Box2d: Elastic rope takes time to get back to its initial position - objective-c

Question

I am creating a rubber band in Box2d. Here is my code.
// init physics
[self initPhysics];
// Create ball body
CCSprite *ball = [CCSprite spriteWithFile:#"rubberband.png"];
ball.position = ccp(100, 100);
ball.tag = 1;
// [self addChild:ball];
//=======Params
// Position and size
b2Vec2 lastPos = b2Vec2(154.0/PTM_RATIO,65.0/PTM_RATIO); //set position first body
float widthBody = 2.0/PTM_RATIO;
float heightBody = 2.0/PTM_RATIO;
// Body params
float density = 0.0;
float restitution = 0.5;
float friction = 0.5;
// Distance joint
float dampingRatio = 0.85;
float frequencyHz = 10;
// Rope joint
float kMaxWidth = 50.0/PTM_RATIO;
// Bodies
int countBodyInChain = 68;
b2Body* prevBody;
//========Create bodies and joints
for (int k = 0; k < countBodyInChain; k++) {
b2BodyDef bodyDef;
if(k==0 || k==countBodyInChain-1) bodyDef.type = b2_staticBody; //first and last bodies are static
else bodyDef.type = b2_dynamicBody;
bodyDef.position = lastPos;
bodyDef.fixedRotation = YES;
b2Body* body = world->CreateBody(&bodyDef);
b2PolygonShape distBodyBox;
distBodyBox.SetAsBox(widthBody, heightBody);
b2FixtureDef fixDef;
fixDef.density = density;
fixDef.restitution = restitution;
fixDef.friction = friction;
fixDef.shape = &distBodyBox;
body->CreateFixture(&fixDef);
if(k>0) {
b2RevoluteJointDef armJointDef;
armJointDef.Initialize(prevBody, body, lastPos);
armJointDef.enableMotor = true;
armJointDef.enableLimit = true;
armJointDef.maxMotorTorque = 1;
world->CreateJoint(&armJointDef);
//Create rope joint
b2RopeJointDef rDef;
rDef.maxLength = (body->GetPosition() - prevBody->GetPosition()).Length() * kMaxWidth;
rDef.localAnchorA = rDef.localAnchorB = b2Vec2_zero;
rDef.bodyA = prevBody;
rDef.bodyB = body;
rDef.collideConnected = false;
world->CreateJoint(&rDef);
} //if k>0
lastPos += b2Vec2(widthBody, 0); //modify b2Vect for next body
prevBody = body;
} //for -loop
[self scheduleUpdate];
}
return self;
}
Problem is that when the app starts, rubber band appears in stretched form in U shape and then it gradually start contracting and coming to become straight horizontally. Can anyone please tell me why it is happening? I want the rubber band to be without being stretched in the beginning.
Best Regards

Answer 1

You don't update lastPos so all bodies occupy the same position initially. Box2D will force them apart and this could lead to the problem.