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Regulating T-Jet Downforce

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  • #76
    Originally posted by NicoRosberg. View Post
    Big events like the Fray should only allow parts that are sold on the open market all year around, in my opinion.
    Lucky Bobs Slot Cars stocks both killer bee magnets and 3-lam 16 ohm armatures.

    Originally posted by RichD View Post
    With respect to the availability of Dash parts the main problem would be getting complete rolling chassis
    For someone building a Fray style T-jet racer, I would assume they would start with the bare tub as opposed to a rolling chassis. The bare tubs and gear plates are publicly available for a total of $4.75. I bought some tubs in January 2020 so they've been available at least since them.

    Originally posted by Maddman View Post
    The lift pin is a much simpler Go-No Go solution. Once designed and accepted it can be easily mass produced and sold for a reasonable price. The lift pin worked originally, was adapted to serve as the gravity car tech gauge and can be brought back to work again for T-Jets.
    Making an accurate and repeatable pin seems much more practical and far simpler than making an electronic gizmo. However, I'm curious as I have never seen the lift-pin test in action. Does the pin test account for variations in chassis-to-track clearance from car to car? Given axle hole height variation (or simply raised with the wiz-jet), and that rear tire diameter is not limited by the rules, it seems like a good downforce test should take these variations into consideration along with magnet strength. Can a pin do that?

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    • #77
      Originally posted by Maddman View Post
      The lift pin is a much simpler Go-No Go solution. Once designed and accepted it can be easily mass produced and sold for a reasonable price. The lift pin worked originally, was adapted to serve as the gravity car tech gauge and can be brought back to work again for T-Jets.
      But a digital device would be adaptable to multiple classes, rule changes etc.

      It would also enable racers to tune their cars, rather than use a yes/no device. We have devices for other things that give us quantities, we should have one for downforce. One that measures actual on-track downforce would be best of all.

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      • #78
        The pin test is a magnet strength test. The pin is placed in contact with the bottom of the chassis. It the pin is picked up the magnets are too strong. The G-Car test adds a plastic spacer between the chassis and pin. This allows for minimum tire height. The spacer is a layer of tape (or tubing) applied to the pin. What is required along with the pin is a procedure that would be used to tech the car. This procedure would indicate where to locate the pin on the chassis and what constitutes a passed or failed test. This is why development of the pin and the pin test is not a one person project.

        The purpose of this thread was to have an easy to use tech tool that was accurate, repeatable and reasonably priced. The pin test checks all these boxes. If you want to tune your car then buy a downforce gauge. Just make sure that the car also passes the pin test.

        Personally I have never used a downforce gauge to set up my T-jets. I make sure the chassis is straight. I bench set the tire heights and pickup shoes, put the car together, do some dyno testing to dial the motor and gears in and then start track testing. During track testing I adjust things like shoe position, shoe spring tension, front weight and motor brush type and tension. I will try different rear tires and rims. As the car is set up for minimum tire height I don't play that much with changing tire height during testing. The intended body is on the car at all times it is on the track. If the intended body doesn't work then its time to try another. The end goal is to have a predictable, fast, easy to drive car with no vices. The lap times and the drivability of the car gives you the final answer. Every time I have met those goals I have ended up with a race winning car.
        Last edited by Maddman; 11-04-2021, 07:05 AM.

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        • #79
          Originally posted by NicoRosberg. View Post
          But a digital device would be adaptable to multiple classes, rule changes etc..
          I think it's important to stress the complexity and cost of such a device. Take some existing examples.

          The Doane DigiTune scale is based on a high volume product that costs $13, and a few hand mods and markups push the price up to $75.

          VPR Slot Racing sells what looks like a similar down force scale, the VRP MDM, which is also likely based on a cheap gram scale. That one goes for $145, sold direct.

          VRP also sells a dyno which goes for $265. This one looks to be scratch built, but all it does is measure how fast a roller is spinning, which is absolutely trivial. It's not a true dyno in any way. It does not measure power or torque.

          For a true slot car dyno, check out the MiniPro Inertia Slot Car Dyno V1. This is far 1:24 and 1:32 cars, but is actually real dyno. It sells for $600. But again, it simply measures the speed of the roller together with time. Making these measurements accurately is trivial with modern electronics.

