[sanddrag]

Well, say you have a motor with 20 ft-lbs of torque and spins at 400 rpm. (just an example by no means a realistic figure) It has a 20 tooth sprocket on it. Your wheel has an 80 tooth sprocket on it that is linked by chain to the one on the motor. You now have a reduction of 4:1. The torque is increased four times and the speed is reduced by a factor of four. The wheel has 80 ft-lbs of torque about its axis and is moving at 100 rpm. Still with me?

For the force, it depends on distance, which in this case is the radius of the wheel. Given perfect traction, a smaller radius wheel will place more force on the ground than a large radius wheel. If there is 80 ft-lbs of torque about the axis of the wheel, and the wheel is 2 feet in radius, 40 lbs of force will be applied by that wheel to the ground.

Now, if you have the same setup on the other side of your robot, you will now have two motors, two reductions, and two wheels. Together, they will provide 80 lbs of force. This means that if the robot had the traction to be driving straight up a wall, it could have a total weight of up to 80 lbs ad still be able to maintain movement in the upward direction.