I'm toying with the idea of building an electrically assisted bicycle and am thinking of using a Sturmey Archer wheel to give 3 speeds.

The idea is if I set up 2nd gear for my cruising speed first will improve get away from standstill and 3rd will give a little boost when I'm feeling super energetic.

Just to make it interesting I'd like the gearchange to be automatic but how to do it?

I guess i could use a pic and determine gear change up and down revs of the rear wheel for each gear but how do I actuate the gearchange mechanism?

As I recall the spring in the gearchange is quite strong would a stepper motor be strong enough? Can stepper motors be geared down easily to give more torque?

Cheers!

Just pedal harder!

Could try to get a stepper motor with a lead screw type shaft (linear stepper motor)
Like these

Linky

You may be able to drive a piece of threaded studding and use the moving nut to push/pull the gear box linkage.

Hope that make sense.

Excellent thanks that's just what I need

Cheers!