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I am considering motorizing my bike for my work commute. However I don't have enough knowledge about motor output as in torque / rpms to be able to determine what kind of characteristic I should be looking for. For many of the electric motors I am finding online, it doesn't really say the torque output. Is there a spec such as watts that I can judge a motor by? What specs should I be looking for in a motor that can assist in moving my weight. Or an links to good knowledge sites would be great too. Thanks.

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## 10 Replies

PKM (author)2008-07-02

I believe watts to be the common measure of electric motor power. One horsepower is about 750W. For reference, the sustainable power output of an adult human cycling is generally reckoned to be 350-500W and sprint power up to 1000W IIRC. Trying to calculate acceleration, top speed etc. from motor specs is fraught because there are a lot of factors (your weight, bike weight, terrain, tyres, wind, ...) but a lot of the DIY electric bike builders have their own rules of thumb for what wattage you need for what performance. I suggest Googling around and seeing what performance others report from their setups, they are usually good about posting details of the motor/batteries used.

Somehow I doubt when us humans are biking, we're using 1 and 1/3 horsepower...

bodo (author)2009-05-08

hi use a capasitor discharging engin you would be able to produse more than 250 Hp in nanoseconds if you can build it that will owercome all your needs

NachoMahma (author)2008-07-01

. Most electric motors are rated in horsepower. Using the formulae at the link, you should be able to compute torque.

NachoMahma (author)2008-07-01

. BTW, for assist, 0.25-1.0 HP should be enough. Guess it depends on how you define assist. For reference, a 50cc scooter motor puts out 3-4HP.

aeneas shrike (author)2008-07-01

But CC is strictly for combustion engines isn't it?

forgesmith (author)2008-07-02

Yep, CC (cubic centimeters) or CI (cubic inches) are for engines. NachoMahma is saying a 50cc scooter engine puts out 3-4 HP, so if you want about the same performance as that scooter you need a 3-4 HP motor. However that size is probably outside the legal definition of "motorized bicycle."

Definition conflict: In PA a "motorized scooter" has no seat, while a "motor scooter" does. And for both practically all of them are illegal on sidewalks and roads.

I'd suggest you read this concerning the legal issues about motorized bicycles and see if you can even use one to get to work, be a shame to put in the effort and find out it's only good for driving around the backyard.

aeneas shrike (author)2008-07-02