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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.

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Vehicle Voltron – Replacement Fist Part 2

My method for these is pretty simple. I just sit down with a pair of calipers and a pencil and paper and make some sketches and take a bunch of measurements.

From there, I model the geometry in Alibre Design. The part that interfaces with the firing mechanism is going to be a bit touchy. So, I broke the model into two parts. This allows me to print the fist in an optimal orientation in the FDM printer and this critical part in its optimal orientation as well. This also makes iterating quicker, as I don’t have to print the whole fist each time.

After a couple of iterations, and a little bit of work with the file, I have a replacement fist that fits and fires similar to the original!

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Vehicle Voltron – Replacement Fist Project

Here’s a toy you don’t come across very often. This is a 1982 Matchbox “Vehicle Force Voltron”

I stumbled across this while going through everything in a move. Overall, the figure is in okay shape and is nearly complete. He is missing a fist, and the propellers are damaged, but present. The damage to the propellers is pretty common in played with examples of this toy.

The fists are of course interchangeable, and I was able to confirm the firing mechanism of both arms is still functional.

The opening in the hand for holding an accessory has a bit of an odd shape. The U.S. version of this toy did not come with a sword.

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Framing and Connections

I like the things I build to have robust connection points. In other projects, I’ve come up with small 3D printable pieces that allow me to “panel mount” Anderson PowerPole connectors as well as 5.5 mm barrel jacks. I reused these concepts here. I use three powerpole connectors for my controller wiring.

In order to give them something solid to be attached to, i framed the foam track with 1/16″ thick 1″ leg angle aluminum. I used Gorilla Glue to attach the aluminum to the foam.

I used a power drill and some files to create the square hole for the barrel jack to protrude through. I also made a little template to make it easy to drill the pair of mounting holes.

I didn’t want the aluminum to be directly in contact with the surface it might be sitting on, so I made 3D printed plastic corner pieces to cover up the corners.

To complete the project, I put a bead of grey caulk around the top edge of the aluminum and foam to fill the gap. I also purchased these variable voltage power supplies from Aliexpress, so it is easy to change the track voltage.

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Copper Tape

I’m using standard adhesive backed copper tape for the rails. I did order the tape with conductive adhesive, and I can report that indeed this stuff works very well.

The testbed oval track used 1/4″ wide tape. There was a lot of pleating and wrinkling in the turns, even though they were a pretty large radius. I asked some questions on the forums and had a very helpful discussion with Ed “HO RacePro” who suggested going to thinner tape. So, for the proof of concept track, i ordered 1/8″ wide tape.

There were a few difficulties. The tape applying tool I found on thingiverse worked quite well with the 1/4″ tape. It didn’t work as well with the thinner tape. The OD of the retainer was smaller than the OD of the roll itself, so the roll wanted to unspool over the sides. i ended up applying the tape by hand, which was tedious.

This is a still shot from a video I took of a car running. At this point, I just had the rails hooked up to a power supply.

The crossover took a little thinking, and in hindsight i wish I would have planned it a bit better.

I just drilled holes through the foam with a 5/32 drill bit and ran the tape to the underside of the foam to make connections. Most connections are just the tape stuck to itself. I did solder wires onto the tape where i would need to make connections to the power supply and controller.


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Slot surface improvements

On the proof of concept oval, I just used flat latex in the slot. This worked fine, and made it easy to judge if any wear was happening, but the roughness of it that is good for tire grip isn’t good for low resistance on the guidepin or flag. If you just pushed the car around the track with your finger you could feel the drag that just isn’t there in a plastic track.

So, I devised the following solution.

  1. go ahead and paint the slot and road area grey.
First coat of grey

This is necessary to blend the slot in with the road color.

2) Coat the slot and area where the car will run with Smooth-On Epsilon Pro Epoxy.

A quick snap in the middle of the process.

Repaint just the area where tires will touch, not down in the slot.

Now you have high hardness and some strength only exactly where you need it. And you have traction where you need it.

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Clean Up

The burning process will leave very long “hairs” of melted foam down in the slot. will be on the walls and some will be on the bottom. I used drill bits to break them out. They may or may not have actually caused a problem, but I didn’t want them coming loose in later steps.

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The Finer Points

The tip used in the soldering iron should be smaller than the slot desired. the tip is so hot, it will burn off the foam without actually touching it. This has a few undesired effects. The width of the slot varies a bit depending on how fast you move through the material. If you stop or hesitate to reset your feet, you’ll be left with a large defect at that point. I cut the slot in the pictures all in one go, as starting and stopping creates relatively large defects. Plan ahead and start and stop in straights, and make sure you wont have to reset your feet in a curve.

I started with a 40W fixed power iron, but that was too hot. I used my adjustable iron in the final iteration. I don’t have a lathe, so I just built up the cutting tips by nesting K&S brass tubing. I tested 2mm, 2.5mm, and 3.0mm diameter cutters and various wattages. I found that 2.5mm and 30 Watts was the best. I wanted the slot a little oversized, as I was planning on a few extra steps to be described shortly.