Cheap motor to push a Go-kart 30mph with 72v lipos

[stanson]

I already have an e-bike, so I am craving a 4 wheel kart build...

I have a little kart frame here, and I only want to go 30-35mph. My plan is to buy a Vevor type 72v brushless motor kit ($250 on Amzn), then attach a 60 tooth sprocket on a live axle. I will power this motor with the 72v lipo packs that I have. The motor claims to be 4,900 rpm, and I will be putting either 11 or 13" tires on it.

The motor comes with a cheap controller. I am hoping that if that controller doesn't work, I can use the QS 80 amp controller that came from the QS205 ebike hub kit...

Does all of this sound reasonable? Or am I setting myself up for disappointment?

 

[stanson]

bummer no responses. Ugh back to reddit

 

[glennb]

The motor’s capable of that speed. They’re happiest below 6500rpm. So, the 60V version with 20S battery, and around 5:1 gear reduction will do what you want. You’ll get there with 50A no problems. Likely only need 30A.

I wouldn’t bother with field weakening, you can just use battery voltage to achieve the desired RPM.

With fixed battery voltage, motor kV, wheel diameter, and desired top speed, you have all you need to calculate the appropriate gear reduction. It’ll be around 5:1 by the looks of those Segway wheels.

 

[stanson]

The motor’s capable of that speed. They’re happiest below 6500rpm. So, the 60V version with 20S battery, and around 5:1 gear reduction will do what you want. You’ll get there with 50A no problems. Likely only need 30A.

I wouldn’t bother with field weakening, you can just use battery voltage to achieve the desired RPM.

With fixed battery voltage, motor kV, wheel diameter, and desired top speed, you have all you need to calculate the appropriate gear reduction. It’ll be around 5:1 by the looks of those Segway wheels.

Thank you Glenn! I appreciate the response!! It certainly helps, but I am not quite grasping all of it. Are you suggesting that I...

1) use the 60v motor with my QS 80 amp controller along with 72v lipos, or

2) that I use the 60v motor along with the cheap controller it comes with, paired with my 72v lipos?

Wouldn't doing the second option overheat the cheap 60v controller with my 72v lipos? Also, I was under the assumption that having a bigger motor was usually preferable to a smaller one. Are you strongly suggesting the 60v vs. the 72v version of the motor? If both achieve my desired speed, I would prefer whichever motor consumes less power (if it works that way?)

 

[glennb]

No worries.

The power requirement is fixed.

It’s just a question of how you want to achieve the required RPM.

13” tires have to spin at 780rpm to propel you 30mph. 11” tires, 920rpm.

The 72v and 60v motor versions differ only in how fast they spin at any given voltage. The 60v version will spin 20% faster at 72v than the 72v version.

The motors are limited in how much current you can supply them, so the only way to improve acceleration without sacrificing top speed is to pair higher motor rpm with greater gear reduction.

If your 80amp controller cannot be current limited to below 50amp, you cannot use it. I think you’ll find 30amp is plenty for 30mph, and 40amp plenty for satisfying acceleration.

And 72v could easily destroy the 60v controller, yes.

So the 72v Vevor kit could well make the most sense.

I’d recommend crunching rpm numbers before ordering motors or sprockets or wheels though.

This one’s a nice simple calculator for that purpose:

RPM Calculator - RPM to mph

The RPM calculator helps you understand RPM (revolutions per minute) and compute the relationship between the speed of a vehicle and the RPM of its engine.

www.omnicalculator.com

 

[glennb]

P.s. the motor specs mightn’t distinguish between “rated” and “maximum” rpm. Either figure will suffice for the purposes of calulating speeds, but you need to know which it is that they’re providing.

 

[stanson]

P.s. the motor specs mightn’t distinguish between “rated” and “maximum” rpm. Either figure will suffice for the purposes of calulating speeds, but you need to know which it is that they’re providing.

Glenn, your advice has been incredibly helpful!

So I'd say you are absolutely right about figuring out rpm before ordering parts. These cheap motors just have such a wide range (4900 rpm continual, 5900 max), that I'm not really sure they are dependable figures.

It seems kind of moot though, since the Vevor type brushless motors seem to be the only option in my price range ($200-$400 max). Unless there is a quality brand motor that is comparable in price, I'll have to get the Vevor type, then experiment with different sprockets and wheels to make it work...

If I am being honest, I am perfectly fine with being over my target of 30mph lol. My equipment/frame would probably start failing at 40+, so as long as its under there, I am happy.

According to my calculations, 58t with 13" tires, with a rpm of 4900 should be 35mph. I think that should be a good starting point, as I can pretty easily swap for smaller tires, bringing the speed down by 5 to 30mph if I need to slow it down.

 

[glennb]

Sounds like you’ve got it all sorted.

The motor you’re proposing is near enough perfect for your application. They’re great value.

If starting from scratch I’d use the 60V motor with 72V battery, but 4900rpm is enough to work with, and likely gentler on the motor assuming you don’t feed it higher current in search of quicker acceleration.

You probably realise you can switch the T8F motor sprocket for 25H if that opens up better options to you. A 12 or 13T with proportionally larger rear sprocket will be quieter and longer lasting.

30mph, sitting so close to the ground, with more road buzz, will feel fast - a whole lot quicker than 30mph on your bike.

I’d be cautious about ground clearance with 11” tires.

A properly designed front fairing could easily knock off hundreds of watts of drag and give you a few more mph.

Good luck with it.