4-wheel drive trouble

Originally Posted by jskene

With 4-wheel, tank-drive, as you are using, your robot must be wider than it is long, or you will have trouble turning. Make sure your robot is 36" wide and 30" long, not 30" wide and 36" long. This will make a huge difference in the ability of your robot to turn.

Actually, its the distances between wheels that matters (technically, it's the distance between the contact points of the wheels that matters). Measure the distance between two wheels one side of the robot and call that y and measure the distance between two wheels on opposite sides of the robot, but both in the front (or back), and call that distance x. If y > x, your turning efficiency is drastically reduced. If x > y, you should be good.

To understand why, let's look at some physics. Let's say your pivot point is the center location between all 4 wheels. Then, the distance to each wheel is called r. To find the torque used to turn the wheel you take the cross product between Force and Distance (both vectors). Look at the atttached drawing. The force that the ground exerts on the wheel (friction) is always parallel to the wheel. Draw a line from the wheel's contact point to the center. When the angle between force and this line is brought closer to 90 degrees, you can turn better. (A cross product between vectors A and B is equal to A*B*sin(d) where d is the angle between the two vectors. So when d is closest to 90 degrees, sin (d) is closest to 1, so the cross product has its largest value.)

Moral: Move the left set of wheels farther from the right set of wheels. And make the wheels on the left closer to each other, and the wheels on the right closer to each other. Or don't use 4-wheel drive. Our team made this "mistake" the first two years - we could hardly turn or maneuver. This year we went with track drives and they turn on a dime...