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Weight Placement & Testing

Discussion in 'Building tips' started by gWebber, May 21, 2018.

  1. gWebber

    gWebber Council Champion

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    I had an idea for being able to put weights in a car, do some testing and then be able to move the weights with (I think) reasonable ease.

    First part of the idea is placing the wieghts so they stay, but can be moved.
    I'm thinking this could be accomplished using "Museum Wax" or "Blue Tac".
    (Blue Tac leaves less residue I think).

    Both are used for tacking an object in place but are removable materials.

    Step two is using foil tape to hold while testing , but placing paper over the wieghts so the tape doesn't stick to the wieghts, just to the body ... this should allow for removing the tape, make even tiny adjustments to the wieghts and put back the tape for more testing.

    Might even eliminate glueing altogether.

    Thoughts ?
     
  2. gWebber

    gWebber Council Champion

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    Also on the subject of wieghts .. I can get 12 tungsten cubes (2.030 oz) behind the Axle easy, but I could also get another 0.510 more adding a tungsten plate on top of them and even a good bit more stuffing the rear gap with tungsten putty (and still clear the track).

    So the question is .. how much is too much behind the Axle?
     
  3. King Racing

    King Racing Rail Runner Pro Racer

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    CN I use(and many others) double side carpet tape in the weight pockets and foil tape to cover the weight.
     
  4. B_Regal Racing

    B_Regal Racing PWD Royalty Pro Racer

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    I would not put any more than 2.25 ounces behind the rear axle. While it gives you more horsepower down the hill, its harder to carry the speed in the flat. At least, that's a theory of mine...
     
    Maglev, FDM, Mike A. and 2 others like this.
  5. gWebber

    gWebber Council Champion

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    I just thought the wax would be an easier option for moving weight around during testing, less grip than carpet tape and nearly forever reusable
     
  6. txchemist

    txchemist National Contender

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    Let's take some data. First let's load up a typical Pro style thin body with enough cubes to get to 5 oz. OK, we needed 26 cubes.
    [​IMG]

    [​IMG]
    Now, we use TxW 99.95% tungsten bars and find we have achieved the 5 oz. with just 24 cubes of volume when it is the denser tungsten.
    [​IMG]

    [​IMG]

    Now let's put our first car on three scales and see what the DFW (Dominant front wheel) weight is, and how much imbalance we have on the back two wheels when the cubes are loaded symmetrically.


    [​IMG]




    We have 12.9 grams on the DFW and one back wheel is 71.6 grams while the other is 56.9 grams. Another way to say that is the imbalance is 14.5 grams.



    So after many years, a few clever racers started moving cubes around to see if they could find a "sweat spot" where they got a faster car.



    If we move enough cubes to the DFW side, we see that the back imbalance can drop to very low, less than a gram-BUT, the DFW weight went up to 13.89 grams, so with the typical cubes we can improve balance, but it increases the DFW weight- in other words, your COG is slightly worse when you move the cubes and so some racers do not find any additional speed when they look at balance.


    [​IMG]



    Now let's look at the same body, the same wheels and axles, but we use TxW 99.95% tungsten bars to weight the car. We already know we only need 24 cubes of volume, so first we put the bars on to minimize COG.


    [​IMG]



    We notice that the imbalance is only  12.9 grams, and bingo! we have reduced the DFW weight to 11.78 grams. This car should be faster than the car with 26 cubes, no matter how those cubes get arranged.  Can we get even faster? How about we arrange the smaller bars and look for good balance.


    [​IMG]


    We have an imbalance of only 5 grams, not bad, and what happened to our DFW weight? NOTHING!, If anything it went down a bit. to 11.74 grams.



    Yes, this car can go faster than than all the cars above. But if we are clever to move the cubes and bars around in front of the axle to look for improvements- why should we not move the weight behind the back axle to the DFW side and see how it performs? So here is a look at the body that produced the fastest Cub time at the Mid American Pinewood Derby in 2018.


    [​IMG]



    We have imbalance down to just over 1 gram, and the DFW weight is only 10.2 grams. As you can see, we have plenty of room to find the right DFW weight to get maximum speed without degrading our good balance. At least with the combination of TxW weights and the TxW body, the serious racer can find some speed.



    Our results are summarized in the following chart


    [​IMG]
     
    Last edited: May 24, 2018
    cmoney atrain likes this.
  7. gWebber

    gWebber Council Champion

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    After taking some suggestions to trim the body to six in a row in front of the axle, I reset all the other numbers to represent the car as accurately as I could.
    P.S I'm measuring the wheel at the hub, not the tread as they are Razor wheels ... is that the correct way?

    Unfortunately now the DFW weights nearly two grams more than it did before and no matter how I place the weights it stays very close to this weight.

    While I could take the risk of removing wood on a fully painted car to get the DFW weight down, it would unbalance the car and reduce the car's overall weight meaning I'd have to add weight to get to 5oz, then rebalance the car yet again (not easy with one scale) and odds are I'd still be heavy on the DFW after adding more weight unless I gut it completely.

    Given that I already cut thought the paint slightly with the x-acto knife, I'm not sure I want to gamble with a dremel tool.

    As the car stands it balances on a 1 mm thick ruler edge right where the diagram shows and I'm off center by a tiny fraction.

    So at this point I think given the tools I have , it would be easier to build a new car from scratch than to modify this one further.

    IMG_7924.jpg IMG_7925.JPG
     
  8. B_Regal Racing

    B_Regal Racing PWD Royalty Pro Racer

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    In this case, I would race this current car as is and apply what you learned in a car that is designed for it (or at least, that's what I do).
     
  9. Alphonso "The Fonze" Peluso

    Alphonso "The Fonze" Peluso Pinewood Ninja Pro Racer

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    Yeah c’mon George send that thing in already!
     
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  10. gWebber

    gWebber Council Champion

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    I need to prep the wheels and axels and tune it. So maybe racing in Sept?
     
  11. Derek Konrad

    Derek Konrad Hammering Axles

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    Hey this is a great visual, a few questions, noticed that your Left side where your DFW is looks as if the car body is thinner than the right side? Is that accurate or is that just the photo? If yes, does that introduce a natural steer angle by doing that? Just noticed this
     
  12. gWebber

    gWebber Council Champion

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    Yes, the body is narrowed on the dominant side.
    It doesn’t create any steer as it’s drilled straight.
    A bent axle is used for steer.
     

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