[Al Skierkiewicz]

Chris you should have come and talked to me at IRI. We do not choose crab drive before the game is announced and we proto-play the game. In our tests, we took one of our old crab drive robots and attached a trailer to it. It was clear after our trials that no one was considering the shift in center of rotation that the weight of the trailer introduced. Crab allowed a more controlled turn with the trailer attached which was deemed better for defense and ball pick up. We later found with practice that crab could be used effectively in scoring by wrapping the robot/trailer around an opponent while our alliance partner scored. It was easy to block two quadrants of movement on an opponent.
Let me say again...We do not choose crab or any function for the robot until we play the game and see what is needed. The Whole team (except for field build) brainstorms from the minute the satellite transmission goes to the First logo and we spend days at it. We have groups of people play the game as a robot to see what the interaction is and what affects scoring.
There are disadvantages to crab. The modules take a lot of space within the base. We have managed to get ours as small as can be so that they can be placed near the edges of the robot. Still the wheels will never be within three inches of the outside edge due to the rotation. Crab uses more motors, we have in the past used CIM in all four wheels, or two CIM and two FP or two FP and two drill motors. We usually use two Globe for steering but software needs to know where the steering is at so a special mount is required for feedback and position sensing. We never allow rotation much beyond 360 degrees so mechanical stops are needed as a backup to position sensing. All the hardware for construction of the crab eats into the weight budget. Crab only puts four wheels on the floor when more might be better.
And above all it requires practice, practice, practice. The first step is the realization for the driver that all of their life experience is out the window. They know how to control a bike, a big wheels or a car by steering. Crab now gives them the ability to move like they do in a hallway at school. Translating that motion takes practice, and a fair amount of software help to integrate the hand controller with robot function.