Not directly Arduino related but hoping to pick the brains of you electric motor experts on this a bit.
For a school project that I'm teaching we're going to build electric cars - soapboxes is a better word I think. Two 775 type motors rated 35W geared down to 200 rpm are to propel a car made out of a 2x4' (62x124 cm) piece of plywood with one student on top. Each motor drives one of the rear wheels; two smaller wheels on the front that take care of the steering, and a simple PWM power controller for the speed. The wheels are directly driven by the motors.
The question I have is related to the effect of wheel size on total speed. I have built a prototype with wheels of some 25 cm diameter, and with my 90-something kg on top it reaches a firm walking speed, about 6 km/h. I'm quite happy with that. Fast enough to be fun, slow enough to be safe (no brakes).
Now (unfortunately no chance yet to do testing) I'd like to know what to expect from changing the wheel size, and why. My guesses:
Smaller wheels: more torque to the ground, giving faster acceleration. At full speed the motors will be running at higher rpm, enough to compensate for the smaller wheels? Higher rpm means lower current through the motors and less overall power.
Larger wheels: less torque to the ground, slower acceleration. However at full speed motors are running a bit slower, more current, more overall power, resulting in a higher top speed.
Too small wheels: motors run easily at (near) full 200 rpm but the small wheels mean low speed.
Too large wheels: motors don't have enough torque to get the vehicle in motion (something I experienced with my wheels already on a not so flat surface, and getting stuck behind a small bump, needed a little push to get started).
There must be a sweet spot somewhere in there, maximum acceleration (for a drag race across a basketball court) and maximum speed (for a race of a few laps around this court).