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Chain keeps coming off big ring over bumpy ground...driving me mad...Help!!


By Chitts - Posted on 04 February 2011

NB: Originally posted elsewhere on the Global Riders Network and appears via syndication.

Hi,

I ride a Scott Spark and whenever the going gets bumpy....downhill at the dam....section down to the hydro lab etc, my chain comes off the big ring in the front. I have checked the length based on the guidance in Zen an d the art of MTB maintenance and am happy that it is correct (one link overlap big cog to big cog without derailleur).

What else can it be?? Any ideas would be appreciated!

Could be a combination of rear derailleur tension and the suspension design. certain designs cause pedal feedback, though that's not a direct cause but only a contributor to the problem.

But especially if you have a "long cage" derailleur, when it comes down to your smaller cogs the derailluer tension starts getting sloppy over rough terrain, hence jumping your big ring. Derailleur cage springs tend to be very lasting, so i dont think you can put it down to wear/tear - which means replacing that spring wouldnt be a solution. May have to get a shorter cage - but still have to make sure the shorter cage can still accomodate the full range of cogs/chainrings that you have.

You could tune your front derailleur - use your high/low limit screws to tune it a bit further outwards so it doesnt skip down too often?

Yeah, if it's going over the chainring to the outside, then bring the top limiter in so that the chain just misses the derailiuer cage in the large chainring/smallest rear sprocket.

.....I would guess that 80% of the time it comes off towards the smaller cogs.

Check the height of your Front Mech, the outer arm should only be 2-3mm above the teeth on big ring.

Hrmm, Maybe try winding it out a little then.

What drive train is it? How old is it?

Are you in the big ring and around the middle of the cassette when this happens?

Have you tried running in the middle ring instead of the big ring and running down the bottom of the cassette? This might make the chainline want to fall towards the bigger chainring, but since it can't get onto it, it might keep it on the middle.

But if you are up in the middle of the cassette with the big ring, this might be making the chain want to pull to the middle ring as the tension unloads due to the way the chain is angled. I don't know if this is what's happening, but it's something to think about.

Drivetrain is in good condition but the front mech may be the issue. I had the Rotor Q-rings on for about 9 months and then moved back to XT. Maybe with the elliptical rings the front mech was raised for greater clearance and is now too high.

Will try and see how it goes.

Thanks!!

Hey mate I have the same set up and bike and I haven't had any issues at all. 5 things to consider : the alignment of the der, how high it is positioned, the max opening screw, cable tension (less likely) and if this does not work then the front mech could be bent.

Good luck

Cheers

.... while they can't be sure, Brad said he did a couple of things which will hopefully help. The spring tensioner on the rear derailleur cage was on the weakest position (did not know there were options), the derailleur was slightly bent, and he also said that with time (2.5 years and a LOT of riding), the sides of the ront derailleur move out slightly so he squeezed them back in again. Holding thumbs that all works ok, but after an exhaustingly hot 3 hours yesterday, I am boycotting the bike until the weather cols down a bit.

Thanks to everyone for the help and suggestions!! Smiling

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