question for the math whiz

deacon

minor bike philosopher
Jan 15, 2008
8,117
3
0
north carolina
I want to try to rig an electric motor to a mountain bike as a hill helper. I'm hoping that I won't use it except to help with the hills because I want to try it first hooked directly to the mt bike drive, and of course it will prolong the battery life, and give me some much needed exercise.

I bought a junker mt bike. It had all the sprockets but the shifters don't work which is fine. I may switch the rear wheel for a coaster wheel anyway. My question is about gearing and speeds.

desired performance.

speeds of 12 to 15 mph
motor to be used as helper for the hills only.
geared for maximum torque.


motor specs 3000 rpm
12 amp motor.
24 volt
sprocket 1 15/16 diameter 11 tooth.

bike has three sprockets on the crank. several on the rear but probably will be a coaster with a twenty inch wheel. I could leave the mt bile wheel on if it will help.., It is 24"

so motor to chain on which sprocket.... small medium or large...
chain to rear sprocket on which front sprocket.

Im more interested in torque than speed.

setup with controller and throttle so I can vary the rpm of the motor.

summary.

motor to which front sprocket...
chain from which front sprocket to which rear sprocket.. should I replace the 24" wheel with a 20"

I have a about a week to decide before the bike sprocket arrives for the motor.
 
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deacon

minor bike philosopher
Jan 15, 2008
8,117
3
0
north carolina
Im thinking motor to large front sprocket to reduce the number of revs. Then second largest to the largest on the rear so as not to increase them. If i use the rear large sprocket I won't be able to go with the 20 inch wheel. Unless I could find a twenty inch mountain bike wheel.
 

Bacon

New Member
Nov 12, 2008
15
0
0
Tempe, Arizona
To do accurate calculations one would need the following additional information:

The number of teeth on each of the front and rear sprockets and the diameter or circumference of the drive tire, installed on the wheel and inflated to normal operating pressure.

An easy way to get the circumference is to position the bike with the valve stem down near the ground. Make a mark on the ground with a piece of chalk that lines up with the valve stem. Roll the bike in a straight line until the wheel makes one complete revolution and the valve stem is back in the same position. Make another mark on the ground and measure the distance between the marks. That should be pretty close to the circumference of the tire.

Or measure the outside diameter of the tire with a tape measure and multiply that times pi. (3.141)
 

comfortableshoes

New Member
Jul 22, 2008
606
0
0
Beverly, MA USA
You can down load a free gear calculator and I can't find the link right now but it's useful especially if you are working with gears.

Here are the different calcs I made with the calculator:
10:39:53:23 with a 24 inch tire and 1000RPM of the motor=35mph
The same gearing as above but with a 20 inch tire= 42mph

Neither of these numbers seem accurate given the electric motor.

Slow the RPM to 500 and you get:
20 inch=18mph
24 inch= 21.1 mph

Slow that again to 250RPM
20inch=9
24inch=10.5

I looked at my chains today and you are going to need to use a rear derailleur as a chain guide to keep it on the front chain ring, unless you are fancy with lining things up. I think you'll be getting out the WD40...
 

Dave31

Active Member
Mar 1, 2008
11,204
29
38
Aztlán, Arizona
If you guys find that link I'll add it to a sticky or in the "classic" so we will have when we need it. It seems to be useful for the DIY builds.
:ride2:
 

deacon

minor bike philosopher
Jan 15, 2008
8,117
3
0
north carolina
Mph = (rpm x tire diameter)/ (Gear ratio x 336)

this is from one of the my google searches.
rpm = (1500x26)39000 /(5x336) 1680 =23.2mph

sounds about right to me. except there is a no provision for load or any other factors. I think that formula works pretty good but I'm not positive. I figured it with the scooter that the engine came from it works about to under ten mph which sounds about right for a childs scooter.

But this only works if the teeth on both gears are the same size. If I use the 25/bike chain combo it come down to a differnt ratio.

Then it comes out to 11mph which is about what I want but still does not account for load.
 
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comfortableshoes

New Member
Jul 22, 2008
606
0
0
Beverly, MA USA
FR31- you have it in the sticky. the downloadable calc is the best one ever it gives you 3 sections for Jack shaft gearing and figures the circumference of the wheel. Very accurate too
 
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