[Rich Kressly]

Please note I'm talking about using "standard" type (kit or similar) wheels for purposes of this discussion. The use of omni wheels, mecanum drive, holonomic drive, etc. all provide very different characteristics.

In general the "hopping" / excessive current draw / etc problem isn't limited to a six (or more) wheel drivetrain. I've seen it in four wheel drives as well - in fact our team made a design mistake in 2007 with our four wheel drive configuration which led to excessive current draw and poor turning at times.

Turning torque (also called turning scrub) is the culprit and is largely affected by the wheel base width vs. length. Yes wheel types, etc can have an effect too, but as a general rule in standard tank style drives (4,6,8, no matter how many wheels) you'll want your drive base width (distance between wheels left to right) to be equal to or wider than the drive base length (distance from center of front wheel to center of back wheel).

The VEX Classroom Competition teacher's handbook, the Inventor's Guide, and the Autodesk curriculum all carry more detailed information on the topic - diagrams, etc.

In an FRC robot, if you build your chassis in the classic long rectangular configuration (ala 254 most years - ~38" long x ~28" wide), a six wheel drop center configuration provides huge maneuverability advantages and cuts out excessive current draw because only 4 wheels are ever on the ground at one time - cutting the effective front to back wheel base in half, making your effective wheel base on a flat surface wider than it is long.

If you build your FRC robot in "wide body" configuration (~28" long x ~38" wide), there's most likely (depending on other variables) no need to drop a center set of wheels as the wide configuration and shorter front to back wheel base/footprint provides for easier turning all by itself. Team 25 has built some great 6-wheel wide body robots and some long ones as well. In those machines (long), as stated before, clearly they are using means other than a drop center wheel to overcome the turning scrub/current draw issue and they'd be best suited to answer the question as to how they get it done.

If your drop distance (my team has always used .125") is in a decent range, rocking should never be a problem. We did encounter an issue this year like one other post here stated. We started out long configuration, drop center and because our CG was so low and our mass was so evenly distributed around the center, we had a difficult time driving the robot because we were effectively turning on two wheels most of the time. So we took our drop shims out in Washington, DC which made us easier to control and almost impossible to turn (dropping to like 7 volts on a fresh battery). After that we put a pair of omni wheel on one end making turning much easier, but maintaining our control while driving/turning.

Whatever you decide, just be sure you understand the way turning torque/scrub changes with the configuration of your robot chassis and wheel position. Those simple factors should enable you to make sound decisions without things getting too very complicated.