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

I have done some calculations, with wheels who have a 75inch circumference, i have calculated that 444 rpm should get me going about 40km/h or 25 mph. I don't believe that this is right. I am also wondering what amount of torque would be good enough for this situation?

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

4 years ago
36v, 1000w Motor = Draws 28 Amps

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.

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
NoNameCola (author)  brian31404 years ago
Since you average car battery has a capacity of about 45 amp hours and i will have three, i should have about a maximum of about 5hours of use at full speed.
4 years ago
Most unlikely - you will probably get less than 1/2 an hour at full speed.

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.
4 years ago
If you add batteries in series - so that the voltage is higher, your 3 12v 45ah batteries are going to act like a single 36v 45ah battery.  That is 1.6kWh.  If you run your 1kW motor at full throttle, you'll only get 1.6 hours out of it.

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.
NoNameCola (author)  brian31404 years ago
The rpm is 3000 and i am planning on using 3 car batteries to power it. I would mount the car batteries onto a bike rack that i have not yet made.
4 years ago
Your car batteries will weigh in at a LOT. This is one problem with using lead acid battery technology and why no electrical vehicle manufacturers use them. Weight = drag so the more weight the more power you need to get going and keep going.
NoNameCola (author)  brian31404 years ago
I will also be using chain and gears to reduce the rpm.
-DOUG-4 months ago

Forget car batteries. Unsuited every which way, especially that they
don't last when used this way. Almost \$300, but you can get the LiPo on
eBay for not much more and one set will last through many replacements
of car batteries. Not to mention much smaller and lighter. If you do use
some cheap lead acid cells, (Cheaper than car batteries, even) use some
better suited to the job, such as Werker or PowerSonic.They won't last as long as lithium, either.

As
mentioned, work at 420rpm at the wheel for a 24" and 387rpm for 26" At
the Razor website you can buy the 11t pinion for the motor and the 80t
sprocket and freewheel for the wheel. The motor would have to turn just
over 7 times the speed of the wheel, so over 2,940rpm. And you have a
3,000rpm motor.

You can get serious help with electric vehicles
online. Endless-Sphere.com tends to offer the more scientific minds and
technology help, DIYElectriccar.com has the serious fabricators. Neither
site slouches at the other's specialty, either.

-DOUG-4 months ago

Forget car batteries. Unsuited every which way, especially that they don't last when used this way. Almost \$300, but you can get the LiPo on eBay for not much more and one set will last through many replacements of car batteries. Not to mention much smaller and lighter. If you do use some cheap lead acid cells, (Cheaper than car batteries, even) use some better suited to the job, such as Werker or PowerSonic. They won't last as long as the lithium batteries either.

As mentioned, work at 420rpm at the wheel for a 24" and 387rpm for 26" At the Razor website you can buy the 11t pinion for the motor and the 80t sprocket and freewheel for the wheel. The motor would have to turn just over 7 times the speed of the wheel, so over 2,940rpm. And you have a 3,000rpm motor.

You can get serious help with electric vehicles online. Endless-Sphere.com tends to offer the more scientific minds and technology help, DIYElectriccar.com has the serious fabricators. Neither site slouches at the other's specialty, either.

cawthornish9 months ago

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

cid23234 years ago
have you tried replacing the pulleys to see which work best at rpm and torque....try getting a larger pulley for the motor to increase rpm and torque...
GTechno134 years ago
I didnt read through everything so sorry if this has been said or this is useless info but I have made a small 350w electric vehicle myself.  If you putting this on a bicycle, 3 car batteries take up a lot of volume and if they are mounted above the axle you may have a hard time even holding the bike up!  Check out  "lifepo4 batteries" aka lithium iron phosphate.  They are designed for EV applications but are a bit expensive even SLA would be a better place to start.  A car battery is for cranking.  My electric skateboard type thing uses a 350w motor and goes 20mph on a 7 in drive tire but has a hard time with uphills combined with 200lbs total weight.
lemonie4 years ago
You ain't going to get much from 1KW, that's equivalent to a small cc petrol (gasoline) engine, so 25-30 mph seems about right.
What sort of transmission do you have?

L
4 years ago
1KW=1.34hp,  1HP=746W
rickharris4 years ago
From a sheer practical viewpoint. see my instructable www.instructables.com/id/Building-an-electric-racing-car/

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.
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.
frollard4 years ago
When you consider a human can generally put 1-200 watts to the pedals sustained and an athlete can put out 500+, its a fair amount of power.

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.