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Originally Posted by nuggetsyl
Y lower the middle wheel. Makes no sence. 
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In order to ensure better turning, the following relationship is key.
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longitudinal coefficient of friction x width of wheel base > lateral coefficient of friction (length of wheel base - 4((distance from center of area to center of mass)^2/length of wheel base))
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Or simply put, there are a few ways to improve turning. Shorten the wheel base, widen the track width, reduce lateral friction while keeping longitudinal friction high, and moving the center of mass away from the center of the wheelbase.
By lowering the center wheel, you effectively create two 4WDs with a shared center wheel. Each of these has a wheelbase roughly 1/2 of that of the same drivetrain with coplanar wheels. Also, closer the center of mass of your robot is to the center wheel, further it is away from the center of area of both of the wheelbases. Additionally, the traction and width factors should be the same as if you had a coplanar 6WD, and you still achieve many of the other benefits of a 6WD.
(Thanks to Chris Hibner for
this terrific white paper on the physics of turning with a 4WD).