I thought I should throw in some more information about our robot since last year we did use a 4-wheel crab system.
Crab steering simply put is omnidirectional steering. On your ordinary car the front two wheels are rotated clockwise or counterclockwise about their Y-axis to give you the ability to steer. Crab steering uses this concept, but instead rotates all the wheels in the same direction to move in a line in that direction.
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Crab steering is accomplished through potentiometers and massive amounts of code. As the motor turns the crab module, the potentiometer is rotated. When it turns to a certain value it is stopped so as not a turn past the limit of the potentiometer.
For our robot last year we added lots more neat features in our code to make the movements smoother. One of my favorites was what we called 'quad drive' or what some refer to as 'monster truck drive'. In this the front wheels turn 45 degrees in one direction and the rear wheels turn 45 degrees in the other.
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The only problem with this is that the modules are put under a lot of stress because the outter wheels tend to drag on the carpet when turning. Your
Ackerman steering would be very useful for this type of turning as it would stop wheels from turning. The best we could do for this is force the outter wheels to drive faster to keep up and drag less and keep the motors from stalling out. Ackerman steering could be accomplished by using four independently steered crab modules.
There's a few different ways to accomplish 'crab'.
1) 4 independent crab modules using 4 crab motors
2) 4 modules separated by front and back crab motors with both motors in either the front or back connected by a chain
3) 2 independent crab modules using 2 crab motors
4) 2 modules with one crab motor connected by a chain
If you have any additional questions please feel free to ask.
Crab Drive Motor,
Crab Module,
Crab Module Placement