Simulate Tank Tracks with Ammo.js Constraints

Simulating tracked vehicles like tanks in 3D physics engines can be challenging due to the complexity of treads. This article provides a straightforward guide on how to simulate a tank using Ammo.js (the JavaScript port of the Bullet Physics engine) by leveraging physics constraints. We will explore how to set up the chassis, configure rigid bodies for road wheels, and use constraints to mimic realistic tank suspension and skid-steering movement without the extreme computational overhead of simulating hundreds of individual tread links.

The Architectural Approach

Simulating every individual link of a tank track with rigid bodies and hinges is highly unstable and computationally expensive. The industry-standard workaround is to simulate the tank as a wheeled vehicle with physics constraints, and then visually wrap the wheels with a shader-animated or bone-animated track mesh.

To achieve this in Ammo.js, you will need: 1. A Chassis: A single rigid body representing the hull. 2. Road Wheels: Multiple cylindrical or spherical rigid bodies on each side. 3. Constraints: Physics joints connecting the wheels to the chassis, acting as the suspension and axles.

Step 1: Creating the Chassis and Wheels

Start by creating the main chassis rigid body. Give it a box shape and an appropriate mass (e.g., 8000 kg to represent a tank).

Next, create the road wheels. For a standard tank, you will want 5 to 7 road wheels per side. Use cylinder shapes oriented along the rotation axis. Give them a high friction coefficient so they don’t slip easily when contacting the ground.

Step 2: Applying the Suspension with btGeneric6DofConstraint

To simulate a realistic tank suspension (like a torsion bar system), use btGeneric6DofConstraint (6 Degrees of Freedom) instead of a simple hinge. This constraint allows you to restrict movement on certain axes while enabling spring suspension on others.

For each wheel: 1. Attach a btGeneric6DofConstraint between the chassis and the wheel. 2. Lock Translations: Lock the X and Z translations (horizontal movement) by setting their limits to zero. 3. Enable Vertical Suspension: Leave the Y translation (vertical movement) with small limits (e.g., -0.2 to 0.2 meters) to allow the wheel to move up and down. 4. Configure Springs: Enable the spring on the Y translation axis using enableSpring(5, true) (index 5 usually represents Y translation depending on frame setup) and set the stiffness and damping to match the tank’s weight. 5. Lock Rotations: Lock the rotation on the Y and Z axes, but leave the X-axis (the driving axle) free to rotate.

Step 3: Implementing Driving Force with Hinge Motors

To drive the tank, you must apply rotational force to the wheels. While you can apply torque directly to the rigid bodies, utilizing the constraint’s built-in motor yields more stable results.

If you used a btGeneric6DofConstraint, you can enable the rotational motor on the free rotation axis. Alternatively, if you prefer a rigid suspension, you can use a simpler btHingeConstraint for each wheel.

Enable the motor on the constraint:

constraint.enableAngularMotor(true, targetVelocity, maxImpulse);

Step 4: Implementing Skid Steering

Tanks steer by spinning their left and right tracks at different speeds, known as skid steering.

To turn left, program your physics controller to: * Set a forward targetVelocity on all right-side wheel constraints. * Set a reverse or zero targetVelocity on all left-side wheel constraints.

Because the wheels have high friction, this difference in velocity will force the chassis to slide and rotate on its center axis.

Step 5: Visualizing the Tracks

Once the physics constraints are handling the movement, you need to handle the visual tracks.

Instead of adding physical objects for the tracks, update the texture coordinates (UVs) of a continuous track mesh in your rendering engine (such as Three.js). Query the rotational speed of your drive sprockets from Ammo.js, and translate that speed into UV offset animation. This creates a perfect illusion of moving tracks that react dynamically to the physical wheels rolling over obstacles.