Raising the center of mass causes vibration and noise

Question

I'm looking to use your library for a truck simulation, and I want to be able to raise the CoM to simulate changes in balance, such as carrying a heavy load that raises the CoM. Therefore I'd like to have some flexibility in how high the CoM can be set. 

With the JPickup prefab, I've noticed that if I set the CoM Y value above about 0.75 (default is 0.6), the vehicle starts to vibrate and the engine noise occurs, even though the engine hasn't yet been started. I'm not able to control the vehicle when it does this - it remains stationary (apart from the vibration).

Do you know why the JPickup behaves this way? Is there a way to modify other vehicle parameters to avoid this problem?

Answer 1

Go to the Advanced section of the VPVehicleController component and reduce the Tire Impulse Ratio parameter to 0.35:

If you raise the CoM more the effect will reappear. Reduce Tire Impulse Ratio slightly more if that happens.

Technical details

It's not engine noise, but rapid suspension bumps caused by the stationary forces. The higher the center of mass (CoM) the easier the tire forces can produce an excessive angular velocity in the car. When this happens the stationary position is "overshoot", so a counter-force is immediately applied that also overshoots the target position, and so on. 

The Tire Impulse Ratio parameter controls the magnitude of the stationary tire forces to prevent that effect. The ideal value is the higher value that doesn't produce any vibration. The main factors that affect this are:

• Height of the Center Of Mass (CoM). The higher the CoM, the easier the vibration to appear and the smaller Tire Impulse Ratio value required.
• Track Width (distance from left to right wheels). The more track width, the less chances of the vibration to appear and the larger Tire Impulse Ratio value accepted.
• Inertia. The wider the inertia volume the less chances of the vibration to appear, as a larger inertia reduces the angular velocity.

Cargo Components

Additionally, the Enterprise edition of Vehicle Physics Pro includes solid and liquid cargo components that handle the cargo dynamically in a straightforward way:

https://twitter.com/VehiclePhysics/status/1094969089259393024