how to reach 50-55 mph?

nickydlax said:

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

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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.

nickydlax said:

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?

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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.

nickydlax said:

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

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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.