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Spiral spring

  1. Sep 14, 2007 #1
    I am planning to build a bicycle with mechanical regenerative braking. I am thinking of setting a spiral spring to wind during braking and use its tension to assist in climbing hills

    Would such a spring give me enough power for this? Are there any other alternatives?
    Is this feasible??
    Any info or tip would be appreciated.
  2. jcsd
  3. Sep 14, 2007 #2


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    It's definitely possible, but I suspect that the additional weight of the necessary 'clockwork' mechanism would outweigh any benefits you might gain.
  4. Sep 14, 2007 #3
    i dont really see more than a few Kgs of additional weight from the 'clockworks', but can the spiral gear serve my porpose.
    Last edited: Sep 14, 2007
  5. Sep 14, 2007 #4


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    Welcome to PF, Ambush.
    Even if you keep the weight down, the sticky part that I can foresee is in the mechanical complexity. If your riding conditions were exactly the same all of the time, it wouldn't be a problem. What troubles me is how you would set up the power-management system so that it both absorbs and releases the energy at rates that are suitable for different situations.
    For instance, my town lies at the junction of three 8° grade highways that are over a km long each. You'd need a spring a metre in diameter to do any good here. Even someplace like San Fransisco has so many different lengths and grades to its streets that getting a 'generic' system to suit all of them seems quite daunting.
    There's also the matter of how it would interact with your gear train.
    As Brewnog said, it's very doable. Practicality, though...?
    It might be worth sacrificing some weight issues and installing a small generator-battery-motor set. It would be easier to regulate.
  6. Sep 14, 2007 #5
    I really don't need it to be practical, its just a project for school. Just a working model for demonstration purpose would do fine..
    Last edited: Sep 14, 2007
  7. Sep 14, 2007 #6


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    Oh... that's different, then.
    If practicality isn't an issue, then you can definitely make one that will work to some extent. You'd need some sort of reversing mechanism, since a spring releases its energy in the opposite direction to which it was absorbed from, plus a 'neutral' to keep it out of the way in level riding.
    Good luck with it. Keep us informed of your progress.
  8. Sep 14, 2007 #7


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    Hi ambush,
    The type of spring you're refering to is called a "constant force spring" also called "constant torque spring" and "negator spring". All you need to do to show the effectiveness of such a project (in principal) is to calculate the amount of energy stored in a given spring, and relate that to the amount of energy you might be able to store during a typical braking application. If you size a spring that might do this, then compare the size and weight of that spring to what you believe is practical.

    It might also be nice to see if the amount of energy stored for a spring could be used to augment the power needed to climb hills or something like that. I'd imagine being able to store up some energy while peddling along flat ground by applying the brake for example, then using that energy to help climb a hill.

    Here's a web site that sells these springs. They might have all the information you're looking for.
  9. Sep 14, 2007 #8
    Will try that out....Thanks..
  10. Sep 14, 2007 #9


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    You can do that calc without worrying about the energy storage mechanism itself.

    Ignoring air resistance etc, the KE that you can store when the bike stops = (1/2)mv^2. The energy needed to climb a hill of height h = mgh.

    Play with those numbers and decide if your idea is going to work.

    A more practical option might be to store the energy decelerating from speed V down to 0, and use it to help you accelerate on the level back up to speed V. That could be useful for cycling in cities. An electric motor/generator and a battery might be more practical than a spring. But Google for inventor Clive Sinclair in the UK to see how not to make money out of this idea. The Sinclair C-5 wasn't exactly a commercial success, and his second try at power-assisted bikes was even worse.
  11. Sep 15, 2007 #10


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    It's too late for me to edit my previous post, so I'm just entering this as a correction for a typo. The highways leading to my town are 8 percent grades, not 8 degrees.
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