Big ring over-shifting problem: flummoxed!

by flobble   November 29, 2011  

Completely bamboozled by this one, so calling on the collective wisdom of the road.cc community.

I recently replaced the chain on my road bike, and now when I shift from small ring to big ring, it jumps off the outside edge of the chainring. Every time.

Things I've tried without success:

1. Adjusting the big ring limit screw so the mech doesn't move so far.

2. Moving the whole mech closer to the chainring so there isn't enough room for chain to pass between mech and chainring. Just falls off the chainring in front of the mech instead.

3. Changing to the big ring slowly.

Most infuriatingly...

4. Putting the old chain back on (not too worn, about 0.75mm stretched) Still happens!!!

The only thing I can think of is that the big ring has worn out and needs replacing, but I'd be very surprised if this would cause such a sudden change in behaviour; I'd expect the overshifting to happen a little at first, then increasingly often, but it happens everytime now!

9 user comments

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I would check that the limiter screw is doing its job first. It may have become cross-threaded or generally gunked up if you've been riding in poor weather. I assume you haven't repositioned the front mech? You need max 2mm between the cage and the top of the chainset teeth.

Have you removed the chainset or BB recently?

arrieredupeleton

posted by arrieredupeleton [585 posts]
29th November 2011 - 16:39

2 Likes

Limiter screw works, yes.
And mech is positioned as close to the teeth as I can get it without fouling. Will double check though.

The problem appeared when I changed the chain - no mech movements involved.

posted by flobble [62 posts]
29th November 2011 - 17:06

1 Like

I may be thinking back to front here, always get mixed up on this one!! what about the tension in the cable??

giff77's picture

posted by giff77 [1117 posts]
29th November 2011 - 17:14

2 Likes

Cable tension is fine as far as I can tell. Sits on the limiter screw with cable just taut when on the small ring, so not over-tight or over-loose.

posted by flobble [62 posts]
29th November 2011 - 18:01

1 Like

I would suggest that it is that your outer chainring is worn, and the chain doesn't seat properly. I've just had the same thing happen to me, and it just suddenly started happening and wouldn't stop.

I've moved the limit screw in which helps a bit, the thing I find helps most is to do the opposite of what you'd expect: keep the power on as you shift, don't ease off. This keeps tension in the chain and helps it to seat on the teeth. Works nine times out of ten for me. Sooner or later I'm going to have to replace the chainring though.

what's the chainset, out of interest?

Dave Atkinson's picture

posted by Dave Atkinson [7743 posts]
29th November 2011 - 22:34

1 Like

Interesting advice from Dave and I'm sure its good for the short-term but continued use wont be good news for your new chain. As this wears it will also accelerate wear on the cassette. I think you'll have to bite the bullet if this is the case. If you've another bike, it would be worth swapping the chainset over to double check that is the issue.

arrieredupeleton

posted by arrieredupeleton [585 posts]
30th November 2011 - 10:44

2 Likes

To change properly the front mech needs to be precisely parallel to the chain ring...Could you have moved it slightly when swapping the chain? Worth checking

posted by SideBurn [906 posts]
30th November 2011 - 13:19

1 Like

arrieredupeleton wrote:
Interesting advice from Dave and I'm sure its good for the short-term but continued use wont be good news for your new chain.

true dat. best to sort it sooner rather than later (note to self: sort it sooner rather than later)

Dave Atkinson's picture

posted by Dave Atkinson [7743 posts]
30th November 2011 - 13:23

2 Likes

also, it's possible for the outer plate of the mech to get bent or bowed a bit so check that it's straight and if not, apply some oomph to wangle it back in line

joemmo's picture

posted by joemmo [1034 posts]
1st December 2011 - 16:14

1 Like