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So building this new bike up running 1 x 11,xtr mech & shifter,m8000 11 speed cassette 11-46,how long does the chain need to be, cos the rear mech is not taking up the slack,any ideas guys?
Depends on the bike surely. I've always gone with the "chain around big cog and chainring but not through mech plus a full link" estimation. I think you add two full links for FS bikes.
it's a Hard tail
RD-M8000 tech doc
Big-Big+2 links, same as this;
Surely by bypassing the rear mech using the 2 big ones it will then be too short,or am I missing something,its not a fixie ?.
When you are in the big cog the derailleur cage is almost in line with the lower chain line as seen on the above diagram and there's very little "extra" length to consider. As you go up the gears the mech swings backward taking up the slack.
The two extra links on top of that length account for the extra chain length you need for the derailleur.
Better add 4 links (assuming the rear derrailleur capacity allows you to do so), just in case you decide to go with a larger chainring.
When the derailler is fully stretched out, the chain runs straight through without wrapping around the jockey wheels. You definitely need two links or so slack to make sure it can shift properly, but you don't need extra for the derailler itself. Basically, if you have a problem out on the road and have to remove two links to repair the chain, you need enough that it will still wrap around the two biggest sprockets.
Surely by bypassing the rear mech using the 2 big ones it will then be too short,or am I missing something,its not a fixie ?.
Thats why you add the 2 links, as specified by Shimano.
You can have the chain longer if you want to change chainrings etc, but it will not shift quite as well (upper jockey wheel is offset from the pivot point), also noiser/more chain slap with less tension.
I set them up as per Shimano's instructions unless I have a good reason not to.
http://www.machinehead-software.co.uk/bike/chain_length/chainlengthcalc.html
Used it recently for new chain for the Wazoo...
Fixed chainstay length
445mm chainstays
38T biggest chainring
34T biggest sprocket
Recommended chain length 55"
LOL at the chainlength calculator. It takes about 1 minute to wrap the chain around the two biggest sprockets and cycle the suspension to find the correct length. That way, you know it will be the correct length. Pissing around measuring stuff and running it through an online calculator will take longer and you still need to check it the low-tech way to make sure that it's ok.
Thx guys