But it is the downward force on the cabane V that causes the inward pull on the longerons. It's a matter of leverage. As an example: If you tie a rope tight between 2 points and then pull at 90 degrees at the center of the rope, you get way more force exerted than if you pulled straight on the rope with the same amount of force. The longer distance between those 2 points and the closer to straight the rope is when you are pulling on it, the more leverage you have. That is why I think tieing to the seat truss would help. If the center of the V can't go down, it can't pull in on the sides. JImChuk