Why no friction unit?!

[Jan Oelbrandt]

Or am I missing it from your build files?

Friction of gears is not anywhere close to keeping the cable from overcoming it and servo has to maintain a constant current draw just to keep it in place! Small wonder you only get a few hours of runtime... 

No that's right, the servo is pulling directly on the spring-returned cable. But I think the ratio of power consumption versus action radius is satisfying enough (5AA 2200mAh batteries lasting +10 hours. The two other AA's are just there to prevent the Arduino from powering-off due to voltage drop below 5V - I know this can be done better, see this thread)
But I agree, to add a friction unit would be a good idea to release tension on the servo so that it would consume less current.

have you thought about torque-sensing BB integration? It will be better than just cadence!

Yes I have. But while I developed Shift4Me, I always tried to make the design as simple and easy-to-install (and modify settings) as I could. That's the reason I never went for (amongst other options) torque sensing. But feel free to implement it!

btw: nice wheels you got there!