Realistic Car Driving Script

using UnityEngine;
public class RealisticCarController : MonoBehaviour
[Header("Engine & Transmission")]
    public float enginePower = 400f;        // Horsepower equivalent
    public float maxRPM = 7000f;
    public float idleRPM = 800f;
    public AnimationCurve powerCurve;       // Power vs RPM curve
    public float[] gearRatios =  3.5f, 2.1f, 1.4f, 1.0f, 0.8f ;
    public float finalDriveRatio = 3.2f;
    public float shiftUpRPM = 6500f;
    public float shiftDownRPM = 2000f;
    public float autoShiftDelay = 0.2f;
[Header("Wheel & Steering")]
    public WheelCollider[] wheelColliders;   // Order: FL, FR, RL, RR
    public Transform[] wheelMeshes;
    public float maxSteeringAngle = 35f;
    public float steeringSpeed = 60f;        // Degrees per second
    public float antiRoll = 5000f;            // Anti-roll bar stiffness
[Header("Suspension & Grip")]
    public AnimationCurve lateralGripCurve;   // Grip vs slip angle
    public float downforceFactor = 0.5f;
    public float brakeForce = 3000f;
    public float handbrakeForce = 2000f;
private float currentSteering;
    private float currentThrottle;
    private float currentBrake;
    private float currentRPM;
    private int currentGear = 0;
    private float nextShiftTime;
    private Rigidbody rb;
void Start()
rb = GetComponent<Rigidbody>();
        rb.centerOfMass = new Vector3(0, -0.5f, 0); // Lower COG for stability
void Update()
GetInput();
        UpdateWheelMeshes();
void FixedUpdate()
UpdateEngineAndGearbox();
        UpdateSteering();
        UpdateWheels();
        ApplyAntiRoll();
        ApplyDownforce();
void GetInput()
currentSteering = Input.GetAxis("Horizontal");
        currentThrottle = Input.GetAxis("Vertical");
        currentBrake = Input.GetButton("Jump") ? 1f : 0f;
void UpdateEngineAndGearbox()
// Engine RPM based on wheel speed (average of driven wheels)
        float wheelRPM = 0;
        int drivenWheels = 0;
        for (int i = 2; i < wheelColliders.Length; i++) // Assuming RL, RR driven
wheelRPM += wheelColliders[i].rpm;
            drivenWheels++;
wheelRPM /= drivenWheels;
float engineRPMFromWheels = wheelRPM * gearRatios[currentGear] * finalDriveRatio;
        currentRPM = Mathf.Lerp(currentRPM, Mathf.Max(idleRPM, engineRPMFromWheels), Time.fixedDeltaTime * 5f);
// Add throttle influence
        if (currentThrottle > 0.1f)
            currentRPM += currentThrottle * enginePower * Time.fixedDeltaTime * 20f;
currentRPM = Mathf.Clamp(currentRPM, idleRPM, maxRPM);
// Automatic shifting
        if (Time.time > nextShiftTime)
if (currentGear < gearRatios.Length - 1 && currentRPM > shiftUpRPM)
currentGear++;
                nextShiftTime = Time.time + autoShiftDelay;
else if (currentGear > 0 && currentRPM < shiftDownRPM)
currentGear--;
                nextShiftTime = Time.time + autoShiftDelay;
void UpdateSteering()
float steeringInput = currentSteering;
        float speedFactor = Mathf.Clamp01(rb.velocity.magnitude / 100f);
        float maxAngle = maxSteeringAngle * (1 - speedFactor * 0.5f);
        currentSteering = Mathf.MoveTowards(currentSteering, steeringInput * maxAngle, steeringSpeed * Time.fixedDeltaTime);
wheelColliders[0].steerAngle = currentSteering;
        wheelColliders[1].steerAngle = currentSteering;
void UpdateWheels()
float motorTorque = 0;
        if (currentThrottle > 0 && currentBrake == 0)
float powerFactor = powerCurve.Evaluate(currentRPM / maxRPM);
            motorTorque = currentThrottle * enginePower * powerFactor;
for (int i = 0; i < wheelColliders.Length; i++)
// Apply motor torque to rear wheels (index 2 and 3)
            if (i >= 2)
                wheelColliders[i].motorTorque = motorTorque;
// Brakes
            wheelColliders[i].brakeTorque = currentBrake * brakeForce;
// Handbrake on rear wheels
            if (Input.GetKey(KeyCode.Space) && i >= 2)
                wheelColliders[i].brakeTorque = handbrakeForce;
// Realistic grip (lateral friction)
            WheelHit hit;
            if (wheelColliders[i].GetGroundHit(out hit))
float slipAngle = Vector3.Angle(hit.forwardDir, hit.sidewaysDir);
                float gripFactor = lateralGripCurve.Evaluate(slipAngle);
                WheelFrictionCurve lateralFriction = wheelColliders[i].sidewaysFriction;
                lateralFriction.stiffness = gripFactor;
                wheelColliders[i].sidewaysFriction = lateralFriction;
void ApplyAntiRoll()
WheelHit hitLeft, hitRight;
        float travelFL = 1f, travelFR = 1f;
if (wheelColliders[0].GetGroundHit(out hitLeft))
            travelFL = (-wheelColliders[0].transform.InverseTransformPoint(hitLeft.point).y - wheelColliders[0].radius) / wheelColliders[0].suspensionDistance;
        if (wheelColliders[1].GetGroundHit(out hitRight))
            travelFR = (-wheelColliders[1].transform.InverseTransformPoint(hitRight.point).y - wheelColliders[1].radius) / wheelColliders[1].suspensionDistance;
float antiRollForce = (travelFL - travelFR) * antiRoll;
        if (wheelColliders[0].GetGroundHit(out hitLeft))
            rb.AddForceAtPosition(wheelColliders[0].transform.up * antiRollForce, wheelColliders[0].transform.position);
        if (wheelColliders[1].GetGroundHit(out hitRight))
            rb.AddForceAtPosition(wheelColliders[1].transform.up * -antiRollForce, wheelColliders[1].transform.position);
void ApplyDownforce()
float downforce = rb.velocity.magnitude * downforceFactor;
        rb.AddForce(-transform.up * downforce);
void UpdateWheelMeshes()
for (int i = 0; i < wheelColliders.Length; i++)
Vector3 position;
            Quaternion rotation;
            wheelColliders[i].GetWorldPose(out position, out rotation);
            wheelMeshes[i].position = position;
            wheelMeshes[i].rotation = rotation;

