After seeing a few Hot Wheels to RC conversions on Youtube, I decided to give it a try. The scale of the parts made FDM printing a challenge, but I was eventually able to produce a working steering rack and stuff all of the necessary electronics into the shell without shorting anything out.
Opening the car was not too difficult, but does require drilling out the rivets used to fix the bottom in place. The wheels and axles will be clipped into the bottom, and can be removed by cutting off the clips or cutting the axle in half, but I wanted to tweak the wheel sizes and stagger the rear - this model is based on a wide-body kit, not the original E46 body.
Unlike the step van, this one uses directly-driven wheels, which was my biggest mistake. Putting the motors under the chassis allowed me to fit a pair of 6mm coreless motors, which are still not nearly torquey enough. They are not strong enough to move the car much without being geared down, which would require moving the motors to the top of the chassis plate, much like I did in the step van. It’s also possible the battery is the limiting factor, but I have tested the same motors with an external power supply and they don’t have much power. The additional weight of the die cast shell makes a pretty big difference, but I think an N20 motor would have enough power for this.
The steering has the most moving parts, but I am relatively pleased with the results. The steering knuckles do not have room for proper bearings, so I used short lengths of PTFE tubing to provide a low-friction sleeve, although I have since found some 1x3x1mm bearing that might work. The front wheels are attached to the knuckles using flat head pins, bent over and fixed in place by the PTFE tubing. There are a few issues with this approach: the wheels are not likely to fall off, but they’re not fixed as tightly as I would like, and develop significant negative camber under the weight of the car. I dug up a small linear servo for the steering, no brand but it has a lot of jitter.
Detailed instructions and the 3D printed parts can be found at: