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OpenSlotCar Design

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  • #16
    Do you know any of the specs of the motors you're testing? Like, how many RPM is it rated, and at what voltage? This information can help you determine what are the best controllers for your various motors.


    • #17
      The first motor I tried was part number 964015 from

      The only provided rating information is:
      Rated voltage: 3V
      Reference current: 0.35-0.4A
      3V speed: 20000 r / min
      6V speed: 40000 r / min

      The second motor I tried, from Solarbotics, their RM3-
      Free running - 3V - 42 mA,

      Stall - 3V -370 mA, 24000 r /min
      unsurprisingly, these performed similarly in the car.


      • #18
        I have an adjustable power supply, so I can run the track voltage up and down. I'll try anywhere from 9 to 15 volts just to see. If I'm running a Go!! car in the other lane, I need to keep the voltage relatively high in that range, or you can just sit there and pin the throttle.


        • #19
          A hypothetical update

          I've been working on this project off and on, and have a few updates.

          I've been working on something that looks more like a race car, and have modeled the body in such a way that printing a very thin shell is straightforward. Basically, it is a shell that is only 0.84 mm thick. I also reduced the chassis to as little material as I could imagine. I resized everything around an FF-030 motor and 2mm shaft material. Smaller diameter wheels and tires are also in the mix. I have printed some proof of concept parts and weighed everything and I believe I should be down around the 45 grams others have mentioned as a reasonable weight. My Vytaflex that I use for tires went bad on the shelf, so I have ordered more and am waiting for it to come.

          Here is the new car next to the old car:

          And a similar view with the bodies hidden. I did another version of the chassis for the Monaco that uses the FF-030 motor but haven't printed it yet to test that all out.


          • #20
            A real update

            I poured a set of tires tonight, and am waiting for them to cure.
            In the mean time, I set about putting the car together.

            Everything is together except the tires:

            I did some more tweaking on the body after I posted the earlier renderings, and I went ahead and added the rear wing as a separate piece.

            Here is the bottom side of everything:

            I used 2mm shaft for both axles and the guide flag pin. It isn't in there all that well, even with glue, so I may have to rework that. I seemed to have gotten away with this design with 1/8" shaft, but may not be able to with the 2mm.

            No tires or paint, I at 34 grams. Hopefully I will be able to put it on the track this weekend and test it out!


            • #21
              Where can we find the models? Have you made them available yet?


              • #22
                I haven't put the models up anywhere yet. I'd like to do some work on how they run first. I don't want someone to go through the trouble to build one and have it be miserable to drive.

                I got a chance to run this car a little bit tonight. There are a few things to be tweaked. The guide flag was bottoming out in the slot, and I think the rear body may have been hitting the track. The front wheels weren't really touching. When I did the redesign, I don't think I accounted for the smaller wheels and tires correctly in the guide flag stack up. I also need to add provisions for adding weight to the car. All in w/ tires I'm at 36 grams. The car is very loose and doesn't want to stay in the slot. I stuck a 7 gram wheel weight into the car on top of the motor and that helped. Obviously, putting it lower in the car would be better, and perhaps more weight over the front would be good too.

                As I've mentioned in this thread before, my track doesn't have any borders, and so any bit of rear end sliding results in a de-slot. I'm going to have to put provisions for strong magnets into this chassis too, in order to provide something reasonable to run on standard Go!!! track.

                I was running the car at 10 Volts with the AFX controller and the car was reasonably controllable.


                • #23
                  I made a few changes.
                  First, I added a 1mm relief in the front end to allow the guide flag to sit higher. You can see the relief on the left side of this picture.

                  I also added a series of bosses to the chassis such that there are places now that are convenient to screw things into the car from the bottom. Things like magnets or just weights. I also designed a couple of magnet holders.

                  The motor I'm using is larger than the stock Go motor. It is very difficult to fit magnets in behind the motor in a similar position to the Go cars. I tried a design with two small circular magnets, but they interfere with the gear. The rectangular magnets I have are close to the Go magnets in dimensions, but are a hair smaller.

