# Electric Bicycle Mod Using an 36 volt, 1000watt motor to make an electric bike. I have questions about rpm and torque.

The motor is 1000watts, 36volts and 3000rpm. I am planning to use three 12 volt car batteries to power the motor. I would be using a chain and gear transmission system to reduce the rpm.

I am looking to have a max speed of from 50 km/h or 31 mph to 40km/h or 25 mph.

I would really apretiate anyones help with this, so i could then finish this project.

thanks

Powerful motor! What kind of batteries do you have for that? Two $100 DeWalt 18v, 2.4 Ah batteries wouldn't even last that long. I don't think you can really calculate how long they would last, but best case scenario I think they'd last 10 1/2 minutes. I could be wrong about that part though.

I put the math at the bottom, (because its a bore lol) but to go 31mph you need to get your 75" circumference wheels spinning at 437rpm.

About your torque concern, 1kw motor is capable of providing more than enough, but it depends on how you gear it. I couldn't tell you exactly what torque you need because there are to many variables, and really it will just determine how fast you accelerate. Your biggest concern is how to power that motor, and some way to get the motor's max rpm (what is it?) to turn your wheels at about 450rpm.

More data is needed:

-Is it direct drive or will you have a transmission? Any gear ratios to consider?

-What is the rpm of the motor?

-Are you saying your wheels are 75in. circumference or is that theoretical?

I posted a graph I found at the bottom. Its a little helpful.

Source

Also I found a bike conversion kit that says their 1000w kit uses a 10Ah battery, goes 30mph and has 63ft/lbs of torque. So it sounds like you are on the right track if you figure out how to power yours.

Link

MATH:

--75" circumference wheels (24" diameter) and you wana go 31mph:

31(miles) x 5280(ft. in a mile) x 12(in. in a foot) = 1,964,160in/hr

--So you wana go 1,964,160 in/hr that means.

1,964,160(in/hr) / 75(wheel) = 26,189rph

--Your wheels need to spin 26,189 revolutions per hour.

26,189(rph) / 60(mins in a hour) = 437rpm

CONCLUSION: 75" wheels spinning at 437rpm will give you 31mph.

Image Source:

http://www.electric-bikes.com/betterbikes/winddrag.html

Our car - see my other answer - use 2 78 Ahr batteries to drive a 250 watt 24 volt motor. At best we could get about 1.5 hours out of the battery set.

A cooling off period would allow a further 1/4 to 1/2 an hour of use.

The battery voltage at that point was perilously close to the danger zone below which you will not be able to recharge the battery.

If you add batteries in parallel - so that the voltage remains the same, your 3 12v 45ah batteries are going to act like a single 12v 135ah battery. Notice that your total kWh capacity (and thus range) is the same either way.

The reason you put them together in series is because the higher voltage is (generally) more efficient and allows for higher speed and more acceleration, but more acceleration does not come free! You will burn 3x the energy at full throttle in series than in parallel - and also accelerate 3x as fast.

how did the car battery thing go? i want to copy you.

What sort of transmission do you have?

L

a 250 watt motor is capable of giving you 30 to 40 MPH. a 24 volt 250 watt motor will do about 30 to 50 miles at that speed on 2 x 70 Amp/hr car batteries.

The wheels were 8 inch in diameter and the motor ran at 2000RPM for best performance. The gearing was 14: 1

I agree 31.25 MPH from your calculations.

1. Think about the Amp/Hr of the battery pack you will use.

2. think about it's weight!

3. A 1000 watt motor will eat 4 x as much power as my 250 watt (give or take)

4. My 250 watt motor cost just short of £200 new (say $400)

5. How you use the power has much to do with how long it will last - we were racing on a flat circuit and for the most part full throttle all the time so the motor ran at it's optimum speed. Look at the commercial electric bikes and see what range they give for some idea of what you might get.

As lemonie suggests, you're not going to spin the wheels with that kind of power, but it will get you to 50k.

According to your numbers, a 24" circumference wheel is 3.8" radius 7.6 diameter...Those are pretty small wheels...I guess you meant 24" diameter.

According to google

/24 inches/rotation = 45.6 rotations/second * 60 seconds/minute = ~2700 rpm50 (km / hr) = 546.806649 inches / sElectric motors are rated a certain rpm at their optimal wattage. You need to come up with a gear ratio that turns the powered wheel at 2700 rpm while turning the motor at its favourite speed.

If you meant 24 DIAMETER then its a bit different...pi*24 = 75.4 inches circum.

546.8 / 75.4 = 7.25 revs/sec = ~435 rpm. If you had a 3000rpm motor, you want a gear ratio of about 7:1 load:power.