Shimano 12sp chain length

Cardy George

Piercing rural members since 1981
I'm running a 34t front for around home, but I have a 28t for going to ridiculous length away races.

Do I need to shorten the chain suit? I'm going to use waxed chains, therefore I have multiple chains, so I do have options. I can shorten one and keep it and the 28t as race only, but that means the chain won't wear with the cassette and could cause problems. Or can I leave it and as long as it doesn't rub the derailleur cage everything is rosy? Or do I take a link out of all of them and aim for the middle compromise?

Ive read the shimano sizing instructions but how much difference does the length actually make?
 

DMan

shawly the least hangeriest guy on rotorburn
For every two teeth dropped on the chainring one full link should be removed. So in your case, the difference is 3 full links.
What about oval? I've just gone from round to oval and almost reckon I could take a link out?
 

ozzybmx

call me Cáitín
Yep yep, I know they should be, the question is do they have to be?
If your chain is the correct length for 34t (I would absolutely assume it would be ;))

Dropping 6 teeth will have the derailleur at full 'spring back' and you will have a loose chain in the smallest 10t cog and possibly 12t.
 
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ozzybmx

call me Cáitín
I use this diagram every time I replace a chain... because people break them, remove the broken link and 'bushman' a pin in so measuring against the old one doesn't work most of the time in this house.

You could run the chain 2 links shorter on your 34t ring but this might cause shifting issues dropping off the 51t cog.

12spd is the most temperamental groupset ever :rolleyes:

387723
 

Cardy George

Piercing rural members since 1981
If your chain is the correct length for 34t (I would absolutely assume it would be ;))

Dropping 6 teeth will have the derailleur at full 'spring back' and you will have a lose chain in the smallest 10t cog and possibly 12t.
I use this diagram every time I replace a chain... because people break them, remove the broken link and 'bushman' a pin in so measuring against the old one doesn't work most of the time in this house.

You could run the chain 2 links shorter on your 34t ring but this might cause shifting issues dropping off the 51t cog.

12spd is the most temperamental groupset ever :rolleyes:

View attachment 387723
Gotcha. And that's the diagram I was planning to go by.

Reckon there will be an XT cassette heading this way next month and I'll keep the entire XTR driveline for big trips. Gives me the option to use a squirt lube mid-race too.

Does any one have an answer for @DMan
 

DMan

shawly the least hangeriest guy on rotorburn
Gotcha. And that's the diagram I was planning to go by.

Reckon there will be an XT cassette heading this way next month and I'll keep the entire XTR driveline for big trips. Gives me the option to use a squirt lube mid-race too.

Does any one have an answer for @DMan
I use that diagram too @Cardy George. I just changed from Shimano to Sram due to now running a Sram cassette. I cut the chain at the same length as the XTR, which was nearly new but it certainly looks like I could drop a link now with the oval.
 

ozzybmx

call me Cáitín
Gotcha. And that's the diagram I was planning to go by.

Reckon there will be an XT cassette heading this way next month and I'll keep the entire XTR driveline for big trips. Gives me the option to use a squirt lube mid-race too.

Does any one have an answer for @DMan

Just an idea, If you were to go 10-46t 12spd XT cassette with a 34t for local duties where you didnt need the big 51t, you might get away with running the 28t and 10-51t for events and keep the chains the same length. It would mean changing cassette and chainring :rolleyes:

Also adjust the b-screw and line up with the derailleur gap adjustment mark, I find it really helps to keep your shifting crisp. This cant be achieved with a chain thats too long or too short, you will run out of screw adjustment.

387724
 

DMan

shawly the least hangeriest guy on rotorburn
Just an idea, If you were to go 10-46t 12spd XT cassette with a 34t for local duties where you didnt need the big 51t, you might get away with running the 28t and 10-51t for events and keep the chains the same length. It would mean changing cassette and chainring :rolleyes:

Also adjust the b-screw and line up with the derailleur gap adjustment mark, I find it really helps to keep your shifting crisp. This cant be achieved with a chain thats too long or too short, you will run out of screw adjustment.

View attachment 387724
Is the idea to keep the big ring teeth about 1cm away from the mark?
 

ozzybmx

call me Cáitín
Is the idea to keep the big ring teeth about 1cm away from the mark?
I line the teeth up on the mark.

There's is a bit of conflicting info online but I believe the teeth are meant to 'touch' the line when adjusted correctly... works for me anyway :)

EDIT: Pic

387726
 
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Cardy George

Piercing rural members since 1981
Just an idea, If you were to go 10-46t 12spd XT cassette with a 34t for local duties where you didnt need the big 51t, you might get away with running the 28t and 10-51t for events and keep the chains the same length. It would mean changing cassette and chainring :rolleyes:
That's a good idea, the 45t had already flashed through my brain but I hadn't considered it might counter the shorter chain.

Reckon it comes with a cup of concrete? There's a couple of pinches around here that I have been using 1st and 2nd get up.

But then if I don't have them I can't use them!
 

beeb

Dr. Beebenson, PhD HA, ST, Offset (hons)
@beeb. Really, it was a legitimate question this time!
Haha, no. The picture was mega-confusing given the context of his post. The idea is to align the guide mark with the very tips of the teeth. They generally shift pretty poorly if this is wrong (in my limited experience).
 
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