I build "X" cars as a way to test out several new (to me) design or build concepts in one car. Aesthetics is not a concern so they tend to be a little rough around the edges. The obvious design elements that stand out are the fender designs, but each car has other more subtle elements too.
The X1 (below) was my first attempt to build a car with fenders. I also had my own ideas regarding the ideal fender design, but didn't know how to build them. So this was a huge learning experience to say the least. I built this in accordance to the PWDR "Limited" rules.
The X2 (below) built on these ideas and tweaked them for the PWDR "Pure Stock" class, which imposes the stock wheelbase and wheel location.
With the X3 (below), I was again tweaking the fender design. But the primary goal of the X3 was to build a lightweight body even with all the fender additions. This was built according to the NPWDRL "Street Stock" rules.
Finally, I'll leave you with a picture of another "X3" based body I built later which was more refined. And you can see that even with all that fender material, wheels and axles, it still only weighs .75 ounces.
I wonder if, after I seal up those chasis cavities with monokote, I could pump them full of helium! 
