Motor that can move possibly up to 4kg

[CJNight15]

I'm designing a robot to compete against a friend who is also making a robot based on the Robot Sumo competitions. We decided on the dimensions of our bots and placed a 2kg limit on their weight. I'm having trouble determining what motor to use that will be able to move my own ~2kg robot around and also push my friends ~2kg robot.

I'm unsure how to figure out the torque required to push (up to) 4kg. There's a stall torque at a particular voltage and then the gear ratio which affects the torque at the wheel and then the radius of the wheels. I've read these from various different sources and am just having trouble piecing it all together.

Any help would be appreciated. Thank you!

 

[Spinnor]

Suppose you have a motor with a stalled torque of 1inchpound. That means if the motor shaft had attached a pulley two inches in diameter and you wrapped a string around the pulley the string could lift up to a pound in weight (you don't want to overload the motor, this is just for an estimate). If you now attached a pulley that was a 1/4 of an inch in diameter to your motor and again wrapped a string around the pulley the string would now lift up to 8 pounds or about 4 kilograms. The motor pulley radius times the lifting weight is a constant determined by the motor.

This means that if you use 2 inch wheels you will need a gearbox that will increase the torque of your motor about 8 times or 8 turns on the input shaft = 1 turn on the output shaft to get about 4 kilograms of force using the motor above. You only have several numbers to work with, motor torque, wheel size, pushing force, and gear ratios . You can only play around with the numbers a few ways.

Don't worry too much about the math, you will pick that up. Hopefully if I made any gross errors they will be pointed out.