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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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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.

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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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OpenSlotCar – The Foundation Design

After working through the Dodge Monaco, I decided to refocus the project on making a no-mag car that could be FDM printed and would actually be fast. I also set out the goal of coming up with something that could be made for a total cost of about $4 (excluding decals and paint). Obviously, you need a little bit of economy of scale to get the per unit cost down to $4. However, if you were making 20 or 30 of these, I believe you could get down to around this price point.

I’ve always liked the Group C JaguarXJR-9. I decided if I was going to make a custom car, it would have to be one of these.

With this price point in mind, and based on the issues I had with the King Crown Gears in the Monaco, I redesigned the chassis and drivetrain around 2mm components. This allowed me to get ball bearings very inexpensively, as well as allowed me to use crown gears that I can buy in bulk in China for a few cents each.

This is the very first version of the chassis. The axles are both 2mm shaft. You can see the wheels have a wide deep recess in the middle (where the setscrews are).

The frame and body have provisions for 4 screws. I typically just use two on the diagonal to secure the body (with a little bit of rattle).

This version still has de-soldering braid for the pickups and screws to secure. I ditched this in the next iteration and just run the motor wires through the guide flag. Here is the fully assembled chassis. I added several mount points to the rails to allow attaching magnets and lead weights. The loose pieces next to the car are plastic versions of the lead weights (just to check space claim/fit)

The magnet that is in the middle of the car works really well. It provides enough extra downforce to mask a lot of performance problems. I tried finding a way to tuck a pair of very small magnets towards the rear of the car (they are near the crown gear). There just wasn’t room to tuck them in.

I entered a pair of these cars into a Pro No-Mag proxy race. They ended up getting a DNQ primarily because I didn’t clue the tires to the wheels. The track they were on in the proxy race is much larger (and much faster) than my home track. Issues with the tires coming off the wheels didn’t show up in my testing.

Stickers on the Jaguar were from a 1/43 model kit. Paint and stickers really bring things to life. The other paint scheme is my personal paint scheme I use on all of my cars. I guess that makes it a fantasy livery.

All of the stl’s as well as a text file with sourcing information and other tips and tricks can be found here:

http://www.lightsout-industries.com/wp-content/uploads/2019/11/Export_20191013_OpenSlotCar_V00.zip

I have done a lot of work on wheels and tires since I made these cars. Stay tuned, as I plan on writing up something on what I believe to be the best overall strategy for making wheels and tires.

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OpenSlotCar – The Beginning

For a while now, I’ve been working on a project I’ve been calling the Open Slot Car. This project really began as a response to the terrible quality of Carrera Go!!! slot cars. I purchased a Carrera Go set for my son and I to play with a few years ago, and while the track system is quite nice, the cars themselves had many problems. Also, in general, there is much less selection of cars in 1/43 scale than in other scales.

So, I thought it would be fun to design a 3D printable chassis and bodies to enable people to build cars that run better than the Go cars, and be inexpensive to make.

The first car I wanted was a 1974 Dodge Monaco. I found some dimensions and pictures from the internet and set about creating this iconic car.

I learned many things in the design of this car. Number one is that modelling car bodies in regular MCAD is quite difficult. Number two, is that you have to be really careful with how you set up the models so that you can shell them out in order to keep the weight down.

Here is a body off shot. I tested many ideas about flag/pickup design as well as evaluated different options for the front axle. This initial flag design didn’t work very well.

I started off using de-soldering braid I had ordered from McMaster. I ordered braid with no-flux, per the suggestions of others. I thought the little screws pinning them in place was a good idea, but the screws ended up damaging the braid, and I had trouble getting a reliable connection.

For this first iteration, I tried to use as many “commonly available” slot racing parts from other scales as I could. The smallest Parma King Crown Gear I could find was still a bit to tall for what I was trying to do here. That also drove me to use 1/8″ axles, bearings, etc. I had a lot of trouble with the gear “high centering” the car on the track when in motion, causing terrible handling.

You can also see that this car uses a 130 can motor. These are so inexpensive and so common, I really wanted to make these work. However, after trying several different motors, I concluded that for 1/43 scale it is worth the trouble to track down a smaller 030 can motor.

I went into this project with the intent of making my own tires. I wanted a wheel/tire design that would make it easy to change/replace tires (no glue required). I also wanted the tires to be easy to cast. The Wheel/Tire geometry I came up with met those goals to a point. On home track, at relatively low voltages and speeds they work fine. If you take care in the process of mold making, you can produce wheels and tires that don’t require a truer (at least for low speeds and home track use). however, in the end I’ve concluded you really need a tire truer to get any real performance out of these parts. The FDM printed wheels are a little rough, the tires are a little rough, and the truer does a lot to correct these issues. I also tested two different hardness of Urethane, with the softer 20 shore being a clear winner.

If you’ve read this far, you should also be aware that I decided early on that I wanted to be able to run with no-magnet. I did design and test a few different magnets and holders on this chassis. Indeed, adding enough magnet would make it run, but I prefer no-mag racing and wanted to find a way to sort an FEM printed car to the point it could run with no magnet.

This car ended up not running very well for a variety of reasons. However, I used this car to test many different materials/parts/concepts and it served its purpose quite well. I still don’t have a 1974 Dodge Monaco in my fleet, but that day may be coming soon.

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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

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3D Printed setscrew wheels – improved

I’ve been working on and off on my 3D Printed “OpenSlotCar” design for a while now. I have two areas that I want to improve before I turn the files loose. One of them is the setscrew wheels (the other is the process for molding the tires).

I had a bit of a break through on the wheels that I’m quite happy with.

The original wheels were just 3D printed with a “cross drilled” hole for a set screw. I advised threading the hole with a tap to reduce the risk of stripping the plastic out with the setscrew.

These wheels worked okay, but could only be tightened and loosened around a dozen times at most before the plastic gave up. It was also difficult to get them tight enough not to slip but not strip them.

The solution I came up with is very simple, and I believe would be useful for many other 3D printed couplings. Essentially, I took the wheel model and added a pocket that was sized to press fit a piece of 6mm OD brass tubing. I pressed the tubing in, then cross drilled it with a drill and tapped the combined plastic/brass hole.

This hoop of brass gives the screw a sliver of metal to grab onto and yields a much stronger joint than just the plastic threads.

Step 1: cut the tubing to length.
Step 2: Hammer the piece of tubing into place
Step 3: Put the wheel in a vice
Step 4: Drill out the hole
Step 5: Tap the threads. I’m using M3 setscrews, so an M3x0.5 tap.
Step 6: Wheels fitted to a scrap of 2mm OD shaft.

This system provides a connection that can be really tightened down hard with a hex driver and grips the axle hard enough that it is very difficult to twist or pull the wheel straight off with your hands.