I'm building a 4WD robot, but I'm a little confused about how to optimize the robot for turning. I need to turn the robot 90 degrees, and I need the center of the robot to stay in the same spot as it turns. Anyway, were should I place the wheels? Should I put them far from each other, or as close a possible? The Robot will be 2ft wide and 3.5ft long.
Hope those motors are fairly powerful. Hope it won't be run on carpet (unless you don't care about your carpet long-term).
What this is called is "skid-steering" (actually differential steering) - meaning the wheels skid sideways as the vehicle turns:http://en.wikipedia.org/wiki/Skid-steer_loader
In fact, your dimensions seem really close (scaled) to that of many skid-steer loaders like depicted in that link above. Regardless, it really tears up the ground, and if used on hard surfaces (like concrete and such) that have no "give" - it can mess up bearings, transmission components, and tires in short order. Keep that in mind for your robot. If you plan on this being used on dirt or grassy surfaces only, all the better. But if being used on hard surfaces (especially with grippy tires and lots of weight), expect the components to fail quicker than normal.
The best orientation (as far as wheels are concerned - tracks are a whole 'nother matter) for the wheels are at the extreme corners of a square base (that is, at the points where, if a circle was circumscribed around the square, the vertices of the square would meet the line of the circle). Ideally, you would also be able to angle the wheels such that they tracked the circle as the vehicle turned in-place. This of course would be more complex to implement.