WheelCollider negative torque vs. brake torque

Hi all,
So I'm curious about brake torque and negative torque to wheel collider.
It is recommended to use brake torque instead of applying negative torque to stop the wheel. But how is that different from applying a negative torque?
How is brake torque applied in terms of the longitudinal tire force function that propel the rigidbody forward?
Thank you very much.

 

The theory is:

- A motor torque accelerates the wheel in either direction, positive or negative. Using negative torque to brake a wheel would end in the wheel accelerating backwards.
- A brake torque aims to stop the wheel, no matter the direction it's currently rotating to, nor any other motor torque being applied. If the wheel is stopped, a brake torque prevents it to begin rotating until an equivalent amount of motor torque (either positive or negative) overcomes it.

This is the theory. I couldn't figure out how the WheelCollider applies them. As for what I've seen, probably PhysX just sums both torques and applies the result, which a simple solution but physically incorrect.

 

Thanks for passing by.
I just took a look again at the clutch strength thread from nvidia, and indeed they use sum of the 2 forces:
wheelTorque = brakeTorque + handBrakeTorque + tireTorque + G*clutchTorque.
Right now I'm imagining the system like this: Assume the car is moving forward, the wheel is producing torque in positive direction. The brake is applied, the brake uses friction torque in the opposite direction from the wheel torque direction to slow the wheel down. What happens when the magnitude of the wheel torque is less than the magnitude of the brake torque, say in a certain step, wheel torque is +100Nm and brake torque is -200Nm, would that causes the wheel to roll backwards if the sum of 2 torques is used?
I found the formula for braking force from this website, and they also use the sum of 2 torques:
https://www.engineersedge.com/mechanics_machines/braking_torque_13635.htm

with Tf being braking torque, Ti being wheel torque.
So what is the "correct" way to apply brake force?

 

Hi! Saw your explanation of brake torque. I had a question in an physics course saying that there was a negative brake torque applied at an object. Is this really possible according to you? Shouldn't it be called "positive torque" in that case?
Apparently I should have interpreted it as a brake torque but that it was going in the other direction since it was expressed as a negative value. For me it just seem counterintuitive to even call it a break torque in that case

 

To me, brake torque is a sign-less magnitude, equivalent to a coefficient of friction (also sign-less). I'm not sure if this is academically correct, but that's how it works.

A "positive" brake torque increases the magnitude of a brake torque value, while a "negative" brake torque reduces the magnitude of an existing brake torque value. The actual brake torque magnitude always remains non-negative. For example, you may apply more or less pressure to the brake pedal in your car. This applies more or less braking torque magnitude to the brake pads.

When a brake torque magnitude is applied to a rotating object, then the actual torque value receives the sign that counteracts the rotation. If the angular velocity is actually zero, then the sign of the brake torque counteracts any other regular torque applied to the rotating object. This is analogous to the standard Coulomb friction, where the friction force opposes the velocity vector.

For example, suppose a wheel that is not touching the ground in a car, and it's rotating forwards (positive angular velocity):

1. The brake pads apply a braking torque of 100 Nm.
2. The actual torque applied receives the sign that opposes the rotation, so it becomes -100 Nm (negative) and the angular velocity starts decreasing.
3. When the wheel is completely stopped then the brake pads keep applying 100 Nm of brake torque. This is sign-less, and it's still there opposing any possible rotation of the wheel. Think about a static friction.
4. The drive shaft of the wheel applies some torque backwards, let's say -50 Nm. Then the brake torque becomes immediately positive and applies +50 Nm to counteract it, so the wheel remains stopped.
5. The torque in the drive shaft raises to -100 Nm. The brake torque increases to +100 Nm, and the wheel remains stopped.
6. The torque in the drive shaft raises to -101 Nm. The brake pads keep applying a brake torque of 100 Nm, which in this case is positive because of the drive torque being negative. The net torque results -1 Nm, so the wheel's rotation starts accelerating backwards.