# Scooter - BLDC motor calculations

I would like to ask you for few question related to BLDC motor. I am trying to build my own scooter, but I don't know how to choose BLDC motor correctly. Actually I understand the electrical aspects of motor, but its hard to get and combine it.

Let's say, that my weight is something around 80kg. How to determine what will be the value of power consumption at the velocity equal to 20km/h? How should I choose motor in order to get the best efficiency there? What should be the torque and KVs? And so on.

Power at a speed on level ground is determined by your frontal area, wind resistance, rather than your weight.

Power up a hill is determined by your weight. How fast do you want to go up a (for instance) 1:10 hill? With a body weight of 80 kg, and perhaps 20 kg for the scooter, that's 1 kN / 10 = 100 N force. To do that at 20 km/h = 5.5 m/s would need 550 watts.

How steep a hill do you want to climb at any speed? To climb a 1:5 would require a force of 200 N. This can be converted to torque on the motor by multiplying by the radius of the wheels. Maybe you don't need to climb a hill that steep, but would simply hop off and walk if necessary?

Once you have the wheel radius, you can convert speed to rpm, or rad/s. When you choose your battery voltage, this will give you the required KVs.

Or just pick a motor in the low hundreds of watts range, and analyse what performance it will give you. It's often much easier to do several analyses of specific configurations, than try to synthesise a good configuration, especially if your criteria are not well thought out.

• Thank you for the answer. How should I calculate forces related to frontal area and wind resistance? As I assume, it is not as easy to get. Is there any "model"/standard value, which I can use in calculations? Or any type of calculators? As I can see there isnt any calculator for e-scooter/e-skateboard that includes required power parameter. Mar 25, 2022 at 21:01
• You can get approximations for wind resistance from drag = drag_coeff x frontal_area x stagnation_pressure. Stagnation pressure = 0.5 x density x v^2. Drag coeff is the tricky one, 1 to 1.3 for a standing person. Mar 25, 2022 at 21:18
• Thank you for help, now I can understand most of things. I have one more question - what is the load factor and how to calculate it? As I can see people include the efficiency in order to calculate required electrical power instead of mechanical power and the load factor to provide the real KV value of motor that is needed. Mar 26, 2022 at 15:05
• @Bratw I don't know, why do you mention Load Factor? It's not mentioned in your question, or my answer, so must come from somewhere else. It sounds like it may be just a general over-design margin, your source should have defined it. Mar 26, 2022 at 15:19
• I don't know if it is correct, but It comes from every online calculator provided by a community. They usually calculate "theoretical" top speed, and the "weighted" one, that consider this load factor/efficiency. There is the example: calc.esk8.it Mar 26, 2022 at 15:26

You questions indicate to me that you will be far more likely to be successful getting kits than DIY from scratch. There is too much to learn.

https://www.amazon.ca/Brushless-Wireless-Transmitter-Skateboard-Longboard/dp/B07FQBCJX5?th=1

or

https://www.amazon.ca/FLIPSKY-SK8-ESC-Electric-Skateboard-EScooter/dp/B07GFB55NV/ref=pd_day0fbt_2/145-4711119-9382822?pd_rd_w=TXN0L&pf_rd_p=acc65436-fc4e-45ed-98e6-1cea96557795&pf_rd_r=85KDDRYSQENDDC91CV59&pd_rd_r=d3f0e420-a953-4753-8de1-1d135d55e5bc&pd_rd_wg=RP86z&pd_rd_i=B07GFB55NV&psc=1

I suggest this kit with the largest motor and 36V battery pack to fit under a longboard with a throttle mounted on T bar.

What problems did you end up with unresolved on your 200A busbar? regarding EMC issues.