Calculate Speedometer Velocity in Ammo.js Vehicle

This article explains how to accurately calculate and retrieve the speedometer velocity from a btRaycastVehicle chassis in Ammo.js. You will learn how to extract the linear velocity from the vehicle’s rigid body, determine directional forward and reverse speed, and convert the physics engine units into standard real-world speeds like kilometers per hour (km/h) or miles per hour (mph).

Step 1: Access the Rigid Body and Linear Velocity

In Ammo.js, the vehicle’s movement is driven by a rigid body representing the chassis. To find the vehicle’s speed, you must first access this rigid body from your btRaycastVehicle instance and retrieve its linear velocity vector.

// Retrieve the rigid body of the vehicle chassis
const chassisBody = vehicle.getRigidBody();

// Get the 3D linear velocity vector
const velocity = chassisBody.getLinearVelocity();
const vx = velocity.x();
const vy = velocity.y();
const vz = velocity.z();

Step 2: Calculate Absolute 3D Speed

If you only need the overall speed of the vehicle regardless of the direction it is facing, you can calculate the magnitude (length) of the 3D linear velocity vector. This is calculated using the Pythagorean theorem in 3D.

// Calculate absolute speed in meters per second (assuming 1 unit = 1 meter)
const speedMPS = Math.sqrt(vx * vx + vy * vy + vz * vz);

Step 3: Calculate Directional Forward Speed (For Reversing)

A standard speedometer needs to differentiate between forward and reverse driving. To achieve this, calculate the dot product between the vehicle’s linear velocity vector and its forward-facing vector.

Ammo.js provides a built-in method on the raycast vehicle to get its current forward vector.

// Get the vehicle's forward vector
const forwardVector = vehicle.getForwardVector();
const fx = forwardVector.x();
const fy = forwardVector.y();
const fz = forwardVector.z();

// Calculate the dot product of velocity and forward vector
const forwardSpeedMPS = (vx * fx) + (vy * fy) + (vz * fz);

If forwardSpeedMPS is positive, the vehicle is moving forward. If it is negative, the vehicle is reversing.

Step 4: Convert to km/h or mph

Ammo.js physics equations typically assume standard SI units, meaning the velocity is returned in meters per second (m/s). To display this on a user-facing speedometer, multiply the value by the appropriate conversion factor:

// Convert directional speed to km/h and mph
const speedKmh = forwardSpeedMPS * 3.6;
const speedMph = forwardSpeedMPS * 2.23694;

// Use Math.round() or toFixed() for display purposes
console.log(`Speed: ${Math.round(speedKmh)} km/h`);