Wheel Moment of Inertia

According to physx vehicle documentation:

mMOI:

This is the component of the wheel's moment of inertia about the rolling axis. Larger values make it harder for the wheel to rotate about this axis, while lower values make it easier for the wheel to rotate about the rolling axis. Another way of expressing this is that a high MOI will result in less wheel spin when stamping on the accelerator because it is harder to make the wheel spin. Conversely, lower values of MOI will result in more wheel spin when stamping on the accelerator.

If the wheel is approximately cylindrical then a simple formula can be used to compute MOI:

MOI = 0.5 * Mass * Radius * Radius
There is no reason, however, to rely on equations to compute this value. A good strategy for tuning this number might to be start with the equation above and then make small tweaks to the value until the handling is as desired.

Does anyone know if this automatically being calculated by Unity?

Thank you.

 

Without having verified it I'm sure it's being calculated internally by Unity. Vehicles wouldn't work otherwise. You may use the wheel's mass in the WheelCollider to configure the effects described in the documentation. The mass property doesn't have any other effect in the vehicle (i.e. it doesn't sum up to the vehicle's mass).

 

Thanks Edy, that makes sense. Speaking of mass, I have another question, if you allow me. When a manufacturer says that the car weighs 1520, is it with or without mass?

Thanks.

 

We can assume the weight is in Earth's gravity, so the weight is equivalent to mass

 

Eheh, I just noticed I missed a word, sorry! I mean with or without tire mass

 

Yes, that weight comes from putting the car on a scale, so it includes all car parts.

 

There's something I don't quite get ... both physx and unity say that a wheel mass should be between 20 and 80, but looking on the internet, it seems like a tire can actually weigh between 6.5 and 80. The tire model i'm using (235/35ZR19 91Y) should have around 9kg. So i'm left a bit confused to which value to put. I've put 20 kg because that's the minimum unity and physx are saying, but still I'd like to understand why.

 

In the simulation the wheel mass affects the angular acceleration of the wheel, which has a strong effect in the stability of the numeric simulation (especially in a "weak" implementation such as PhysX's). Therefore they recommend to use unrealistically large masses in the wheels to reduce the chances of numeric instabilities.

 

Thank you very much for the explanation.

 

Wheel mass in PhysX appears to be used for both the wheel frequency calculations and the rotary inertia (although in RL these have different contributors).

20-40 kg for a wheel collider sounds reasonable to me, even if we only consider the un-sprung and rotating parts you need to remember that it is the mass of the tyre + wheel + brake disc.