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Homework Help: Motorcycle Intertia Dyno - Moment of Intertia for various objects?

  1. Dec 30, 2004 #1
    Hi all,

    Firstly - I'm not a collage student but this seemed a good place to ask the question! Any help welcome :smile:

    As a fun home project, I've been building a motorcycle dyno, connected to a PC via a custom made PIC based device. Have most of it worked out with regards to data collection, ignition reading from the bike, etc.

    Was planning to make a dyno drum from one solid piece of steel, thus making the physics easy with regards to moment of Intertia (simple solid cylinder) - but steel is (now) very expensive, so I wanted to make a steel hollow drum, but filled with 'something' to give it weight for the motorcycle to work against. I was thinking possibly concrete because it's cheap & heavy - with the drum sealed as a solid unit once the concrete was solid.

    I would know the exact dimensions & weight of the steel used to create this hollow drum - but how would I use the dimensions of the concrete in the physics?

    Would I have to weigh the drum before filling? Then fill it & allow the concrete to set & weigh again? Then seal the end of the drum and weld it shut & wiegh again for a final total?

    That would give me weights before and after - but what does concrete weigh per cm3? Arggh! (they told me I would actually use physics once I left school & I didn't believe them! :wink: )

    Umm...advice welcome!

  2. jcsd
  3. Dec 30, 2004 #2


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    can be quite handy.

    Regarding the density of concrete, you're more or less on your own. How much concrete weighs after setting can vary a whole lot since concrete is, after all, a mixture of various things.

    Really, I would suggest that you worry about balancing the drum first. Once you have that, you can use a known force to measure the moment of inertia.
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