[patrickrd]

Before you go ahead and design a multiple-motor drive system, think it through. Multiple motor systems do not achieve what most suspect.

The physics:
1) The amount of force that can be transferred to the ground is limited by the effective coefficient of friction of your wheels (or treads). Once this force is exceeded, your wheels are now slipping and provide a smaller, constant force with the ground.
2) The amount of force that a motor can transfer to the ground is a function of gear ratio and wheel size (and applied voltage). This force is limited by the voltage your battery can provide (~13V)

Combining points (1) and (2) yield my main point (3):
3) There is a physical upper bound on how much "pushing power" (by that I mean force your robot can apply at stall) your robot has! As long as you choose an appropriate gear ratio for your robot, and you have wheels or treads that maximize your traction, you CAN NOT improve your pushing power.

Other lessons to be learned:
4) For given motors, you can maximize your pushing power AND speed simultaneously by measuring your coefficient of friction. Once that number is obtained, you can gear your robot such that at stall, it provides exactly the amount of torque required to make the wheels just begin to slip. This ensures that you can push as hard as your robot is capable of, and at the same time, maximizes your robot velocity.
5) The only thing to be gained by adding multiple motors in a drive system is velocity. For example, if I put two drill motors in series instead of one, I can now cut my gear ratio in half, doubling the acceleration and maximum velocity, without sacrificing any pushing power. If geared properly, you CAN NOT improve pushing power by adding motors.

I have seen many teams over my the years add multiple motors only to see their wheels spinning non stop in a pushing war, while their opponent is moving them with ease. Multiple motors in series can be used to give you speed. But if pushing power is your objective, look elsewhere (i.e. get better traction with the ground). Once you do that, then you can recalculate your gear ratio, and if your robot moves too slow, only then consider multiple motors.

- Patrick

P.S. There are three assumptions I make when choosing the appropriate gear ratio for maximizing your pushing power. You can actually get more pushing power than my point #4 solution above (without changing the coefficient of friction). Can anybody figure out what my assumptions are, and how you might actually get better pushing power than my solution?