I wanted to bring
this helpful "white paper" to the attention of people who are busy designing their robots in the mistaken belief that turning on a slippery surface will not be an issue.
If you build a 4WD skid steer robot in the "traditional" or "narrow" (wheels aligned with the long axis of the robot) orientation there is a very good chance that your robot WILL NOT TURN.
I know this might seem counterintuitive, but run the spreadsheet in the whitepaper with the coefficients of friction for the wheels (KoP list / section 10.2.4.1 of the manual as I recall) and look at the results.
Now keep in mind that in order to spin the trailer about your robot you will need to apply about a 3 lb force (30lb trailer weight estimate x .1 transverse dynamic coefficient of friction) at a distance a couple feet away from your robot's centre of rotation. So, say about 6 lb feet of torque.
In this case even a 6wd robot (with the middle wheels dropped a bit) will have difficulties turning in place... it will do it, but slowly.
And thank you to Mark from 1189 for posting the white paper and spreadsheet.
Jason