[sanddrag]

to answer the second part of your first question, take a bathroom scale, put it up against a wall, and drive your robot into and keep the joysticks at full throttle for a coupel seconds so you can take a reading from the scale. Then divide the reading by the weight of the robot including battery. That is your COF.

To answer question 6, the advantage of smaller wheels is that you can often direct drive them saving you a couple more chains you have to tension. Also, they save weight and space and give you a longer wheelbase, which is a good thing if you have a wide robot but probably not such a good thing if you have a long robot.