# Vehicle Moment of Inertia? How?

1. Oct 21, 2011

### jprecinos

I have been playing with several vehicle dynamics simulators for a while now, and many of them require vehicle inertial properties to create realistic simulations.
There is very limited published data out there so I was wondering how could one calculate these properties experimentally? ....Other than hanging it on a huge pendulum or creating a big rotating table is there an easy way to estimate the principal moments of inertia of an actual size vehicle?
Any conceptual ideas would be helpful. Thanks JP

2. Oct 21, 2011

### Naty1

It would help if you specified just what inputs are required for your models.

Try checking wikipedia for INERTIA and MOMWNT OF INERTIA as a start.

See the reference at the bottom of each article like this one:

http://en.wikipedia.org/wiki/List_of_moments_of_inertia

Also, seems like an auto is very much like a spring a damper system (coil springs,struts,etc).

so check out the standard formulas for such system dynamics.

3. Oct 21, 2011

### Naty1

4. Oct 21, 2011

### jprecinos

Inertial paramaters of a dynamic object:
1. CG location - easy to get. no problem
2. Moments of inertia: Yaw, Pitch, Roll
how to calculate the moments experimentally?

Yes, most simulators model vehicle dynamics as a spring, damper system, some use a more complexe model than others (with more DOF). Inertia parameters however are always needed and that is more of a geometrical and mass distribution problem. It wouldn't be accurate to model the system with the Moments of Inertia of a box for example...the dilema is how to get the Ixx, Iyy and Izz for a vehicle.

5. Oct 22, 2011

### Naty1

Well if people have developed vehicle simulators, I would hope there were input parameters to make them useful.

All I can suggest is google type searching....or post your issue in an engineering forum if nothing further turns up here.

6. Oct 22, 2011

### AlephZero

Make a model of the vehicle split into pieces where you can estimate the mass and CG position of each piece. Probably the most massive single "piece" would be the engine, which will have pretty much the same inertia properties as a block of material with uniform density.

You will have to "tweak" the complete model to get its total mass and CG in the correct position. Then work out the inertia from the components, using the parallel axis theorem etc.

That's the way it is done in "real life" at the design stage, though obviously it is easier if you are starting from a detailed CAD of the complete vehicle.

7. Oct 22, 2011

### jprecinos

Well the purpose of most simulators is to give you a plattform to recreate dynamics of a variety of vehicles during different scenarios. They do have some starting imput parameters but they wont have real properties on all vehicles out there. I do have access to some engineering databases and the National Higway safety has published some numbers but not on all vehicles either. I will keep on searching :)

Last edited: Oct 22, 2011
8. Oct 22, 2011

### jprecinos

yes CAD and FEA is my plan B. But I was hoping that there would be an experimental way to find a solution .... Like those little intertia apparatus they have at the physcis labs. An experiment along those lines I was hoping :)

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Last edited: Oct 22, 2011
9. Oct 23, 2011

### mender

Seems to me that a set of car scales could be used to find the various moments experimentally.