Checking Plank Stiffness

Since a DN plank is actually a wooden leaf spring, it's stiffness is measured in a term called spring rate (SR). SR measurements are in pounds per inch or how much weight would deflect the spring one inch. Different plank builders use different perameters to figure how much their planks should bend for the weight of the skipper. I like to use a deflection of 1 3/4 inches for bodyweight plus 20-30 pounds (depending if I'm looking for soft or stiff plank). I know another builder who uses a deflection of 80% of the crown of the plank for the skippers weight. There is no right or wrong here, it's dealers choice, however no matter what method you're using, you are trying to come up with a specific SR.

With experience comes the knowledge of how thick the plank needs to be or how much glass must be added to get the deflecion/SR you want when building the plank. After the plank is finished, the builder will typically put wax paper under the ends, step on the plank and measure the deflection. To get this measurement in SR you divide your weight by the deflection and this gives you the SR. I have found that if you want the true SR, this method won't work. I have found that at weights much over 110 pounds the friction of the planks ends starts building greatly and the results get skewed to give much too stiff a result. I graphed the results on a number of planks using a caliper to measure the deflection. I tried number of different materials under the plank ends and at 120 pounds the graph started to show a flattening of the line indicating an error in the results. I now use 110 lbs. on the plank when checking a deflection. I hope this helps.

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