# correct chain length going from 50/34 11-25 to 52/36 11-27?



## veloci1 (Nov 29, 2005)

is there a formula or accurate way of knowing how much longer a chain needs to be when going from 50/34 11-25 to 52/36 11-27?

i think i have the perfect length now and i would like to keep the same tension i currently have on my new semi compact set up.

any help will be appreciated.

regards,


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## tom_h (May 6, 2008)

I would just re-determine length from scratch, in the small chainring/small cog combination.
Make chain as long as possible, such that chain does not rub on the derailleur cage.

Keep in mind that nearly all rear derailleurs have an adjustment for spring tension on the cage. Generally you want sufficient tension for top pulley to be as close as practical to the largest cog, without touching or rubbing.


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## JCavilia (Sep 12, 2005)

If by "perfect" now you mean it's at the minimum length to shift safely into the big-big combo, then you'll need the chain to be one link longer. But most likely you have a little extra slack now, so you wouldn't need to change it at all.



> i would like to keep the same tension i currently have


This is sort of a meaningless statement, since the actual tension on the chain is determined by the derailleur springs and varies widely depending on the gear combination you're in. Proper chain length doesn't affect tension, except to insure that the derailleur can take up all the slack in the small-small, and not be over-extended in the big-big. 

Fixed-gear and single-speed bikes are another matter.


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## cxwrench (Nov 9, 2004)

veloci1 said:


> is there a formula or accurate way of knowing how much longer a chain needs to be when going from 50/34 11-25 to 52/36 11-27?
> 
> i think i have the perfect length now and i would like to keep the same tension i currently have on my new semi compact set up.
> 
> ...


tension? think again...you can't change it, it is what it is, controlled by the pivot spring tension.


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## Kerry Irons (Feb 25, 2002)

veloci1 said:


> is there a formula or accurate way of knowing how much longer a chain needs to be when going from 50/34 11-25 to 52/36 11-27?
> 
> i think i have the perfect length now and i would like to keep the same tension i currently have on my new semi compact set up. any help will be appreciated.


You are "adding" 4 teeth to the big-big combination. Since only half the teeth are engaged, that means you would need to add one full link (one inch, one outer half link and one inner half link) if your current chain is at the limit. A better way to size chains is using the small-small method (derailleur should just take up slack when on the small chain ring/small cog).


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## wim (Feb 28, 2005)

veloci1 said:


> is there a formula


There is, but I don't think anyone will actually use it:

CL = 0.157a + 1/2 T1 + 1/2 T2 + 2

CL = chain length in links (1 link = 1 pin)
a = chainstay length in mm (center of BB to center of rear dropout)
T1 = number of teeth of biggest chain ring
T2 = number of teeth of biggest cog

Example:
chainstay length a = 420 mm, T1 = 52 teeth, T2 = 27 teeth
CL = 0.157 x 420 + 52/2 + 27/2 + 2
CL = 107.44. Round off to 108 links.

Important: always round off so that the final number is divisible by 2 so you can close the chain.


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## oldandtired (Aug 2, 2011)

> There is, but I don't think anyone will actually use it:
> 
> CL = 0.157a + 1/2 T1 + 1/2 T2 + 2
> 
> ...


Yep, if all else fails try math


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## looigi (Nov 24, 2010)

It's even simpler given a chain and ring/cog configuration that works. Add "links" equal to the sum of the largest ring and largest cog change divided by two. (you divide by two because the chain only goes halfway around the ring and cog). 

So going from 50/25 to 52/27 its (52-50+27-25)/2= 2 "links" or 1" longer. 

FWIW, +/- 1" is the smallest change you can make in chain length.


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## veloci1 (Nov 29, 2005)

Thank you for all the replies. i figured it out and the chain slack (not tension as i incorrecly called it ) is perfect.

i should have been more specific about what i meant by tension.

i do not like the chain too lose becasue it creates a situation where i could drop it if i go over rough terrain or roads. in other words, i try to keep it as short as i can can without affecting the rear der.

again, thanks to everyone for the feedback.


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## cxwrench (Nov 9, 2004)

veloci1 said:


> Thank you for all the replies. i figured it out and the chain slack (not tension as i incorrecly called it ) is perfect.
> 
> i should have been more specific about what i meant by tension.
> 
> ...


you're still not correct. if the chain is 'correctly sized' which could be within a range of 1 full link, usually, it is no more likely to 'drop' or bounce off the cogs or chainrings if it's longer. the amount of spring tension on the pulley cage is the same as long as there is tension on the chain. 
by 'correctly sized' i mean that there is no slack in small/small and it is possible to go big/big. the longest chain possible will run the smoothest and minimize the risk of problems if you install a wheel w/ a larger cassette than you normally run. 
let me repeat..._there is NO increased risk of your chain dropping if it's sized like this_. as long as there is at least some tension on the chain in small/small, that amount of tension won't increase if the chain is 1-2 links shorter.


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## veloci1 (Nov 29, 2005)

thank you for the feedback.


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