Knot tying: https://www.hippocketaeronautics.com/hpa_forum/index.php?topic=3650.0 See reply #2 in this Hip Pocket Aeronautics thread. A couple possible changes to the pictured knot tying: you don’t need to pull the first knot all the way to the end of the rubber “ends”. You can leave approx 5/16″ “tails”. Then, after the second knot step and everything pulled tight, you can cut 1/4″ off the tails, leaving very short 1/16″ tails. If you weigh the motor with o-rings as you trim it to length/weight and trim it till it’s 1.52 grams, and then cut 1/4″ off the tails, you will have a 1.485-1.49 gram motor. Perfect for 2023-24 Science Olympiad rules. Also, not shown in the pictures, after tying the first knot and “pulling” it to within 5/16″ of the ends, pull it very tight for about 15-20 seconds. Then after tying the second knot and pulling it against the first knot, also pull both knots (holding loop end and also holding tails) very tight for 15-20 seconds. Also, when the knot is completely finished, you can put a very tiny drop of CA glue between the 1/16″ tails for super security.
Although Tan Super Sport rubber band is sold in “inch widths”, this is not the best measure of a rubber band motor. The best measure is average linear density (ex. grams/inch). This is simply the weight of the motor divided by the length of rubber strip that makes up the motor (ex. from above 1.49 gram motor minus o-ring weight of 0.09 grams divided by the length of rubber strip 11.8″ times two plus 0.5″ of rubber in the knot = 1.4g/24.1in = 0.0581 g/in. Density is best as rubber strip actually varies up up to 5% in thickness or density inch by inch over its length. So, if you measure only by the vendors published “width” you actually have as much as 5% variance between the various motors you are flying with (ex. 0.094″ rubber is commonly used for SO actually ranges in density from .062 g/in to .065 g/in and a these two densities will fly your airplane dramatically different). It only takes a couple of minutes to weigh and measure motor length and calculate density.
Equations to calculate rubber motor density and maximum turns (metric version):
https://drive.google.com/file/d/1k75syu_GsARgzKrATm9-7CwYNg1iMAc3/view?usp=sharing
Calculator for rubber motor density and maximum turns:
https://docs.google.com/spreadsheets/d/1f6ImQiyC75cJG7yd9hIGlB2ptOzFty_I/edit?usp=drive_link&ouid=116913500593842120617&rtpof=true&sd=true
Download this calculator and use it on your PC or phone.
To use the calculator, enter motor loop length and weight and o-ring(s) weight (the green highlighted cells) and this “calculator” uses the above equation to determine density in grams per inch, the equivalent “width” and the number of motor turns and winder turns it will take to break the motor (and the 80%, 85%, 90% of breaking turns numbers).