Chain tension woes

[fjpshaw]

Took my new build out for a maiden ride yesterday. On rough speedy sections, the chain was flapping about all over the shop, and slipped down to the granny ring at least twice. You're probably thinking 'chain too long', but it's not. Set up to length using the standard 'big ring to big ring' method (avoiding rear derry), and right enough if I shift into low gear on the outer chainring, the rear mech is flat out. So chain really can't go any shorter.

Thoughts? Am I missing something obvious or is this a job for a chain guide?

 

[Bullpup]

Have you adjusted the rear mech tensioner - i.e. the screw that pushes on the little lip on the mech hanger?

 

[mattr]

You mean B tension screw.

Also, if you are using the middle ring and the middle/top end of the block, 34/16 ish for example, you might not have enough chain tension. All is not lost, that gear is duplicated on the big ring, use that instead.

Might also be worth looking at your front mech height.

 

[fjpshaw]

Re:

Have you adjusted the rear mech tensioner - i.e. the screw that pushes on the little lip on the mech hanger?

The B-screw? Hmm, I had a fiddle with it, but I'm not sure it was doing much. I think the screw was pushing past the lip rather than on to it, but I'll look again. Does it make a noticeable change? Cheers for the suggestion.

 

[fjpshaw]

You mean B tension screw.

Also, if you are using the middle ring and the middle/top end of the block, 34/16 ish for example, you might not have enough chain tension. All is not lost, that gear is duplicated on the big ring, use that instead.

Might also be worth looking at your front mech height.

Yeah, I'm not convinced the front mech is quite right either :facepalm: ! The High limit screw is fully out and even then the chain only just makes it on to the outer ring. I'm thinking the bb needs looking at to try and bring the chainset in towards the frame by a couple mm. But even then I'm sure all is well alignment wise (112 axle for a M952 chainset?).

 

[jimo746]

what rear mech is it? and is it still "springy" enough? if the spring that puts tension on the chain becomes tired by age etc then it could let the lower run of chain flap about on rough terrain, enough to let the chain hop off the chainring perhaps.

Its unlikely, but after you've tried the easier options try swapping over the mech if you have another that would work, just to see if that is the problem.

 

[02gf74]

How many chainrings you got?

Are you shifted in smallest sptocket and smallest chainring when it drops off?

 

[fjpshaw]

what rear mech is it? and is it still "springy" enough? if the spring that puts tension on the chain becomes tired by age etc then it could let the lower run of chain flap about on rough terrain, enough to let the chain hop off the chainring perhaps.

Its unlikely, but after you've tried the easier options try swapping over the mech if you have another that would work, just to see if that is the problem.

Its M953, and the spring feels nice and tight. I might swap out another mech though if I get no joy in the next round of tinkering.

How many chainrings you got?

Are you shifted in smallest sptocket and smallest chainring when it drops off?

Triple up front. Twice I noticed it was when I was in the outer ring. Don't get me wrong, the chain isn't coming off altogether...just dropping into the middle or granny. Not the end of the world, but not right, and very annoying!

Off work today though and it's meant to shell it down later, so will visit the man cave later. I'm going to have a closer look at the B-screw... :?

 

[JamesM]

Re:

Is the cage long enough?

Also the b spring/mounting bolt can get all bunged up over the years and become tight, it then won't rotate freely so won't spring back all the way and take up all the tension.

 

[02gf74]

3 ring front = long cage rear mech.