Wiring two objects together - convertion of rotation for one object to waveform

[stevesideas]

I am creating a cycle rig. The cyclist feet are attached to the pedal dummies. The wheels rotation and pedals are attached to a slider which drive the entire bike down a path

The bike goes at different speeds along the path and so the pedals change speed. I was hoping to create some fake effort sub animation for the cyclist (spine moving backward and forward) by wiring the rotation of the pedals to the spine but with a converstion to a sine waveform so that the cyclist bends back and forward in time with the pedal movement.

So basically I need to know how to convert a wired rotation into a sine waveform that drives the other object. Any ideas ?