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  • #31
    Fascinating discussion! Gets my electrohead firing for sure. One thing I noticed in the pictures on the tech block is it doesn't appear the front wheels are on the ground. I know M-Murdering mentioned the shoe tuning issues, but there may be too much spring tension as well. I could be wrong (often am, ask my wife) but that what the pic looks like.

    Scott

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    • #32
      My wife and yours must be related

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      • #33
        Mr Murdering is now in possession of said Twins.... and it is definitely in good hands. I'm sure he'll update on the status.
        Thanks to all for the assistance!

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        • #34
          I'm going to try to clarity el Gecko's comment about motors in series -- that the downstream motor sees less power due to the voltage drop of the motor upstream. This is true in a sense. The driving voltage gets divided between the two motors. But if the motors are properly spec'd out, that is no disadvantage.

          To oversimplify things, motors in series should each be rated for half the driving voltage. Motors in parallel should be rated for the full driving voltage. Properly designed, each setup could produce the same power.

          But let me unsimplify things...

          First off, electrically, there is no "upstream" or "downstream". All there is are voltage drops.

          V = I x R is the relevant equation. The voltage drop "V" is equal to the current "I" times the armature resistance "R". That is, when the motors are stalled.

          But when the motors are turning there is another factor causing a voltage drop. That is "Reverse EMF". It is the voltage the motor generates as its armature turns inside its magnetic field. That voltage is always opposite to the voltage driving the motor. At zero RPM the reverse EMF is zero. At maximum speed it is very close to the voltage driving the motor. In between it is proportional to the speed of the motor.

          So we have to modify the above equation...

          V = (I x R) + EMF

          Or figuring in motor speed...

          V = (I x R) + (C x RPM)

          Where "C" is a constant that relates the RPM of the motor to its Reverse EMF.

          So two motors in series can have two different voltage drops IF they are running at two different speeds.

          But none of this has any reference to which motor is "upstream" or "downstream". Even if you can decide which motor is upstream or downstream, it just isn't a factor.

          If we want to get extra picky, there are losses in the connections between the motor brushes and the commutator. Those are not easy to quantify. Things like the eccentricity of the commutator play a big part. But in keeping with traditional engineering practice we'll just say those losses should be small, and we'll ignore them. Saves a lot of problematic modeling to no great effect.

          Once again, a close look at the details of our little toy cars take us quickly into the technical weeds. So much fun!

          Ed Bianchi

          PS - The term "EMF" stands for "Electro-Motive Force", which can be thought of as a more general term for voltage. Physicists seem to feel its a necessary part of the technical nomenclature, since "voltage" refers specifically to "volts", which is a measure of electro-motive force. They don't want to confuse a physical force with a unit of measure for that force. As for confusing college students and civilians, they seem to have no problem with that.

          Here endeth the goldarned lesson.
          Last edited by HO RacePro; 08-11-2019, 06:58 AM.

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          • #35
            If you guys really want to speed up the twins (brilliant project and brilliant name btw), stick a couple of traction magnets in.

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            • #36
              Um, traction magnets actually slow a car down. The cars can just corner faster.

              Same effect as wings on an F1 car. You trade off increased drag for higher downforce. Otherwise DRS (Drag Reduction System) would be pointless.

              The only difference is the downforce and the drag produced by traction magnets is, almost, constant. Wings, on the other hand, produce more downforce with increased speed. It is a square relationship -- double the speed, four times the downforce. And also four times the drag.

              Which is kinda backwards from what would be desired. Wings produce their most downforce in the straights and the least in the curves. That's why the very first wing cars -- Jim Hall's Chaparrals -- had mechanically actuated wings that increased their angle of attack at low speeds and decreased it at high speeds. They produced the most downforce in the corners and the least in the straights.

              I said traction magnet downforce is almost constant. That's because when the rear tires spin up they will expand just a little in diameter due to G-forces, lifting the car and its traction magnets a bit more clear of the track.

              How much will that reduce the magnetic downforce? I remember reading somewhere, quite a few years ago, that the attraction of a magnet to a piece of steel varies as the 7th power of the distance(!!!) Frankly I have never seen an exponent that big in any other physical equation. I'm still not sure I believe it. But I do know a magnet will suck awful darned hard when you get it close to steel.

              If that 7th power relationship is for real, then it becomes very plain to see that even a few thousandths of distance could make a substantial increase or decrease in downforce.

              Some tires for magnet cars have been deliberately designed to provide more lift while powering down the straights, and so reducing the downforce, where it is not needed, and reducing the drag where it is absolutely not needed.

              As for that bit about cornering speed, that only works up to a point. Misjudge a corner just a teeny tiny bit and you fly cross-country, into a wall or onto the floor. I'm happier having a margin for error, where I can use my skill to pull it back in and go on with the race. Gravity cars are quite fast enough for me, thank you.

              I do make some very fast gravity cars!

              Ed Bianchi

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              • #37
                I meant speed up in terms of better lap times.

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                • #38
                  I knew what you meant. It was designed without traction magnets and I think that's a good thing.

                  Ed Bianchi

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                  • #39
                    Yep, there's a plethora of magnet cars about... With the Twins.... I was shooting for gravity only. Now....as soon as my K motors arrive.... hmmmmm.

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                    • #40
                      I just thought of one advantage to having two motors wired in series. As I mentioned earlier when you only have one motor you usually need a resistor or diodes wired in series, even when you are running at 12 volts. I find that I need to choke back to about 9 volts. If you were running the track at 18 volts and had two motors wired in series each motor would see 9 volts.
                      I see that someone owes me a bottle of water, but I am not gloating or anything.

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                      • #41
                        If you don't mind luke warm, I will send you one!

