ios – Sprite Kit销接头似乎有一个不正确的锚点

我想要的设置是：一个宽,平的盒子和两个圆圈;圆圈通过SKPhysicsPinJoints连接到盒子,因此它们可以作为轮子.

这是我的代码我试图使其尽可能简洁：

- (SKNode*) createWheelWithRadius:(float)wheelRadius {
    CGRect wheelRect = CGRectMake(-wheelRadius,-wheelRadius,wheelRadius*2,wheelRadius*2);

    SKShapeNode* wheelNode = [[SKShapeNode alloc] init];
    wheelNode.path = [UIBezierPath bezierPathWithovalInRect:wheelRect].CGPath;

    wheelNode.physicsBody = [SKPhysicsBody bodyWithCircleOfRadius:wheelRadius];

    return wheelNode;
}


- (void) createCar {

    // Create the car
    SKSpriteNode* carNode = [SKSpriteNode spriteNodeWithColor:[SKColor yellowColor] size:CGSizeMake(150,50)];
    carNode.physicsBody = [SKPhysicsBody bodyWithRectangleOfSize:carNode.size];
    carNode.position = CGPointMake(CGRectGetMidX(self.frame),CGRectGetMidY(self.frame));
    [self addChild:carNode];

    // Create the left wheel
    SKNode* leftWheelNode = [self createWheelWithRadius:30];
    leftWheelNode.position = CGPointMake(carNode.position.x-80,carNode.position.y);
    [self addChild:leftWheelNode];

    // Create the right wheel
    SKNode* rightWheelNode = [self createWheelWithRadius:30];
    rightWheelNode.position = CGPointMake(carNode.position.x+80,carNode.position.y);
    [self addChild:rightWheelNode];

    // Attach the wheels to the body
    CGPoint leftWheelPosition = leftWheelNode.position;
    CGPoint rightWheelPosition = rightWheelNode.position;

    SKPhysicsJointPin* leftPinJoint = [SKPhysicsJointPin jointWithBodyA:carNode.physicsBody bodyB:leftWheelNode.physicsBody anchor:leftWheelPosition];
    SKPhysicsJointPin* rightPinJoint = [SKPhysicsJointPin jointWithBodyA:carNode.physicsBody bodyB:rightWheelNode.physicsBody anchor:rightWheelPosition];

    [self.physicsWorld addJoint:leftPinJoint];
    [self.physicsWorld addJoint:rightPinJoint];
}

我期望的是针接头锚定在他们的中心点;然而,当我测试这个,关节的锚点似乎是远离的.

我错过了一些很明显的东西吗？