How to calculate speed from motor kv/voltage

Looking to build a friction drive ebike. I have a 63mm 245kv motor and 12s battery and will run on the top of a 27" tire. How do I do the math to get max speed of that?

KV* battery voltage = maximal motor unloaded RPM.

12s gives you 44.4V nominal or 50.4V hot off the charger.

Wheel diameter (at roller contact point) / roller diameter = Transmission ratio.

Maximal motor unloaded RPM/ Transmission ratio = Maximal wheel unloaded RPM.

Maximal wheel unloaded RPM*60* tire circumference (converted to Km or miles) = Maximal unloaded speed (in kph or mph).

For a 27" tire the circumference should be around 0.00215 km or 0.0013 miles.

Multiply maximal unloaded speed by 0.7 to get a rough estimate of actual max speed.


Avner.

ferret said:

Multiply maximal unloaded speed by 0.7 to get a rough estimate of actual max speed.

Avner.

Click to expand...

That's the max speed the bike's rear wheel can achieve sitting on a bike stand. To calculate real world speed, you need to look at power and drag. And then use the formulae you wrote to get appropriate gearing for that top speed.

Power is proportional to drag * speed cubed. Or, speed is proportional to the cube root of power / drag. I'm pretty sure we can ignore non-air drag on a high powered ebike.

I don't have all the numbers, though. Drag = cD * frontal area. cD for a motorcycle is about 0.8 and bikes are probably similar, but what's the area? And what's the proportion coefficient?

(BTW, Hi, everybody! I've been lurking for about a month since I saw tangentdave's Zerode on Reddit. This is my first post.)

http://www.recumbents.com/wisil/e-bent/rc_drive/rc_drive_calculator.asp

I don't know if that will help but it's a great tools

Do my calculations seem right? If so I will have to get a lower kv motor since my scooter will barely run at 35mph the way it is geared with 12s


KV* battery voltage = maximal motor unloaded RPM. = 250 * 44 = 11k

12s gives you 44.4V nominal or 50.4V hot off the charger.

Wheel diameter (at roller contact point) / roller diameter = Transmission ratio. 700mm/63mm = 11.11

Maximal motor unloaded RPM/ Transmission ratio = Maximal wheel unloaded RPM. 11k/11.11 = 990

Maximal wheel unloaded RPM*60* tire circumference (converted to Km or miles) = Maximal unloaded speed (in kph or mph). 990 * 60 * .0013 =77mph

For a 27" tire the circumference should be around 0.00215 km or 0.0013 miles.

Multiply maximal unloaded speed by 0.7 to get a rough estimate of actual max speed. 77 * .07 = 54mph

Your calculation is correct.

As kbob and me wrote, the 0.7 factor is only a rough estimate but the 250 KV motor does seem too fast.

BTW you can use the calculator boisrondevens posted by entering the diameters instead on tooth count in the reduction fields:

Reduction1 - primary: 63 Teeth
Reduction1 - secondary: 700 Teeth
Reduction2 - primary: 1 Teeth
Reduction2 - secondary: 1 Teeth
Reduction3 - primary: 1 Teeth
Reduction3 - secondary: 1 Teeth

Avner

ferret said:

Your calculation is correct.

As kbob and me wrote, the 0.7 factor is only a rough estimate but the 250 KV motor does seem too fast.

BTW you can use the calculator boisrondevens posted by entering the diameters instead on tooth count in the reduction fields:

Reduction1 - primary: 63 Teeth
Reduction1 - secondary: 700 Teeth
Reduction2 - primary: 1 Teeth
Reduction2 - secondary: 1 Teeth
Reduction3 - primary: 1 Teeth
Reduction3 - secondary: 1 Teeth

Avner

Click to expand...

I was having trouble with that. Tried that and it gave me 55mph, so it is correct. Too fast and probably not enough torque.

Is there any kind of clutch type wheel thing that I can mount to the shaft and have that roll on the tire? So the wheel will spin freely pedaling they engage when the motor is on.

I am not aware of an off the shelf part, you will have to design and build your own.

Avner.