we usually put about 12-14 inches between the main hoop and these bars.just use commonsence if it looks safe and your helmet cannot pass between these bars you will be fine.3/4 049 bars can be added if needed to retain your helmet.
http://www2.propichosting.com/Images/421613022/0.jpg
http://www2.propichosting.com/Images/421613022/0.jpg