4 wheel drive vs. 2 wheel drive

[Bongle]

Originally Posted by Betty_Krocker

Edit: All wheel drive might help but might be wasting power, motors, and battery life...
(i have done some tests and this is what i get, but its up to you)

All wheel drive absolutely helps.

The physics reason for all wheel drive is this:
1) Let's say your robot has 4 wheels
2) Each of these wheels supports some fraction f_i of your robot's weight. The sum of all F_i = 1.
3) Each wheel will thus have a normal force of m_Robot*f_i*g = 9.81*m_Robot*f_i
4) Each wheel will thus be able to generate (if powered) a frictional force of
F_f = u*F_n
F_f = 0.06 * (9.81*m_Robot*f_i)
F_f = 0.5886*m_Robot*f_i
5) Your robot's total potential frictional force is the sum of the frictional forces of all of its driven wheels.
6) For an all wheel drive robot, F_fRobot = 0.5886*m_Robot (because F_i sums to 1)
7) For a 2WD robot with its weight evenly distributed, we know all the F_is are 0.25 (due to equal weight), but only two of them are driven. So F_fRobot = 0.2943*m_Robot, or half of the AWD's robot full force.

So based on 5), 6), and 7), we know that by choosing not to drive some of your wheels, you are throwing away potential friction. Wheels that are touching the floor while the robot is driving and are not powered are completely wasting potential friction. A robot with its weight evenly distributed over 4 wheels that only drives two wheels will be two times slower than an identical all wheel drive robot. The two wheel robot could potentially employ a very complicated suspension so that its weight shifts onto its driven wheels when acceleration or decelerating, but it would be a waste of time when you could just drive all four wheels.