Chain jumping from big ring to small

Michael-B

Hi all,

I thought if someone had experienced an identical issue they might be able to help...

I have SRAM Force 10-speed and when I'm in the big ring the chain will occasionally jump without warning to the little ring as I'm changing gear on the rear mech. It tends to happen when I'm towards the top end of the cassette, often when I'm on 7th or 8th cog. It even happened today when I went over a rough stretch of road and wasn't changing gear, but that's the first time it's happened in that scenario.

It's now been into three different workshops and none have diagnosed the problem, although I've had quite a bit of work done including a full service to try and identify it:

- New freehub (there was play in the body so needed doing)
- New BB (again, there was play so it needed changing but hasn't solved the issue)
- In the time since this problem started I've been through multiple chains
- Tightened bolts on chain set in case they were loose (they weren't)
- Changed cassette (thought it might be a worn Shimano cassette causing the issue but it's happened with the newer SRAM one too)

It's driving me mad, to the point where I think I might have to split the bike or change the entire groupset which I can't really afford and otherwise I like the bike.

Help!

keef66

I don't understand how the chain is dropping to the small chainring if the FD cage is over the big ring? Does it not then jam everything?

How is front shifting otherwise?

What bike? Sounds like a chainline issue to me.

i.bhamra

I have exactly the same issue with a 10 speed campag veloce set up on one particular bike. Have just put it down to bad chain line (frame specific) when in the big ring and towards the lower end of the cassette.

The cage on the front mech is quite wide and allows room for the chain to drop back. Can almost tune it out by tweaking the high/low limits on the mech but it can still happen occasionally and if you set the high limit far enough to avoid it you can get overshifting of the outside instead...

It's annoying but I'm just living with it for now as it's on bike I don't use much this time of year. Generally, avoiding the lowest two or three sprockets when in the big ring means it doesn't happen.

figbat

Is the FD too high relative to the large chainring?
Is your chain too long?
Is the RD spring tension too slack?

svetty

Sounds like rear mech isn't keeping chain tension high enough (or chain is too long?). Pivot bolts gundged up??

i.bhamra

Chain tension is fine in my case. OP has had his bike checked by multiple LBS by the sounds of it so you'd have hoped one of them would have spotted if that was the issue!

Michael-B

Thanks for all the suggestions, I’ll pass some of those on to the LBS if they’re at a loss. It definitely isn’t chain tension as it’s been changed a few times by me and the LBS. My suspicion is a mix of wide front mech, chain line and frame size as it’s a 59cm frame. It still shouldn’t happen but there you go.

PS - it’s not jamming when it jumps as there’s still enough room for it to spin on the little ring, albeit while rattling the cage. I’m even tempted to swap SRAM front mech out as a last throw of the dice... or it’s a new bike.