how to reach 50-55 mph?

8451 Views 44 Replies 7 Participants Last post by nickydlax, Jul 25, 2014

nickydlax

Discussion Starter · Jul 23, 2014

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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arklan

· #2 · Jul 23, 2014

need more information on the 72v motor
72v will get you to the speed u want
accelleration will be similar to normal car

a kart doing 90kph/55mph wont be stable at all, you might want to take a page from forkenswifts book or convert a bike

nickydlax

Discussion Starter · #3 · Jul 23, 2014

Good to , ill see if I can get some more info on it, and make it more steady, like a dune buggy or something

skooler

· #4 · Jul 23, 2014

Your gearing needs to match your motors top rpm.

So with the motor at its peak rpm, the gearing you have only allows for that top speed.

nickydlax

Discussion Starter · #5 · Jul 23, 2014

What should I think about for gearing though

Yabert

· #6 · Jul 23, 2014

A 72v system will be cheap and easy to build. As example, my 1900 lbs car reach over 60 mph with only 72v (but I've 5 ratios transmission).

Is your goal is performance or cheap build?
Do you need a reverse?

nickydlax

Discussion Starter · #7 · Jul 23, 2014

How would I match ratio with rpm though?

nickydlax

Discussion Starter · #8 · Jul 23, 2014

Oh I wasn't going to have a transmission. Well. Id like to drive it to work everyday, but also drift in it. Money isn't a huge obstacle, but still in my mind though. Reverse is nice but not a requirement, but still nice. What wattage is that motor? Remember this is a buggy not a car. I guess performance is more important

dougingraham

· #9 · Jul 23, 2014

nickydlax said:
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.

skooler

· #10 · Jul 23, 2014

Doug has it spot on.

So what are you building? A go-kart, buggy, car? Any pics?

nickydlax

Discussion Starter · #11 · Jul 23, 2014

Are motorcycle tires really that big? Yeah that's twice as much rpm as some 72v go kart motors im looking at, some beast ones go up to about 3700

nickydlax

Discussion Starter · #12 · Jul 23, 2014

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

skooler

· #13 · Jul 23, 2014

Road legal?

nickydlax

Discussion Starter · #14 · Jul 23, 2014

Yeah that'll happen eventually

dougingraham

· #15 · Jul 23, 2014

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

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

Discussion Starter · #16 · Jul 23, 2014

Yeah ill plug in my numbers into your stuff when I get home, thanks for the help man

nickydlax

Discussion Starter · #17 · Jul 23, 2014

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?

dougingraham

· #18 · Jul 23, 2014

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?

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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slimdawg

· #19 · Jul 23, 2014

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

dougingraham

· #20 · Jul 23, 2014

slimdawg said:
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.

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