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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.

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On Wheels and Tires Part 1 of ?

I’ve been quietly plugging away on making wheels and tires lately, and have some thoughts/progress to share.

The very first 3DP slot car I build used printed wheels with “conventional” fitted tires. The wheel had a raised “center” and the tires had overhangs for the sidewalls. This is similar to how most RTR wheels and tires work.

Indeed, these were the very first tires I tried casting in Urethane. I went about it similar to the way one makes copies of an existing tire. I printed in PLA the tires, glued them to the bottom of a cup, and poured Silicone on them to make a mold.

The green mold, with what looks like a tractor tire form in it is one of my early test molds.

I printed a simple cup that I could glue positives into the bottom of, using a regular glue stick.

This technique works very well, and it is possible to yield a dozen or more runs of a given mold, depending on how thin the sections are and aggressive the overhangs.

There are downsides though. If you are wanting to iterate, and try a bunch of differnet things, the cost of the silicone adds up fast. The additionalsteps and curing of the silicone take a bunch of time. Each cycle takes a couple of days to run through.

The shelf life of the Urethane is quite short after you open the two parts. It reallly is a use it or loose it situation. To date, I’ve thrown out 80% of the Urethane I’ve purchased because it has gone bad before I can use it. I’ve had good luck storing unused Silicone however.

I redesigned the OpenSlotCar wheel to have a large tapered recess in the center of the wheel. This means the tire has a large ridge in the center, going into the wheel. ALl of the cross sections are relatively thick, and this shape is easy to cast. The rib keeps the tire on the wheel. A pair of wheels of this geometry are shown (printed in grey). Positives for tires are shown, printed in yellow and a translucent grey. The red tires in the background are finished Urethane tires.

Originally, I was hoping to produce a tire that wouldn’t need to go across a tire truer to be usable on a car. I was also trying to avoid gluing the tires to the wheels.

With regards to the tire truer, it ended up being unavoidable. There are so many irregularities in the printed wheels and the cast tires, the truer is needed to hide al the sins.

On my small home track, I was able to run fine without gluing the tires. However, in the proxy race that I sent the car to, the higher voltages and speeds showed that there is no way around gluing the tires!

I highly recommend joining a proxy race. It is a great experience, and you’ll learn a lot getting direct feedback from the guys who run the races. Here’s the full write up on the proxy race I took my lumps in this year:

https://www.slotforum.com/forums/index.php?showtopic=184483&page=2