I have had a few people asking component select questions, so thought I would post the basics here.
Volts & KV, and assist speeds
You need to select a battery voltage, motor diameter & motor KV to give you no load speed slightly in excess of you required maximum assist speed.
For example I recommend a 5-6s Lipo (20-24v) 63mm motor with 200kv. This gives a no load speed on 20v of 47.5kph, and 57kph on 24v. This means I still have a level of assist up to these speeds, and keeps the motor and ESC happy. Running at a higher voltage would require more expensive ESCs, that would just end up having to PWM the phase current more to keep power levels down at the same level, putting the ESC under more stress. Picking a motor with a higher KV will give less torque, and have the motor running at lower efficiency for the normal operating speeds, resulting in more waste heat and lower sustainable power levels.
A quick equation for the no load speed is:
kph = battery volts * motor diam in mm * motor KV / 5305
mph = battery volts * motor diam in mm * motor KV / 8538 [Edit: corrected]
Note: The no load speed is where the motor provides no work, so for the real assist speed drop this number down a bit, and calculate it at the depleted battery voltage.
I recommend 5s or 6s LiPo batteries. These are cheap, energy dense, easy to parallel up, easy to balance with an RC charger, and don't require the high voltage RC ESCs. You could also pick 4s, but these will require high KV motor to compensate, and require high battery currents for the same power.
Apart from picking the right diameter and KV as mentioned above, getting the heaviest motor will allow it to handle more power. As it has physically more mass to absorb the heat and larger surface area to dissipate it to the air.
A motor with a skirt bearing will resolve the tire contact forces better, and give longer life.
A small diameter motor (ie. 50mm) will fit smaller frames, but will not be able to deliver as much power.
If you are happy with ~500w power levels you could probably get away with a 50mm diameter motor. But I will usually recommend the 63mm motors
A normal RC ESC and motor combination does nothing to limit power levels. This results in lots of power but is also likely to overheat, loss sync, wear tires due to higher forces. That is why I recommend either using my Brain Box, or the Cycle Analyst -LRC. These have closed loop controller of power/current and enforce a minimum speed to maintain the health of the system, amongst other advanced features. Some people (most notably Lyen) have been experimenting with the sensorless ebike controllers so these may also be an option soon.
So please feel free to roll you own system design and electronics, but also be aware of the limitations of friction drives, RC motors and ESCs when doing so.
Last edited by adrian_sm
on Wed Mar 07, 2012 12:28 am, edited 1 time in total.