Matter.js Constraint Stiffness 1.0 vs 0.1 Explained

In Matter.js, the stiffness property dictates how rigidly a constraint maintains its resting length when subjected to external forces. This article breaks down the mechanical and visual differences between configuring a constraint with a stiffness value of 1.0 versus 0.1, explaining how the engine resolves positional offsets, how the bodies behave in real-time simulations, and when to use each setting.

How Constraint Stiffness Works in Matter.js

In Matter.js, constraints act as virtual springs or linkages between two bodies, or between one body and a fixed world coordinate. The stiffness property accepts a value ranging from 0 to 1 (though values greater than 1 can be used with multiple engine solver iterations). This value represents the fraction of positional error the solver corrects during each tick:

• Correction Rate: A value of 1.0 attempts to resolve 100% of the displacement error in a single step, whereas a value of 0.1 resolves roughly 10% per step.
• Elastic Response: Lower values introduce elasticity, while higher values create rigidity.

Behavior of Stiffness: 1.0 (Rigid Linkage)

When a constraint's stiffness is set to 1.0, it behaves like an inflexible, solid rod.

• Immediate Correction: The physics solver enforces the defined length of the constraint with maximum priority. If a body attached to the constraint experiences an impulse, the constraint immediately pulls or pushes back without stretching.
• No Springiness: The connection displays virtually zero oscillation or "bungee" effect. Attached bodies move synchronously according to the constraint's pivot points.
• High-Stress Scenarios: Under extreme forces or collisions involving heavy masses, a stiffness of 1.0 may exhibit slight jitter or deformation if the engine's constraintIterations setting is too low, but it will never intentionally stretch.
• Common Use Cases: Ragdoll limbs, structural trusses, mechanical arms, pendulums, and fixed-distance linkages.

Behavior of Stiffness: 0.1 (Soft Spring)

When a constraint's stiffness is set to 0.1, it behaves like a loose, elastic spring or rubber band.

• Delayed Correction: Because the solver resolves only a small portion of the positional displacement per tick, the constraint yields easily when external forces (such as gravity or collisions) pull against it.
• Visible Elongation: The distance between the connected points will noticeably stretch well beyond the original length property when under tension.
• Oscillation and Bouncing: Bodies connected with a stiffness of 0.1 will bounce back and forth around the resting length. Unless the damping property is increased, this oscillation will persist for a noticeable duration.
• Soft Acceleration: Rather than abruptly jerking connected bodies, a stiffness of 0.1 transfers momentum gradually, resulting in smoother, organic motion.
• Common Use Cases: Vehicle suspension systems, bungee cords, elastic ropes, soft-body approximations, and floating UI elements.

Direct Comparison