How long to cut the chain

[mnichols]

Building a bike from scratch, mixture of new and old bits, but the new bits are chainset, rear mech, cassette and chain

Normally when I'm replacing a chain I compare it to the old one, but as this is a new configuration I don't have a guide

So, how do I know how long to cut the chain?

[richardfm]

Have a look a this https://www.youtube.com/watch?v=O0YibMDWBAw It should help

[PH]

Wrap chain round large chainring through front derailleur and largest sprocket with rear derailleur shifted out of the way, find the rivet you could join the chain at, then cut two rivets longer than that. This is the method from Park Tools

https://www.youtube.com/watch?v=O0YibMDWBAw

EDIT - richardfm beat me to it, even the same link...

[Brucey]

run the chain big-big, measure the direct chain length necessary, and add at least two links for the derailleur.

If the total capacity of the derailleur is insufficient for the combination you are running, you can choose to have slack running in small-small or not to be able to use big-big. The former is the correct choice IMHO.

cheers

[richardfm]

Isn't it good when we all agree?

[Mike Sales]

Isn't it good when we all agree?

Boring. And unproductive of new ideas.

[richardfm]

Isn't it good when we all agree?

Boring. And unproductive of new ideas.

Do you have a different method of sizing a chain?

[Mike Sales]

Isn't it good when we all agree?

Boring. And unproductive of new ideas.

Do you have a different method of sizing a chain?

I thought you were generalising about the benefits of all agreeing.
But yes I do.

[richardfm]

Boring. And unproductive of new ideas.

Do you have a different method of sizing a chain?

I thought you were generalising about the benefits of all agreeing.
But yes I do.

I had the thread about Britain First and the Tories in mind and thinking that this is a nice contrast.
Are you going to share your different method?

[Mike Sales]

Do you have a different method of sizing a chain?

I thought you were generalising about the benefits of all agreeing.
But yes I do.

I had the thread about Britain First and the Tories in mind and thinking that this is a nice contrast.
Are you going to share your different method?

Unnecessary. And there is no handy link to refer to.
I would think that the participants in these fractious threads get something out of their "discussions". Those who don't like them can easily avoid them.

[mattsccm]

About 10 seconds if not rushing.

[andrew_s]

Do you have a different method of sizing a chain?

The alternative chain sizing method, as recommended in the leaflets that Mr Shimano gives you with a new mech (and maybe chain - I buy SRAM), is to put the chain in the largest chainring and smallest sprocket, and then set the chain length so the two jockey wheels are vertically aligned.
However, even Mr Shimano now recommends the large/large method if your cassette goes larger than 28T.

For info:

The large/large + a pair of links method, as described by Brucey, PH, and richardfm above, is the "safe" mathod.
It will always give a chain length long enough that you can change into large/large.

Large/large crosschaining is to be avoided if possible, but should you accidentally attempt to change into large/large when your chain isn't long enough, you could break something and have to phone for help.
With down tube or bar end levers, you can tell what gear you are in from the lever position, but with brifters you can't, and if it's after dark and you can't just glance down, it's quite easy to lose track of what gear you are in and change down once too often.

The downside of large/large chain sizing is that, if you exceed the capacity of the rear mech, the chain will run slack when crosschained small/small (also to be avoided). This however is merely an annoyance - it's noisy, and you are more likely to drop the chain. You hear the noise, say "oops", change to the middle of the cassette and to a larger chainring, and then adjust to suit your speed.

[horizon]

What is actually determining the chain length? Since you know the length of chain (1/2" x number of links), all you need to know would be the largest sprocket, largest chain ring and some standard sizes for the derailleur. So a chain length table should be possible - quick and easy.

So what is the sticking point in this? Is it the length of chain stay? Or another variable?

[scottg]

Put the chain round the big cog, thru the derailer and around the big chain ring.
Make sure you have enough chain not to rip off the derailer, bottom pulley
should be at the 4 o'clock position more or less.

[Brucey]

What is actually determining the chain length? Since you know the length of chain (1/2" x number of links), all you need to know would be the largest sprocket, largest chain ring and some standard sizes for the derailleur. So a chain length table should be possible - quick and easy.

So what is the sticking point in this? Is it the length of chain stay? Or another variable?

you can calculate it easily enough

http://www.machinehead-software.co.uk/bike/chain_length/chainlengthcalc.html

and the calculation for derailleur gears even allows for the extra 1" or 2" of chain (which they call 'one link' or 'two links') you need for the RD. However it does rely on an accurate measurement for the chainstay length. Note also that for non-derailleur gears, it doesn't believe that half-links exist.

Having fitted very many chains, I can say with some confidence that I'm more likely to mis-count links than I am to break the chain in the wrong spot having offered it up, so offering it up is the better method for me.

cheers