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!