The Science Olympiad rules limit the maximum rubber weight to 2 grams. Because the quantity of rubber is limited, we need to be sure that we are using the most rubber allowable per the rules. Your motors should weigh between 1.90gr and 2.00gr, any motors less than 1.90gr should really not be used because precious turns and energy are lost.

Depending on your model, it may fly on rubber anywhere between .080" and .100". And since the weight is limited, the thinner the rubber the longer it will be. Here is a rubber chart to help you make your motors. This chart gives you the approximate length of a 2 gram loop of rubber with o-rings. These values are approximate and really depend on the specific type of rubber you have. Some rubber is thicker and has slightly different densities than others, but this will give you a starting point. This chart also gives a 'safe' and 'maximum' amount of turns expected for each of the different sizes.
 
Rubber width (inches) Loop length for 2 grams
(inches)		 Safe number of turns Maximum number of turns Safe turns per inch loop Max turns per inch loop
.080 18 2095 2335 116.4 129.7
.085 17 1920 2140 112.9 125.8
.090 16 1755 1955 109.7 122.3
.095 15 1605 1785 106.8 119.0
.100 14 1455 1625 104.1 116.0

I would highly suggest using a pair of o-rings on your motor to help with the winding and flying. For the Science Olympiad models I would use a -005 o-ring. You may be able to find these at a hardware store but if you have difficulty then I would buy some from a tool supply house or hydraulic/pneumatic fitting supplier. I would suggest McMaster-Carr, part number 9452K13, Buna N O-ring size -005, $3 per 100pcs.
www.mcmaster.com    Ph: 330-995-5500

  

To make the motor, cut enough rubber to make a loop about 3/4 to 1" longer than the desired final length. Slide the rubber strip into the o-rings and then tie a knot as shown in the picture sequence below.

Now pull the rubber slightly to create a loose knot. If you have a scale, now would be a good time to check the weight of the loop. Because there is some extra rubber on our loop, the motor will weigh over 2 grams, like about 2.05 grams. When we finish the knot and cut off the excess, the motor should come in just under 2 grams. This process will require a few trial and errors on your part to see how much weight is subtracted when you do this process.

If the rubber weight is too much loosen the knot a little and shorten the loop and cut off the excess rubber so that there is again about 3/4 to 1" of excess rubber. If the loop is 2.05 grams or less then pull the knot tight.

Then tie three overhand knots into the rubber ends to secure the knot. Pull the knot tight each time.

Then cut off the excess rubber.

The motor is now finished, check the weight to make sure it is less than 2 grams.

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