that's the case, you have a few options, including of course, not using the problem combination.

    1- try a more effective chain lube to try to help the tooth glide by more smoothly, but this will only help in a borderline case.
    2- change to a chain with more inside bevel or bellmouth, to widen the target for the entering teeth.
    3- trim the FD to push the chain over, but doesn't work under high load, and eventually saws through the cage.
    4- if there's room, bring the crank inboard more to improve chainline.
    5-and if none of the above work, and you still want to ride this combination, consider re-profiling the teeth of the outer chainring to bring their points more inboard and improve pickup from that angle. The easiest way is by holding a file against the outer bevel and spinning the crank. Do this only as a last resort and by degrees only enough to cure the problem. BTW- though modification is frowned upon these days, filing chainring teeth was SOP in the dark ages of derailleurs, when chains falling off was more common than staying on.
    Last edited by FBinNY; 05-02-10 at 09:06 AM.
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  5. #5
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    I don't have SRAM Red anything so I may be speaking out of my blank, but aren't Red rings known for their flexiness? Have you tried another ring?

    cdr

  6. #6
    Senior Member WCroadie's Avatar
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    All great suggestions, thanks guys I really appreciate it. I will check the chainring bolts tonight

    1. I don't think the chain is too short, it came with the bike, I did check for stretch and there isn't much yet.
    2. The FD should be aligned perfectly, not saying it isn't but it should be. I had several wrenches look at it.
    3. I will dbl check the trim but it seems to be working as it should
    4. The chain is a SRAM PC1070, I have a KMC chain in waiting, I could throw that on and give it a try
    5. As for lube, possibly, I notice it doesn't happen all the time, the drive train definitely feels a lot smoother after a nice lube (duh). I can't get it to happen at will, the other day it did not happen the whole ride, the following day it did, I took it to the LBS, he made a few minor tweaks to the FD, I took it for a test spin, could not make it happen. Raced yesterday and after a few laps it would do it every time I got in that gearing and was under load. Maybe I'm just pushing harder in the race and that pressure causes it???
    6. I'm thought about giving the Force crankset a go and see what happens.

    Thanks again, this is my first visit to this section of the forums and I got great suggestions already!

  7. #7
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    Quote Originally Posted by WCroadie View Post
    My bike has 2010 SRAM force with red cranks, when I am in the 53 in the front and 21 or 23 in the back and really putting pressure on the cranks, the chain will drop down to the 39. I know this is cross chaining but sometimes in races I have to ride like that. Took the bike to the LBS friday, the wrench looked at it, adjusted it a little, I took it for a quick spin and it seemed ok. Then in yesterdays race the chaing started dropping again.

    The FD seems to be lined up correctly, any suggestions as to what may be causing this?

    Thanks
    A worn out big ring can cause spontaneous down-shifts under load.

    That could come from much mileage on your chain before replacement (1/16" per foot being the recommended limit) with the increased pitch quickly eating cogs and rings.

    Some sort of bending under load which causes interference between derailleur and chain ring seems most likely though.

  8. #8
    Senior Member WCroadie's Avatar
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    I believe FBin NY is on the right track, upon careful inspection last night, it looks like when the FD is trimmed the outside edge in over just slightly too much and under load flexing just enough to guide the chain into the 39. I rode to work this morning, never use the trim and the chain didn't drop down. Hopefully that is it. I will adjust the FD so I can use the trim. Or maybe I will have the local wrench adjust since I am a newb with adjusting the drivetrain.

  9. #9
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    Der cable is too loose.

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