How do I select a suitable motor?

I'm looking at replacing the motor in my scooter. Right now it uses a 48V 8.5kW Lynch LEM-200 motor overclocked to 72V. (yes that is factory default. And yes, it overheats)

I'm looking for a brushless motor that achieves around 15kW at 72V with a diameter of not much more than 205mm.
So far so easy, many options come to mind.

But here is where I'm stuck. How to I calculate the correct KV range and desired max RPM?

I guess naively, I could calculate back from expected cruise speed (let's say 120km/h) tyre diameter and belt teeth ratio.
Does this approach make sense? How can I tell if I overdid it? I will sacrifice top speed for acceleration, I just don't want to hit a brick wall at speed with power to spare.
What is a sensible approach to select the right motor?
My goal is great acceleration and fun. Sensible efficiency at normal use.
Allowed power for the class of vehicle is 11kW. But I'm not concerned about bending the rules here a bit. I'm even considering re-classifying the scooter as a motorbike.
Speed is not the prime focus. But above 110km/h is must to flow in traffic.


I suppose I will need the gearing ratio and tyre dimensions. I will find those out next...

is it not possible to go direct drive? if not then i would get a mid drive QS motor. that should be able to handle 15kW peaks just fine.
fitting a hub motor would remove the mid motor from the frame and make more space for batteries and more power. that way you can also for a much bigger motor capable of running 120kph all the time without heating up at all. and no more chains/belts.

Honestly, I just assumed hub motos would have less torque. I only see them on toy bikes.
Wrong assumption?
Also, it would require quite a bit of modification to my swing arm which I'm not too excited about.
Having someone cut an adaptorplate withthe right bolt pattern for the drop in replacement motor is abou the depth I had planned to go into with this.
Also, the battery is about as big and heavy as I want it to become.

I only see max 3kW from QS. That is a long way towards my expected +10kW range.

I'm lusting GoldenMotors 20kW motor a bit. But it's too large. I don't even have enough space between swing pivot bolt and tyre.

Their 10kW motor fits perfectly. I can probably even get the right adaptor plate off the shelf. (Other people use the 5kW version in my scooter with great success which I suppose has the same bolt pattern)
The performance at 72V looks nice. https://www.goldenmotor.com/eCar/HPM72-10000Curve.pdf
But how can I tell that this thing makes sense in my scooter?

Just frock it and do it? And change gearing later if it's a problem?
I'm tempted to do that. I just don't know how much thought I should put into this before.

But I always prefer to think before not after I have a problem.
Goldenmotor offers different kV winding options. I suppose that refers to the different voltage options of the product? If the desired RPM don't work out get a custom job?

PS: Another option I had considered http://www.thunderstruck-ev.com/me1507.html
I could move the motor back a bit to make space.
But really, I didn't open this threat to drool over theoretical performance. I wanted to know how I select the right motor for a given sitution.

yes, your assumption is wrong.

a small 10" 205 hub motor can push out 200Nm for example. but a mid motor can do 50Nm and with a ratio of 4:1 you still end up with 200Nm at the tire but with the added losses and headache you get with chains or belts. noise is also a lot less and cooling is much better as a hub motor has direct airflow.

if you can have a 17" rim or so you can mount a QS 273 with even more torque.
http://www.cnqsmotor.com/en/article_read/QS%20Motor%2017inch%208000W%20273%2050H%20V3%20E-Scooter%20Hub%20Motor/248.html.

dropout mods is basically only welding on a flat steel plate with a 15mm slot in it for the axle to slide in. you can do that with a hacksaw and a welder, no CNC needed.

if you go for the goldenmotor then note you need a additional radiator and pump or be prepaired to have a really loud fan. it does have the abillity to change sprockets but that is about the only advantage it has.

Kv of the motor can be simply calculated by taking the desired max rpm you need to get the top speed you want, adding 10~15% to that for the rpm loss under load and adding the battery voltage to that formula. also note that you need to take the voltage when the battery is almost empty. otherwise you only get the top speed when the battery is full and you lose more and more speed when the battery drains. so limiting the top speed in the controller is mandatory for a nice and consistent experience.

Thanks for answering all my questions.
I have to admit the hub motor setup is beginning to appeal to me. I will have to still disassemble the whole back end of the scooter to get good measurements. I will pay attention to that option as well.

Hm, liquid cooling. I have been thinking about it for a while. Could be done.

Mechanical dimensions will make all the difference to me.
I just got word from Motenergy about the ME1304.

Noting that they also have a slower speed winding on a motor called the ME1302. It goes 25% slower, but also provides 25% less continuous power.

I like this motor a lot so far. Fits like a glove and is a beast of power.

flippy said:

Kv of the motor can be simply calculated by taking the desired max rpm you need to get the top speed you want, adding 10~15% to that for the rpm loss under load and adding the battery voltage to that formula. also note that you need to take the voltage when the battery is almost empty. otherwise you only get the top speed when the battery is full and you lose more and more speed when the battery drains. so limiting the top speed in the controller is mandatory for a nice and consistent experience.

Click to expand...

Did I got that right?
My max RPM is 5000 RPMs
adding 10-15%
low voltage at my project is 60V

So is this correct:?
5000*1,1*60V = 330000 = 330Kv ?

DasDouble said:

My max RPM is 5000 RPMs
adding 10-15%
low voltage at my project is 60V

Click to expand...

60 volts * 100 kV = 6000 rpm.

Volts * rpm per Volt = rpm.

So a 100 kV motor will spin 100 rpm for every volt applied to it.

My ebike is around 50 volts and 80 kV and peaks at 4000 rpm. (as an example)

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The "k" means it's a constant... it's easier than if it was rpm/V.

There is a "kT" constant too which relates to torque.