Uses the normal pertubation method, described by Gino van den Bergen: http://www.bulletphysics.com/Bullet/phpBB3/viewtopic.php?f=4&t=288&p=888#p888 Made btRigidBody::getInvInertiaDiagLocal const, thanks to abhikp (http://code.google.com/p/bullet/issues/detail?id=183 )