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OpenSlotCar – Foundation Design – Wheels and Tires

Based on what I learned on the Monaco, i rethought my wheel and tire design. I still wanted geometry that would retain the tire onto the wheel in a turn. I also wanted the tires to be easier to make in a silicone mold.

I also decided to just thread a setscrew into the plastic wheel and hope for the best.

I used the tried and true process of making siicone molds from “positives” of tires, and then pouring Urethane. I use Smooth-On Vytaflex 20 for the Urethane. I’ve used several different Smooth-On Silicone products for mold making and all have worked well.

The grey parts in the right of this picture are just tires printed in PLA plastic. I glue them to the bottom of this container with purple glue stick. You can see rings where I’ve done this.

From there, mix and pour the silicone per the instructions and pour into the container. Submerge the tires by about 1/4″.

Here are some examples of several different molds I’ve done. Early on, I wasn’t using mold release compound, and although the tires weren’t difficult to remove, after half a dozen or so cycles, the molds started taking damage.

The bright red parts in this picture are the first 20 shore tires I produced.

This method works well enough. The tires come out pretty good, however as with any car, you still need to go across a tire truer in some form to really bring them in. The files to print these positives of the tires and these wheels are in the currently available zip file for the Foundation Design.

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OpenSlotCar – The Beginning – Tires

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.

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On Wheels and Tires Part 2 of ?

I decided I wanted to utilize the latest batch of Urethane I purchased and try to cover myself for tires for the foreseeable future. The conclusion of my previous work with silicone molds left me with the conclusion that the process is too slow, and that running across a tire truer is unavoidable.

With that in mind, I decided to make 3D printed molds. The question then is, what sizes make sense? My focus is 1/43 scale cars. The smallest crown gear I have found is a 20 tooth gear, with an OD of around 10 mm. A minimum practical clearance for the gear is about 1 to 1.5 mm. So, the smallest tire it makes sense to use is about 13mm. This translates to a rolling diameter of about 22 inches. This is a bit larger than some cars would really run, but it is what it is. The largest tire diameter that is common in road racing that I could find has about a 28″ rolling diameter (NASCAR). 24″ rolling diameter tires show up on sports cars with some regularity. So, this narrows down to 22″, 24″, and 28″ tires. Of course, you could always true a larger tire down to something smaller.

From the standpoint of wheels, the smallest relevant wheel I can find is a 10 inch diameter. This is impractical at 1/43 scale for production on a home FDM printer (at least for my wheel design which uses a setscrew). The smallest wheel I can make translates to about a 13″ wheel in full scale. 14″, 15″, 16″ and 18″ wheels are all relatively common. At 1/43 scale, 15″ and 16″ wheels are only different by about 0.5mm. I’ve rationalized wheel diameters down to 3, translating to 13″, 15/16″, and 18″.

Depending on the era, tire widths change substantially. The tires on a vintage Mini Cooper would scale down to just 3mm wide. The tires on a new Corvette scale down to around 5mm width. I rationalized common tire widths down to 3mm, 5mm, and 6mm.

At this point, I took a step back. This was going to turn into a whole bunch of different molds. Overall, I think I’ve come up with a good solution.