[SFML + Box2d] Budowa autka (problem wheel jointami)

class Body
{
    b2Body b;
   
    //...
};
class Game
{
    b2World world;
    RenderWindow win;
    vector < Body *> objs;
    //...
    void addDynamicBody( /*...*/ );
};

void createCar( Game * g )
{
    Vector2f ltPos( 320.0f, 100.0f ); // pozycja left-top autka
   
    // karoseria
    b2PolygonShape sh;
    sh.SetAsBox( SFtoB2( 50.0f ), SFtoB2( 20.0f ), SFtoB2( ltPos + Vector2f( 50.0f, 20.0f ) ), 0.0f );
    Body * car = g->addDynamicBody( Vector2f(), sh, "car" );
   
    // koła
    float whRad = 10.0f;
    b2CircleShape wh;
    wh.m_radius = SFtoB2( whRad );
   
    wh.m_p = SFtoB2( ltPos + Vector2f( 20.0f, 60.0f ) );
    Body * lwheel = g->addDynamicBody( Vector2f(), wh, "left wheel" );
   
    wh.m_p = SFtoB2( ltPos + Vector2f( 80.0f, 60.0f ) );
    Body * rwheel = g->addDynamicBody( Vector2f(), wh, "right wheel" );
   
    // amortyzatory
    b2WheelJointDef spr;
    spr.collideConnected = true;
    spr.dampingRatio = 10.3f;
    spr.enableMotor = true;
    spr.motorSpeed = 0.0f;
    spr.maxMotorTorque = 100000.0f;
   
    spr.Initialize( car->b, lwheel->b, SFtoB2( ltPos + Vector2f( 20.0f, 60.0f ) ), b2Vec2( 0.0f, 0.1f ) );
    g->world.CreateJoint( & spr );
   
    spr.Initialize( car->b, rwheel->b, SFtoB2( ltPos + Vector2f( 80.0f, 60.0f ) ), b2Vec2( 0.0f, 0.1f ) );
    g->world.CreateJoint( & spr );
}