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Thread: Chain Drop

  1. #1
    that bike nut BikingGrad80's Avatar
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    Chain Drop

    I need some advice. I can't seem to get my FD so that it doesn't drop my chain on to the BB shell.

    A year ago I switched from a standard to a MegaEXo 50/34 compact the FD is a 6500 double. I remounted the FD and shortened the chain. It seemed to work OK for a while but I keep getting chain drop about once per ride a few percent of the downshifts. I've tried adjusting the limits and even with the L screw set so tight I have chain rub on the lowest gear it still happened. I also took off the FD cleaned and lubed all the pivot points and remounted it as low as possible without hitting the chainring. It still does it.

    Any tips? Could it be the Housing is worn? Should I try raising it or rotating it to a different angle?

  2. #2
    Senior Member andrewluke's Avatar
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    angle will do it as will height. I always ran my 6600 FD very close and my Force FD now almost rubs - it shifts like butter though.

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    Does your chain drop during shifts, or while just running on the inner? Each has a different cause.

    If it happens during shifts, it's possible you're keeping too much tension on the chain during the shift, especially if it happens when you've already shifted to the inner cassette sprockets. The chain is coming to the outer chainring from an inside angle. If you disengage it under tension, it'll look to jump inward because of the tension and overshoot the sprocket. If you relax and lower the chain load during the shift, gravity will drop the chain straight onto the sprocket.

    If it happens randomly when using the inner chainring, especially off the innermost rear sprocket, it's a chainline/tooth profile issue.

    Imagine a pulley for a moment, when feeding straight everything is fine, but if you feed the rope from too much of an angle it'll run up and over the flange and fall off. Same thing with chains and chainrings, except the pulley guides run inside the chain. The teeth are pointed, and pick up and guide the inner links of the chain (outer links don't count here). As the chain angle increases it reaches a point where the upcoming teeth touch the inner plate instead of sliding in smoothly. If they bump the plate enough they'll pick it up and let it run over the side.

    There are a few fixes.

    1- some chains have more bellmouth or chamfer in the inside of the inner plates. These act as wide mouth funnels to pick up teeth and guide them to the center. Next time around look at various chains and buy the one with the most inner bellmouth.

    2- many modern chains have plates that are about the width of the rollers, others use plates that are wider than the rollers, forming sort of a "V" pulley, which helps keep the chain on track. This design has gone out of fashion, so save weight, and to reduce tooth damage from hard shifting under load. If you can find a chain with the older big plate design, it'll be less likely to derail.

    3- you can re-profile the inner sprockets teeth so the points are a bit more inboard. This will make them more able to meed the gap between the inner plates coming from the inside. Set a medium file at an angle against the outer taper of the inside sprockets teeth. Spin the crank to file all the teeth down a bit as if you had a lathe. Stop and check progress often because if you get carried away, you'll cause problems when the chain comes from the outside.

    Or, you may simply buy one of a number of chain keeper devices which catch errant chains and guide them back home.
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    +1 Having the cage nose angled inward too much, mounting the derailleur too high, or the outside of the cage being damaged/distorted can all cause inward misshifts. You've eliminated the height issue, but the angle could still be causing a problem. Some chains hang on better than others, but I have not kept track of current chains well enough to tell you which ones, perhaps someone else can. High pressure on the chain on a downshift can also cause problems but I somehow don't think that's your issue.

  6. #6
    that bike nut BikingGrad80's Avatar
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    Quote Originally Posted by cny-bikeman View Post
    +1 Having the cage nose angled inward too much, mounting the derailleur too high, or the outside of the cage being damaged/distorted can all cause inward misshifts. You've eliminated the height issue, but the angle could still be causing a problem. Some chains hang on better than others, but I have not kept track of current chains well enough to tell you which ones, perhaps someone else can. High pressure on the chain on a downshift can also cause problems but I somehow don't think that's your issue.
    Well I reangled it last night. I put a straight edge on the top outer part of the cage and got it exactly parellel to the chain on the highest gear. If that doesn't work I guess I'll just have to get the chain watcher. Every so often when I downshift it seems the FD snaps back more violently than normal and the chain overshoots the small ring even though the L screw is properly adjusted. Happens once or twice a ride perhaps 1 out of every 15 to 20 shifts.

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    Quote Originally Posted by BikingGrad80 View Post
    Every so often when I downshift it seems the FD snaps back more violently than normal and the chain overshoots the small ring even though the L screw is properly adjusted. Happens once or twice a ride perhaps 1 out of every 15 to 20 shifts.
    That's chain tension, ease off for a half second just as the shift gate comes around and it'll drop down smoothly.
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  8. #8
    Senior Member Retro Grouch's Avatar
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    Quote Originally Posted by BikingGrad80 View Post
    Well I reangled it last night. I put a straight edge on the top outer part of the cage and got it exactly parellel to the chain on the highest gear. If that doesn't work I guess I'll just have to get the chain watcher. Every so often when I downshift it seems the FD snaps back more violently than normal and the chain overshoots the small ring even though the L screw is properly adjusted. Happens once or twice a ride perhaps 1 out of every 15 to 20 shifts.
    Try dialing HIGH limit screw out about 1/4 turn. Sometimes the front derailleur binds up in the big ring position. When this happens it tends to shift real hard when it finally releases.

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