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Chain comes off chainring when shifting in the rear

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Chain comes off chainring when shifting in the rear

Old 12-15-11, 08:43 PM
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Chain comes off chainring when shifting in the rear

Hello all,

So I have researched my issue to death all over the internet before finally breaking down and making this thread. I bought a scott sub 40 from a friend which had a very worn out 24 speed drivetrain. All shimano acera stuff. I replaced the following parts:

Sram 10 speed cassette 11-28
sram rival short cage rear derailleur
sram 10 speed chain with kmc removable link
sram flat bar double tap shifter
FSA gimondi crankset 48t 3/32"
shimano square taper bb 113mm

So after lots of blood sweat and tears, I got the bike up and running. Almost everything works swimmingly. Upshifts are smooth and fast, downshifts are quick and snappy, no noises in any of the gears.... the only problem is when I upshift the chain has a terrible tendency to come off the chainring. It only happens when I upshift, and it only drops off to the outside. I can minimize the occurrence by being very careful to slow my pedalling way down when I shift (I know you shouldn't really shift at power anyway). But sometimes, even when there is pretty much no load on the cranks when I am shifting, it still jumps off. When I watch what the chain is doing when I shift, I have noticed that it can deflect horizontally quite dramatically between the top of the cassette and the chainring, sort of like plucking a guitar string. It doesn't happen everytime, I am guessing it is when I initiate my shift at a less ideal time based on the ramps and all that jazz on the cassette. I am almost certain that it is these times when the chain is jumping around that if I were to put a bit more pressure on the crank, the chain would come off.

My chainline is decent, the chainring is almost in line with the center of the cassette. it probably lines up with 4th or 5th gear. my b angle screw thingy is set so I have just over 6 mm of clearance between the derailleur pulley and the bottom of first gear, as per sram's suggestion. i clean and lube the drivetrain regularly.

soooooooooooo.................................... how do I prevent this from happening?

from the tech docs I have read it doesn't seem like I can adjust the tension of the spring in the derailleur, but if I shorten the chain will that increase the tension and prevent the chain from jumping around while shifting?

Will fiddling around with the B screw and decreasing the separation help with shifting?

Are all chainrings created equal?...... could the geometry of the teeth and the ring be adding to my woes? the ring is has no ramps or pins or anything like that to aid in shifting ... it is meant to be used as a single.

Other than those 3 things, I can't think of anything else that could contribute or fix the issue.... since the bike is nicely set up with the exception of this problem, I don't really want to be screwing around with it all willy nilly without first getting some good, educated feedback. And I don't want to take it to my LBS. They kind of suck, and I would really prefer doing the work myself (with your help). Anyway, sorry about the length, just wanted to make sure I provided all the relevant info!

Last edited by Lance Power; 12-16-11 at 08:04 AM. Reason: changed 3/16 to 3/32.....oops
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Old 12-15-11, 09:27 PM
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The usual cure for chain dropping off a single chainring to install a front derailleur as a guard. Set the limit screws to center the cage over the chainring.

Also, is your chainring intended to be used as a single or part of a double or triple crank? A purpose built single chainring is "flat" with no shift-promoting ramps or pins. If the chainring was designed to be shifted, replacing it with a suitably designed one will probably solve the problem.
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Old 12-15-11, 09:43 PM
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I was hoping to avoid putting the derailleur back on as a guide, mainly due to aesthetic reasons. However, the chain continually dropping off is more than enough reason for me to slap it back on.

The chainring is meant to be a single. It is just the generic one that came on the crankset. Here it is in all of its gaudy chromeness

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Old 12-15-11, 10:31 PM
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A Paul Components Chain Keeper would keep the chain where it belongs with minimal impact on your bike's aesthetics.
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Old 12-16-11, 01:43 AM
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Cyclocross often sets up single rings with a chainguard on both sides,
or one on the outside , and a chain minder /Jump stop, etc. on the inside..
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Old 12-16-11, 02:32 AM
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So it looks like some sort of chain keeper is my best and only option? I was not sure if this was a problem that was inherent with a single chainring and a rear cassette, or if my issue was unique to my set up. Thanks for the feedback!
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Old 12-16-11, 03:04 AM
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A MTB rear derailer might help keep it on. Those Sram 10-speed RDs start at a $100, though. That's a lot to drop on a maybe.
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Old 12-16-11, 04:11 AM
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hmm, yah, I wouldn't want to swap the mech. the rival rd is brand new and I am loving it other than the chain flying off all willy nilly. I would definitely rather a chain keeper if I can't adjust the problem away.
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Old 12-16-11, 11:08 AM
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You have that [#3] crank?
At least it does not have a derailleur [shifting easily, off/onto it] chainring on it.

