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1/32nd Direct Drive

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  • #31
    That is so strange. The photos showed up after I posted my reply.


    • #32
      About the motors...

      The pre-1962 car has an H&K "Jack Rabbit" motor, which is the only motor allowed for that class.

      The unlimited car has a Mid-American Products "The Phoenix" motor. IHRS guru Sherman "Doc" Collins suggested that motor to me. It has rare-earth magnets, which should help with braking -- an area where direct-drive cars are notoriously deficient.

      While the Phoenix motor demonstrates the 'notchiness' typical of a rare-earth motor, it is not so clear the braking is significantly stronger than a more conventional motor. I noticed that during testing. The car with the Phoenix motor did not seem to brake more powerfully than is typical of a direct-drive car. On reflection, the braking power of a motor is more a function of the armature wind than the magnets. Stronger magnets should make a motor more efficient, which will help braking. But electric motors are typically very efficient to start with, so the stronger magnets may not improve their efficiency, and braking, all that much.

      Both motors deliver enough torque to chirp the tires on takeoff. Which I believe is evidence that gearing is not required for adequate acceleration. Excessive torque on takeoff just leads to tire spin, which not only wastes power, but can impair handling.

      And there is strong evidence from the history of direct-drive in HO cars that top speed is greatly increased by not requiring the motor to spin 2 or 3 times faster than it needs to. The top speed of a permanent-magnet DC motor is limited by the reverse-EMF it generates, which is directly related to the motor's RPM's.

      I do realize that the relative size of the motors versus the size of the cars is very different in 1/32nd and HO scale. An HO scale car is pretty much a body wrapped around a motor, while a 1/32nd scale car can hide the motor back under the trunk. So the power on tap versus the size of the car may make a real difference in how the cars perform.

      Does the reduced torque of a 1/32nd direct-drive car limit the top speed to the point where a geared car will have an advantage? This is still to be determined.

      The race tomorrow on the Hellanbach track may give an answer. The track is legendary for being huge and fast. I've never raced on it myself, but from what I hear it should be just the venue for demonstrating the top-end performance of my cars.

      Ed Bianchi
      Last edited by HO RacePro; 12-14-2018, 06:04 AM.


      • #33
        Originally posted by HO RacePro View Post
        About the motors...

        The pre-1962 car has an H&K "Jack Rabbit" motor, which is the only motor allowed for that class.

        Ed Bianchi
        Hey Ed. Actually it's not the only motor allowed in the class. Any motor (other than slim can, they're banned) up to 14.7K rpm can be used. The Jack Rabbit is the motor of choice for most, but it's not mandated.

        Anxious to see what you're running tomorrow at HellenBach!



        • #34
          Well shucks, for technical reasons I did not get to race either of my direct-drive cars at Hellanbach yesterday.

          The pre-1962 chassis didn't pass the ground clearance test. I coulda, I shoulda. Oh well.

          Prior to the tech inspection I did get to drive that car in practice. It was fast, and for the most part handled nicely. But it did have a tendency to roll over in corners. It definitely needed some weight added down low. Unfortunately I had no way to do that on site. Anyway, it failed tech, so that issue was moot.

          My issue with the unlimited chassis was far stranger. It had gobs of ground clearance. Too much, really. But when I put my car on the track during practice, nothing happened. Turned out the track fuse for that lane had blown. It got replaced, and that fuse blew too. Checked the pickup braids -- they looked good, but the next fuse blew.

          Larry was running out of fuses. I decided, rather than risk shutting down the track, I withdrew the car, and my controller to boot. I borrowed a controller from Sherman and used that the rest of the day. I ran all of the races, but my direct-drive cars stayed parked.

          On a powered test block the car ran fine. I have yet to test my controller. I need to see if the motor draws more than 5 amps at startup. But the car never moved before those fuses blew. I'm confused.

          It just wasn't a stellar day for me. I don't think I was dead last in any of the races, but not by much. Still, it was fun to race. I did turn in a few good laps. And it was great to see everybody again.

          Those two chassis may never get to race. All day I was thinking about how I could improve them, and by the end of the day I was certain my best move would be to build two new chassis, incorporating a number of different design features, including ways to adjust the heights of major parts of the chassis.

          The next race is on Saturday January 12th, so I have almost a month to fabricate those two cars.

          To be continued...

          Ed Bianchi
          Last edited by HO RacePro; 12-16-2018, 01:37 PM.


          • #35
            So I tested the car and the controller. The controller is fine.

