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Equalizing cars with different motors

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  • Equalizing cars with different motors

    As noted in other posts I'm working on building a group of 1:32 scale sprint cars. I bought a bunch of Predator motors from SCC and 4 LVJ sprint car chassis from Professor Motor before finding out they are incompatible due to differences in how motor mount holes are aligned. The LVJ (Parma clone) chassis have vertical holes and the Predator motors have diagonal (FF-050) or horizontal (FC-130) holes in the can. But, when I looked in my motor bin I found several Professor Motor PMTR1500 Hot Rod motors that should easily fit in the LVJ chassis.

    I'll build up a few LVJ chassis with the PM motors, then probably build some simple brass chassis using SCC motor mounts that fit the Predator motors.

    Click image for larger version  Name:	Brew & LVJ chassis.JPG Views:	0 Size:	6.31 MB ID:	75658 Brew chassis design (top) & LVJ chassis (bottom).

    The issue will be if and how to equalize chassis powered by 26K PM motors with chassis powered by 18K Predator motors? The LVJ chassis with 26K motors will be heavier than the chassis I build using Strangebrew's lightweight chassis design, but I don't know if that will be enough. On the 1:1 dirt modified racing circuit when an event was open to both big block (467 CU) and small block (358 CU) cars the big block cars generally had a 200 lb weight disadvantage. This seemed to make the cars relatively even resulting in good racing.

    Is it likely doing something similar with the sprint car chassis can have the same effect? Or, can the motor differences also be addressed with different gear ratios? Any suggestions on ways to equalize these cars with different motors are appreciated.
    Last edited by slothead; January 15, 2021, 08:33 PM.

  • #2
    The simplest way to "equalize" the motors would be to wire a resistor in series with the more powerful motors. What value resistor? To be determined by experiment.

    Or you could wire a diode in series, which will commonly result in an 0.7 volt voltage drop. For a bigger voltage drop add more diodes.

    The equivalent in the real world is "restrictor plate racing".

    Ed Bianchi

    PS - You can find the right value resistor using a "half interval search". Start with a high value resistor. Too high? Use half the resistance for the next try. Too low? Split the difference. Keep cutting the difference in half on each try until you get the value you need. Mathematically this strategy is a very fast way to zero in on an optimal value. It is surprising just how fast.

    PPS - A good way to create a resistor with half the value is to wire two equal resistors in parallel.

    Comment


    • #3
      If you put a resistor in the car to lower the voltage that the motor will see the car's brakes will also be reduced, if you would normally have to turn the brakes down a lot that might not be a problem. If you put a diode in the car there would be no dynamic brakes at all, which might be an issue. A better place to put either a resistor or diodes would be between the controller's white connection and the track white connection.
      One way to determine the value of the resistor that you need to use is to use a rheostat, you can adjust that to get the voltage reduction that you need, measure the resistance, then buy a fixed resistor that is close to the same value.

      Comment


      • #4
        I would think you could do the same thing with gear ratios.

        Comment


        • #5
          Gearing the motor speed down will increase the torque at low RPM. Any tendency for wheelspin on takeoff will be increased. Also there will be more tendency for the rear wheels to lock and slide under braking. Overall the handling of the car will be affected in ways that may not be good.

          RichD is right about the impact on braking of wiring either resistors or diodes in series with the motor. His recommendation for reducing the power at the controller will avoid that.

          In HOCOC gravity class racing it is common to use a controller "choke box" to reduce the track voltage. The reduction in top speed is traded off versus excessive torque that causes wheelspin. At the lower voltage gravity cars tend to be much easier to drive, with reduced spins and deslots.

          Getting things to be "equal" is always a trick. Problem is supposedly identical cars are almost never equal. The best you can hope for is a close approximation. Good luck with that. I expect there will always be disagreement as to what adjustment makes things equal.

          Ed Bianchi
          Last edited by HO RacePro; January 16, 2021, 12:13 PM.

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          • #6
            I see the motor mounting slots are vertical rather than horizontal. Can you turn the motor 90 degrees and screw it to the chassis that way?

            Or solder it in place.
            Let us know what you come up with...

            Scott
            Why doesn't my car run like that?

            Scott

            Comment


            • slothead
              slothead commented
              Editing a comment
              I hadn't thought of trying to rotate the Predator motor to see if the holes in the motor can match the chassis, but worth considering. Soldering them in was my default approach before finding the PM motors in my stash.

          • #7
            Why not just drill the LVJ chassis to accept the Predator motor?
            I seems like you’re making the proverbial mountain out of a mole hill.

            Comment


            • slothead
              slothead commented
              Editing a comment
              I'm not sure the motor mounting plate on the LVJ chassis is wide enough to drill new mounting holes. Soldering the Predator motors in was going to be the default before finding several new PM motors that will easily screw in place. Now it seems sensible to put the right motor in each chassis and see if they are, or can be made to be, relatively 'equal' lap time wise.

          • #8
            Thanks for all the feedback. By equal, I mean in terms of lap times. If that means different driving styles are needed that's okay. It will be a fun challenge to have to adapt to each car's performance. A lot of time trials would be needed to characterize the cars and compare average lap times. I accept no 2 cars can be made truly equal given all of the factors involved, but if I could get the averages close I think that would be sufficient.

            Once built I'll see how much they differ regarding handling and speed and go from there. For starters the cars will have identical guides, gear ratios, and tires. Any guesses which chassis & motor combo will have what initial advantage?

            If there is a significant difference in average speed then I'll start tweaking a single factor at a time to narrow the performance gap. While my first notion was to slow the faster version down, maybe I'll be able to speed up the slower version (gear ratio?). If I do resort to reducing voltage to faster cars I'll see if I can fit a small rheostat on the chassis and make small adjustments to zero in on the goal. If this works I'll leave the rheostat in the car.

            Comment


            • #9
              Soldering is an easy way to mount a motor.

              I tried slowing cars down with lower gear (higher number) . It didn't really work for me. I could get the same speed on straights, but the better motors pulled stronger out of the corners. So that didn't work out for me. I ended up checking rpm and just using motors that all turned about the same rpm. There was still a small difference, but gearing could even that out and make lap times within 3-4 hundreds. I was using $2 motors that were either 26k or 29K. At that price I could but them 10 at a time with no postage charge.

              What frame did Parma make that is the same as the LVJ? I thought the LVJ was a design all it's own.
              Matt B
              So. In
              Crashers

              Comment


              • slothead
                slothead commented
                Editing a comment
                The LVJ chassis at the rear end looks a lot like the Parma Womp chassis. They both take the same type of motor. I have a bunch of 'dirt' modified cars made from narrowed and extended Womp chassis that use the PMTR1500 Hot Rod motor and run great. Womp chassis don't have adjustable wheelbase like the LVJ chassis does.

            • #10
              I like the Piranha motor, fits the LVJ chassis and works good at 12 volts.

              Comment


              • slothead
                slothead commented
                Editing a comment
                Thanks for this info. I bought some Piranha motors a few years ago to build a Showdown car then never did. Don't remember what I did with them, they aren't in my motor bin. I must have put the Showdown parts in their own storage box figuring I'd build a car at a later date.
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