Ideal Gear sizes?
Noah Van Bramer
Jeremy Ashinghurst
Not sure what you're asking for. minimum gear pitches, chain,
and belt sizes to handle our power requirement? gearing ratios?
top speeds?
for chain, teams have had success with everything, even #25 chain
(though I recommend #35 and up). If this is your first build, and
your motor's rated power is <1500W/1HP, you should probably
gear your car for 14-16 mph of top speed at full battery charge.
if your motor is rated for 2kW+, gear for 18mph
I am working on my first car, and I'm in charge of the drivetrain. Does anyone have any advice for what size gears to use?
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Scott Young
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Jeremy Ashinghurst
OK, let's start with some basics. I apologize if I've gone too basic and you can skip some of this stuff.
if you're starting out with 750W/1HP, gear for 12mph (I derp'd
and said 1500W = 1HP before. I am a dirty, rotten liar. 2HP
should be able to gear for 14-16mph). You can make your car
faster later if it turns out it's reliable, but the first step is
to finish a race without burning up a motor.
I'll assume you're using a 10" OD tire since those seem to be the most common. To get to 12mph, that tire's OD has to go 12(mi/hr)*5280 (ft/mi) = 63360feet in an hour, or 1056ft/min. Since the tire is 10"dia, it has a circumference of about 31", so for it to travel 1056ft*12(in/ft) = 12672in/min, it has to spin 12672(in/min) / 31(in/rev) = 409 rpm.
So you want to gear your motor down so the rear axle spins ~400RPM on a fresh battery.
If you're using a Permanent Magnet Brushed DC motor, it will
probably tell you on a label what its output speed is, maybe
2000RPM @36V (for example). if you're running it at 36V nominal,
it will give you close enough to 2000RPM on a fresh battery. So
you'd want to gear it down 5:1 to get to your 400RPM.
If you're running the motor at a nominal voltage other than its rated voltage, just scale its rated output RPM on the input voltage: higher voltages make motors run at higher speeds, and the increase is linear (running the 200RPM@36V motor at 72V would make it run to 4000RPM, and running it at 18V would make it run to 1000RPM.
If you're using a brushless motor, it will have KV rating. to find out how fast your motor will spin at full charge, multiply your max battery voltage by the KV rating, then calculate your gearing ratio from there.
Generally you can do up to ~6:1 in a single chain reduction stage, so in the case of the 2000RPM@36V motor run at 36V you'd be OK having a 10-tooth chain sprocket on the motor and a 60-tooth chain sprocket on the wheel. If you have to run multiple stages of gear reduction (My car needs ~44:1 reduction to run correctly), keep in mind that reduction stages multiply so a 4:1 chain stage going into another 4:1 chain stage is a total of 16:1, not 8:1.
Good luck!
I was hoping for some gearing ratios for a motor rated about 1HP if you have any recomendations?
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