Problem to to implement Hooke´s Law

Unity Engine
1

Hi!

I am trying to apply hooke’s law to my box.
The problem here is that this “car” not remains in the ground, it starts jumping.

I set this values based on streetcars:

Mass: 1000
Suspension length: 1
Stiffness: 8000

This is my code:

 void FixedUpdate()
    {
        RaycastHit hit;

        for (int i = 0; i < wheelPoints.Length; i++)
        {
            var wheelPoint = wheelPoints[i];

            if (Physics.Raycast(wheelPoint.transform.position,
                                -transform.up,
                                out hit,
                                suspensionLength,
                                layerMask))
            {
                // Spring force
                float contactDepth = hit.distance - suspensionLength;  

                float springForce = - stiffness * contactDepth;
               
                // Final force or suspension force
                float suspensionForce = springForce;
               
                rigidBody.AddForceAtPosition(transform.up * suspensionForce, wheelPoints[i].transform.position, ForceMode.Force);
                vehicleInAir = false;
            }
            else
            {
                vehicleInAir = true;
            }
        }
    }
2

Is there drag force somewhere? Drag and inefficiency in the spring are what prevent real springs from endlessly oscillating.

3

Nop, drag force is 0 and Angular Drag is 0.05.

Sorry, the screenshot that I added before is very small.

4

Easy. You are simulating springs without damper. An ideal spring alone just keeps oscillating forever. What you have there is a Harmonic Oscillator.

Real car suspensions have springs and dampers. Dampers are critical as they dissipate the oscillating energy in the spring allowing them to come to a rest pose. The damper can be implemented easily as a force that opposes the movement of the spring. Check out the mass–spring–damper example in the Wikipedia’s Damping article.

5

Before I added the damper force, the car has been crazy but it is even worse now. I don’t know what am I doing wrong…

https://www.youtube.com/watch?v=iACL-2KlU18

Current values:
Mass: 1000
Suspension length: 1
Stiffness: 8000
damping: 800 // 10% of the stiffness

Current code:

 void FixedUpdate()
    {
        RaycastHit hit;
        for (int i = 0; i < wheelPoints.Length; i++)
        {
            var wheelPoint = wheelPoints[i];
            if (Physics.Raycast(wheelPoint.transform.position,
                                -transform.up,
                                out hit,
                                suspensionLength,
                                layerMask))
            {
                // Spring force
                float contactDepth = hit.distance - suspensionLength;  
                float springForce = - stiffness * contactDepth;
               
                //Damper force
                float compressionSpeed = contactDepth - lastContactDepth;
                float damperForce = - damping * compressionSpeed;
              
                // Final force or suspension force
                float suspensionForce = springForce + damperForce;
              
                rigidBody.AddForceAtPosition(transform.up * suspensionForce, wheelPoints[i].transform.position, ForceMode.Force);
               
                vehicleInAir = false;
               
                lastContactDepth = contactDepth;
            }
            else
            {
                vehicleInAir = true;
            }
        }
6

1m is a large and unrealistic suspension.

For Mass=1000, I have a vehicle where these suspension settings work rather good:

Suspension length: 0.25
Spring rate: 25000
Damper rate: 1500

These are the settings at the WheelCollider. I’m not sure if/how they work implemented with direct forces.

Also note that in your code vehicleInAir depends on the latest evaluated wheel only, regardless all other wheels.

7

Just add wheels it works

8

Damper force is damping coefficient * speed
In your formula, I think compressionSpeed is not actually a speed, because you forgot dt
Try

                float compressionSpeed = (contactDepth - lastContactDepth)/Time.deltaTime;
                float damperForce = - damping * compressionSpeed;