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how to reach 50-55 mph?

8458 Views 44 Replies 7 Participants Last post by  nickydlax
Hey guys im brand new, I need some help figuring out my first build
My goals are to have an electric kart that can get up to 50-55 mph at the most. Acceleration would be awesome too (anything more than a standard car) but id like to take this to work every day, it's not too long of a drive.
Anyways. Would a 72v motor get the job done? If I use a 72v, what should the gearing be? Im thinking about using motorcycle wheels fyi. Thanks in advance!
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What should I think about for gearing though
This depends on the diameter of your tires.

Lets just assume your tires are 24" diameter and your motors max RPM is 6000 and you want to go 55 mph at that max motor RPM point.

A 24" diameter tire will advance 75.3982 inches per revolution (diameter * pi.) 55 miles per hour is 3484800 inches per hour (55 * 5280 * 12.) 3484800/75.3982 inches gives 46219 revolutions per hour. Divide by 60 to get an rpm of 770.3. To get from 6000 motor rpm to 770.3 wheel rpm means you need a reduction ratio of 7.79 : 1 (6000/770.3).

You are probably going to want to set the 55 mph point at a little lower motor RPM. This would be to keep the motor operating within its torque curve.
Doug do you mean a motor that has a lower top rmp?
Skooler im building a go kart that may be a little on the big size, but barley
You need to apply the numbers for your build and work through the equations. I am pretty sure I gave enough detail so you could punch in your tire diameter and desired max motor rpm. If you know the tire size you can go to one of the calculators on the internet and it will tell you the diameter. Or I can compute it for you. (example 185/70-13 tires have a diameter of 23.2 inches.) A go cart would be smaller. That size seems about right for a dune buggy which was why I chose 24 inches for the example. 6000 rpm is reasonable for motors in this size.

Depending on voltage and current motors have a fairly flat torque curve that extends from 0 rpm to some rpm and after that point the torque tapers off in a fairly linear fashion with an increase in RPM. To a point you can simply increase the voltage and widen the torque band. Similarly you can control the torque by limiting the motor current.
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I got a ratio od 4:1 with 20 inch tires abd at 3700 rpm.... so, shouldn't this take into consideration power? Or is this assuming it can even get that fast?
The only thing you asked about was gearing. In my example where the rpm limit of the motor was 6000 and your top speed was 55 mph I gave you a gear ratio where that would happen. Assuming the motor would put out enough power to exceed the rolling resistance and air drag this would be the best reduction ratio for a low end torque situation. Without a transmission the reduction ratio is going to be really important in the feel of the vehicle. My reduction ratio was nearly 8:1 which given the same motor and tire diameter would provide twice the torque at the face of the tire.

With a given motor the torque is proportional to amps. You want a lot of torque given you don't have a transmission which acts as a torque amplifier. The motor will have a current limit and will be rated for a certain torque specification at that current. You will need a motor controller that can provide that current and you will need batteries that can put out that amount of power. The reduction ratio combined with tire diameter is the other part of all this. Gearing for higher speeds will reduce the torque at the face of the tires. Decreasing the tire diameter will also increase the torque at the expense of top speed.
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First off I disagree with the statement the it will be unstable above 55mph we run my daughters cart at 72 and it's completely stable shooting for higher top end we run 7 inch dork lift motor @ 60 volts 1.21 to one gear ratio 400 amps 71.9 mph

Had to detune it for drag strip due to her age 60 foot time was 1.8sec 1/8mile 9.6 @60.48 mph
What diameter are the tires? If they were 8 inch diameter at 72 mph that would be a motor rpm of 3661.
I was going to try to keep it simple and not use a transmission, as most go don't have one.

As long as I can beat most things on the road up to 60mph ill be happy

Could I get a fancy controller that has a reverse or does it not work like that? I wish I had pictures but im still in the design phase
If you don't use a transmission you need a LOT more torque or you won't be happy with the acceleration.

Reverse depends on the motor. With a Series DC motor it takes three full power contactors to flip the field winding polarity. With a permanent magnet DC motor it takes two double throw relays or four full power contactors to switch the direction. With an AC motor the controller can simply tell it to run the other direction.

Lets say your vehicle ends up weighing 1000 lbs. If you have 1000 lbs of force at the face of the tires you would be able to accelerate at 1G assuming enough tire grip. This would probably make you happy. You could make white smoke and squealing sounds would come from the tires easily. A WarP9 motor at 1000 amps will do 240 lb-ft (according to Tesseract's testing). So to get your 1000 lbs at the face of the tire you need a 1 ft radius tire and a 4.16 : 1 final drive. I think you indicated that your tires would be 20" diameter. This is a 10" radius which gives you a small mechanical advantage of 10/12 meaning your final reduction would need to be 3.47 : 1. So now you have a respectable amount of power on the road what does this do for your top speed? The WarP 9 has an official redline of 6000 rpm. At 6000 rpm with 20 inch tires and a 3.47:1 the vehicle should be traveling 102.88 mph. Of course this depends on the battery voltage because with only 72 volts the torque starts to taper off at around 1500 RPM (25.7 mph). Double the voltage and you should have full torque to 51.4 mph. This will take a pretty impressive battery.

As always, your mileage may vary. I am not a professional and I don't even play one on TV.
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Lighter is better. Look at what other people have done in a similar sized vehicle and then try to decide if the performance will be what you are looking for.
So voltage doesn't make power? Wattage does?

Actually I found a 20kw 72v (26hp) .....would this be powerful enough
Volts * Amps = Watts

Watts/746 = horsepower.

Watts = power.

I don't know how much you would need to be happy. If you had a 26hp ice on your go cart would that be enough? And torque is the important thing for driver satisfaction.
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