[Camerzn]

The best example of 6wd I've ever seen did have the dropped center wheel (about 2 cm). The center wheels on each side were powered directly from the gearboxes, and belts drove the other two wheels on each side; even if all four belts broke, the robot would have still been powered. The robot wasn't terribly fast, but it had excellent power and traction on the low pile carpet. It was definitely a snazzy 'bot, and if you want to try it, go ahead. Like Cory says, however, remember that six wheels weigh 1.5 times as much as four.