For Roblox (Luau)

• Do not use built-in VehicleSeat. It is not realistic.
• Use BodyMovers (BodyVelocity, BodyPosition) or AlignPosition.
• Constantly raycast sideways. If lateral velocity > 5 m/s without steering input, you are sliding. Apply opposite force to simulate self-aligning torque.

6. Decision Logic Examples

• Lane change initiation:
  1. Check need (slower vehicle ahead or planned turn).
  2. Check gap acceptance using projected trajectories over horizon T_pred (2–4 s).
  3. Signal for t_signal (0.8–1.5 s), then execute steering profile (smooth ramp of δ) while modulating speed.
• Following behavior:
  • Maintain time gap T_gap = 1.0 + 1.5*(1-α) s (aggressive drivers shorter gaps).
  • Use model predictive adjustment: predict lead deceleration and adjust a to avoid TTC < 1.0 s.

Abstract

We present a modular, physics-informed scripting approach for realistic car driving in interactive simulations and games. The script combines vehicle dynamics, driver behavior models, perception, and control heuristics to produce believable driving patterns across maneuvers (lane following, lane changes, turns, parking) while remaining computationally efficient for real-time use.

1. The Core Physics Model

To move away from "arcade" floating mechanics, a script must account for four main physical forces: realistic car driving script

• Engine Torque & Traction: The engine generates rotational force. This force is transmitted through the gearbox to the wheels. If the force exceeds the tire's grip, the wheels spin (burnout). If grip is higher, the car moves.
• Drag & Rolling Resistance: Cars don't coast forever. Air resistance (drag) increases with speed ($Force = v^2$), creating a natural speed limit. Rolling resistance is the friction from tires touching the road.
• Slip Angle: This is the secret to realistic handling. When a tire turns, the car's inertia wants to keep moving in the original direction. The angle between the wheel's facing direction and its actual movement direction is the "slip angle." Tires generate lateral force (cornering grip) based on this angle.
• Weight Transfer: When you brake, weight shifts forward, increasing grip on the front tires and reducing it on the rear. When accelerating, weight shifts back. This determines if a car understeers (front slides out) or oversteers (rear slides out).

2. Driver Characteristics

• Driver Experience and Skill: The driver's experience and skill level influence their ability to control the vehicle, navigate complex scenarios, and respond to unexpected events.
• Driver Emotions and Distractions: The driver's emotional state and distractions (e.g., fatigue, phone usage) impact their attention, reaction time, and overall driving behavior.

2. Building the Suspension Model (The "Feel" of the Road)

Most realistic car driving scripts replace standard collision detection with four independent suspension raycasts (one per wheel). Here is the logic you need to implement: For Roblox (Luau)

The Hooke's Law Equation: (Force = SpringConstant * Displacement) Do not use built-in VehicleSeat

• Cast a ray from the wheel hub downward.
• If the ray hits the ground, measure the distance from the rest position.
• Calculate the spring force and damping force.

Realistic Implementation Tips:

• Spring Constant: Heavy cars (SUVs, sedans) need stiffer springs; light cars (hatchbacks, sports cars) need softer springs to absorb bumps.
• Damping: Without damping, the car will bounce forever. Critical damping (ratio of 1.0) stops oscillations immediately.
• Sway Bars: To prevent the car from rolling over in corners, your script must link opposite wheels. If the left wheel compresses, the right wheel should artificially extend slightly.

A realistic script makes the chassis lag behind the wheels. When you turn left, the car should lean right.

B. The Powertrain (Engine & Transmission)

A realistic script calculates speed based on RPM (Revolutions Per Minute) rather than just setting velocity directly.

1. Calculate Engine RPM: Based on current speed and current gear ratio.
2. Clutch Logic: When shifting gears, power delivery is momentarily cut.
3. Torque Curve: Engines don't produce linear power. A script should reference a "Torque Curve" (an animation curve or array) where torque peaks at mid-range RPM and drops off at high RPM.