How can I calculate the elasicity in a rubber rod.

See:

http://en.wikipedia.org/wiki/Torsion_(mechanics)

You can determine the material's shear modulus by clamping one end and applying a torque at the other, and seeing how miuch the rod twists. The basic equation is:



where:
= Shear modulus, which is basically the torsional modulus you are looking for,
= torque applied at one end of the rod,
= length of the rod,
= the torsion constant for the rod - for a round or tubular rod this is the same as its polar moment of inertia, and
= angle of deflection, in radians.