Help with motors and hub motors

[Will172]

Hi everyone,

I'm looking to convert an old petrol scooter to an electric one, and I'm looking at all the same bike conversion kits etc.

However I can't seem to figure what I 'actually' need motor wise. I can see 3000w 72v motors and 500w 25v 10inch hub motors.

Ultimately, I want this thing to take me up a bit of a hill, off-road and I'm 110kg.

Can someone give me a bit of advice in what I'm looking for here?

 

[vfr400]

You'll need at least 2 kW of output power to maintain a reasonable speed up a hill. The power quoted for motors is their rated power, not their output power. It's the motor controller that decides how much power you get, and the battery that provides the power.

Another important characteristic is the motor's KV (RPM per volt). Let's say you want a modal speed of 30 mph. You get best efficiency and power at about 75% of maximum rpm, so you need a motor that maxes out at around 40 mph. You then have to convert that to rpm by dividing by the wheel circumference. If your wheel diameter is 20", that would be about 700 rpm if I've done my sums right. So, if you wanted to use a 72v battery, you'd need a motor with KV of around 10, and a KV of around 14.5 for a 48v battery. Unfortunately, motor suppliers don't normally tell you their KVs. Instead, they tell you the max speed, but they never make it clear whether that's the no load speed or real world travelling speed or at what voltage and current you can expect that.

 

[Will172]

Okay that makes sense, thanks.

I've looked at a few 2kw/3kw motors and the main complaint seems to be people not being able to get enough voltage into it. If I'm looking at making a 72v battery what's going to be the best route? 18650s or?