I wrote an earlier article about building the IROC hot rods. That article took into account how to build identical cars based on measurements... you know, is the chassis flat, does the top plate match the chassis, are the armature Ohm readings close, are the magnets matched and close in strength etc. If you would like to catch up and read about that you can do that here. Well as good as that sounds that's just the basics... there is more and that is the fine art of tuning the IROC chassis. If you're interested then click the read more link below.
The first thing I did was adjust the pick up shoes. What I found out was the closer I got the wear pattern on the pick-up shoes to match between cars the closer they ran. They were closer but not there yet there were other issues lurking.
This was a lesson in how tuning is just as important as building. I would highly recommend building several IROC car sets. The more you build the more you notice and learn!! Happy Building/Tuning!!! :-)