Back when I ran ZD's (ZD21 and the ZD326), I would run the front tires at max pressure. They seemed to pivot better. In the rears, I would run low pressure, if I remember right, around 10psi. I know they looked kind of like a radial tire looks like on a car. Like you should add some air to them. This seem to give me the best traction. Of course you could load the rear tires with fluid for max traction and a lower center of gravity on hills.