Physics of robot base vs. width

[Vikesrock]

The idea behind the rocking 6wd is to shorten the wheelbase while maintaining the same stability as the longer wheelbase.

To turn a "skid steer" system the wheels have to slip. Imagine a robot with 4 wheels on the corners, a level 6wd will behave pretty similar to this system. The force trying to turn the robot is applied at a distance equal to 1/2 the robot width. The friction force preventing the robot from turning is applied at a distance equal to 1/2 the robot width.

If the length is longer than the width, and the wheels have similar coefficients of friction in the two different directions, the torque about the center of rotation being applied by the wheels trying to spin is not enough to overcome the torque being applied by the friction of the wheels being pushed sideways.

[IKE]

Chris Hibner wrote a great paper on 4wd turning.
http://www.chiefdelphi.com/media/papers/1443

I would recommend reading through it. Basically, a 6wd drop center is essentially 2 4wds (as others have stated).

An interesting phenomenon can occur with 6wd with a reasonably flexible chassis. Even with stiff rails, if the two sides are not torsionally stiff relative to each other, then diagonal outer wheels can end up touching at the same time. This will cause it to act like a longer wheel base which can reduce maneuverability.