Thing is getting the chassis right is actually an exercise in CAD. The CAD we get shown isn't actually what is used to cut the tooling, but a product all the parts that cut the tooling, fitted together, and at that point you can generally also then do things with that model. You can check the fit of the parts, how all the moving parts interact and run the chassis around on some virtual track. The constraints of the physics simulation will help immensely there. So basically, you don't prototype until you've got the CAD behaving properly. By its nature, a chassis is quick to produce CAD for as there aren't too many fiddly details, there will be tons of already designed bits on file as well that can be dropped in (bearings, wheels, bits of motion etc). Sorting out a scanned set of geometry into something that can be manufactured is probably a more time consuming job. It needs a whole bunch of rationalisation, sectioned, and finessing (sometimes the physical scan resolution just isn't high enough to get the subtleties of a shape etc), but it does give a much better starting point than an incomplete set of measurements.