Thank you, gzumwalt, for this incredible design!
While gzumwalt's idea to hide the spring within its gear is genious it was too subtle for my not so fine motor skills: Trying to get the maximum distance out of the car I destroyed 3 springs.
Therefor I combined the pull back system with the larger and open spring from gzumwalt's previous work. As I needed to counter the spring somewhere I built that frame around the front axle. And asI started it I changed a few other things that came to my mind:
I designed new wheels that work with tires I printed using TPU. I found that easier and cheaper (a 50 g sample will cost you 3 € and give you an unlimited amount of tires).
All the disassembling and assembling for test purposes wore down the axles and wheels, so the friction sometimes didn't make them stick together anymore. So I built axles with screws at the end and the matching screw nuts. I designed an o-ring version, too.
My tire design is actually the result of quite a bit of testing in order to resolve the problem of the wheels trending to spin causing the car to spin itself. My finding was that the problem is less the friction between tires and ground, but rather the light weight of the car. So I added an additional thread to add some steel nuts (M10). These steel nuts are not mandatory but recommended.
- edit 19/01/13: I added two alternative tire designs: a tire with some kind of profile - I tested it and it showed no advantage over the flat design, but maybe it works for you. And just now I created a new tire which I can't print as I ran out of TPU (50 gr sample, see above...). The idea would be to print it 0 % infill (I know bridging TPU isn't easy, that's why I didn't add fillets at the top), so you have a very soft tire. Maybe the weight of the car is enough for the tires to cave in a bit augmenting the surface and the grip. I also uploaded the F360-file, so feel free to play with it. And please tell me if it works.
Das Filament PLA or PETG, (TPU)
I recommend printing the axles and the spring with 100 % infill.
The last part of the files' name tells you how often you have to print that part. The wheels for O-rings and custom tires are to be printed alternatively, of course.
The longer axles sometimes fell during printing. A perfect first layer might help - or a brim.
"Elephant foot compensation" (0.2 mm), an advanced setting in Slic3r, helped me a lot to make the parts fit easier.
I allow myself to refer you to gzumwalt and the pictures of the print for assembly instructions. Please pay attention to fit in the spring in the right direction: the last part of the arc faces downwards (see picture).
You might need to sand and trim the parts a bit (despite the elephant foot compensation).