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On Wheels and Tires Part 5 of ?

Hopefully this is the last post about tire making I write for a very very long time! Coming full circle, the simplest solution is to pour tires one at a time into an open top mold. I simply decided to print a two piece mold with separate halves for the rolling diameter and the wheel diameter.

There are simple provisions to clamp the two halves together with a screw in two of the corners.

Here is the rolling diameter mold:

Here is the Wheel Diameter mold:

I made the molds all interchangeable, however you have to have a pair for each tire width.

Based on my earlier research, I concluded I only needed to worry about wheel diameters equating to 13″, 15″, and 18″. Similarly, I only needed worry about tire rolling diameters of 22″, 24″, and 28″. From the standpoint of tire widths, it didn’t make sense to make a tire narrower than 5mm or wider than 8mm. I decided 1mm width increments was enough.

I made 8 sets of these molds in different diameters and widths. I proceeded to cast a few sets of all of the combinations of tires possible with the molds I had made. I now have a library of 20 Shore tires of easily 400 tires. All of these were yielded from a 2 pound trial size kit of Smooth On Vytaflex 20. I failed to get pictures of this process, so no glamour shots of this system.

The key to success regardless of the mold design selected is producing a large number of molds and being able to pour many tires in each sitting. The Urethane spoils quickly once the containers are opened. It has a 16 hour cure time, so you can only pour into a given mold about once per day as a practical matter. Two pounds is about 900 grams. If you are only pouring 10 or 15 grams at a time, and can only cycle the mold once a day at best, they urethane will go bad long before you can use it all.

The pot life is relatively short, just a few minutes, so I found that working in batches of 20 to 30 grams at a time was ideal. For mixing such small amounts, I used a high precision digital scale that reads out into the 1/10th gram. The urethane can be mixed in equal volume or equal weight, but in such small volumes, it is difficult to eyeball the volume accurately enough. I found disposable 1 ounce medicine cups to be ideal for this sort of work. Something like these on Amazon. For stirring, I used cocktail toothpicks, available at the grocery store.

In a single sitting, I’d mix 6 to 8 20 gram batches and pour 80 to 110 tires. Once I hit critical mass on the molds, I effectively used more of the Urethane in two week period than I had used in the previous two months. I was fighting with the Urethane at the end, as it was trying to go bad on me as I was working through the final pours. I had some curing issues with some tires towards the end of the run because of it.

Files for all of the flat open top molds can be found HERE.

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On Wheels and Tires – Part 4 of?

Hopefully all of the electrons I’m expending documenting all of the ways I can think of to cast tires will help someone else along the way. After the success of the “fixed width” mold, I decided to try a similar approach to make “tubes of tires” that I could cut to whatever width I needed.

I simplified the inner mandrel to just have printed cylinders of different sizes that slip fit over 4mm brass tubing. In order to cast different wheel sizes (tire ID’s) I just need to print different sizes of cylinders. I settled on 1mm undersize for the tire to get some “stretch” over the wheel at installation.

I made outer molds that correspond to a 22″ rolling diameter, a 24″ rolling diameter, and a 28″ rolling diameter. I oversized the cast parts by about 1mm on the OD to give room to true them.

The mandrels and outer molds are interchangeable.

I experimented with cutting them to width freehand but the results weren’t good. I needed a fixture to assist.

By swapping out the indexing block, I can cut different widths. I made blocks for 4mm to 7mm wide tires.

The mandrel just slips into place, and is retained by the plunger on the left.

The mandrel is rotated by hand, and the pressure on the blade is also applied by hand. It is a bit tedious, but the results are good.

I stopped cutting midway and pulled off the tires to show the process.

Overall, this process is pretty effective. Pouring the tires is relatively quick, as you are pouring around 10 at a time. It takes a few runs to get used to eyeballing how much Urethane to pour into the outer mold before putting in the mandrel. Inserting the end of the mandrel into the guide in the bottom of the mold is blind operation and can take a few attempts to land.

Cutting the tires to width, even with the fixture I designed is slow and difficult. The Shore 20 compound is so soft, it wants to stretch and bend instead of cut. Still, with a little time on the truer, they all come out very good in the end.

The files needed to print your own molds and mandrels may be found HERE.

The files needed to print your own tire cutter may be found HERE.

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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.