    // Create the joint and set it's max force property.
    breakableJoint = cpSpaceAddConstraint(space, cpPinJointNew(body1, body2, cpv(15,0), cpv(-15,0)));
    cpConstraintSetMaxForce(breakableJoint, 4000);

    // In your update function:
    // Step your space normally...
    cpFloat dt = 1.0/60.0;
    cpSpaceStep(space, dt);

    if(breakableJoint){
      // Convert the impulse to a force by dividing it by the timestep.
      cpFloat force = cpConstraintGetImpulse(breakableJoint)/dt;
      cpFloat maxForce = cpConstraintGetMaxForce(breakableJoint);

      // If the force is almost as big as the joint's max force, break it.
      if(force > 0.9*maxForce){
        cpSpaceRemoveConstraint(space, breakableJoint);
        breakableJoint = NULL;
      }
    }