chain length guide

[Blodwyn Pig]

In readiness for the probable collection on N+1, in Feb,  {a bent},  how does one assess the correct chain length. I'm 6'2", but the seller is 5'8",  and the chain has been shortened. Now, obviously the boom will need to be extended, although I 'Think' I may prefer a slightly bent knee, rather than a fully stretched knee,  but how to calculate how many extra links. Chain has only done about 2k (miles) ish, so extra links with an extra quick link is correct...oui?...rather than a new chain (s).  But how many, is there a rule of thumb,ie middle sprocket, middle cog, rear mech hanging straight down?   is it guess work, ?  Now assuming 6" difference in height between us,  do you halve this and add 3" of chain?

I'm thinking too much I know, but that's just me.

[Tigerbiten]

It's roughly double the difference in height.

On a bent you're normally running towards an extreme gear range.
A 22-26 tooth difference at the front and similar at the back for a total of close to 50 teeth is normal.
So I'd first set it up so big-big works, as this is a must, plus a couple of spare links .
Then change it to small-small and see how badly the rear derailleur wrap around is.
If the wrap around is to bad and the chain rubs on itself then back to big-big and see if I can remove a few links.
But depending on your exact setup, it may not be possible to run small-small without the rear derailleur being pushed past it limits.

Luck ..........

[Kim]

As with any other bike, the correct chain length is the amount that doesn't jam in big:big.  It's a recumbent, so you're likely to use big:big deliberately now and then, rather than just by accident.

But if you're still at the boom length fettling stage, a little bit of slack is probably prudent.

(Note that you need to adjust for your riding position after you've been going steadily for half an hour or so, not where you sit when you first get on the bike.  Most people settle in the seat a bit.)

[Phil W]

I've done this setup recently on my new recumbent.

Once I'd got the boom in the right position.  I put it in a big / big with the chain through the derailleur.  Then shortened it, so the derailleur bottom arm pulls forward at about 45 degrees from vertical.  After further riding and boom positioning that proved not short enough as derailleur would mesh with small cog if in small / small. So I took about another 6 links out.  I can now go from small / small to big / big. Note I have a long cage derailleur though, so it might not be possible with a lower capacity  derailleur. It's now three sections of chain joined by quick links.

[PhilO]

It's roughly double the difference in height.

Roughly exactly double the difference in boom position, Shirley?  Which will be somewhat less than the height difference between riders, being bottom-half of the body only.

But yes, trial and error is probably the only way to get there.

[Nightmare-1]

Set the boom first then add links as required on Big-Big as that would be putting on the most amount of chain links & it is easier to shorten it if needed.

Wrap the chain around big-big and add 2 links is the normal way, then thread it through the rear derailleur and connect with a Quick link.
That's how I do it and it seems to work out fine with a 55-42-30 front and 9-32 capreo rear on my ICE Qnt.

On a recumbent you will be using Big-Big normally because of the extra long chain it doesn't matter about crossing the gears.
It also makes setting up the front derailleur a lot easier as you don't get chain rub from crossed gears (i.e. big-big/small-small).