Chain length with a 6870 derailleur??

londoncommuter

I've just built up a bike with a Ultegra di2 groupset and sized the new chain using the tried and trusted method of looping it round the largest front (50) and rear cogs (25), bypassing the derailleur, and then adding one complete link (2 half links if you know what I mean).

It seems hopelessly short though when routed through the derailleur and in fact I can't get the largest three gears at all when on the big ring. Here's a picture when on the large chainring and smallest cassette cog.



I've either accidentally cut the chain in the wrong place or the above rule of thumb isn't right for some reason. Does it always hold or is that only for short cage derailleurs and the 6870 isn't one?

Bit lost really.

Hopefully I can split the chain, get a couple of connecting pins and add some links in without weakening the chain too much. How much should I be adding though?

wongataa

Download the Shimano tech docs for your kit. Follow the instructions there for setting the chain length.

ElectronShepherd

The "big to big + 2 links" certainly works for long cage derailleurs. That's what I use on an Ultegra triple (52/39/30 rings and 25-11 cassette) and a GS rear mech. To be honest, the most likely explanation is that the chain's been cut in the wrong place. If I was just shown the picture without the accompanying text, that would be my thought.

You could check without cutting the chain by removing the rear derailleur from the hanger, which will then allow a straight chain path, and then manually looping the chain over the largest cog to see if it fits.

londoncommuter

wongataa wrote:

Download the Shimano tech docs for your kit. Follow the instructions there for setting the chain length.

Shimano instructions are to use the big/big + 2 link (looks from the picture that they mean 2 "half links" so the same as I did) for cassettes of 28T and over.

For cassettes smaller than that they say use the big chainring and smallest cassette cog and then add enough chain so that the two jockey wheels are immediately above each other.

I've got a 25T so should have followed the latter but I'm just wondering why I'm so far off.

ElectronShepherd - good idea, at least that would rule that out. Mind you it's wrong now so I'll have to split the chain so I may as well do that rather than taking the mech off (not sure about shoving cable back down...).

pippi_langsamer-2

Your total chain length should be 339 inches (or 339 links)

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

Based on your Foils 405mm chainstay length and your chainring/ sprocket combo...

londoncommuter

Pippi Langsamer wrote:

Your total chain length should be 339 inches (or 339 links)

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

Based on your Foils 405mm chainstay length and your chainring/ sprocket combo...

Thank you so much, that looks a great site. Slightly confused though as when I drop in 405 and 50/25 it's showing 52 links.

pippi_langsamer-2

londoncommuter wrote:

Pippi Langsamer wrote:

Your total chain length should be 339 inches (or 339 links)

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

Based on your Foils 405mm chainstay length and your chainring/ sprocket combo...

Thank you so much, that looks a great site. Slightly confused though as when I drop in 405 and 50/25 it's showing 52 links.

Probably my typo then. I am nearing the end of the bottle of red

Go with what you get.

londoncommuter

Pippi Langsamer wrote:

londoncommuter wrote:

Pippi Langsamer wrote:

Your total chain length should be 339 inches (or 339 links)

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

Based on your Foils 405mm chainstay length and your chainring/ sprocket combo...

Thank you so much, that looks a great site. Slightly confused though as when I drop in 405 and 50/25 it's showing 52 links.

Probably my typo then. I am nearing the end of the bottle of red

Go with what you get.

Thanks again for posting the link. Just counted and I've only got 50 so am a couple short.

pippi_langsamer-2

Yep, I reckon an additional 2"/ links would have the two jockey wheels more or less in-line vertically.

Not sure about the best solution- either new chain, or buggering about with more quick links to splice the required links back in?

dj58

The Shimano tech doc shows that for an assembly with a max cassette sprocket of up to 27T, the chain is the correct length when in the big chain ring/small cassette sprocket, the RD guide and tension pulleys line up vertically forming a 90 degree angle to the ground baseline.

londoncommuter

Just to conclude this, looks like I just mistakenly cut the chain in the wrong place originally as adding the two links back in took it up to the big/big and one complete link which I'd intended. That agreed perfectly with the length calculator above so the "old rule of thumb" is right after all.