Actually, in 3.5 and I presume Pathfinder, vehicles DO have explicit facing rules; they have a defined front and back and turning capabilities, usually it depends on it's size. As for lightcycles, try using a 2 long and 1 wide vehicle and make it impossible for them to move diagonally.
So bascially, let's say you can move 30 feet. If you move 25 feet forward and 5 feet left, the vehicle will be facing left, and if you traced it's path, it would make a very tall L shape. If however you moved the 30 feet in a straight line, you would have to maintain the forward facing, you would have to turn left on your next turn.
