[dricks]

The problem is that the combination of the length of your drive train and the friction of your wheels on the driving surface is not allowing you to turn effectively. To solve this you can shorten the steering base. Which is what is being suggested by the recommendations for six wheel drive with a raised center wheel. You could also move your four wheels closer together, but this may give you stability problems.

Another option is to reduce the friction by changing 2 or all 4 wheels to wheels with less friction. You could replace two of your wheels with slicker wheels or Omni wheels. Obviously this option surrenders a little traction.

Changing from arcade drive to tank drive( controlling each side of the drive train with a different joy stick) would not solve the problem but it might give your driver a little more control.

If your speed controllers are set to coast, you may be able to improve control slightly by setting them to brake.

You have to evaluate what will work best for you with the time, space and material constraints your team has.

Good Luck!