Why does FixedJoint break under fast motion?

Default Break Force is finite. High-speed collisions exceed it instantly. Set Break Force and Break Torque to Mathf.Infinity for unbreakable joints.

Joint slop on heavy bodies?

Solver Iterations on each Rigidbody (default 6) may not be enough for tight constraints. Bump to 12-30 for joint chains; cost scales with iterations.

Joint Pre-process true = baseline; false skips solver pre-step which can introduce slop. Keep true for clean fixed connections.

Fix: Unity FixedJoint Tears At High Speed

Quick answer: Break Force = Infinity. Break Torque = Infinity. Solver Iterations 12+. Enable Pre-processing.

Trailer attached to truck via FixedJoint. Truck hits a wall fast. Trailer detaches and flies. Joint exceeded its break threshold.

The Fix

FixedJoint:
  Break Force:        Mathf.Infinity
  Break Torque:       Mathf.Infinity
  Enable Preprocessing: true

Connected Rigidbody:
  Solver Iterations:    12
  Solver Velocity Iter: 4
  Collision Detection:  Continuous Dynamic

// Code: programmatic config
var joint = ...;
joint.breakForce  = float.PositiveInfinity;
joint.breakTorque = float.PositiveInfinity;

Infinity makes the joint unbreakable. Solver iterations stop visible slop. Continuous CD prevents tunneling that would also stress the joint.

Verifying

Smash truck at speed: trailer stays attached. Without infinity: detach within a few crashes.

“Infinity force. More iterations. CCD. Joint holds.”

Related Issues

For trigger stay sleep, see trigger stay. For 2D slope controller, see slope.

Infinite force. Iterations up. Joint holds.