I have a chainring disc on the outside, and a chainminder on the inside of my single ring bike..

[FWIW, a bike friday with a chain tensioner and an IGH ] works well..
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Old 12-16-11, 11:13 AM
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Originally Posted by Lance Power
hmm, yah, I wouldn't want to swap the mech. the rival rd is brand new and I am loving it other than the chain flying off all willy nilly. I would definitely rather a chain keeper if I can't adjust the problem away.
Adjustment avenues include shortening chain (be careful you don't go too far or your RD will go boom when trying to get into the big cog) and screwing in the B-screw.
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Old 12-16-11, 11:20 AM
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just so you know...

it's possible to run an 8 speed derailleur 12,000 miles on paved surfaces with no problems involving derailing off the front chainring. this involved a 130mm spaced rear wheel with an XTR rear derailleur.

if i were spontaneously derailing the chain, i would suspect insufficient tension on the chain or possibly a poor chainline favoring low or high gear end.

causes of insufficient tension are among others. bent cage. weak springs in cage. gunked up chain/derailleur pulleys.

Last edited by hueyhoolihan; 12-16-11 at 11:23 AM.
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Old 12-16-11, 12:15 PM
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Originally Posted by hueyhoolihan
just so you know...

it's possible to run an 8 speed derailleur 12,000 miles on paved surfaces with no problems involving derailing off the front chainring. this involved a 130mm spaced rear wheel with an XTR rear derailleur.

if i were spontaneously derailing the chain, i would suspect insufficient tension on the chain or possibly a poor chainline favoring low or high gear end.

causes of insufficient tension are among others. bent cage. weak springs in cage. gunked up chain/derailleur pulleys.
As I recall (and I may be wrong) XTR rear derailleurs are/were designed with an extra heavy spring controlling the action of the jockey wheel cage. I don't remember if this was to give better shifting or eliminate chainslap, but it creates higher chain tension and makes it about the best rear derailleur for single chainring/multi cog setups.

To the OP - many older serailleurs (old suntour, simplex, campagnolo) are considerably nicer looking than the evil robots mounted on modern bikes, and they will work just as well as a chain keeper.
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Old 12-16-11, 10:28 PM
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I was thinking it might be chainline at one point, but I ruled that out because it seems to happen more in the middle of the cassette (5 to 7).

I should be able to safely shorten the chain a couple of links. Right now when I am in first gear, the rear mech isn't in any sort of extreme angle, so I should be good on that.

I am going to put my fd back on today and give that a go. I like the simple look without it, but I really don't like having to constantly be super careful when I shift, and having oily hands. Function trumps form this time around
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Old 12-17-11, 12:05 PM
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Rear-derailleur spring-tension has little effect on the top-run of the chain. Since you're upshifting, going from larger to smaller rear-cog, the chain is going from a higher tension to lower-tension state. This introduces some slack into it and can cause the oscillation you've observed that causes lateral movement and falling off the front. Try ADDING some tension to the chain when you upshift with a little extra pedal-pressure and see if that keeps the chain tight and prevents the extra movements.
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Old 12-18-11, 03:00 PM
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Very true, good point. I tried more pressure while shifting and that definitely helped, but it still came off on occasion. I just broke down and put my fd back on, and presto, no more issues. I am going to get a slightly more streamlined purpose built chain minder, and then I will be a happy camper.
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Old 12-18-11, 06:19 PM
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Lance Power Hi,
Just a quick thought on your Chain jump problem, try and put a thin Spacing Washer between the Chainring and the Five arms . This will move the Chainring out just a fraction and give the Chain a better line for the cost of 5 SPACE WASHERS.
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Old 12-19-11, 10:05 AM
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??? - Bottom bracket spacer...

https://store.velo-orange.com/index.p...t-spacers.html
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