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                        • #42

                          Minnesota Twinn 1: Sleeping Beauty

                          H0Bro's pix dont do this build justice. At a glance, it appears to be a beast; but actually, she's very refined. I proceeded with some reverence for it's originality, and tried to make the platform work as delivered.

                          On the finger roll, the dually set up meshes smooth as silk, but the first voyage was a sputtering no go. The motors ramped up nicely on the gator clips, with no weird oscillation or vibration in the rear components, and I'm glad. That rear section is a tricky little bit of fun, so it was good news to have the tail be skookum.

                          Nice motor ramping when energized from the shoes is good indication that the shoe geometry requires some adjustment, as was shown by the lack of lineal burn we saw on the contact patches in the original pic. Per earlier observation, the contact patches needed heeled down, with an equal amount of toe toe-up rolled into the shoes. As mentioned previously, the intention is to have the shoes parallel to the rail, with the chassis's front shoe hanger centered in the shoe's hanger window. This subtle angular adjustment is frequently over looked, when folks raise or lower their sprung hard-shoe cars.

                          The second voyage resulted in some consistent motation, but was a jerky clatter fest. It was obvious that we werent up on plane yet. Re-inspection revealed three different, but related issues, that were scrubbing speed/performance. The Twinns sit on a flexi design chassis using a traditional, longitudinal piano wire, amidships. The "twang" is held in check laterally by slip pins in the port and starboard frame rails, at the rear bulkhead. I studied on the aftermath for a bit.

                          1. The motors were dropping through the pan and dragging the rail with the chassis at rest. Evidenced by the racket, and obvious scuffing on the cans. The drag is exacerbated by the motor's magnetic attraction to the rail. Merely a lack of pre-load on the piano wire, allowing the motors belly down too far. A slot car hernia of sorts. Easily remedied by a calculated bend, so that the motors had clearance over the rail, at the maximum downward hinge of the chassis. This subtle adjustment made the chassis functional.

                          2. At first glance the shoe module always had a bit much back angle. Even though I wrestled the shoes around, the burn was still off. We were still plowing, evidenced just by the sound. Skkkkkkkkkuh .... is not a happy shoe. Closer inspection revealed that the shoe module was epoxied in, but had slipped the clamp at some point, and sagged down at the rear. I just broke it open, surgically removed the excess epoxy; and re-epoxied the shoe module into a more level position, relative to the chassis line. This brought the contact patches into a location where one can fine tune the shoes fractionally, for any rail or profile height differentials.

                          The re-test was sweet silence, other than the metal guide pin banging around the slot as the chassis side loaded in the turns. This chassis is noticeably clean and flat through the turns. With a build triangle height of 1.60", one would expect a little more "on throttle" push coming off the apex, but typical of drop through arrangements exerting some anti-gravity effect, this chassis holds the exit until the bitter end, then comes right with a quick shake of the tail. (Think "drop through" Riggen.) I did not open the cans, but based on the lack of ferrous objects "porcupined" to the motors after being on my bench, and the easy finger roll, it's safe to assume the motor magnets are ceramic ... but one has to remember to double the width of the magnetic effect, because its a dual motor.

                          3. There was still some abnormal wiggle and burn when coming back on the throttle, considering the silly foams fitted were virtually new; indicating she was still plowing, if ever so slightly. The cause? Random front hub drag. Fairly common. It will generally give you a twitch where the exit transitions into a straight. On the bench things turn freely when you flick them with your finger, but add a little side load and things start grinding or bucking. The independent front wheels are 'loominum of course, in contact against a brass alloy frame rail. Not my favorite materials to be side loading in the first place. The axle carriers were filed back a skoshe on the vertical line. Then some Wizzard nylo spacers were squeezed in to eliminate the hub scuffing up front. The frame spacing is right at the ragged edge, so you have to center and set the axle between the spacers yourself. H0bro can take a little off the back of the hubs, if he wishes.

                          All work was minimal and non invasive. Although H0bro covered the bases and sent extra motors, I made the choice not to violate her. Personally, I wouldnt change one dang thing about this build. It's a wonderful project, that should have a clear or tilt nose lexan mounted on it; in order to show off it's unique and intricately delicate construction. She's not a blistering missile, but plenty fast enough to light them up, slide the turns, and now able get out of her own way at 18v.

                          *********

                          The instructive bit is, that other than a few degrees up on the hinge wire, and few degrees back on the shoe carrier; this build was essentially RTR.



                          Last edited by model murdering; 08-12-2019, 08:23 PM.

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                          • #43
                            Bill, cannot begin to thank you enough for taking a look at the Twins! I had been happy using the conventional shoe module (a sawed off BRST G chassis secured to a brass pan), and on this iteration... I went a bit overboard on ensuring that the module was bullet-proof enough to stay secured to the brass...hence the brass posts that ultimately caused the front wheel snagging. Regarding that, back to the old drawing board. I am going to try using this method again, but perhaps constructing my own shoe module...maybe with Plastruct. One thing that was mentioned by another poster...was the appearance that the front tires were not actually touching the track...from what it appeared to me... the spring tension was just too strong for the weight of the front end (I used .007 springs) does anyone know if there is a .006 made? Otherwise, I could try removing a coil or two. Thoughts?

                            Bill, thanks again!! Mucho appreciated!

                            -Jeff

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                            • #44
                              I'd like to read MM's opinion on how well controlled the rear axle is, given its unconventional inboard bushings.

                              Also, there is a brass post mounted at the rear of the chassis. I suspect its function is to keep the body from contacting the rear tires. True or not?

                              The dual motor drive might result in more powerful braking. Zat so?

                              Ed Bianchi

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                              • #45
                                Hi Ed. The brass post was just something put in to firm up the axle when I installed it. And that reminds me... I need to read through your soldering tutorial a ba-gillion more times. Thanks for that!

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