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    power, torque

    Do i understand it correctly that higher power engine will have higher torque, and lower power one will have lower torque; when torque is not enough we should shift to lower gear combination where it would be enough, however speed of bicycle would be slower. What happens when torque is not enough to turn the wheel - do engines have any protection from overload, will engine's parts be damaged when it works in that mode when it hardly turns the wheel or it doesn't matter? How much bigger should the torque be (compared to the counterreacting) to move the bike and give it acceleration?
    I need an engine for a bike to be able to go up long (10 -15 km), moderately steep ascends (10-15 deg), with allowed rider weight (110 kg or so). Speed is not important; i do not want it to be greater than 25 kmh on flat portions and priority is slow, smooth riding with the longest battery life. Would lets say 250 w cope with the task at the lowest gear combination without overheating or breaking down or a higher power should be chosen?

    #2
    Originally posted by miken View Post
    Do i understand it correctly that higher power engine will have higher torque, and lower power one will have lower torque; when torque is not enough we should shift to lower gear combination where it would be enough, however speed of bicycle would be slower. What happens when torque is not enough to turn the wheel - do engines have any protection from overload, will engine's parts be damaged when it works in that mode when it hardly turns the wheel or it doesn't matter? How much bigger should the torque be (compared to the counterreacting) to move the bike and give it acceleration?
    I need an engine for a bike to be able to go up long (10 -15 km), moderately steep ascends (10-15 deg), with allowed rider weight (110 kg or so). Speed is not important; i do not want it to be greater than 25 kmh on flat portions and priority is slow, smooth riding with the longest battery life. Would lets say 250 w cope with the task at the lowest gear combination without overheating or breaking down or a higher power should be chosen?
    I think a clarification of terms might help.

    To get a measurement of "Power" you'll need three different elements, and they're all important:

    1: Force. This refers to the amount of energy applied -- measured for our purposes usually in pounds or kilograms.

    2: Distance. This refers to how far something has been pushed.

    3: Time. How long it took to accomplish both 1 and 2.

    Torque is a combination of 1 and 2. Force multiplied by distance = torque

    Power is a calculated by multiplying torque, by the measure of time...

    So--here we go:

    Story goes, way back when, somebody just wanted to figure out a standard way to measure power. They dropped a 1000 pound weight down a well, with a pulley at the top, and harnessed up a horse to try to pull like heck and lift that weight.

    Well--according to the story, that (average or not?) horse was able to struggle that weight a whopping 33 feet over the course of one minute...

    So--go back to the formula: 1000 pounds force, x 33 feet, x 1 (for 1 minute). And so, 33,000 foot-pounds per minute--is our common measurement used today--for one "horsepower" (for our e-bike world, it's worth noting that 746 watts is equal to one horsepower).

    The calculation for torque in Newton Meters is less of a story, but the same basic deal. Force x Distance.

    Unfortunately, it's not always a "given" that the correct terms will actually be used by everyone--so there remains some room for confusion.

    However--to your questions as best I can:

    While we can assume the one-minute time-frame is a constant in the formula, it's not too hard to imagine how two motors with identical "Power Output Ratings" could still have radically different "Torque Output Ratings". A good example of this, would be to compare direct-drive hub motors to geared hub-motors.

    Electric motors like to spin fast. They're most efficient when they do. The faster an electric motor is spinning, the greater its torque, (and its power). DC electric motors can be geared for more torque at low-speeds, but they can also be wired internally in different ways, for either more torque, or more top speed. So, if you are climbing hills, at a max of 25 kilometers per hour, I think a geared-motor is definitely the way to go.

    How much power? That's a difficult thing for anyone but you to decide. But to help you begin to make comparisons, it can be helpful to know a baseline: A good strong healthy bicyclist should be able to put out around 250W of power. So--if you choose a 250W motor, you're basically adding another strong person's power to your bike. If you choose for a motor with a high gear-ratio (in the neighborhood of 1 to 10 would be good), you will end up with a lower top-speed (like your desired 25 KPH), but your motor will be under a lot less stress, because it can spin much faster inside.

    Now--as far as acceleration characteristics, geared motors, with their higher initial torque, usually provide much greater amounts of "pull" at lower speeds, when compared with a direct-drive motor. You'll see sharp acceleration with a high-torque geared motor. They produce actually pretty good power across their operating range, but once they reach their preset "max speed" that's all you're gonna get from a geared motor--while the direct-drives are more like steam-engine locomotives. The faster they go, the more powerful they get--and they sometimes seem to want to keep speeding up until you've just flat run out of juice to feed them, or the self-generated headwind is just too great... But this difference is especially helpful if you will be using your bike frequently on hills. For example, if you should stop on a hill, and must pedal uphill from a stop in order to get going again, this situation will be far less stressful on your system, the greater the gear-ratio of your motors is. Direct-drive motors wouldn't like that situation so much. It wouldn't hurt them in any way, but from a standstill, they wouldn't have the torque available to help you very much. Direct-drive motors don't want to climb steep hills, they want to set land-speed records!

    I've read that many motors (internally, or through their controllers) do have overload and overheat protection, but I'm sure there are also many which do not. I'd ask your vendor once you're ready to start choosing products.

    As far as gear-combinations, that depends on whether you go mid-motor, or hub-motor. For hub-motor projects, I'd say it doesn't matter a lot--you'll just shift into whatever gear feels right for the circumstance--same as you always have.

    For a mid-motor design, I would opt for a large rear-sprocket, and a smaller front (crank-set) sprocket. This will give you much more power at lower speeds. It will of course reduce your top speed, but you said you want that to be slow anyway--so yeah. Go for the low-end torque, and get the big sprocket in back with a mid-drive. Set it up with some "soft start" or the like, and it'll still be gentle enough on your chains and other components.

    I hope this is helpful to you, and that if I've said something incorrect that a more knowledgeable poster will correct me!

    Best of luck in your adventures!

    Take care,

    Tklop
    Last edited by tklop; 1 week ago. Reason: to fix a typo and for clarity

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