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B Power from two sources, electrically assisted bicycle

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  1. Mar 31, 2016 #1
    So I'm just doing a project where i have a electrically assisted bicycle and im struggling working out the power when the bike is going uphill.

    If i have a certain force provided by the rider, and a force provided by an electrical motor; can I do [tex]Power=F_{motor}* v_{motor}+F_{rider}*v_{rider}[/tex]

    where the [itex]v_{motor}[/itex] and [itex]v_{rider}[/itex] are the speeds that the given force will generate, or would it be [tex]P=F_{total}*v_{total}.[/tex]

    sorry if this is a basic question I've just been struggling and needed some reassurance.
     
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  3. Mar 31, 2016 #2

    BvU

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    Both ! Does that offer some reassurance ?

    There is a small snag: because of wind resistance, power is not a linear function of ##v## (the force to go twice as fast (##2v##) is more than twice the force for ##v##).
    So if you on your own go 15 km/h and the motor would give you 10 km/h, the two of you together won't achieve 25 km/h, but for example 20 km/h. But the relative contributions would still be 15/25 (60%) from you and 10/25 (40%) from the motor.
     
  4. Mar 31, 2016 #3
    Oh that does, I didn't think of it like that!

    So if we are travelling at a set speed (constant) say 40kmph for both the motor + rider contributions it would be better to work the power out using the second method i mentioned by working out the sum of the forces acting on the bike?
     
  5. Mar 31, 2016 #4

    BvU

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    Yes. The ##v_{\rm rider}## and ##v_{\rm motor}## aren't very meaningful. ##F_{\rm total} = F_{\rm rider} + F_{\rm motor}## is much more sensible.
     
  6. Mar 31, 2016 #5

    jbriggs444

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    Well, it might be more sensible if one had a definition for the three terms in that formula. No such definition is evident for any of the terms here.
     
    Last edited: Mar 31, 2016
  7. Mar 31, 2016 #6

    BvU

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    I plead guilty to the well-meant reproach in post #5. ##F_{\rm total}## is easily defined: the force needed to maintain the speed ##v##. It can be measured, e.g. with a spring balance. And then the other two have to be loosely defined as the (average) fractions times ##F_{\rm total}##. Perhaps ##F_{\rm motor}## can be recovered from the specifications.... (or from the time it takes the battery to go empty, an efficiency, etc. etc.). Even exact is relative :smile: .
     
  8. Mar 31, 2016 #7

    jbriggs444

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    I'll buy that. Though this model has limitations. It seems to assume that one has a single-speed bicycle, a constant torque motor and a rider who will exert a constant average force on the pedals throughout a relevant range of speeds.
     
  9. Mar 31, 2016 #8

    russ_watters

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    I don't see how it makes such assumptions. I only see force, not torque or rpm, so the drivetrain issues just seem to not be addressed. It might just be that the OP hasn't gotten that far yet.

    If I were designing a motor assisted bike, I might put the motor on the input side of the drive so it can take advantage of the gears along with the rider.
     
  10. Mar 31, 2016 #9

    BvU

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    Matt, I've seen what other threads you initiated and I'm wondering about this project. I would almost think one of your kids hijacked your account, but there must be a better explanation ?
     
  11. Mar 31, 2016 #10
    I do have equations for those terms, I was just curious which way was best to approach given my situation where I'm trying to stay at a constant cycle speed.


    Thankyou, it works out better this way!

    Yeah I haven't got that far yet, i've just been roughing out some physics first thanks for the suggestion.





    Matt:
    Yes my friend borrowed my account, they wanted to ask for a second opinion. Maybe i didn't explain it well enough thanks all! I need to up my skills as a tutor haha
     
  12. Mar 31, 2016 #11

    jbriggs444

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    You cannot add force determined by a spring scale attached to a bike at one speed and force determined by a spring scale attached to a bike at another speed and get a number that is meaningful for the purpose at hand if the speeds are different without such an assumption.
     
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