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Gearing Your Motor to a Track

The team competes at many tracks over the course of the year, on varied surfaces, sizes, and shapes. Between offroad and oval, dirt, carpet and paved, all these factors affect how your motor will respond for you. Even your driving style will affect how you gear your motor (that is, set the ratio between your motor and transmission). Brushless motors work best within a range of power output. When a motor is being worked too hard (too much power is being demanded or the motor is not in an efficient range), the driver notices diminished performance. When the motor temperature is tested after the car is removed from the track, it will be higher than what makes it efficient, even to the point of damaging it beyond repair. The goal in this initial gearing setup is to place the motor into a workable zone from which we can fine tune the car for best performance.

 

Use this formula to calculate the overall gear ratio:Spur Teeth, divided by pinion teeth, times internal transmission ratio. The bigger the overall number, the lower the gear ratio will be. OK, you are at the track, you have a new motor, where do you start?
    1. For this method to be accurate, you will need fans running on both the motor and ESC and measure the temperature as soon as you remove the car from the track. With fans running, the motor and ESC will cool quickly.
    2. Ask around to see if anyone has used a similar motor to use as a starting point, this will give you a baseline, a starting point. Make sure the gearing suggestion is based on the same internal transmission ratio you have. Serious damage will be done, for example, if you take the gear suggestion for a car with a 2.6 internal ratio when your transmission has a 2.4 internal ratio!

 OK, we are ready to do preliminary gear testing:

  1. Put your car on the track, and as best you can, run at racing speeds for two continuous minutes, NO MORE! If the gearing is way off, we do not want to melt down the motor!
  2. Within 30 seconds, measure the motor temperature (at the timing end gives highest temperature) and ESC temperature. Use a non-contact temperature tool. I do not recommend using the touch method at this point!
Armed with the temp information (Note: Consider a motor with a temp above 140 degrees as hot, an ESC with temp above 130 degrees as hot): 
    • Motor Temp: High - ESC Temp: High = The motor is geared too high for the track, decrease the pinion tooth count or increase spur gear size. 
    • Motor Temp: High - ESC Temp: Low = The motor is geared too low, increase pinion size or decrease spur gear size.

    After a few trials, you will have the motor in a range you can now optimize for your car and this track. What I recommend, and with tires which have decent grip for the track (if the tires, diff, or slipper are slipping excessively, we will not be accurate with the gearing), is to listen for when the motor reaches its top speed on the longest straight. The motor should be at max RPM about 90% down this straight. Again, monitor motor temperature during your testing, measure every time you come off the track promptly.

    We are not finished yet. This is where the driver will need to decide how the car pulls out of a variety of corners. If the car is lazy, reduce pinion tooth count one one tooth and test again to see if acceleration is better. Check temperature.

    Give this process a try and see if it improves your laps or car feel. And always measure that temperature!

     

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