Re: 2 motor drive VS 4 motor drive
 

There's a simple-to-use motor calculator here which handles different voltages easily.

You can even enter a desired operating point (speed and torque) and it will tell you the voltage.

Re: 2 motor drive VS 4 motor drive
 

Thanks to James for pointing out that I hadn't actually uploaded the version of the tool that generated those screenshots. It's up as "Version 3.0" now.

http://www.chiefdelphi.com/media/papers/2469

Re: 2 motor drive VS 4 motor drive
 

it all depends upon the application and drivetrain. we use mechanum drive, so we use 4 independent motors and gearboxes. we have used mechanum every year but '09, and we honestly have no problem w/ pushing power or inclines. we also have not had any chatter problems, although, in 2010, crossing the bumps took it's toll on the bot. we even managed to drive surprisingly well on 3 motors when a gearbox stripped out a pinion.
if your not gonna be pushing a whole lot, 2 motors will work fine. we use 2 motor RWD tank drive on our t-shirt cannon that is probably 50% heavier than a competition robot, and it works well.

Re: 2 motor drive VS 4 motor drive
 

For students interested in the math, attached are the equations of motion¹ and plots² of distance, velocity, and acceleration vs time, for the 2 and the 4 CIM vehicles, at 73% and 100% drivetrain efficiency, assuming:

- assume constant 12V supply to the motors (not a good assumption, agreed)

- "speed loss constant" 100% (e.g. vehicle top speed corresponds to motor free speed. may not be a good assumption)

- motive force varies linearly with vehicle speed

- wheels are not traction limited

With the given assumptions, the equations of motion can be solved analytically to obtain a closed-form solution (no need for numerical integration).

¹ http://maxima.sourceforge.net/

² units converted to feet for plotting