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OpenSlotCar Design

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  • OpenSlotCar Design

    I've been working in starts and stops on a slot car design that maximizes what can be made with an inexpensive FDM style 3D printer.

    My goal is to give away the files and designs similar to Daniel Noree's OpenRC project ( , which I think is a great thing by the way. There's been several projects inspired by him, like the openRC Tractor project:

    I have a hypothesis that there are a lot of schools and other organizations with FDM 3D printers that are not being used all that much. Filament is so cheap, that anything that is printed is basically free. So, if you can keep the cost to get the car running down to a couple bucks, I think you have a viable option to see a TON of slot cars get made.

    The beauty of slot cars is that so much of the cost of making the car run is tied into the track, and thus can be bought once and used with many different cars (with very little effort). Again, I'm comparing slot cars to RC cars.

    I've got one car built, that actually runs, although not very well. I tried running NinjaFlex tires, but they didn't provide much traction. I've been messing around with casting my own urethane tires, and they look and feel great, but I haven't had a chance to run them on the car.

    The first car I did was modeled after a 1974 Dodge Monaco. Not a traditional "race car" choice, but I had a bunch of normal Go!! racecars, and I love the Blues Brothers, so I started there.

    I need to shoot a little primer on the parts so they will photograph better (I printed them in bright yellow, because it was what I had laying around).

    So, the chassis, body, wheels, guide flag, and motor mount is all 3D printed in PLA. I used 1/8" drill rod for the axles as well as a little piece of it for the guide flag to pivot on.

    I had a bunch of king crowns laying around, so I used one. I think the final spec would be one of the generic crown gears, because they are so much cheaper.

    I've been experimenting with different motors. I have ordered more different motors than I've had time to test, but I've been focusing on inexpensive motors from ebay and banggood.

    Where I get really hung up is trying to model the bodies. I'm not so good at modelling the organic shapes. I have a few ideas on how to overcome this, but I want to get the first car working right first.

    Does anyone have any good references on scratch building a slot car controller? I'd like to come up with files and instructions to allow someone to make their own wirewound controller. Not necessarily because it saves money, but because it seems like it is difficult to consistently procure high quality controllers with the range of adjustment that you'd like to have for matching to a variety of cars. For example, when I was looking for controllers for my track, all I could really find were vintage AFX controllers. They are nice enough and a big upgrade over the stock controllers, but they feel like they will break pretty easily if they get dropped.

    So, that was a lot of words. I need to get some pictures together and start posting.

  • #2
    Yes, please. All of that.
    Sorry I can't help with the modeling, though.


    • #3
      I got a chance to work on this today, and I wanted to share my progress.

      Here is the car prior to putting any primer on:

      In the background you can see a pair of tires and a lead weight. Originally, I printed all 4 tires in Ninjaflex. I found that it is too hard and doesn't provide much traction. So I made molds and poured Urethane tires. This was easier than I thought it would be. I wish I would have just bought the stuff and tried it sooner. I'll do a little write up on the tires later.

      Here is the body with no primer. The yellow printed plastic doesn't photograph well:

      So, I sprayed the body with grey rattle can primer:

      You can see the ridges in the part from the printing process. These surfaces definitely need some sanding.

      Here is the guideflag.

      I used 1/8" drill rod for the front and rear axles. I also used a piece of it for the guideflag post. The flag itself is printed. I just used a file to open up the hole in the chassis for the drill rod to rotate in. I print the guideflag in an odd orientation, up on it's side, so the printing ridges are in the direction of travel of the flag in the slot.

      Here is the bottom of the assembled car. I only had one screw in place to hold the body one while testing.

      The car isn't much of a runner. It is difficult to get it around the track, although the new rear tires made a big difference. With the printed tires, it was like a drift car!

      The motor seems to not modulate very well with the AFX controller I'm using. At 90% throttle, the car is barely moving, and at 100% throttle the car launches out of control. It doesn't matter which way I turn the sensitivity knob. The length, lack of magnets, and high heavy body don't help things either. m

      Overall though, I feel like this is a good start. The list of things I can do from here to make the car run better is long, but it is good to have a starting point. I have a few other motors to try, and I may run some larger wheels to improve the ground clearance a bit. I'm going to stay away from magnets, as I'd like to come up with something that will work in a no mag proxy.


      • #4
        I like it.
        For the power, it sounds like the controller and/or power supply are not appropriate for that motor/gearing. Try different voltage. It's also possible the sensitivity knob isn't working properly. A power supply and controller meant for larger scale cars (like 1/32) would probably work better.


        • #5
          Yes, I turned the voltage down to 9 Volts (instead of 14 like we run the Go cars at) and that helped. I have a Parma 4 Ohm controller I may hook up just to see.

          The motors are a bit of a crap shoot, as the specs available for the cheap motors are very incomplete. In order to keep the cost down though, I'd very much like to find something I can buy in bulk that isn't a "name brand" thing.

          Right now, the most expensive single component on the car is the Parma king Crown gear.....


          • #6
            4 ohm? that's a light switch. I don't "do" Ohms, since I use an electronic controller, but I know 4 would be WAY too little.

            Good point about the gear. It might be worth experimenting with different ratios.


            • #7
              Hello Cdub, I may have missed it but cant see any mention of the motor you are using, could you tell us what it is please?

              Regards, Lloyd.


              • #8
                Yes, I have that controller laying around from racing 1/24 FCR's with 16D motors. It seems like I need something in the middle (between the little AFX controller and the 4 Ohm).


                • #9
                  The motor in the car right now is a 130 size motor from banggood. 10 pieces for $5.
                  The rating information is 3V to 6V and 8000 rpm. The part number is 964015.


