Chain Length question
 

I know I can plug the numbers into an on line calculator but I like to know the formulas.
So it appears to me half of the largest front ring tooth count + half of the largest rear sprocket + two times the crank to rear axle length in inchs gives you the min chain length. I assume the angle distance of the chain between front to back doesnt really amount to enough to worry about. How much do you add to that for the bends around the derailleur wheels and to keep the derailer from being pulled up parallel to the chain in the large front and back combination?

One more question, look at Park tools site, Repair and help section under Chain length sizing
In their example they say the chain for a 32 rear , 42 front with 16.375" axle to crank should be 52", I am thinking more like 69. Is it wrong?

For normal bikes forget about measuring. Just wrap the chain around the big ring and big cog without going through the derailleur and cut the chain one full link (inner and outer half-links) past the first point where the two ends could be joined.

Or put it in small/small and set the chain so it just clears the rear derailleur and doesn't rub. Either way works but that's how my boss does it and i find it easier than big/big

1slowride, There are so many combinations of derailleur drivetrains/chainstay lengths that the only practical method is to use either the big-big or little-little method. Both have their strong advocates and while for me the little-little method is my personal favorite, the big-big may prevent sucking the RD into the spokes with a particular combo.

Brad

+1 for big-big plus one link*

*that is only for hard-tail bikes. If you have a full suspension bike you have to un-hook the rear shock and cycle through the travel as some bikes, the Chainstay measurement will "grow" as it cycles through its travel.

For a new build I thread the chain carefully and check the big to big and the small to small combinations making sure that the rear derailleur does to reach its forward or backward limits (does not "bottom out").

When replacing a that I know is right I use the same number of links as the old chain.

I use the simplified formula on the Park Tool website. Big-Big does not work for me because I normally ride 11-25 and keep a 13-29 arround for special occaisons. I don't use little-little for the same reasons Sheldon doesn't (didn't, RIP).

Big-big is the more conservative approach, particularly if you set it using the largest cog you will ever use on that chain. Small-small assumes you have a rear derailleur capable of wraping up the entire tooth count and if you do, it does mean big-big will work too.

The problem with small-small is if you have a total-tooth count setup greater than your rear derailleur can handle. For example, I routinely substitute a 26T granny for the OEM 30T granny on road triple cranksets and that gives me a total tooth count greater than the rd will handle. I set the chain at big-big and realize it will go slack in small-small but that not damaging and not a problem. Not being able to handle bib-big can do a lot of damage if an inadvertant shift to that combination is tried.

Indeed. I recently performed this exercise on a full-suspension frame I've been riding all summer. Turns out my chain was one link short. I probably got away with that all summer because I avoid big-big when riding, and if I do fall into that combination, it's usually inadvertent during a climb when I'm unlikely to hit a big enough bump hard enough to bottom the suspension. Next time I put a chain on a suspension bike, I will be sure to cycle the suspension afterwards. (It's a bummer that I can't cycle the suspension *before* I cut the chain).

You have got to be kidding! The idea of a formula strikes me as rather much ado about nothing.
Shimano recommends that the rear pulleys be verticle when in the large small combo. I just make sure that I can shift to the large large combo without tearing off the derailuer.

As the responses here have illustrated, there are several valid ways to size a chain assuming the rear derailleur is compatible with the gearing. If it is, any of them will work. If it isn't (as in my case) big-big+1" assures you won't break anything.