Rear Derailleur Capacity.

     Any derailleur will work back there, right? Wrong... I have lost count how many times someone has come into the store after swapping out or, upgrading a rear mech., only to find that it now does not work properly. When I ask if the derailleur capacity matches their setup, I usually am greeted with a vacant stare.
      So, before yanking off chains and clipping cables let us take a moment to work out what you need to put back there.
      Before we do the math, (yes, there is always math), it might be helpful to clarify exactly what a rear derailleur does. I imagine most people can see how it operates to change gear, you make a gear change at the shifter and the cable that connects to the derailleur pulls it side to side and derails the chain from one cog to another, simple. The other thing it does, that is equally important but possibly not as readily understood, is to take up the slack in the chain and keep it tensioned correctly.
       Why the hell is there any slack in the chain you ask? Because each gear combination requires a specific chain length to wrap around the selected cog and chain-ring. When you select the big ring at the front and the big cog at the rear, the chain wrap is at its longest. Conversely, when on the small ring and small cog it is at its shortest. The difference between these two extremes is the amount that the rear derailleur has to be able to handle. The Chain Wrap Capacity.
        
The math;

(No. of teeth on big cog - no. of teeth on small cog) + (No. of teeth on big ring - no. of teeth on small ring) =

For example;
cassette 12-25 and crank-set 53-39
25 - 12 = 13
53 - 39 = 14
add them together 13 + 14 = 27.
So 27 is your chain wrap requirement that the derailleur needs to be able to handle.

      The way that the derailleurs are made to handle larger capacities is primarily by increasing the distance between the jockey wheels. This is often referred to as cage length and you will see terms like, short cage and long cage used to describe different models. However do not just assume a mid cage will handle what you have, always refer to the actual chain wrap capacity figure that is published with every model and make out there to be sure

       Happy shifting..