GS derailleur and chain length.

ben@31

I've seen ways of calculating chain length

Simple Equation: L = 2 (C) + (F/4 + R/4 + 1)
L = Chain length in inches. Round the final result to closest whole inch figure.
C = Chain stay length in inches, measure to closest 1/8". Use chart above to find decimal measurement.
F= Number of teeth on largest front chainring.
R= Number of teeth on largest rear cog.


However if you have a medium sized GS derailleur rather than the usual smaller SS derailleur, do you need to add any more links to the chain to compensate for the longer derailleur cage? Thanks.

arthur_scrimshaw

ben@31 wrote:

I've seen ways of calculating chain length

Simple Equation: L = 2 (C) + (F/4 + R/4 + 1)
L = Chain length in inches. Round the final result to closest whole inch figure.
C = Chain stay length in inches, measure to closest 1/8". Use chart above to find decimal measurement.
F= Number of teeth on largest front chainring.
R= Number of teeth on largest rear cog.


However if you have a medium sized GS derailleur rather than the usual smaller SS derailleur, do you need to add any more links to the chain to compensate for the longer derailleur cage? Thanks.

That's a bit complicated, I use largest chainring/largest sprocket as below.

http://www.parktool.com/blog/repair-hel ... -section-3

And no, you don't need to add any more links.

arlowood

Or just use this online calculator (which may or may not be based around your equation above

http://www.machinehead-software.co.uk/b ... hcalc.html

To answer your other question I would say - No - you don't need to incease your chain length when using a GS rear derailleur. The reason you would fit a GS rear mech would be to accommodate larger rear sprockets - say 32t and above. In those situations it's the largest rear sprocket size combined with largest chairing size that determines the optimum chain length rather than the fact that you have a GS rear mech.

ben@31

Thanks for the advice. Maybe I was confusing myself with the lower jockey wheel being further away, but would this make any difference if the cage pivots?
Arlowwod you're correct I have a GS derailleur as I do use bigger 30t or 32t cassettes. I think 28t is the limit for the short SS derailleur.
Trying to get it indexed now, think the cassette could be worn or somethings bent?

arthur_scrimshaw

ben@31 wrote:

Thanks for the advice. Maybe I was confusing myself with the lower jockey wheel being further away, but would this make any difference if the cage pivots?
Arlowwod you're correct I have a GS derailleur as I do use bigger 30t or 32t cassettes. I think 28t is the limit for the short SS derailleur.
Trying to get it indexed now, think the cassette could be worn or somethings bent?

Or cables, start with the most likely cause - Occam's Razor.

ben@31

I understand what you mean by the cables but mines a Di2 groupset. That's why in the other post I mentioned replacing just the rear dérailleurs cage rather than the entire dérailleur. I think I can buy a mechanical Ultegra dérailleur for fraction of the price of a di2 and just swap the cage and jockey wheels over.
Also the indexing is at the limit of di2 adjustment (16 out of 16 settings) hold a button down and the shifters will cause the rear dérailleur to move one millimetre in or out to index, hold the button again to get back to normal shifting.

Edit... The shifting could have been out for a while, I only noticed it wouldn't go into the bottom smallest gear when I had the bike on the bikestand after I fitted a new chain and watching it. Then when I tried to fix this, I found It wouldnt re-index it properly .