                  • #10
                    a small update

                    With regards to gearing, my original intent was to use the set screw style gears so it would be easy to play with ratio. However, in order to deal with the smaller tire size of 1/43 scale, I have to use the smallest available king crown gear, which is a 26T (this is the smallest I seem to be able to find).

                    Even the O.D. of this gear was great enough, that I had suspicion it was hitting the track in some situations.

                    So, I printed a new set of rear wheels that are 1mm in diameter larger. I put the same tires back on, they easily had enough stretch for this. The car ran much better this way, and was more predictable on the track. I think the additional ground clearance helped. I think what was happening before was when the rear would start to drift out, the crown gear would hit the track and cause all rear traction to go away.

                    These pictures are with the larger wheels and the nominal 0.5 mm increase in ground clearance.

                    Based on all of this, I'm going to drop down to 2mm shaft and cheap gears from ebay that are 24T 0.5 mod. I ordered a bunch of bits and pieces (gears, shafts, motors, etc) from the bay, coming on the slow boat.

                    I also did a little investigating on my AFX controllers. They are the adjustable sensitivity ones from ho scale racing. They seem to work well with the Go!! cars, but they don't work well on any sensitivity setting with this car. Directly measuring the resistance with an ohm meter, and moving the trigger shows resistances that are all over the place. One controller still has nearly 10 Ohms of resistance present just before you hit the full power contact and the end of travel, the other controller has 4 Ohms. These measurements are quite rough, because the values are jumping all over the place.

                    I have two other motors, both from Solarbotics that I need to try. One is a drop in for the banggood motor. The other is a smaller motor that looks more like a Go!! motor (their part number RM1A). It only has a 1.5mm shaft, so I need to come up with a pinion to fit it.

                    Parma really needs to resume production of the economy controllers. I can't find them anywhere, and I know they would be so much better than what I'm using. The bay has some used stuff, but not much and most of it looks rough or is crazy expensive.

                    We need a good, simple, open source controller project. I was looking at wire resistance values and nichrome wire on amazon, and I think it would be doable. I just don't want to take it on.
                    Last edited by Cdub; 03-29-2018, 06:05 PM.


                    • #11
                      That motor you described with the 1.5mm shaft sounds like an FF030. Similar in shape to the Scalextric slimline but shorter. It is one of the 2 types of motor I use the most and is widely used in robotics. To use up the 2mm bore pinions i just use brass tube with an id of 1.5mm and od of 2mm.


                      • #12
                        Thanks for the tip on the tubing. I'll have to give that a try. I need to take measurements of the smaller motor and model a chassis that fits it before I can try it.
                        Here's a picture of the motors:

                        I printed an updated chassis, and put in provisions for a magnet. I didn't put the lead weight on the new chassis. I installed the Solarbotics RM2 motor, which is the same form factor as the 50 cent banggood motor. It came with a plastic pinion on the shaft that seemed to fit the Parma crown gear, so I just went with it.

                        The new motor had the same lack of control. I had previously tried turning the voltage down, which didn't seem to help. I realized I had never tried turning it up. So, I bumped the track up to 20 Volts. The motor was much easier to control, I could actually modulate the throttle. Then, the little AFX controller started smoking!

                        I just ordered 25, 35, and 45 Ohm turbo resistors direct from Parma to fit my old 4 Ohm turbo controller. I suspect I will have much better luck with that. Whichever one works best, I'll just order another turbo controller and that resistor and call it done. I'd love to see a "how to" article on making your own wire wound controller, but my searches have come up short.

                        The magnet was pretty far from the track, but it definitely helped. My goal is to develop a no-mag car, but I really want to get something that actually goes around the track in a reasonable manner sooner rather than later. How much weight do you guys typically end up adding to your 1/43 scratch built no-mag cars? What is a typical "running weight"? I just threw that quarter ounce of lead on the back of the first car as a guess. Was I way off? Right now, the car weights 88 grams. My old International 32 weighs 112 grams.
                        Last edited by Cdub; 03-30-2018, 05:11 PM.


                        • #13
                          My no-mag cars typically weights around 45 grams, printed or brass chassis.
                          Here is a link to Kleskun Hills Fun Run carfact,
                          Dan did a great job putting together all the data.



                          • #14
                            I used to change all my rear tyres to urethanes, until I painted my Carrera GO track with what we call matt Emulsion wall paint in the UK, I think it is called latex elsewhere. Transforms the grip!

                            Many 1/43 scratchbuilt cars use no extra weight these days.

                            Regards, Lloyd.


                            • #15
                              A few small updates. I tried the 25 Ohm resistor, and that was still too high. I ordered and installed a 15 Ohm resistor and that seemed to be the ticket. I actually had some modulation and could "roll the throttle" on. That really helped with the perception of traction.

                              Based on our discussion about weight, I also 3D printed an other Dodge Monaco body that looks identical to the first one from the outside but only weights half as much. I was just more aggressive with turning down the infill. The original body weighed 40 grams, and this body weighs 22. I believe that weight reduction really helped lower the CG, which in turn made the car run better.

                              The remaining issue is that my track doesn't have borders, so when this long slow car does slide out the rear falls off the track. Once it begins that vertical pivot it deslots pretty easily.

                              I've been working on ideas for a simpler layout that would make it easier to put borders on my track. I've also begun work on a shorter chassis and a Jaguar XJR-9 body.

                              I still have a variety of motors to test, but I think I'll wait and test them in the XJR-9, as my big Dodge is going to be limited in what it can do on the track until I get the border thing figured out.

                              I may order a 10 Ohm resistor and try that out too, but I'll wait until I have a reason to order something else at the same time.