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Old 10-10-08, 12:49 AM   #1
thefultonhow
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Chain falling off 7th gear under load

I just replaced my old three-speed crankset with a 42-tooth largest chainring (don't know the other ones, and I never used them) with a single-speed crankset with a 50-tooth chainring. I also replaced the cassette on the rear with a new one for two reasons -- one, the 7th gear (smallest chainring) on the old one was worn and so the chain was skipping, and two, I wanted a larger ratio spread with lower gearing on the first couple of gears due to the larger chainring up front. I tried seventh gear once, in rush-hour traffic, but I missed shifting down when I had to brake suddenly, and when I tried to accelerate and put pressure on the drivetrain in order to do so, the chain fell off the rear chainring. When I hadn't been loading it up, it had pedaled fine and with no skipping. I also had replaced the chain at the same time, so I had basically a completely new drivetrain that should have meshed fine.

I was wondering if it's possible to block the chain from coming off. Can I put any kind of guard in the way that will prevent the chain from moving any farther outboard? (In the current setup, it can fall onto the spindle that attaches the wheel to the frame.) Are there any other methods? TIA.
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Old 10-10-08, 01:11 AM   #2
meb
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Originally Posted by thefultonhow View Post
I just replaced my old three-speed crankset with a 42-tooth largest chainring (don't know the other ones, and I never used them) with a single-speed crankset with a 50-tooth chainring. I also replaced the cassette on the rear with a new one for two reasons -- one, the 7th gear (smallest chainring) on the old one was worn and so the chain was skipping, and two, I wanted a larger ratio spread with lower gearing on the first couple of gears due to the larger chainring up front. I tried seventh gear once, in rush-hour traffic, but I missed shifting down when I had to brake suddenly, and when I tried to accelerate and put pressure on the drivetrain in order to do so, the chain fell off the rear chainring. When I hadn't been loading it up, it had pedaled fine and with no skipping. I also had replaced the chain at the same time, so I had basically a completely new drivetrain that should have meshed fine.

I was wondering if it's possible to block the chain from coming off. Can I put any kind of guard in the way that will prevent the chain from moving any farther outboard? (In the current setup, it can fall onto the spindle that attaches the wheel to the frame.) Are there any other methods? TIA.

Your semantics appear to be off.

If you are referring to the rear sprockets as the place the chain came off, you probably need adjust the limit stops on you rear derailleur.

If the chain is coming off the front chainring, you probably have too much chainline angle from the rear sprocket to the chainring. That would be likely scenario. Another possibility is the front derailleur is in the wrong position and you may need adjust it to the right -particularly if you have a friction shifter up front-you could do it with the shifter.

You could but an idler in between the sprocket and chainring to keep the chain on the chainring, but it would rob you of power. Also, the front derailluer could be used to hold it onto the chainring-but that friction also robs you of power. Another option might be to go back to a triple front (or even a double would give you a better chainline).
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Old 10-10-08, 01:29 AM   #3
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Yeah, I'm pretty sure my semantics are off... I really am just getting into bikes and am still struggling with terminology. I've learned my other hobbies (computers, cars, photography) very gradually, but never paid attention to bikes until I realized I was actually starting to enjoy riding beyond just getting to class in less time. So I'm having to learn all the terminology all at once.

The chain did come off the rear as far as I can tell. I'm running a front derailleur and it's adjusted properly and the shifter (yes, friction) is tightened so it can't move. I had problems at the front when I first put on the chainring, and I adjusted it very carefully, so I'm pretty sure that's not the problem. The issue is that the chain fell off when I was really going fast, so by the time I looked down, it was off both the front chainring and the rear sprocket.

Would the limit stops really help? I'm not sure I understand exactly how they function -- I thought they limited the range at the outside edge of 7th and the inside edge of 1st?

FWIW, I believe the chainline angle is such that the way it fell off (towards the outside on the sprockets, and towards the inside on the chainring) would be counterintuitive if that were the issue.
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Old 10-10-08, 02:30 AM   #4
Bill Kapaun
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I've swapped just the cranks and had to tweak the RDER limit screws slightly. Same with just a cassette. Doing both, I'd EXPECT to have to do minor adjusting as a matter of course!

You may also want to loosen the front shifter, so that you can use it to "trim" the FDER slightly, depending on which side of the cassette you are using. You can set the front limit screw so that you have minimal side to side travel, but still enough for a trim function to help prevent chain rub/noise when on the extreme ends of the cassette.
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Old 10-10-08, 12:00 PM   #5
thefultonhow
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So, any advice on how the limit screws work? I guess I could learn by trial and error, but it's easier to ask on here.
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Old 10-10-08, 01:46 PM   #6
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Screwing IN either screw reduces the travel of the DER.
You would probably want to try about 1/4 turn on the H screw.
Keep track of HOW MUCH so you can revert, in case you turn the wrong one. TOO MUCH, and it won't shift from 6 to 7.
Screw the L screw in too far and it won't go from 2 to 1. Too far out and the chain goes into the spokes.
For "fine tweaking", turn a little bit and RIDE the bike, going through ALL the gear combos. I like to hit a couple SMALL bumps also, just so things are fully "settled in".
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