First: The bobble: it is there with all cars but most don't react so quickly or turn so sharply on their axis. The bobble occurs because of a mistake I made when building the road. I inadvertently got two sections of roadway misaligned, so that the wire is not centered perfectly as the car crosses from one to the other, but 1/8 inch to the side. What you see is the magnet snapping 1/8 inch to the side as the car passes from one section of roadway to the other (you can't see the seam between sections because I sanded, filled, and painted it). On bigger cars with longer wheelbases, they cars steering reacts but the car does not really jog quickly as that happens. The short-wheelbase Cobra, like a real race car, reacts instantly.
Second: the length of the steering arms: One thing I am learning in my R&D on making my own is that there are subtle interactions going on between tiller arm length, magnet power, steering geometry, tire traction (some model car tires, like those on the White Mustang, are so hard they have limited traction, others like those on the red Camaro, are so wide, and soft, that they stick like glue), and weight distribution front to back. It is surprisingly complicated how these factors interact and dictate that some cars can have short arms and others need longer arms, that some cars need two magnets stacked one above to other, and some just one. I'm not sure I understand it all and I can't explain it well yet because I really still have a lot to learn - hence my focus on more R&D making different scratch-built cars and seeing what works or not, and why. What I do know is that AutomotionFX apparently does know it all because out of sixteen cars I have from them so far, all sixteen run perfectly, whereas with cars I try to make, only one (but the most recent one!!) dependably follows the wire all the time.
Thanks for the explanation Lee, more than meets the eye with these cars. I hope you write a book on making wire guided cars, because that will be another one (or a set) that I add to my bookshelf! There's obviously considerable engineering that has gone into smooth reliable operation.