[Craig Roys]

Pushing force is basically your static friction force between your wheels and the ground. The max static friction force is equal to your [Normal Force (the weight of the robot on a level surface)]x[coefficient of static friction].

The max weight of a robot including bumpers and battery is around 150 lbs.

Assuming a coefficient of static friction between the wheels and carpet of ~1.5, you can create about 225 lbs of pushing force before your wheels start to slip. Once they start to slip, that number will drop.

Keep in mind that you also can only draw 40 amps per motor before you start popping breakers. The trick is to get your gearing such that your wheels will begin to slip just before you get to 40 amps to avoid popping your breakers. Once the wheels slip, the resistance force goes down and your current draw should decrease.

That's an over simplified look at it, but it's a place to start. There are many other factors such as type of drive-train, number of wheels, type of tread, weight distribution of robot, etc. All can affect your pushing power.