[Waynep]

So we've played around with 8 wheel configurations and here is what we found:
-As compared to a rockered 6WD system we have more consistent power delivery to the ground when pushing. Given that (hopefully) at least 6 full wheels are in contact at any point.
-Rockering the center will definitely help the robot rotate.
-Centralization of mass is a must, you really want the majority of your weight over the center of your robot for turning. The 8WD system gives you a larger range to spread out the weight than 6WD where you pretty much have to be spot on otherwise you have a forward or rearward weight bias.
-The other trick is using wheels with a different coefficient of friction. We have noprene and gum rubber treads on our robot. Ideally, you put the grippier ones in the center and the ones that slide more towards the outside. (The picture that Joey Gannon pointed to has a mismatch due to the fact that we were replacing wheels with whatever we had, we got in a good deal of pushing matches and driveoffs at Pittsburgh)
-Choose the appropriate gear ratio for such a system so you don't trip your breakers.
-There's something magical about the diameter of the wheels (this is a phenomenon I have tried to explain and still don't have a definitive answer), but it goes something like this: If you have the choice smaller diameter wheels will actually give you more bite given they will sink more into the carpet and what you get is a larger contact area than a larger diameter wheel whose contact patch can be thought of more as a tangential line. This also means that you have to correct more for the increased traction to get it to turn. If I remember correctly 229 uses smaller diameter wheels on their setups for Denominator and Irrational.

Downsides:
-Added weight and parts.
-More chains to tighten.
-Added current draw due to increased resistance and when turning
-Friction hop is an issue if turning at full speed from a stop, this was tuned out in software.
-More points of failure and things to go wrong.
-Smaller diameter wheels limits, limits sprocket size, and ground clearance .

Fellow Engineers please do correct me if I'm wrong. But these are the conclusions that my students have drawn after our testing and why we choose to go with the 8WD setup as opposed to 6WD.

Best,
Wayne