Is there a formula for calculation chain length from capacity?

Quoted message said:

Or more precisely, for caculating chain length difference from capacity
difference.

I am planning on getting a smaller big ring and a bigger granny ring
and I want to know how much shorter I can make my chain.

The chain length is basically a function of the chainstay length and the
sum of the sprocket sizes.

The rough formula would be 1/4 the sum of the sprocket teeth, plus 2 x
the chainstay length. This gives the chain length in complete links
(inches.)

This assumes the top and bottom run of chain are reasonably parallel.
In practice, the angular difference is usually small enough to be ignored.

You could get fussy with trigonometry, but this isn't necessary in practice.

For derailer gearing, the relevant sprockets are the largest ones front
and rear.

The same chain length that works with a 52/12 would work with a 42/22.

For actually determining the chain length, it's easier to just do a
trial fit, rather than try to calculate it.

See: http://sheldonbrown.com/derailer-adjustment

Sheldon "Chains" Brown
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| explanation. --H.H.Munro ("Saki"😉(1870-1916) |
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Phone 617-244-9772 FAX 617-244-1041
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Quoted message said:

Quoted message said:

For actually determining the chain length, it's easier to just do a

trial fit, rather than try to calculate it. <<

but I wanted to know before i bought the new chainrings whether I would
be getting the correct sizes.

the specifics: I have a 53 big ring and a 24 granny (on a triple) with
a 12-27 cassette. i started with the minimum chain length that can
accomodate 53x27. this chain length is too long and is slack when i'm
on 24x27. so I eventually removed 4 links (2 inches) to make it so that
I can use my 24 granny with the 4 largest cogs without slack. This
consequently made the chain too short for the big-big. I can only use
up to 19 (or maybe 21? i have to check again) with the 53. not a big
deal except there's the danger that i may inadvertently shift to a too
big cog-chainring combo. since 53 is too big anyway, i'm thinking of
getting a smaller big ring. and except for special occassions 24 is too
small anyway so i'm thinking of getting a larger granny. i want to know
if switching to a 50 big ring and a 26 granny is enough to make the
chain fit the new big-big (50x27) -- i may have to add a couple of
links -- AND still let me use the granny with the 4 largest cogs
without slack. I have other options for example going to 49 on the big
ring or going up to 27 or 28 on the granny.

btw, this is only an issue because I'm using a short cage rear
derailleur which i want to keep.

maybe i should just ask: what is the biggest *actual* capacity that can
be accomodated (fit big-big and no-slack on smallx19) by a short cage
campy rear derailleur?

Quoted message said:

btw, this is only an issue because I'm using a short cage rear
derailleur which i want to keep.

Yeah, that's your problem. Get a real rear derailer and you should have
much less chain length issues.

Jasper

Wouldn't the size of the rear mach's cage a;so play a factor in this?

Just curious...

- -

"May you have the winds at your back,
And a really low gear for the hills!"

Chris Zacho ~ "Your Friendly Neighborhood Wheelman"

Chris'Z Corner
http://www.geocities.com/czcorner

earth... said:

Maybe i should just ask: what is the biggest *actual* capacity that can
be accomodated (fit big-big and no-slack on smallx19) by a short cage
campy rear derailleur?

Seems like you've already determined the effective capacity since the
smallest combo that works currently is 24 x 19 and the largest that
works is 53 x 19 (maybe 21). So that gives you a capacity of 29 teeth
(maybe 31). You'd like to go from 26 x 19 to 50 x 27, so you need a
capacity of 32 teeth. So if the 19 is really as big as you can go now
while on the 53 then you don't have enough, but if the 21 is ok and
looks like the chain still has some extra length (i.e. it's not pulling
the der. cage out as far as possible) then it may work, but without
much margin.

I agree with Jasper that the proper solution is to leave the rings
alone and get a proper derailleur for your wide-range gearing.

A trial run is rather easy to perform. Just shift the bike onto the
big front ring and big rear ring. Then look at the derailleur. Chain
length should be adjusted so that the jockey pulley cage - bolt pair -
lines up at a direction of 315 degree or less, roughly.

Then try the small-small ring and see if the jockey pulley cage can
hack it. If the jockey pulley cage bolts line up at 225 degrees or
more, you can probably add a link with impunity.

The problem with a formula is that chainstay length and angle is not
constant and can vary by up to 3 inches across many road bikes and so
there can be no formula for chain links, unless it incorporates both
chainstay length and bottom bracket height - more trouble measuring
than its worth.

- Don Gillies
San Diego, CA

(Donald Gillies) said:

The problem with a formula is that chainstay length and angle is not
constant and can vary by up to 3 inches across many road bikes and so
there can be no formula for chain links, unless it incorporates both
chainstay length and bottom bracket height - more trouble measuring
than its worth.

Uhm, chainstay angle is irrelevant. Unless you measure the chainstay
length as a 'virtual horizontal chainstay'.

Jasper

peter said:

Seems like you've already determined the effective capacity since the
smallest combo that works currently is 24 x 19 and the largest that
works is 53 x 19 (maybe 21). So that gives you a capacity of 29 teeth
(maybe 31). You'd like to go from 26 x 19 to 50 x 27, so you need a
capacity of 32 teeth. So if the 19 is really as big as you can go now
while on the 53 then you don't have enough, but if the 21 is ok and
looks like the chain still has some extra length (i.e. it's not pulling
the der. cage out as far as possible) then it may work, but without
much margin.

you're absolutely right! i didn't realize i have already measured the
actual capacity! dang! thanks for pointing this out.

so i can actually go with 50x27 big-big and 28x19 small-small = 30
capacity which is 1 over the 29 actual. I think this is ok since 53x19
right now is not yet maxed out stretched. i'm not really planning to
use the big-big combo anyway but just wanted a safety margin that if i
accidentally shift there it won't destroy my derailleur.

Quoted message said:

I agree with Jasper that the proper solution is to leave the rings
alone and get a proper derailleur for your wide-range gearing.

yes that's an option too. but a rear derailleur costs almost 3x the
chainrings, and like i said except in the couple times a year i ride a
super-hilly event i don't really need a super-wide gearing.