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Minnesota Twins

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  • #16
    Thanks all! I will post some more pictures ASAP. OK, when I first put it on the track, the first thing that was noticeable was the lack of rear-end hoppage when accelerating (at least compared to the few brass cars I have built). Not sure if that has anything to do from the twin 'screws'. Sadly... the second thing I noticed was the severe lack of top end..talking wounded animal slow! Granted, the motors I used are an unknown entity...have not used them before. My first thought was to remove one of the motors, and see the result. However, if I do that, I lose the axle centering. So I am considering this.... replacing one of the pinions with a 1.5MM pinion.... that way the axle stays centered. Thoughts? My other option is to just build a new (single engine) chassis using one of these motors.


    • #17
      OR..... replacing the motors with two different ones.


      • #18


        • #19


          • #20
            Oh, and the pinions are LL. The crowns are Viper.


            • #21
              A suggestion...

              I'd reverse the positioning of the two crown gears, having them inboard of the pinions instead of outboard. That would allow you to have the rear axle bushings outboard, in a more conventional position. From what I can see I believe there is adequate space between the pinions for the crown gears.

              As for rear axle hop -- it shouldn't happen. Sloppy bushings, a bent axle or out-of-round tires are the culprits. Excessive power on takeoff can cause the car to break traction and misbehave, but once rolling axles should not hop.

              Twin motor setups have never seemed to live up to expectations. Don't ask me why. I can think of no theoretical reason why that should be. Do the two motors end up fighting each other? Is there some sort of resonance happening? I really don't know. It would be very interesting to get some answers.

              Ed Bianchi


              • #22
                Thanks Ed. I am going to try and determine if the two motors are coming to blows with one another.


                • #23
                  Lost again, drinks are on me.


                  • #24
                    Thanx for the pix!

                    Runs like a wounded animal? Yeah, I've been there many times.

                    At a glance, your shoe geometry is raked too far forward. See where it is immediately cleaving the toe right at the upward roll, after very low minutes? To get her off the leading edge and up on plane, those shoes will have to be heeled down AND perhaps even some simultaneous toe up; in order to stretch the burn along the shoes intended contact patch. As it sits, you're using very little of the shoe's available contact patch. Think of a water or snow ski. That's the face plant position. Max friction, erratic current transfer, and most importantly? No glide! Mash the back of those slippers down, and hopefully that should wake her up a little. (keep in mind that any plowing condition up front will exacerbate wheel hop and chatter) You have to have the contact patch parallel to the rail with the shoe in the middle of it's travel range. Easy to type, shoe voodoo is not always easy to execute; but it gets a bit easier with every build.

                    I've toyed with a similar rear arrangement in the past. While a centered, single, rear axle bearing looks mega cool, my concern was always that the any excess tolerance at the center is factored up by the time you get down the axle length to the ends. Essentially, there is no triangulation at the critical point(s). Which is kinda what I think Ed is driving at.Typically bearings are always shouldered up close to the load. With the dual drive, in a straight axle configuration, using a centered single bearing; any available tolerance can and will be used against you. When one side is driven in any particular direction the other side executes the mirror opposite. Think of what a compass needle does when you bump the glass.

                    Ya got this O><O, a teeter totter with pointed fulcrum providing the opportunity for unwanted oscillation at the rims

                    Hence the wabos and bouncing.

                    Ya want this ol------lo, a park bench stabilized at both ends.

                    I never got past the playground. ; )


                    • #25
                      That 1/24th twin-motor chassis has three bushings on the rear axle. Not at all normal practice. Unless all three bushings are precisely lined up they will bind on the axle and create drag.

                      That assumes a conventional single axle in the rear. But if the axle is split -- half and half -- then that center support makes sense. It would have four bushings, with two in the center, which actually appears to be the case.

                      De-coupling the two motors by giving each a separate half-axle, and a separate wheel, might obviate (your word for the day) any issues with the motors interacting with each other. On the other hand, having two drives operating independently of each other, each driving one wheel, may make handling unstable. It would also give the car a differential effect during cornering. The inside wheel will be able to turn slower than the outside wheel, which could improve traction overall.

                      As noted above, the crown gears are not installed in opposite directions. So both motors turn in the same direction. This allows both motors to have a timing advance in the same direction. The motors can be identical.

                      Finally -- though I am not sure of this -- it appears to me that the chassis is split and pivoted somehow. So that the front axle and guide can rotate in the 'roll' direction versus the rear axle and drives. Pivoting chassis like this have been shown to have advantages on tracks with banking.

                      I'd love to hear more about this chassis, and any others like it. It is the kind of outside-the-box engineering that I find so fascinating. How well it worked, or didn't work, is something I'd dearly love to know.

