My initial design for wheels and tires had many problems. In order to reduce the number of parts, my original plan was to thread the axles themselves and screw the wheels on to the axle. I purchased left hand thread and right hand thread dies to do this.
I practice,, threading the axles tended to bend them, or at a minimum make them less straight. Of course this is a problem. I also had a lot of trouble with marring the axles trying to hold them while putting on the threads. I did get a test car put together this way, but one of the sources of vibration in the car was clearly the bent axle.
The final issue I discovered was that since you are counting on the wheels to help set the gear mesh, the fact that the wheels are getting “tightened” onto the axle while running causes binding.
My initial tire design was based on the tires on Carrera Go cars. The wheel has a “rib” around the middle and the tires have small “flanges” that fill that rib. The intent of this geometry is to keep the tire on the wheel in the turns.
These small side flanges are difficult to cast in a silicone mold. It can be done, but it isn’t as easy as I would like. The undercut in the mold is severe, making filling the flange and removing the tire both difficult. mold life was a bit of an issue.
