Walter & Group.......
Some casting physics by Walte Simberski :-
Gordy - the bend/flex in the rod has nothing to do with the speed/velocity of the line or of the rod tip. It is determined by the force applied to the rod. Remember - force = mass x acceleration. Since the mass of this system remains constant (unless we slide load) we need acceleration to bend the rod. Accleration is the rate of change of velocity (or speed for a straight line path). If the speed/velocity does not change (acceleration = 0) then the rod will be straight.
When we haul on the line we increase the speed of the line which means we accelerate the line which means the rod will bend more.
Now let's take a look at a typical cast with a haul. The rod tip may move as much as 20 feet for an extreme cast. In the same time the line hand may move about 5 feet. If the line hand mirrors the rod hand then these movements will take the same amount of time and the more significant portion of line accleration obviously comes from the rod. If we wait until later in the casting stroke, say about the point where loop formation is started, to start the haul then the rod tip may only be moving a few feet while the line hand is moving 5 feet. Now we could actually get an equal or greater amount of acceleration coming from the line hand, which translates into greater line speed, which translates into greater distance cast.
The increase in force on the rod may double during the haul but the amount of bend won't be anywhere near double because the rod is tapered. Think of two springs, one very weak and one very strong, attached end to end. If we attach a weight to the end of this joined spring the weaker portion will be almost straight before the stronger portion starts to stretch noticeably.
Cheers
Walter