Test Weighing Your Car

After the car is painted and the wheels and axles are prepared, it is time to determine just how much weight needs to be added to bring the car up to 5.000 ounces. It is easiest to do this while all of the parts are still loose.

Tools and materials you'll need:

  • Safety goggles for both you and your son
    (If you are using a band saw or grinder for this process)
  • Electronic scale or balance with 0.1-gram or better resolution
  • Weight stock (We used 0.40" diameter brass rod)
  • Band saw (easy) or hacksaw (safer but slower)
  • Grinder (easy) or file (safer but slower)
  • 5 each 1/2" #4 flat-head brass wood screws

First of all, let's discuss what we are trying to do, and why. As stated in the introductory sections of this writeup, we want the car to be just over 5.000 ounces or 141.643 grams with all of the weight-tuning screws in place and just under that with all of them out. We'll shoot for a setup that needs two or three screws to be exactly on, allowing about a gram of "slop" for the official weigh-in.

Steps in the process:

  • Place the car body, wheels, axles, and all of the tuning weight screws on the scale, and measure the mass.
  • Subtract what you got from 141.643. This is how much mass you will have to add, plus a little more.
  • If you are using rod stock or some other "cut-to-fit" weight inserts, cut a piece exactly an inch long and measure that alone on the scale. Divide the "weight needed" from the previous step by the "weight per inch" and you'll know how many inches of weight stock you need to get to 5 ounces. If you are using the car design in the drawings, you will want the weight in three roughly-equal pieces, so divide your "inches of weight" by three.
  • Cut several pieces of your weight stock to the length computed above. Don't worry if you are off by 1/16" or so, and do make a point of cutting one or two just a bit over the calculated length. Grind or file the ends smooth.
  • With the car, wheels, axles, and screws on the scale, experiment with your pieces of weight stock until you come up with a combination of three pieces which bring the total mass up to just under 143 grams, or about 1.0 to 1.5 grams over the limit. You may have to file a little off some of the pieces of weight stock to get exactly where you want to be. You should stop when the total is between 142.5 grams and 143.0 grams.