Vehicle Physics

# CHAPTER 10

Vehicle Physics #

1. Introduction #

2. Learning Objectives #

• Understand the architecture of a physics-based vehicle setup.

• Utilize the WheelCollider component to simulate suspension and friction.

• Apply MotorTorque to propel a vehicle forward.

• Apply SteeringAngle to turn the vehicle realistically.

• Manipulate Center of Mass to prevent cars from constantly flipping over.

3. The Architecture of a Virtual Car #

1. 1. The Chassis: The main 3D model of the car. It has a single massive Rigidbody and a BoxCollider to represent the body.

1. 2. The Wheels (Logic): Four empty GameObjects, each containing a WheelCollider component. These simulate the suspension and tire friction.

1. 3. The Wheels (Visuals): The 3D tire meshes. The C# script must constantly copy the position and rotation from the logical WheelCollider and apply it to the visual mesh so they spin.

4. The Wheel Collider Component #

• Suspension Distance: How far the wheel can travel up and down.

• Spring Force: How violently the car bounces back up when it hits a bump.

• Damper: How quickly the bouncing stops. (Without damping, the car will bounce endlessly like a pogo stick).

5. Driving and Steering (Torque) #

• MotorTorque: The rotational force applied to the wheel to move the car forward.

• BrakeTorque: The force applied to stop the wheel from spinning.

• SteerAngle: Turning the front wheels left or right (in degrees).

public WheelCollider frontLeftWheel;
public WheelCollider frontRightWheel;
public float motorForce = 1500f;
public float steerForce = 30f;

void FixedUpdate()
{
    // 1. Get Input
    float vertical = Input.GetAxis("Vertical");     // Gas pedal (W/S)
    float horizontal = Input.GetAxis("Horizontal"); // Steering wheel (A/D)

    // 2. Apply Gas to Front Wheels (Front-Wheel Drive)
    frontLeftWheel.motorTorque = vertical * motorForce;
    frontRightWheel.motorTorque = vertical * motorForce;

    // 3. Apply Steering Angle
    frontLeftWheel.steerAngle = horizontal * steerForce;
    frontRightWheel.steerAngle = horizontal * steerForce;
}

6. The Center of Mass (Preventing Flips) #

• To make a car handle like an arcade racer, you must lower the Center of Mass artificially using C#. By putting the weight closer to the floor, it becomes almost impossible to flip.

7. Drifting and Friction #

1. 1. Forward Friction: How much the tire grips when accelerating.

1. 2. Sideways Friction: How much the tire grips when sliding horizontally.

8. Visual Learning: Vehicle Setup #

       [ Center of Mass ] (Artificially lowered)
              |
[ Body BoxCollider & Rigidbody ]
     /                  \
[Wheel FL]          [Wheel FR]  <-- SteerAngle applied here
  (Spring)            (Spring)
     |                  |
================================== (The Road)

9. Best Practices #

• Separate Logic from Visuals: The WheelCollider is invisible. You must write a C# loop that calls wheelCollider.GetWorldPose(out pos, out rot) and applies that exact position and rotation to your 3D tire models every frame so they spin visually!

10. Common Mistakes #

• Mass Discrepancies: A real car weighs 1,500kg. If you leave your Rigidbody mass at 1kg and apply a MotorTorque of 1,500, the car will instantly accelerate to the speed of light and break the physics engine. Always use realistic mass values (e.g., 1000kg to 2000kg) when tuning vehicle suspension.

11. Mini Project: Build an Arcade Drifter #

class HoverCarDrifter
{
    public Rigidbody rb;
    public float speed = 2000f;
    public float turnSpeed = 100f;

    void FixedUpdate()
    {
        // 1. Move forward
        rb.AddRelativeForce(Vector3.forward * Input.GetAxis("Vertical") * speed);

        // 2. Rotate car (Steering)
        float turn = Input.GetAxis("Horizontal");
        transform.Rotate(0, turn * turnSpeed * Time.deltaTime, 0);

        // 3. Fake the Drift
        // A real car has friction stopping it from sliding sideways.
        // We calculate the sideways velocity and kill 95% of it!
        Vector3 rightVelocity = transform.right * Vector3.Dot(rb.velocity, transform.right);
        
        // Subtract the sideways slide to emulate tire grip
        rb.velocity -= rightVelocity * 0.95f; 
    }
}

12. Practice Exercises #

1. 1. Why must the Center of Mass be artificially lowered in a racing game?

1. 2. What specific physics component is used to simulate a car's suspension spring and tire friction?

13. MCQs with Answers #

14. Interview Questions #

• Q: Explain the mechanical difference between a standard BoxCollider and a specialized WheelCollider in a game engine. How does a WheelCollider utilize Raycasts to simulate a suspension spring?

• Q: A racing game requires an "e-brake" drift mechanic. Explain how you would manipulate the Sideways Friction curves of the rear WheelColliders via C# to initiate a drift.

• Q: Why is tuning realistic vehicle mass (e.g., 1,500kg) critical for a WheelCollider-based car, whereas a kinematic platformer character can remain at 1kg?

15. FAQs #

16. Summary #

17. Next Chapter Recommendation #