How many chain links longer for long cage 105 derailleur?

[Custom24]

Hi
My bike came with short cage 105 derailleur
Early on, while trying to fix an issue (noisy drivetrain), LBS replaced it for long cage one, because that's all they had in, I think.
But they didn't lengthen the chain. There was enough slack in the chain that they didn't need to, apparently. So I stuck with keeping the same chain lengths since. Large/large is just about OK, not that I use that combination, of course.

Now it's time for new cassette and new chains, and I am thinking I might try a longer chain, because surely they should have done this at the time?

So, how many links longer should I use?

Thanks
Mark

 

[Spoked Wheels]

When working out the length of the chain, deraiileurs aren't taken into account, hence you don't need to lengthen it.

When you add a larger cassette or chainring then yes.

 

[Custom24]

Did not know that. Thanks.

 

[slowmotion]

I just use this online chain length calculator.
http://www.machinehead-software.co.uk/bike/chain_length/chainlengthcalc.html
It's always been spot on. If you don't want to measure it, the bike manufacturer will have published the chainstay length somewhere in its technical literature about the frame geometry.

 

[Spoked Wheels]

Another way would be to wrap the chain around the biggest sprocket and the largest chainring + 1 link.

 

[Hacienda71]

I always thought you added two full links on the large to large wrap method.

 

[Ajax Bay]

Seek Sheldon's advice.

"The best technique for setting chain length is to thread the chain onto the large/large combination, without running it through the rear derailer (sic). Mesh the two ends on to the large chainwheel so that one complete link (one inch, -- one inner and one outer half-link) overlaps. In almost all cases, this will give the optimum length."
Pictures:
http://sheldonbrown.com/derailer-adjustment.html#chain
http://sheldonbrown.com/derailer-adjustment.html#chain

 

[Spoked Wheels]

I always thought you added two full links on the large to large wrap method.

No, just one full link is enough.

 

[Dogtrousers]

So ... (and this never occurred to me before) ... the conclusion is that the derailleur cage length does not have any effect on the Sheldon method. That's a lesson learned. Not all that much practical use personally though - I'm never going to be using a short cage mech due to my love of dinner plate sprockets.

I can never remember if it's two full or two half links. I always have to look it up.

 

[Ajax Bay]

No: long cage v short cage is an issue with the gearing capacity needs: smallest to largest chainring delta PLUS smallest to largest sprocket delta. If you think about it, if you're careless enough to go large-large the 1" extra two links chain will work but the RD will be stretched out so the chain is close to direct (not under much tension). But if you make the chain too long the cage won't wrap it up and the jockey will interfere with the smaller(st) sprockets. You can run a chain shorter than the method above requires if you 'promise' not to go large to large. This can allow you to run a chainwheels/cassette combination (eg needing capacity of 40) on a short cage RD but is a risk and not recommended (and there may also then be a 'b' screw issue).