[JesseK]

I wouldn't suggest 4 Omnis + 2 traction. Even an omni bot or mecanum drive bot can play excellent defense against you. Actually, they see you and salivate at the defense they can play against you since they can also outmaneuver you.

Last year 1885 did a 6WD rocker with 4" wheels. It is my opinion that we turned "too well", and the center of rotation kept changing from one side of the bot to the other. This proved to be detrimental with sudden shifts in speed and direction, as we could very easily tip if we did the incorrect sequence of movements. We tried to limit max turn speed via the gyro but that went south pretty quickly as it was a last-minute "feature".

For this coming year, we're toying with 4 traction wheels + 2 omnis. This is so the center of gravity and center of rotation are more stable. It also allows us to adjust the distance between the rear and middle traction wheels so we can adjust our traction wheel base, whereas with a rocker the wheel base is always set in stone. This will play a key role in weight placement because the students have already derived the advantages of trying to keep the center of mass just behind the middle traction wheel axles.

So, rocker pros:
It turns very well.
More than likely you have 4 traction wheels on the ground at all times for good enough pushing.
It does not inhibit other design decisions, such as transmission types, overall forward speed/acceleration, etc.

Rocker Cons:
Others can still turn you very well when you're up on only 2 wheels.
Your center of gravity and center of rotation change, constantly.
It's hard to control how well it turns (well, it's not something that can be programmed spur-of-the-moment)
You need more advanced drilling techniques for the axle holes, otherwise if you weld the frame you will definitely see some warping, leading to wierd rocking or cambered wheels.

On March 9th, 2008 I will tell you what I really think about 4 traction wheels + 2 Omnis.