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.

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.

On Wheels and Tires – Part 3 (Improved Foundation)

After a lot of thinking about how to avoid the silicone mold cost, I concluded the least expensive way out was to find a way to 3D print the molds. After thinking through many options, I decided to try something a little different.

I call this the “fixed width” design, as due to the inner diameter geometry, you have to match the inner mandrel and outer molds.

The lowest part of the mandrel has a taper and a shoulder to set the position in the mold. The mold is comprised of two halves. The are indexed by short lengths of 4mm aluminum tubing. The mandrel pieces are also on a 4mm aluminum shaft.

The process is to pour the urethane into the open mold, around 3/4 full, then plunge the mandrel in.

Once cured, the mold is spit, and the mandrel comes out with all of the tires bonded together. They must be cut to width.

The outer molds have ridges in them to mark where the tires need to be cut. These grooves are enough that the tires can be cut to width with just a razor.

As the tires come off, you can pull the mandrel parts off one at a time to ease removal.

One pour yields 8 tires:

The zip files for these molds are available here –

Click for for zip file of STL’s

This exercise proved to me that I could pour tires directly into printed molds. They require some cleanup but are definitely usable.

The downside of this design, is that you’d have to print a unique set of outer molds and inner mandrels for each combination of Diameter and Tire width that you want to produce. This isn’t insurmountable by any means, as filament is cheap.