I am wondering what they are, mainly because two Unity-made sources are contradicting each other. In the Unity 5 WheelCollider tutorial, it states: “Add a Physics 3D Rigidbody component to car_root. The default mass of 1 kg is way too light for the default suspension settings. Change it to 1500 kg.”
and yet on the docs, it states: “You should strive to keep mass close to 0.1 and never more than 10. Large masses make physics simulation unstable.”
I do not know whether the docs just haven’t been updated or what, but I would definitely like to know.
I can’t see any logic on that statement. I can only guess it’s a legacy doc coming from the times where games used very limited physics (i.e. puzzle games with falling bricks or something like that). I remember an early version of Unity 3 where the Editor allowed a max of 10000 Kg in the mass. It was raised soon after.
The numeric limits for the rigidbody’s mass are: greater than zero and less than infinity.
Good values are those that physically represent the physic object you are simulating: 1500 Kg for a car, 80 Kg for a person, 15 Kg for a table, 1 Kg for a bottle… The mass of the object determines the forces and impulses that would be required for moving that object, as well as its behavior when colliding with other objects. For instance, a car colliding with a table will throw the table away without the car being affected.
It’s got to do with the ratios of masses in a physics sim, not the actual masses. There are always stability problems (especially when doing constraints) when objects interacting with each other have wildly different masses. I’m guessing whoever wrote that was just keeping it simple for everybody by telling them to keep all the masses small so they’re about the same.
It really ought to be changed in the docs to instead say something about the ratio of masses that are connected by constraints or that might collide with each other. Like Edy said, the actual value of the mass is irrelevant. I always use real mass values. People faking those numbers is why they end up with arcade type games that don’t respond realistically.