Think of the fork on a bicycle. The frame tube is canted forward at the bottom, and the rake of the fork continues this trend. If you were to remove a front wheel and the steering knuckle, and draw a line through the center line of the balljoint, it would be tilted to the rear at the top, and toward the front at the bottom. Now think of too much caster, and each wheel wants to be the dominant one. They start arguing, tugging back and forth. Or, not enough, and neither wheel is inclined to do anything, especially track straight. Caster is what makes the steering wheel come back when you've turned a corner. That's why the proper amount of caster is the best. Don't let the appearance of the pumpkin fool you into thinking that it's favoring any particular caster setting. The real thing to be concerned with is the angle of inclination of the ball joints/knuckles. And, I've had alignment guys flat out tell me that the caster can't be adjusted. That's when you show them the shims at the rear of the lower control arms, and the elongated mounting-bolt holes.