            But when I tested the motor of the unlimited car I got ohm readings in the 2 ohm range. At 12 volts that would mean the startup draw would be about 6 amps. The track was fused at 5 amps, so that should explain why I kept blowing the fuses.

            Two things bother me. One is I always seem to get flaky readings when trying to read the ohms of a motor. I use a Fluke multimeter, which is considered a professional-grade instrument. The best I can figure is carbon dust on the commutator -- from the carbon brushes -- complicates the issue. This is an old problem. I'm never confident of the resistance of an armature unless I take it out of the motor and probe the commutator directly.

            The other thing that bothers me is that the car did not even twitch before the fuses blew. There should have been some response. I might theorize that the motor's super-strong magnets held the armature locked in place for the fraction of a second it took to blow the fuse. But that's a guess.

            I probably should not be using a low-ohm motor for a direct-drive car anyway. Low ohms mean few windings, which results in high RPM's but low torque. But I do like the rare-earth magnets. Maybe I'm going to swap a higher-ohm armature into that motor, assuming it can be done.

            Ed Bianchi
            Last edited by HO RacePro; 12-16-2018, 01:48 PM.


            • #36
              Originally posted by HO RacePro View Post
              . Maybe I'm going to swap a higher-ohm armature into that motor, assuming it can be done.
              It can be done. The Phoenix motor is a standard size, pretty much any arm from that size of motor will fit and run. There's a huge range of motor specs available to experiment with. That size motor normally has the the end-bell held on with 4 bent over tabs. It's not normally practical to bend back and reuse the tabs, but they can easily be ground off with a Dremel. That gives a problem with fixing the endbell back on. Sticky tape will work but maybe that's not robust enough for direct drive. If that turns out to be a problem screws or soldering the plate on would be worth a try.
              (Armatures from S can and many 16d size motors will physically fit, but won't run properly unless the comm is rotated to suit the different brush gear.)

              Originally posted by HO RacePro View Post

              Two things bother me. One is I always seem to get flaky readings when trying to read the ohms of a motor. I use a Fluke multimeter, which is considered a professional-grade instrument. The best I can figure is carbon dust on the commutator -- from the carbon brushes -- complicates the issue. This is an old problem. I'm never confident of the resistance of an armature unless I take it out of the motor and probe the commutator directly.
              That's a standard measurement problem, nothing to do with the quality of the meter. As you say, probing direct onto the comm gives more reliable measurements. That works best with the standard 4 probe connection for low resistance measurements.


              • #37
                Yesterday I opened the Phoenix motor and tested the armature, probing the commutator directly. I was surprised to see it ohmed out at 0.7 ohms! That means, at least in theory, the startup draw of the motor on 12 volts would be a bit more than 17 amps! No wonder the 5 amp fuses popped so quickly!

                I am falling back to using the JK Product M20 Hawk DD motor, at least for now. Tested without disassembly it ohms out at about 6 ohms. That would give it a startup draw of 2 amps -- possibly more, but hopefully less than 5 amps.

                To be continued...

                Ed Bianchi


                • #38
                  Are you sure that the commutator is not shorted? 0.7 ohms sounds low, however I am not familiar with that motor. In any case since the motor is not geared down it will tend to pull more amps on the track at start up than it would in a regular car.


                  • #39
                    Around O.7 ohms is about right for the hottest slot racing winds used in that size motor.
                    The lower performance ones typically used in plastic chassis cars will have considerably more resistance.
                    Really quick slot racing motors will have considerably less than O.7 ohms resistance, but they have brush gear capable of handling higher currents and are run on tracks that can supply the 20+ amps they need.
                    Last edited by Al's slotracing; 12-17-2018, 08:32 AM.


                    • #40
                      A regular 16D motor has a 0.7 ohm armature, that is about the mildest motor that is used on commercial 1/24th tracks. At 12 volts the starting amperage would indeed be 17 amps, that is why my 20 amp Pyramid power supply screams when I first turn one of those motors on. You could try slow blow fuses, start at maybe 5 amps and go up if that blows, or down if it doesn't. If my calculations are correct the car will be using about 2.6 amps once it gets rolling. If the track uses a power supply with a digital display you can use a cell phone camera to do a video while the car is running, like this one:
                      In that case I was running a car with a 4 ohm armature at 18.5 volts for a calculated stall current of 4.6 amps, but on the track the amps peaked at 0.7, except at the end when I did a standing start that took 1.2 amps.


                      • #41
                        Direct Drive Mark 2

                        I spent a whole bunch of time getting a pair of new direct-drive chassis built for the IHSR race in Rockville Maryland today. But with the weather so iffy -- a good chance of significant snow later today, I decided to stay home. Rockville is a about a two-hour drive from here, and I didn't want to risk a long drive home in dangerous conditions. There is another race next Saturday and, weather permitting, I'll debut both of these cars there.

