Motorcycle Intertia Dyno - Moment of Intertia for various objects?

[PJ Adams]

Hi all,

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

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! )


Umm...advice welcome!

Paul

 

[NateTG]

http://scienceworld.wolfram.com/
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.

 

[wais]

Hi Paul,

I'm not a physics expert, but I'll try my best to answer your question. First of all, it's great that you're taking on this project and incorporating physics into it. As for your question about the moment of inertia for various objects, it is indeed based on the object's mass and distribution of mass. In your case, using a solid steel drum would make the calculations easier, but using a hollow drum filled with concrete is also doable.

To calculate the moment of inertia for your drum, you will need to know the mass and dimensions of the drum, as well as the mass and dimensions of the concrete filling. To do this, you can weigh the drum before and after filling with concrete, as you mentioned. However, you will also need to know the density of concrete in order to calculate its mass from its volume. The density of concrete can vary depending on the type of concrete and its composition, but a general average density is around 2400 kg/m^3. So, if you know the volume of the concrete filling, you can calculate its mass by multiplying the volume by the density.

Once you have the total mass of the drum and concrete filling, you can use the formula for moment of inertia for a hollow cylinder to calculate the moment of inertia for your drum. This formula is I = mr^2, where m is the mass and r is the radius of the cylinder. You will need to use the parallel axis theorem to account for the fact that the mass is not distributed evenly in a hollow cylinder.

I hope this helps and good luck with your project! Remember, physics is all around us and it can definitely come in handy in unexpected situations.