Archive for April, 2010

Surfboard flex

April 11, 2010

Surfboard flex has been a topic of interest in surfboard design in recent years. Let’s take a look at this and see what we can learn. Basically if a surfboard flexes, the rocker changes because of bending of the board. From this we can see that surfboard flex gives us variable rocker. Is this desirable? Yes it often is but not always.

The surfboard is going to flex only when pressure is put on it; the more pressure, the more flex. Therefore, the harder you push your turn, the more flex and rocker curve you will force into the board. This will make for tighter turns, but it might slow your acceleration if you are trying to get a jump on a section ahead of you. Too much flex will definitely slow your acceleration. Note that the only part of the board that will flex is the part that is in the water. Also the flex will be controlled by where the most pressure is placed on the board and by the thickness and its placement.

If someone wanted to flex test surfboards, the task could be a bit complex. Supporting the board at both ends, putting weight on it, and then measuring the deflection is not going to duplicate what happens when that board is being ridden. When surfing, the flex will be caused by the surfer’s weight; it will be input to the surfboard where his feet are and it will be input in proportion to how much weight is on the lead foot and how much weight is on the trailing foot. Note that the force input will be greater than the surfer’s weight, while the surfer is rising from a crouch (pumping) in a turn. I don’t know how much extra force (above the surfer’s weight) can be put into the board, but leg strength, timing, and balance really count. Yes, if another surfer is getting more acceleration than you, he’s doing it harder (assuming your boards have similar acceleration/speed characteristics).

I’ve accounted for the force input, but that is only the top half of what is happening. That force is being countered by the water’s pressure against the bottom of the surfboard and since water will flow away from higher pressure, the water pressure would be evenly distributed across the contact area. Obviously trying to duplicate all this in a lab would be extremely difficult. However, by placing sensors on a surfboard and collecting the data while the surfboard was being surfed, the force and flex relationship could be studied.

Here are some recommendations to consider about flex: A heavier surfer would need a stiffer surfboard and a lighter surfer more flex. A stronger surfer would need a stiffer surfboard. A more flexible surfboard would be more useful in slower (down the line) waves. A stiffer surfboard would be better for faster waves. Tight vertical surfing would benefit by more flex, down the line surfing by more stiffness.

Bob

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