blinky

I have a favorites folder dedicated to cycling info and one of my favorite entries is a BF post with video on threading the cable to its' anchor on the Shimano FD-8000 - but for chain length I rely on Keith who has way more know how and experience than I - a few months back I felt that my shifting could be smoother and played around with cable tension , etc. but still fell short - dropped the bike off with Keith and when I picked it up there were 2 chain links attached by twist tie to the aero bar - should ask Keith what method he uses but if I were to pick a method I think it would be the one favored by Sheldon Brown .

cyccommute 

I use this method often. If all you are changing is the chain, this is the best way to go. It doesn’t work if you change gearing, however.

If I don’t have the chain…a frequent occurrence when referbing bikes…or if I’ve changed gearing, this method is the way to go. It’s quick and easy to use and, perhaps more importantly, easy to remember. Nomenclature and miss understandings can cloud the water a bit but if you have a picture (see above), it’s relatively easy to understand.

This can work within certain ranges. However, there are pitfalls with this method. For example, I have used triples with 48, 46, and 44 outer rings with a 20 tooth inner ring on some of my triples which are mated to a 36 tool low gear on the cassette. Small/small with just enough tension so that the chain isn’t rubbing on itself will result in chain that is too short, especially for ring larger than 44 teeth. In the case of the 48 tooth ring, the chain can be too short in the second gear on the cassette.

One caveat: The large/large method with the 46 and 48 tooth can result in a slack chain for the small/small combination.

It can work but can result in slack chains in the small/small combination or in too short a chain if you have the rear derailer at the wrong angle. You also have to hold the chain in place, hold tension on the derailer, gauge the angle of the derailer, and then mark the chain where you need to cut it. If you were an octopus it would easier.

Frankly, I didn’t even know there was a mathematical formula until just recently. I’ve never counted links in my life. It’s just not worth the effort to calculating the chain length when a simple, reliable method is available.


The only caveat that I can find is for 1x systems. I don’t have experience with those but from what I can find it looks like the chain is sized using 4 rivets (2”) instead of one. SRAM’s chain instruction show this but they say nothing about cassette size. For a triple using a large rear cog, I would start with the big/big method and either shorten the chain while avoiding using the big/big combination or just live with a slack small/small combination and avoid using it. Neither combination should be used anyway.

By the way, SRAM’s rear derailer manual provides the same advice as their chain manual does for sizing the chain.

cyccommute 

Okay, perhaps I was wrong. But the illustrations in that 24 year old manual is kind of simple and not terribly informative. It doesn’t say that the Dura Ace needs a different chain length sizing method from other derailers. Here’s what Shimano says now and, yes, it’s in their derailer dealer’s manual. Oh, look, it’s the big/big with added links. The method suggested in a 24 year old manual would likely result in the same length as the big/big + 2 rivet method.

Okay. Shimano doesn’t address chain length in their chain manual. But SRAM does. So does KMC. That covers a very large part of the market. And, again, Shimano’s method uses the same large/large + links method. They use 4 rivets but that’s just details and doesn’t change the basic method.

mitchmellow62

Cyccommute: I looked up the manual for the XTR M972 and shimano recommended the big-big plus 2 back then: https://si.shimano.com/en/pdfs/si/5W...0B-001-ENG.pdf . But that was for mountain bikes. Shimano still recommends the big-small with derailleur cage vertical for newer road derailleurs up to certain max cog sizes (27 for double; 30 for triple): https://si.shimano.com/en/pdfs/dm/RD...003-09-ENG.pdf .

mitchmellow62

I should have added: Leave it to Shimano to add a little complexity.

theapodan

Here are the criteria I start with:
A. The chain must be long enough for big/big with rear derailleur capacity to spare.
B. The chain should be short enough to maintain acceptable slop in small/small.

There may be multiple workable chain lengths that meet the two criteria above, depending on your combination of frame, rear derailleur, chainrings, and cogs. A longer chain weighs more, insignificantly extends drivetrain lifespan, and may enable swapping in a larger cog or chainring. A shorter chain weighs less, insignificantly reduces drivetrain lifespan, and may enable swapping in a smaller cog or chainring.

There are drivetrain configurations where the shortest possible chain for criteria A above will still be too long for criteria B. Avoid small/small selections in that case or revisit drivetrain component selection.

Last week I installed a new 12-36 cassette and new chain on my size 57 Jamis aurora touring bike with 26-36-48 crankset and mountain long cage derailleur. I installed the chain full length routed though the derailleur normally, carefully confirmed it worked in big/big, then checked for acceptable slop in small/small. It met both criteria so I left it uncut. 114 links.

Last time I sized a chain I used the big/big+2rivets park/sheldon method, and maybe if I was a bike mechanic in a hurry that's what I'd do, but since I have the time to screw around, I like my iterative approach.