When I was 7, the coolest toy I'd ever had was a car that would drive into a wall, climb up it, flip over, roll over, and then drive off unscathed in the opposite direction. Now, more than 5 decades later, I have an excuse to make myself another one!

Required store-bought parts:
2 HN-GH12-1324Y gearmotors
1 Philmore BH363 Battery holder (6x AA)
1 NTE 54-872 switch
2 10mm wide x 50mm long rubber bands (like the ones used to bundle broccoli)
6 6-32 x 3/8" socket cap screws
2 3mm wide zip ties
Hookup wire

Note: I used parts I had lying around (given the limited contest design window), but the motors are a little pricey for a project of this caliber. I've posted the STEP file for the car if anyone is interested in re-working the design to use cheaper gearmotors, like those from Sparkfun.

Note: This works best on low-pile carpet. On smooth floors, the wheels might slip if the floor or the wheels aren't clean. Wiping down the wheels and floor with a damp cloth should be sufficient.

UPDATE: Fenders! They don't just look cool, they also enhance the roll-over by preventing the front wheel from kicking the ground when the car is inverted. Just print them out and glue them on.

Printed parts (PLA recommended):
1 Car Bottom (no supports needed)
1 Car Top (supports are needed, but can be very coarse ~10mm)
4 Wheels (no supports needed)
2 Shafts (no supports needed)
2 28t pinion gears (no supports needed)
2 28t drive shaft gears (no supports needed)
4 Fenders (2 right and 2 left)

Assembly:

1. Test fit the shafts in the axle holes in the bottom. Rotate them back & forth in the holes until they are running pretty smoothly. After running them in, apply some light grease or vaseline to the axel holes.
2. Press one end of the axle into one of the wheels so that the end of the axel is flush with the hubcap side of the wheel. Take a short piece of filament (~15mm) and tap it into the little hole in the end of the axle. The hole is tapered so this will lock the wheel onto thte axle. Trim the end of the filament leaving about 2mm sticking out so you can pull it back out if needed.
3. Slide the free end of the shaft through one hole in the bottom. Slide the gear over the shaft with the gear hub facing the wheel. Continue pushing the shaft through the hole in the other side of the bottom. Place a second wheel over the other end of the shaft and secure it as in Step 2 above.
4. There is a small tapered hole through the shaft adjacent to the gear hub. Insert another small length of filament into the hole to trap the gear against the inside wall of the bottom.
5. Press the pinion28 gear onto the motor shaft, taking care to align the “D” shape of the bore with the flat on the shaft. Press the gear all the way down so that the shaft sticks out the end a bit.
6. Place the motor in the semicircular cradle and rotate and align it such that the motor gear is engaged and aligned with the shaft gear. Use one of the zip ties to secure the gearhead portion of the motor to the cradle.
7. Repeat the steps above for the second shaft, wheels and motor.
8. Insert the switch into the side of the top. Wire up the motors, battery pack and switch to that the two motors are in parallel, but with reversed polarity. Current should flow from the battery (+) through the switch, through the two motors, and into the battery (-) terminal. The battery pack should be oriented so that the terminal side of the pack is on the same side as the switch. Test the direction of the wheels – make sure that as the car drives for with the switch towards the front of the car.
9. Secure the top to the bottom with the 2 6-32 screws. And insert the roll bar. If the roll bar is loose, add a couple drops of superglue.
10. Lastly add your rubber band tires to the wheels. If they aren't tight you can secure them with a few drops of superglue.
11. As per the update, glue on the fenders with superglue.

Happy crashing!