Cutting chain to length

[Chris M]

Cheers jags, the gears run up and down fine but still the slack.

Peter, it's a 11-34T cassette worked fine before.

I'm tempted to take a couple of more links off the chain.

[Chris M]

I just removed another inch from the chain and it still hangs slack when on the smallest chainring and sprocket. I'm convinced it's a GS cage and I should be using an SGS cage. I've had this set up on my other bike for a couple of years with no problems but it got me thinking and I can't remember a time when i actually used the 'granny' ring so it's possible that it's always been like that and I never realised.

Luckily, Cyclesurgery are open tomorrow and they have the SGS in stock just down the road in Holborn.

[phopwood]

Chris,

I agree it looks like the RD you have simply does not have enough capacity for the setup you have. 

Looking at the tech doc
https://www.google.co.uk/url?sa=t&source=web&rct=j&ei=-MrDUveFF8yihgfEzYCgBA&url=http://www.bike-components.de/bedienungsanleitungen/2011/shimano/mountain-bike/deore_xt/rd-m772.2.pdf&cd=4&ved=0CDUQFjAD&usg=AFQjCNGeGgfbHAEBssTH8Gj4pjszK-829A&sig2=povxoQ1dYgXU0KQlYczNBA

The total capacity of the SG is 33 and when you are in 48 / 11 you need a capacity of 37 so I feel you need an SGS.

But that does not explain how it ever worked.

Peter.

[Chris M]

Thanks Peter

Looking at the back plate in the tech docs; the cut outs on the GS backplate match the ones on mine. I spent a bit more time last night trying to get it to work. By removing another inch from the chain and bottoming out the B tension screw, I can just about make it work although the shifting isn't nowhere near as good as it should be. I think I could have made it a bit smoother with time but at 2am I'd had enough. I'm popping out soon to get a new RD, same model but SGS, I'll take this one with me just to check the cages against eachother.
I read that the GS has a cage length of 83mm and the SGS a cage length of 100mm, I'm not sure where the actual cage is measured from though. Measuring mine from centre to centre of the jockey wheels gives 80mm.
Hopefully a new RD will sort the problem for me as I'm itching to get it set up and ride it.

Thanks for the help mate, much appreciated.

Cheers

Chris

[Chris M]

I've just returned from the shop and can confirm that I had the wrong RD.
I had my other bike converted by the LBS to all XT a couple of years ago and they fitted the GS rear derailleur alongside a  26/36/48 chainset and 11-34T cassette. I just assumed that everything would work together on my new bike.
It makes sense now why it used to take me ages to adjust the gears to run smooth whenever I carried out any work on it.
For comparison the photo below shows the new RD M-772 SGS on the left against the old one, M-772 GS.

Thanks for all the help everyone and sorry for all the questions.

Cheers

Chris

[phopwood]

Well done Chris you will be up and running in no time,  but the LBS were lucky to make the GS work.

Peter.

[jags]

good stuff your that bit wiser now 
glad you got sorted happy cycling.


jags