How do you calculate the correct chain length

ridgeracer

Is there any accurate way of calculating the correct chain length for a given situation without measuring the old one.
I have just fitted a chain(first diy time) and just cut the new chain down slightly shorter than the old one - making an assumption that old one had stretched (checked with a chain gauge). It's running ok but when on the big front ring(53) and big back ring (25) there is no slack at all.


In other words is there an accurate calculation one can do using cassette/chain ring details?


Thanks.......

sungod

no

it isn't just the rings that affect the length, it's the rear mech and the distance front-rear etc.

the manufacturer's instructions for setting up the rear mech should include how to get the correct chain length, for shimano look on http://techdocs.shimano.com

otherwise see...

http://sheldonbrown.com/derailer-adjustment.html#chain

btw normally you wouldn't use the big front+big rear due to the chain angle, but does sound like it might be a bit tight

hopper1

I posed the same question recently.
I did a basic search and found it again here

nicklouse

around the two big cogs missing the rear mech and add a full link.

ridgeracer

Thanks everybody - very helpful!

andrew_s

As short as possible whilst still being able to change onto big/big.
Nicklouse's method is the normal way of judging it.

If you can't change onto big/big, you run the risk of breaking something if you select it one dark night when you've lost track of what gear you are in.

John.T

andrew_s wrote:

As short as possible whilst still being able to change onto big/big.

I disagree. I prefer as long as possible without the chain catching the rear mech cage when on small/small. It gives better chain wrap and IMO a smoother change in all gears. I also hate to see a mech looking 'stretched'.
This usually is about the same as the Shimano recomended jockey wheels vertical when on big ring and small sprocket.

redddraggon

Little-Little - set so that rear mech just about takes up the slack.

Gives the longest chain.

andrew_s

If you size on small/small with no rub, I recommend that you refrain from exceeding Shimano's rear mech capacity.

HellsCyclist

nicklouse wrote:

around the two big cogs missing the rear mech and add a full link.

http://www.parktool.com/blog/repair-hel ... gth-sizing

In the "Big cog to Big cog method" it suggests 2 links. Does this really make much difference? namely for shimano?

drlodge

John.T wrote:

andrew_s wrote:

As short as possible whilst still being able to change onto big/big.

I disagree. I prefer as long as possible without the chain catching the rear mech cage when on small/small. It gives better chain wrap and IMO a smoother change in all gears. I also hate to see a mech looking 'stretched'.
This usually is about the same as the Shimano recomended jockey wheels vertical when on big ring and small sprocket.

Me too. I do the chain as long as possible in small-small - if its then not long enough for big-big you will need to change your rear derailleur! Since I have the max gearing for my RD (34-50 with 12-29), I need the chain as long as possible anyway.

bianchirider

Built another bike this year and finally got to grips with gears and chain length. Simplest way i've heard and used is put the chain on the big ring at the front and the small ring at the rear and pull the chain until the arm of the rear mech is vertical. Measure the links and join it there.

drlodge

bianchirider wrote:

Built another bike this year and finally got to grips with gears and chain length. Simplest way i've heard and used is put the chain on the big ring at the front and the small ring at the rear and pull the chain until the arm of the rear mech is vertical. Measure the links and join it there.

IMO that's a rubbish way of doing it, since you don't know if the chain is too short or too long.

StillGoing

Big big and missing the rear derailleur and add 2 links including the power link if you use them.