This CO2-powered crash test car lets you design and iterate crumple zone bumpers in order to protect a passenger egg from injury. Use a CO2 car launcher from someone like Pitsco or Kelvin, wheels from a Lego Technic motorcycle (I got mine on eBay), and create a flat wall for the car to crash into (I put a board against paint cans). The car is guided by monofilament, and I put the wood wall at 50' from the launcher. At that distance, a 12 gram CO2 cartridge will shoot the car fast enough to break an egg without a well-designed crumple zone. Use a slo-motion camera app to capture and analyze the destruction. See "CO2-Powered Eggmobile" on YouTube for my 2 min. video. https://www.youtube.com/watch?v=liyXWWYGgdg My students copied and tinkered "Bumper Posts" in Tinkercad. This way, the connection to the car is consistent and correct, but the crumple zone element is their own invention.
Kelvin launcher: http://kelvin.com/kelvina-hotshot-raceway-a-co2-dragster-launcher-1-lane/
Pitsco luancher: https://www.pitsco.com/EZ-Start-Gate
The axles need brim, but not the other components. I use an exacto blade make the axle shafts round where they will contact the car body, and I shave down the ends so they fit into the Technic wheels properly. Also, the axle rings are important in order to reduce friction between the wheels and car body. They also help reduce play between the wheels and body. Finally, you may also need to shave the opening where the CO2 cartridge goes because it spreads just slightly on the print surface.