                        I decided to build all-new chassis for my two direct-drive cars. I had too many improvements (I hope they are improvements!) to make to try and salvage my Mark 1 chassis.

                        For the pre-1962 vintage IHSR class I picked on a J&R Products 'Jack Rabbit' motor -- which is recommended for the class -- and solid brass bar stock 1/16" thick by 1" wide as the basis for the chassis. I also decided to create independent front wheels. I chose to bolt the whole chassis together, as much as possible, rather than solder everything. I wanted to be able to shim the heights of the front axle and guide. I used a whole lot of UNC #4-40 flathead screws, with matching machine nuts, all in 18-8 stainless steel.

                        One nice thing about using 18-8 stainless is it is non-magnetic. Makes things easier to assemble in the presence of motor magnets.

                        At some point in the future I plan to trim the length of all those screws. For now, I am leaving them long so I can adjust the shimming.

                        First, the pre-1962 vintage car -- a Cooper with a single direct-drive motor:

                        The motor mounting system -- common to both cars. has a flat brass plate glued to the side of the motor, first with superglue, then with JB Weld epoxy. That plate is fastened to an L-shaped bracket with Bo-Link servo tape. The servo tape is a foam tape, rather thin, extremely strong, and gives a bit of flex and shock absorption to the chassis. The body mount is a thin brass plate, loosely mounted to the chassis, so the body has a little bit of freedom to move.

                        The front axle assembly is bolted on so its height can be shimmed. The wheels rotate independently, held in place by tiny brass washers soldered onto the ends of the axle. The axle sits right above the guide post, which created a clearance problem. Said problem was solved by grinding a bit of the axle away directly over the guide post, and also by using a tiny screw and washer to hold the guide on, rather than the normal machine nut.

                        For the unlimited class I have built a dual-motor direct-drive chassis, covered by a Mazda sports car body. Both motors are JR Products "M20 Hawk DD" -- a motor used in their own direct drive chassis. I measured their armatures as 4.3 ohms. Assuming perfect, zero-ohm contact and circuitry, that makes the car's start-up current draw 5.2 amps. In the real world it will be a scootch less, hopefully less than 5 amps.

                        The construction of the chassis is similar to the other car, with a few major differences. For one, the front motor is mounted so it can rotate a bit in the 'roll' direction. (Don't understand roll? See below.) And, inspired by space constraints, the guide is mounted on a drop-arm. The guide is held on by a conventional machine nut.

                        The pivoting hardware for the front motor is shown here. The thick brass plate has a flat-head screw mounted in a tapered through-hole and soldered in place. A pair of short brass tube sections form a bushing assembly, and everything is held together with a pair of machine nuts. This setup allows the clearance inside the assembly to be adjusted for free movement with minimal clearance.

                        The drop arm is a soldered-up assembly of 1/4" x 1/16" brass bar and a short length of brass tubing. This all pivots about a length of UNC 2-56 threaded rod, towards the rear of the chassis, secured with machine nuts, which rotates in a close-fitting length of brass tubing soldered to a brass plate. That brass plate is bolted to the chassis.

                        The above photo shows how the front motor pivots (That's roll, see?). The point of designing in that pivot is to keep all four tires in contact with the road while negotiating banked sections of track, which are quite common in the routed tracks we race on. The ability of the front axle to pivot has proven to be a significant advantage in the custom HO chassis raced in the HOCOC unlimited class. Early testing seems to indicate that this chassis does indeed corner well on banked curves.

                        That same pivoting action has a second, potential advantage, that I am not going to disclose here. At the moment I have not integrated that feature into this car, and I may never do that. Right now I don't think it is needed.

                        Both cars have been tested on my 4 x 12 foot high-banked oval HO track. The slots on that track are not deep enough for the guides -- the bottom of the guides drag on the bottom of the slots. Also, the braids on that track are only 1/8" wide, which makes power pickup touchy.

                        Excellent electrical contact appears to be essential to these cars performance. If the braids are not adjusted just right there is a notable drop-off in acceleration and speed. I am hoping when these cars are run on a 1/32nd scale routed track the wider track braids will obviate this problem.

                        Regardless, at the moment, from what testing I have done, I feel both of these cars have potential. We'll see how they do in a race, hopefully next weekend.

                        Ed Bianchi
                        Last edited by HO RacePro; 01-12-2019, 02:19 PM.