                  The 4 loose yellow pieces are just plastic slugs to claim the space of weights. I will use these to make a silicone mold (same stuff as for tires) and then pour lead weights. For now, I just attached them to the car to check out the fitment of everything. If they were lead they would weight around 4 grams each. So, there are provisions on the car to add around 24 grams of weight if running no magnet.

                  All parts together:

                  I haven't gotten to run it on the track yet. I may get to yet today.


                  • #24
                    The back of my head set an alarm when I read about your plans to cast lead weights in silicone rubber. It said something about silicone breaking down at temperatures above 700 Fahrenheit, and emitting toxic gases.

                    That sent me online trying to find references to back that up. I did not find much that was relevant. But I did find some information from a MSDS (Material Safety Data Sheet) for a particular brand of silicone rubber, quoted below:

                    Section 5 - Fire-Fighting Measures

                    5.1 Extinguishing Media:
                    Water Fog, Dry Chemical, and Carbon Dioxide Foam

                    5.2 Special hazards arising from the substance or mixture:
                    None known.

                    5.3 Advice for fire fighters:
                    Use water spray to cool fire-exposed surfaces and to protect personnel. Shut off “fuel” to fire. If a leak or spill has not ignited, use water spray to disperse the vapors. Either allow fire to burn under controlled conditions or extinguish with foam or dry
                    chemical. Try to cover liquid spills with foam.
                    Because fire may produce toxic thermal decomposition products, wear a self-contained breathing apparatus (SCBA) with a full face piece operated in pressure demand or positive-pressure mode.

                    I admit that last bit refers to fire, and not exposure to molten lead (621 Fahrenheit), but at the very least I would recommend taking precautions.

                    I also found an article about casting lead in silicone molds. It states that you must use a high-temperature silicone. It does not say what happens to other silicones, but I assume that thermal breakdown occurs. See the link posted below:


                    The end of that article includes this boxed warning:

                    Uncured silicone is toxic to humans. Do not expose your skin to it and do not inhale the fumes. Keep it away from children and pets and work in a ventilated area.
                    Do not attempt lead casting if you are inexperienced. Lead is toxic, and hot metal is dangerous to work with. If you are new, work with it only under the close supervision of someone who is skilled in casting.

                    No other references I saw said anything about uncured silicone being toxic. But I suppose it is better to be careful about exposure.

                    That warning about lead and hot metal? No quibbles there.

                    Ed Bianchi


                    • #25
                      Pouring lead into silicone molds is a common technique across a variety of hobbies.

                      I started doing it a few years ago for Pinewood derby. I've had success with OOMOO and MoldStar (both from SmoothON).

                      The lead melter I use is similar to this one:

                      But the one I use is likely older than I am.

                      Yes, you should wear welding gloves, wear eye protection, jeans, and closed toe shoes when doing this. Definitely go into the pour with plan for what you are going to do with the melter before, during, and after the pour, and take care to not have any moisture in your molds. Molten lead makes steam very quickly.

                      As far as the interaction with silicone goes, it's not a problem. You can get dozens of good pours from a silicone mold before it begins to breakdown. On paper, the temperature capability of the silicone and the melted lead seem close, but in reality several things go your way. The lead begins cooling the instant it leaves the pot. The silicone starts at room temp, and every bit of energy the lead puts into the mold to warm it up takes temperature out of the lead. Usually, the largest surface area of the part is the open top where you are pouring, so all that heat just goes into the air. So, from a practical standpoint, it just isn't a problem.

                      The most annoying part is the lead shrinks considerably as it cools. There is an art to "overpouring" your part with a little dome to it so when it cools it is relatively level. The great thing is that if you mess up, just throw the part in the melter and try again.