          So what does a fully custom downforce meter that is truly accurate, repeatable, and calibrated cost? One that is not likely to sell more than 1000 pieces in total, even if it were the same price as the Doane DigiTune? I think $600 would be a nice goal, but I think the person who made them would be giving them away at cost or at al loss at that price. And would there still be a market at all at that price?

          That's why I say a metal pin looks like a very practical solution.


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          • #80
            Maddman, I'm a fan of your GT-Jet rules on your Siberia Racing page. I have built one car to your GT-Jet rules and I think it's a great idea.

            You mention the GT-Jet chassis would follow Nitro rules, but the page you link to is no longer accessible. I believe you've also mentioned in the past that you helped to refine the Fray rules at some point.

            If you were asked to fully define your own special class, say as a unique HOPRA class, and write out the full chassis rule set for the GT-Jet, how would you address the various issues with magnets, downforce and "production" parts we've been discussing?

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            • #81
              The lift pin works well for Gravity cars. We use it for our local group that races Gravity. I didn't see the pin on Scale Engineering's site, but it is offered on Lucky Bob's site. If there was a desire to tech magnet strength, this would be the way to go. It's affordable and is a go/no go test. From what I recall, the pin is a John Deere part. Instead of worrying about what it is made of, just use the John Deere part. I agree with Maddman that a group with HOPRA and representatives would need to determine how much tape the pin would need to be wrapped with. Other groups would most likely adopt what these groups define.

              Regarding tire height, unless the car is slammed low like a Fray or E-Fray car, downforce is going quickly decline as the car is raised up. The majority of classes that use skinny tires that I have seen typically have minimum tire heights that put the chassis too high to get any noticeable downforce. Unless you are running low ohm arms, strong magnets are not going to help. Plus, the magnets that really need to be monitored are the AW/JL blue and white magnets, the DASH blue and white magnets, and the killer bee magnets. I haven't seen a huge adoption of the molded magnets OS3 makes, but if they are, the test would already be in place. If it becomes needed different pins could be created for different classes, but in the case of the Fray and HOPRA it would really only be the Fray style cars that would need to be tested. For the clubs I race T-Jets with, all the non-fray classes require stock magnets, so there is no downforce issue there.

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              • #82
                NITRO and the GT-Jet rules were a long time ago. The GT-Jet car was a Fray chassis of the day with spec body mount and a Lexan body, They were quick. In practice my car was over half a second faster than a Fray car running the same front/rear tire setup. NITRO has evolved into the Great Lakes Slot Car Series. Their rules can be found on line and are very similar to the old NITRO rules. The purpose of the NITRO rules was to document the Fray rules of the day which had a number of very vague concepts. I was able to dig down and better define the rules. Some of that was fun as in one case I contacted Tony, Bob and Gary to define the difference between armature balancing and truing. Seems that the Fray mixed the two together. Tony, Bob and Gary disagreed and the NITRO rules specifically defined what Truing and Balancing was. The Fray thought my rules were so good that they borrowed them and put some of their loopholes back in. IMHO they didn't do that great a job on the rewrite but it is what it is.

                If the GT-Jet class was brought forward I would mandate Blue and White ceramic magnets and spec 325/350 tire heights. Front tires and rims would be RTHO. I would allow both Delrin and Brass front rims. Rear tires would be Supertires on 0.170 double flanged rims. One thought is to base the class on the current AW chassis with the traction magnet removed. The AW motor has more than enough power for a Lexan body.

                It might be fun to hand the rear tires out just before the race starts and prohibit rear tire changes during the race.

                It might be necessary to work on the spec rear body mount to make it a bit more robust. That being said the four cars in the race did not experience a body mount failure during the six-hours. After I wrote this post I went back and reviewed the GT-Jet rules and would recommend that the rear mount rules remain as originally written.

                Getting back to the Pin test. One thing I did in NITRO was develop a simple tool that was a precision 16 ohm resistor. It was used to check the DMM before the DMM was used to check armature resistance in post race tech. All DMMs are not make equal and it was good to know that the DMM to be used was accurate before tech started. There were 10 gauges made. Each gauge was numbered and sealed. Each gauge was well within 0.1% of 16 ohms. Cost of the 10 gauges was probably under $10 total. That cost range is what I was thinking of as opposed to dropping $25 - 600 dollars per gauge.
                Last edited by Maddman; 11-06-2021, 02:05 PM.

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