# Center of Mass Problem

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Hey people,

I heard this forum has a lot of really talented and technical oriented people who can help me out of my problem.

At work I have to do some research and create a little scene where a car SLOWLY falls down a cliff. Imagine someone would push a car down a hill without a driver and it falls down to hell.

So as a Houdini-Newbie I imported an OBJ of a car and declared it as an RBD-Object. My simple-geometry-plane is a static object. Through the gravity tag my car "rolls" down that hill and jumps over the cliff. But there is one problem. The center of mass should be more in the front of the car. Is there any easy way to move the CoM-point to the position where the engine sits? I tried different things like pushing the pivot aso. but nothing worked. It seems like Houdini tries to calculate the center of mass in the Object-Level with a bounding box and ignoring my custom moved pivot.

Is there an easy way to define an exact point, where my center of mass is? Or is there any way to show it at least?

My goal is, like you will see in the screenshot, that my car falls down a little bit faster in the front. Like it would behave in real world. Now it is pushing itself over the cliff until it gets over the center point of the OBJ.

Thanks,
Bastian

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marty    574

RBD Packed Object Physical tab/ uncheck Compute Center of Mass and Inherit Pivot from Point Position. Then set your pivot CoM

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Hey marty, thanks for the quick reply. But is there any way to show the pivot that I'm moving after disabling this both options? I did that already and my car started to spin.... Maybe this indicates other problems I should solve first?

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marty    574
Posted (edited)

easy - create a Null at Obj level and copy and paste relative references of it's transform parameters. Make the handle persistant so you can see it and use it.

EDIT: if you turn on point display then the CoM becomes visible without the Null shenanigans!

Let us know if you need help!

Edited by marty

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On 8.4.2017 at 0:03 AM, marty said:

easy - create a Null at Obj level and copy and paste relative references of it's transform parameters. Make the handle persistant so you can see it and use it.

EDIT: if you turn on point display then the CoM becomes visible without the Null shenanigans!

Let us know if you need help!

Hey marty,

I found out that the bullet dynamics makes a bounding box around the complete mesh. Had to change the dynamic to RBD and merge the tires with the cars body and it seems to work now. Is there an easy way to group different parts of a mesh for dynamics on object-level? Because a problem was that when I was importing my mesh into the scene, the complete car broke into pieces (I still don't know why and how I fixed it. I think it was inside the cars SOP in the assemble-node). Any suggestions here?  I'll link a screenshot of the car node-setup. Maybe this helps.

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marty    574

hey - not too sure of what's happening there - if you can upload a test file would be better - just lock/freeze, the file node so the geo stays in the .hip.