ArPee IV This is my fourth attempt at a wheeled vehicle with rack and pinion steering. It has no bumpers; in theory it doesn't need them, because it negotiates its way around a track using a light sensor mounted on the front which continuously scans back and forth to establish the position of the edge of a track on the ground.
	Here is ArPee IV with the RCX removed and the wires folded out of the way. You can see the construction of the gearbox and the rubber band method used to control the steering (as in the other rack & pinion cars). The same method is used to move the light sensor back and forth.
This is the car stripped right down, affording you a good view of the wheel mountings. Note the non-traditional mounting of the rack below the steering arms. This was to lower the steering motor into the chassis, which allowed the RCX to be mounted closer to the ground. (I don't like having that brick making the models top-heavy!)

Here is some NQC 2.x code to make the car follow the track on the Lego Mindstorms test track. It may not be a lot of use unless you make an accurate replica of the car, but it might help to glean an understanding of what I was trying to acomplish. It is (typically) very well commented.