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Bicycles and their mad skillz

  1. Nov 22, 2009 #1
    Alright. So I'm froob and must admit I don't quite understand the bicycle and how it seems to give you more for less, so to say. I figured the shape of a wheel and how the curves stand off from the ground sorta enable gravity to give an extra pull on it. How ever , when I went to look it up it said the wheels acts like a lever. Hmm, So how does a lever work? Couldn't find a very exact reason for that. Somehow the lever and the wheel-lever definitions need to match...

  2. jcsd
  3. Nov 22, 2009 #2


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    More what for less what? :smile:
  4. Nov 22, 2009 #3
    Well for like one step may only get you two feet, while going down on the pedal may take you several
  5. Nov 22, 2009 #4


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    This is mainly due to rolling. But assuming no rolling (high friction) the bike would be just a lever: you foot moves less than the tire, but with more force, depending on the gearing.
  6. Nov 22, 2009 #5


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  7. Nov 22, 2009 #6


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    Gear wheels and cranks are both, effectively, levers. They move around a pivot and the Velocity Ratio is the ratio of the two radii involved. The 'perpendicular distance' as referred to in lever calculations, is just the radius for a wheel and the length of crank (assuming you are pushing on it at right angles).

    What you get "more for less" is distance traveled. This involves more force, of course, because you can't get more energy (/power) out than you put in. The Mechanical Advantage is the actual ratio of the forces involved and the Efficiency is given by:
    Efficiency = MA/VR
    this is always less than unity but pretty good in biycles, the majority of the loss being due to the rolling resistance of the tyres which are constantly having to raise you 'up hill' as the tyre is always flattened against the road slightly. That's why you pump up racing tyres to insane pressures. Chains and sprockets and ball races are pretty damned efficient by any standard.
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