How Ammo.js Roll Influence Affects Vehicle Stability

This article explains how the rollInfluence parameter affects the stability of a btRaycastVehicle in the ammo.js physics engine. We will explore how adjusting this value alters body roll, prevents rollovers, and changes handling characteristics during sharp turns.

In ammo.js, which is a direct port of the Bullet physics engine to JavaScript, the rollInfluence parameter is a wheel-specific setting that determines how much of the lateral suspension force is translated into roll torque on the vehicle’s chassis. When a vehicle corners, lateral forces act on the tires, creating a rotational force that threatens to tilt and roll the vehicle’s body.

By default, the rollInfluence value is set to 1.0. At this default setting, the full physical forces are applied, resulting in a realistic simulation of body roll. However, because virtual environments often struggle to perfectly replicate complex real-world variables like tire slip and precise center-of-mass distributions, a setting of 1.0 usually makes vehicles highly unstable and prone to flipping over during high-speed maneuvers.

Reducing the rollInfluence value toward 0.0 artificially reduces the roll torque applied to the vehicle’s rigid body. This action effectively lowers the vehicle’s virtual roll center. If you set the parameter to exactly 0.0, lateral forces will exert absolutely no rolling moment on the chassis, meaning the vehicle will remain completely flat and level during turns, regardless of how sharp or fast they are.

For most game development and simulation purposes, a value between 0.1 and 0.3 is ideal. This range provides a compromise: it retains a small, visually convincing amount of body roll that communicates weight transfer to the player, while significantly lowering the physics-based rolling forces to keep the vehicle stable and glued to the ground.