                      Ed Bianchi


                      • #26
                        Usually those can style motors have have more power than you can use in a gravity type car, you either have to include a resistor or choke back on the power some other way to have a car that you can drive. In any case that is an interesting build, but I am not sure if the extra power would be of any benefit. The ultimate answer is to build the car and give it a spin.
                        Someone mentioned wiring the motors in series, I was trying to figure out what the advantage to doing that might be. If the motors were essentially identical and were wired in parallel they would both see the same voltage and draw the same amperage. If the motors both had 6 ohm armatures at 18 volts they would each pull 3 amps at startup for a total of 6 amps. A DC motor is only a resistor when it is not turning however, my own measurements indicate that on the track the car would pull far less than that, between 0.25 and 0.5 amps for a car with one motor. The effective resistance of the motor goes up as the RPMs increase. A motor that is turning generates a voltage that opposes the voltage from the power supply, if the motors are turning in opposite directions that effect might get cancelled out. With two motors in series there is only a single path for electricity, so the resistance when the motors were not turning would be 12 ohms and the current draw would be 1.5 ohms. Once the motors were turning I am not positive what the effect would be, especially if one motor was turning backwards. It would be easy enough to do a bench test with two separate motors, but what would happen when the motors were pushing a car around the track would probably be a different matter.


                        • #27
                          I have been trying to puzzle out the interaction between two permanent magnet DC (PMDC) motors when wired in series or in parallel. Consider the following facts...

                          The reverse EMF generated by such a motor is directly proportional to its RPM's. At zero rpm there is zero reverse EMF. At no-load maximum rpm the reverse EMF is very close to the voltage driving the motor. The difference is due to the power needed to keep the motor spinning that fast, countering windage and frictional losses.

                          The torque output of a PMDC motor is directly proportional to the current passing through it. Given that fact, two PMDC motors wired in series will have the same torque output, since they both pass the same current. However, the POWER output of those motors is proportional to their rpm's times the torque output. So it is entirely possible that, if the motors are spinning at different speeds, their power outputs will be different, with the faster motor producing more power than the slower one.

                          However, if those two motors are wired in series they are driven by the same voltage, but can pass entirely different currents, again depending on their rpm's. The slower motor will pass a higher current. The power output of a PMDC motor is a curve that peaks near the middle of its speed range, so which motor is producing more power gets complicated. A motor running at the middle of its speed range will produce more power than one running either faster or slower, given the same driving voltage.

                          Now gather up all these facts and mush them together, and you should be able to come up with predictions about how those motors will behave under load.

                          And that is what has had me mentally running in circles for the last few days. There is something counter-intuitive in that mix which sets off my illogic alarm. Is there a feedback loop here? Is it stable or does it slam against the stops?

                          Further cogitation may ensue. Insights may be acquired. Or not.

                          Ed Bianchi
                          Last edited by HO RacePro; 08-09-2019, 04:59 AM.


                          • #28
                            If the motors are wired in series the same current has to go through both of them. If both of the motors had armatures that were the same ohm value the voltage drop across each one would be the same, at least while they were in a stalled condition. With mismatched motors each motor would have a different voltage drop. Once the motors are turning things get more complicated. Motors that seem to have the same measurements often perform differently on the track. As far as the back EMF goes I did an experiment with a pair of similar 1/32nd motors. I used one motor to drive the other one, I applied 12 volts to the drive motor and measured 8 volts at the driven motor, that was with the motors turning at about 20K RPM. If I applied a short across the driven motor the RPMs went down by a considerable amount. A number of years ago someone posted about a similar experiment that he had done, he looked at the output of the driven motor with an oscilloscope and found that the voltages generated by the individual poles were not necessarily the same. The ohm value of each pole could be different, the mass of each pole could be different, and the number of turns of wire on each pole could be different. The wire itself could be distributed differently on each pole as well. All of those factors would alter the strength and shape of the magnetic field around each pole as it was energized.


                            • #29
                              R/C guys generally think parallel is better, since in series the "downstream" motor sees a voltage/current drop. This makes sense because all of the current/voltage from the positive terminal runs THROUGH the upstream motor first, before it gets to the downstream motor, with a bit of loss through the brushes/comm.

                              Parallel wiring gives both motors the same voltage, but doubles the current requirement on the power supply. If each motor pulls 3A (for example) at start, you'd need a 6A minimum power supply to compensate.

                              Series only allows 1/2 of the track voltage to each motor, and only 1 motor worth of current is required of the power supply. You'd have to double the track voltage to get the same top speed. (speed=voltage, torque=current)


                              • #30
                                "Series" may not be a negative in this particular case. The RC motors used in RC are rated for the available power. The RC motors used by slotters are not matched to the available power, so more advanced slotter use trickerations. Except for the Mega Gee Plus, all the mini motors are under rated for H0's standard 18 volts. If your cutting the track voltage in half to the motors, by wiring in series, one may actually be encroaching on the sweet spot. One might not need a choke, an on board resistor, or variable power.
                                Last edited by model murdering; 08-09-2019, 08:45 AM.