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 –
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.
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.
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.
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:
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.
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.
