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Ideal Gear ratio? Answered

hello i am competing in a design challenge at school. were were each given a 9v 12000 Rpm Motor, the object of the challenge is to build a land vehicle that can go 10m as fast as possible. we will be holding a race on november 3rd to see who has designed the fastest vehicle.

anyway my question for the community is using a 12000 rpm electric motor what would be the best gear ratio to use between the motor and the axle to optomise take off speed as well as having a high top speed. (if the gear ratio is to high then the car wont accelerate fast enough to low then the car wont have a fast enough top end) so basicly im looking for that perfect med ground. if anyone here has had much experience with gears and help would be great thanks

(PS the vehicle cant be any biger then a vhs case but can be as tall as i want it)



9 years ago

Wheel slip is going to be the limiting factor in the design, because once you lose traction, you are basically lost.

I'd try and calculate a likely maximum speed for my given diameter of tyres, translate that into shaft RPM, and make a simple box to give that final reduction.

Then I'd use a microcontroller, and build a ramping PWM controller.

Then I'd try to see what the maximum ramp I can get without getting wheel slip is.


9 years ago

Remember, there's also the question of how much torque that motor can produce. Knowing that it spins at 12,000 RPM under no load tells you nothing about how it responds under load, and if you actually want to calculate this you need that curve.

Probably simpler to set things up so you can experiment with different gear ratios.

Of course the ideal would be to change gear ratio to optimize for different conditions -- for all the same reasons that cars have several gear ratio settings in their transmissions. If you're really ambitious you could go looking at various automatic transmission designs; I can think of one or two that might be adaptable to this scale (though I'm not at all convinced they can be built in the time you have available).

As others have said, there are also questions of minimizing friction in the axles and power train. And air resistance can matter even on this scale. Remember to optimize the _whole_ system.

Have fun, and good luck.


9 years ago

Here is a link to a drag racing calculator using known gear ratios and RPMs.  Plug in the values and it will give you an estimated top speed.
It may not be perfect, but it will get you a starting place to begin testing.


Make sure you have javascript enabled or it won't work.


9 years ago

Well there's really to many variables to give you an answer. The wait of your car and drag from your drive train play in as well as the amount of power your motor has. You really need to use trial and error to find the ratio that works best for you.