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Originally Posted by Evan Austin
Here is another question:
I know that some teams with 4wd robots will use 2 standard wheels on one end and use 2 omni wheels on the other end to enable them to steer easily. I wonder if anybody is using 6wd has done something similar with the 2 standard center wheels and omni wheels on all 4 corners? I know of one team which used a 2wd center drive and had small omni wheels on all corners, and they were very maneuverable and stable. However, they were not powering these wheels on the corners. If this would work, you would not have to lower the center wheel, thus keeping a flat footprint.
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This could work, however, you might as well power
all of the wheels in contact with the ground. You're wasting normal force if you don't (recall that static friction is a function of normal force, and that the pushing ability of a robot is based on the friction of the driving wheels on the playing surface).
If you go this route, also be aware that your robot will rotate more easily when it experiences a sideways force; like John pointed out, that can be a good thing or a bad thing, depending on your driving style, the needs of the autonomous mode, and how often you expect to be pushed from the side. Also, consider that 3 points on a surface define a plane; 4 is overconstrained, 6 is very overconstrained. Consider how you will provide for keeping many drive wheels in contact with the ground at any given time—if you don't, you're back to the problem of wasting normal force. (A little frame flexibility, and/or the shape of the omniwheels will often conceal this in a static state, but when turning, beware of the moments acting on the frame, which can twist it, and perhaps raise a corner off of the ground.)