# Parenting a rigid body: Physics problem

I made a car using the Vehicle Controller (demo file at the end/bottom of the page) concept.

It's a "rigid body" chassis, that attaches four "no collision" wheels at run-time. It works pretty good, except for this :

I modeled a "car body" and parented it to the chassis, but no matter what settings (mass/radius/form factor) I try (for both the chassis and the body) I cannot seem to make the car body a "rigid body" (or anything else than "no collision") : When I press "p" my car flies frantically up like a balloon...?

• Exactly what is the force that makes my car fly up like this ?
• The child's mass, radius and form factor values should generally be smaller than those of the parent's, right?
• Do I have to make sure the parent and the child don't intersect?
• Are there other variables that I should look for?
• I can't answer how to fix, but what makes it fly away are the two physics properties fighting for priority. one wants to fall, and another wants to ... do something else. you didn't specify. Commented Mar 4, 2015 at 15:46
• But... What would not want to fall? The chassis wants to fall (rigid body mass 10.000) but the body car should "want" to fall too, being of the same nature and slightly inferior mass..? Commented Mar 4, 2015 at 15:57
• It's just a thing that happens. Their courses could be off by a few decimals of a millimeter and they would explode. Honestly I've been waiting for this question a Looooong time. thanks! Commented Mar 4, 2015 at 16:00
• Well, I ended up joining Ctrl-J the chassis and the body. That worked good enough for the level of detail I'm aiming for. Commented Mar 4, 2015 at 20:04
• Yeah, but it creates problems if you're trying to get bounding boxes to be happy with their children. See this question if you didn't understand that sentence.. Commented Mar 4, 2015 at 20:05