                      If you just want to prototype something, and only need a few pours, you can make a mold from a pine 2x4. Just drill the hole or holes and do whatever you need to make the cavity you want and then pour away. The drier the wood the better, as you'll get less steam. The lead will char the wood which ends up protecting it. My point is that wood should "burn" around 500F, and I've never managed to set a kiln dried 2x4 on fire with lead.

                      There are many youtube videos out there about pouring lead weights for fishing. You can watch them and judge for yourself the level of danger involved (and get a chuckle out of the decision making of some of the folks making the videos).

                      Hope this helps.
                      Last edited by Cdub; 07-12-2018, 05:51 PM.


                      • #26
                        Well, I got a chance to run the car, and it runs very well. The magnet is very close to the track, and so the traction is quite high. Indeed, it screams around the track glued down like the mag Carrera cars. My previous attempt at putting a magnet in the Dodge Monaco was flawed, as I just put a pocket into the chassis, and didn't really lower it closer to the track. Where it is on the car now, it doesn't seem to really do anything. Now I know.

                        The magnet more in the middle of the car behaves interestingly, as the car will "step out" if you overcook it, but not just totally fly off. So, you have a chance to pull it back in.

                        The gear mesh is a little rough. I'm hoping it will wear in if I can get out and run it more.

                        The plastic I'm using is PLA, which is notorious for having difficulty bonding with glues. My 2mm guide pin is glued into the printed flag on a very small area and this joint just isn't very strong. I'm going to rework some of that geometry to make it stronger. Other than that, I'm quite happy with how this is coming along. I want to pour the lead weights and run that car as a no mag a bit more to see if I can figure anything else out there.

                        I'm in the process of putting a more flowing layout in place to make evaluating a no mag car a little easier.

                        I hope to have files I'm confident enough in to share soon.
                        Last edited by Cdub; 07-12-2018, 05:52 PM.


                        • #27
                          For anyone that is interested, I put together a write up about pouring lead weights in the scratch-building section:



                          • #28

                            After much more experimenting and troubleshooting, I have made a few more changes and finally have a car that runs!

                            A few notes on motors:
                            I built a second car and used a different motor. The link is below.


                            I cut some of the shaft off from the end I wasn't using, and pressed the provided gear off and replaced it with a my own (different pitch).

                            This motor works great with my track at about 12 volts. The power delivery is nice and smooth, and the car runs really well. So, the 13,100 rpm runout speed at 12Volts is a very good match for my plastic track and nomag car. I'm using Carrera Go! track, but all turns are either R2 or R3.

                            The motor in my first car was a solarbotics RM1A. This motor is rated at 7400 rpm @ 4.7V. Running the track at 11 or 12 Volts makes this car extremely hard to control. Turning the track down to around 8 or 9 Volts is about right.

                            A note on braids/brushes/pickups:
                            The first car I built used desoldering braid for the pickups. It was soldered to the ends of the motor wires and appeared to be pretty robust. I don't know exactly what had gone wrong, but I was getting intermittent power through my braids to the motor. This, along with the too high voltage was making the car a bear to drive. I cut them off and switched to just using the motor wires splayed out for pickups and suddenly the car ran much better.

                            I believe I have the chassis, pickup, and tires sorted now, however, I think the body is still hitting the track in some conditions. I have a little more refining to do in that regard. Overall though, I am much closer to having a formula ( and files) for a car that i'd be comfortable sharing.

                            More soon.


                            • #29
                              Desoldering braid has dry flux on it. Did you do anything to remove that, such as soaking in acetone? If not, then that's why you had problems with it. Either clean it, or use clean braid meant for slot cars.

                              Great work, though. I look forward to printing up a car or two.


                              • #30
                                The braid I'm using claims to be "without flux core", according to McMaster-Carr. However, I did nothing to treat it or verify that there wasn't something on the braid.

                                I believe the issue was with my retention method. I was using a screw to "pinch" each braid into the guide flag. I believe I over tightened these and tore through the braid (at least one of the two). The damage was concealed in the flag, so it wasn't obvious what had happened.