Rubber Motor Turns Tables Formula

Motor turns tables are limited to the lengths and cross sections listed. Those of us who cut and strip rubber to other than standard size need something more general.

I use a formula.

Caution; the breaking turns are affected by many things. Every batch of rubber has different properties, warmer temperature stresses the rubber, lubrication, stretching, rate of winding, cooling, previous stretching, braiding, contamination and other things affect how many turns a motor will take. Winding heats the rubber, it heats faster as torque increases and every turn puts in more work. Slow your winding as you get close to full to let it dissipate the heat.

Formulas can be a useful guide to winding under standardized conditions.

My formula is:

T = 10.64/Sqrt(S)

T is breaking turns per inch. Multiply that by the length of your motor to get breaking turns for that motor.
S is motor cross section in square inches. Multiply strip thickness (0.042″) by motor width times number of strands.
Sqrt( ) is square root. Look on your pocket calculator. Most computers have a calculator window.
10.64 is an empirical coefficient. It is the result of testing some pretty good Tan II. Lou got exactly the same for some Tan SS. Purely coincidence.

You can establish your own formula by winding a short test motor under standard conditions and using the formula to calculate a new coefficient. “Good” rubber will run from 9 to 13.

Here is an example of a turns table I made up for a motor.

Denny Dart II, 7″ NP Prop, 17″ loop of 0.083″, 127.4 tpi

% Breaking Turns        Number

100                                  2166

95                                  2058

90                                  1950

85                                  1841

80                                  1733

75                                  1625

70                                  1516

If you are using a 15:1 winder, divide those turn numbers by 15 to get the number of cranks on the winder.

Gary Hinze

4 thoughts on “Rubber Motor Turns Tables Formula”

1. dlwiener says:

Denny Dart II, 7″ NP Prop, 17″ loop of 0.083″, 127.4 tpi

If 0.083″ is the width of the rubber you used in your example table, I can’t get 127.4 tpi using the empirical coefficient of 10.64 that is listed in your formula, I’m confused? I get 10.64/(.083*.042)^.5 = 180.2 tpi.

Is it possible that your table is based on a empirical coefficient of 7.52?

1. gary says:

I am pleased that someone is reading this closely enough to try the numbers. It makes me feel that my effort is justified. Sometimes I make a mistake, so I carefully reviewed the calculation.

Your discrepant number is the result of an easy oversight. The width of 0.083″ is the width of a single strand of rubber. There will be two strands in the loop used in the motor. So the cross section is twice the width times the thickness.

This number is a rough guide. Every batch of rubber is different and motors cut from the same batch may differ noticeably. The width and thickness may vary slightly. When winding a motor, you can wind up to about 90% of this number, then you can stretch check. You will find that energy may be stored in two ways, by torsion and by tension. The sum has a limit. As you approach the limit in torsion, you will feel the motor stiffening in tension. As you approach maximum turns, you should wind slowly and stop occasionally. Let the motor cool and then give light tugs to the motor to see how it responds to stretching. When it doesn’t want to stretch with a light tug it is a good time to stop winding. The decision is a matter of judgement based on experience. You wind this hard only in competition. If you wind this hard you probably will prestretch the motor once and fly it once, then discard it. It will likely break the next time you wind that hard. For sport flying, 70% to 80% of maximum turns is preferred. That will allow many flights from the same motor.

1. dlwiener says:

Ughhh, it never dawned on me that a loop is two strands. I appreciate your feedback, I couldn’t let a clever concept like this go, Thanks!!!

(10.64)/[(2 strands*0.083*0.042)^.5] = 124.7 tpi…. Sweet : )