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Old 03-31-11, 08:38 PM   #1
engstrom
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Throwing chain off outside in 50 / 11

I was having all kinds of shifting problems on my 2010 Trek 4.7 which has a full Rival groupset (crank, shifters, front/rear derailleurs, brakes). I replaced the chain with a KMC X10SL. Now everything shifts great. In fact all of my shifting problems have been solved except for one - I'm throwing the chain off the outside of the big ring when I'm in the smallest rear cog.

With the old chain I was actually throwing the chain off the big ring in the last two smallest cogs and very occasionally when in the 3rd smallest cog but with the new chain it just happens on the smallest cog. No matter how I adjust (and I have adjusted the rear derailleur high limit screw so that the guide pulley is as far inside as possible and still have it shift to the smallest cog) I can't get the chain to stay on the large chainring up front.

The rear derailleur appears true and straight but a couple months ago I did have the bike fall on the expensive side when a gust of wind decided it shouldn't be leaning again the rail where I had set it. The only things that makes any sense to me are that somehow the tension pulley is too far to the outside, the tension on the slack side of the chain is too low, or the big ring is too worn to keep the chain from falling off. I did remove links on the new chain so that it had the same number of links as the old chain so I guess the only way the tension should be too low is if the spring in the rear derailleur has worn and seeing as the bike is only 9 months old that seems unlikely.

So am I missing something? Anyone have any ideas? Could it just be because there's no resistance on the rear wheel (the rear wheel is up in the air while I'm spinning the crank with my hand to test)? Is it just the nature of trying to cram 10 gears into a single cassette - there's always going to be a pretty severe angle trying to throw the chain off when in the smallest and largest cogs on the cassette.

Honestly 50 / 11 is not a combo I'm likely to use - at least not around the flat lands of North Texas. I'm guessing that I'd only need it going down steep hills and there just aren't any around here. Still, it's as much a matter of understanding the ins and outs of the drive line as it is a principle thing that makes me want to understand and hopefully fix the condition.
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Old 03-31-11, 09:05 PM   #2
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Start by focusing your efforts in the right place. The RD has absolutely nothing to do with chainring issues when pedaling forward. The chain is coming off the top of the rear sprocket which totally insulates it from whatever the RD is doing at the bottom.

Disregard this post and clarify your problem of the chain falls off only when backpedaling.

Your problem most likely relates to FD issues. Start by checking the height, alignment and outer limit setting. Correct adjustment should keep the chain on.

From your description of this as a long term problem, I suspect that you may also have a chainline issue, with the chainrings lining up somewhat inboard of the center of your cassette creating greater angle when using your outboard sprockets.

You can check this simply enough, with a straightedge about 15-18" long. Measure from the outside of your outer chainring to the center of the space between the rings. This should be about 8mm or so. Now lay the edge of your straight edge against the outer ring on a secant, to get a line and extend it back to the cassette. It should line up the same 8mm or so distance to the outside of the space between the 5th and 6th sprockets. If it's inboard center by more than 2-3 millimeters it explains why your chain won't feed properly coming from the 11t sprocket.

BTW- when you're ready for your next chain, give it a careful eyeball exam. You want to find one with more than average bellmouth formed into the inner plates. Bellmouth on the inner plates acts like a funnel picking up sprocket teeth and guiding them into the chain as they engage.
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Old 03-31-11, 09:35 PM   #3
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You may need to lower the front derailleur. It should be as low as possible so long as the you can shift the chain onto the big ring smoothly.

In addition to checking the front derailleur high limit screw you can try reducing the tension on the FD cable slightly.

Go to: www.parktool.com for derailleur setup and adjustment instructions.
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Old 03-31-11, 11:18 PM   #4
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Sounds like front derailleur needs some fine tunng thats all while you shouldn't be cross chaining like that it should still not drop off it will rub inside FD a little.
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Old 04-01-11, 06:58 AM   #5
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Thanks FB, Al1943 and JTGraphics. Looks like I'll spend some time on the FD tonight getting it adjusted. I can believe it's not where it should be because when my shifting problems started I adjusted it to help shift up to the big ring.

Ah well, the more I learn the more I realize I don't know. Maybe someday I'll get good at this bike wrenching thing. Until then I'm sure I'll have no end to practicing.
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Old 04-02-11, 11:51 AM   #6
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Just wanted to say thanks again. I lowered the FD last night (too far, ran into a host of other problems) and this morning I raised it up a little (split the different between original and last night) and adjusted limit screws / cable tension and now have a bike that works in all front/rear combinations...well, except for rubbing between the FD and chain when cross-chained in the small chainring and small cog but I'm never going to be that cross-chained so I don't really care.

I appreciate the advice in pointing me in the right direction so I could get things working all spiffy.
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