Can someone confirm these maths? I'm thick.

The Maestro

Esteemed Pedelecer
May 15, 2008
296
0
I have a bike with a wheel circumference of 2164mm, I want to buy a motor and want to work out how many rpm the motor must deliver to give a speed of 20 mph.

The way I've worked it out is:

1609 meters in a mile
/
2.163 wheel circumferance
=
743.5 revolutions necessary to travel one mile


20 miles = 14870 revs
/
60 minutes
=
247.8 rpm required


The reason I ask is that I'm trying to work out which one of those 8Fun motors to buy but the ones for the 26" wheel size (which I'd imagine is the only sensible one to fit to a 700c bike) all seem to do around 210 - 220 rpm. Now I've heard that bikes with these motors such as the Ezee Torq can get up to over 20 mph. So how is this possible? Probably my maths completely wrong.
 

frank9755

Esteemed Pedelecer
May 19, 2007
1,228
2
London
Your maths is right. Suppliers such as Suzhou Bafang and Tongxin make motors with speeds up to 260 rpm
 

The Maestro

Esteemed Pedelecer
May 15, 2008
296
0
Your maths is right. Suppliers such as Suzhou Bafang and Tongxin make motors with speeds up to 260 rpm
I've looked through the specs of all their motors on their website and the only ones that do 260rpm and above are listed as for 20" wheels and smaller. All the 26" ones are ~ 210 rpm. So are people buying motors for the smaller wheels and putting on the bigger wheels or are their some products I'm missing?

e.g.

http://www.szbaf.com/en/product_detail.asp?ProductID=506
 

ITSPETEINIT

Esteemed Pedelecer
Dec 11, 2006
492
0
Mere, Wilts
Right maths - wrong wheel size

I have a bike with a wheel circumference of 2164mm, I want to buy a motor and want to work out how many rpm the motor must deliver to give a speed of 20 mph.

The way I've worked it out is:

1609 meters in a mile
/
2.163 wheel circumferance
=
743.5 revolutions necessary to travel one mile


20 miles = 14870 revs
/
60 minutes
=
247.8 rpm required


The reason I ask is that I'm trying to work out which one of those 8Fun motors to buy but the ones for the 26" wheel size (which I'd imagine is the only sensible one to fit to a 700c bike) all seem to do around 210 - 220 rpm. Now I've heard that bikes with these motors such as the Ezee Torq can get up to over 20 mph. So how is this possible? Probably my maths completely wrong.
The Torq has a 700c wheel and that translates as virtually 28 inch.
I know from experience that the type of tyre can may a substantial difference to the "Nominal" size of the wheel.
Since a 28inch wheel is 7.7% bigger than a 26 inch wheel, it follows that 20 mph on a 26 inch becomes 21.54 mph on a 700c (28 inch) wheel.
Peter
 
Last edited:

The Maestro

Esteemed Pedelecer
May 15, 2008
296
0
The Torq has a 700c wheel and that translates as virtually 28 inch.
I know from experience that the type of tyre can may a substantial difference to the "Nominal" size of the wheel.
Since a 28inch wheel is 7.7% bigger than a 26 inch wheel, it follows that 20 mph on a 26 inch becomes 21.54 mph on a 700c (28 inch) wheel.
Peter
Yep, its just that I'm trying to find the best motor and worried that all the ones with an rpm rating in the area I want are listed for 20" wheels. Maybe this could strain the motor or something if I fit to a 700c.

I think I remember a post a while ago on this forum that mentioned the model number of the motor on the Mk 1 Torq. I'll try and dig that out.
 

flecc

Member
Oct 25, 2006
52,760
30,348
The Torq 1 motor was the Quando one, designed for 20" wheels, hence the speed but poor hill climbing in the Torq wheel.
.