如何实现一个独轮车功能？

[system]

想用cocos2d-iphone+box2d做独轮车游戏目前用b2PolygonShape 建立一个矩形 然后一个b2CircleShape建立轮子 用b2RevoluteJointDef吧他们连接起来 能够转动，但感觉不自然 请问有没有什么好的方法啊？

- (void)createUniCycleWithCoords:(CGPoint)pos
{
    b2BodyDef bd;
    bd.type = b2_dynamicBody;
    bd.position.Set(pos.x/PTM_RATIO, pos.y/PTM_RATIO);
    bd.angularDamping = 5;
    bd.linearDamping = 5;
    truckBody = world->CreateBody(&bd);


    b2PolygonShape shape;
    b2FixtureDef fd;
    fd.shape = &shape;
    fd.density = 1;
    fd.friction = 0.6;
    fd.restitution = 0;
    fd.filter.groupIndex = -1;
//
//    // front axle and front spring
    bd.position.Set(pos.x/PTM_RATIO, pos.y/PTM_RATIO);
//
    frontAxle = world->CreateBody(&bd);
    shape.SetAsBox(5.0/PTM_RATIO,
                   20.0/PTM_RATIO
                   );
    frontAxle->CreateFixture(&fd);

    // add two big wheels
    {
        float radius = 20.0f;
        b2BodyDef bd;
        bd.angularDamping = 5;
        bd.type = b2_dynamicBody;


        b2CircleShape shape;
        shape.m_radius = radius/PTM_RATIO;

        b2FixtureDef fd;
        fd.shape = &shape;
        fd.density = 0.5;
        fd.friction = 10;
        fd.restitution = 0;
        fd.filter.groupIndex = -1;


        // front wheel
        bd.position.Set(frontAxle->GetWorldCenter().x,
                        frontAxle->GetWorldCenter().y-14*sin(M_PI/3)/PTM_RATIO);
        frontWheel = world->CreateBody(&bd);
        frontWheel->CreateFixture(&fd);


    }

    {
        b2RevoluteJointDef rjd;
        rjd.enableLimit = true;
        rjd.enableMotor = true;
        rjd.upperAngle = 180*M_PI/180;
        rjd.lowerAngle = -180*M_PI/180;

        rjd.Initialize(frontAxle, frontWheel, frontWheel->GetPosition());
        frontMotor = (b2RevoluteJoint*)world->CreateJoint(&rjd);
    }

}

-(void) createMenu
{
        // Default font size will be 22 points.
        ;
       
        // Reset Button
        CCMenuItemLabel *reset = [CCMenuItemFont itemWithString:@"Reset" block:^(id sender){
                ;
        }];

        // to avoid a retain-cycle with the menuitem and blocks

        // Achievement Menu Item using blocks
        CCMenuItem *itemAchievement = [CCMenuItemFont itemWithString:@"Ceate Truck" block:^(id sender) {
               
                CGSize winSize = winSize];
                ;
        }];
       
        // Leaderboard Menu Item using blocks
        CCMenuItem *itemLeft = [CCMenuItemFont itemWithString:@"left" block:^(id sender) {
                frontMotor->SetMotorSpeed(10  * -1);
        frontMotor->SetMaxMotorTorque(12);
        }];

    CCMenuItem *itemRight = [CCMenuItemFont itemWithString:@"right" block:^(id sender) {
                frontMotor->SetMotorSpeed(10  * 1);
        frontMotor->SetMaxMotorTorque(14);
        }];

       
        CCMenu *menu = ;
       
        ;
       
        CGSize size =  winSize];
        ;
       
        ;       

}