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Originally Posted by dlavery
Uhhhmmmmm, not so fast there. As a general statement, this is not true.
It all depends on how you are going to define a "typical six wheel drive" and what assumptions and caveats you are putting on that definition. I will maintain that in a "typical" six-wheel drive, all six wheels are co-planar. In that case, the wheelbase of the six-wheel drive is established by the Conservative Support Polygon, which is defined by the four outer wheels. As specified in the original statement, the wheels are arranged along the long axis of the robot. The four outer wheels determine the resulting wheelbase, which by definition is longer than a four-wheel drive arranged along the short axis of the robot.
This fundamental truism is only modified if the definition of a "typical" six-wheel drive is altered to promote non-standard configurations. For example, the common practice of moving the middle pair of wheels into a non-planar configuration. On a hard planar surface, this causes the robot to ride on the middle set of wheels and one of the "end" set of wheels. Only four wheels support the robot, not six. Thus, the wheelbase becomes approximately half of the long dimension of the robot. But this is actually no longer a true six-wheel mobility system. It is a set of two four-wheel systems that share a common pair of wheels (the center ones). In this case, you lose many of the advantages of a six-wheel drive (ie. distribution of weight across more points of contact with the floor, greater stability across the longer wheelbase, etc.).
-dave
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Yes, this is entirely true -- and what I get for posting at 3 AM or some ungodly hour like that. In FIRST, I consider a 'typical' 6WD arrangement to include a lowered center wheel, but I can see that in another application, this definition would be incorrect. Trucks or airplanes, for example, may ride on six wheels rather four to, as you mention, better distribute their weight across a surface. I meant to speak only with the confines of FIRST use -- and even then, perhaps it's erroneous to assume that most consider the 'typical' 6WD arrangement to have a lowered center wheel.