Flex numbers are derived by using the universal flex system invented by Herb Jenks of Sky-Pole. The universal flex system was shared with Cata-pole, then with Pacer/Sky-Pole, and then adopted by UCS/Spirit. Over time, each company has made alterations to the system.
Pole manufacturers have developed devices and methods for measuring the stiffness of poles. Most measuring devices hold both ends of the pole on a frictionless support and a 50 pound weight is suspended from the middle. Since poles come in various lengths, each span will produce a reading based on the relative stiffness of the pole in relationship to its structure.
The flex number of a pole is a direct measurement of how much it sags when you suspend a standardised weight from its middle, as it is being supported from the ends at a pre-determined distance from the weight.
The flex spans of the system were changed by Pacer/Sky-Pole in 1996 to reflect a more precise measurement. Working with flex numbers, one should never rely on this number being an exact. Many conditions alter the results such as altitude where the measurement is being made, if there is friction on the supports, humidity in the testing area, and many other little things to alter the number found by at least .2! Some methods use 6 inches from the butt for a measure point. Earl Bell and Tye Harvey's flex systems use 12 inches from each side as a measure. Many forget that the method of measure is measuring the sail wrap only, not the whole pole. Measuring the whole pole provides a reading of stiffness in the grip area contributed to flex number an area vaulters do not bend the pole. So if this type of reading may be considered more accurate, it will be very much different by .4 than a butt sail measurement. Both methods do provide a relative stiffness if you are measuring the pole on the same machine.
Flex numbers from brand to brand will vary due to the following inconsistencies:
Accurate digital pole flex scale When you are moving from one pole to the next, you expect it to be only stiffer but to react the same. One manufacturer uses so many changes to the pattern, it is very necessary that they have a more precise system to measure the butt flex number.
This chart has changed by each manufacturer and is totally different from one pole maker to the other. Because UCS has the most poles out in the field used, one needs to be as close to them as possible to help in transfer of brands. UST-ESSX and Altius are about .2 stiffer than UCS and .2 softer than Gill flex system. While all poles seem to be 8 lb. softer than Nordic as they are on the oldest flex chart originally made by Sky-Pole in 1968.
Each brand has a marking that tells you the load at which the pattern of the pole will hold the load. This is called the pole rating. All brands used are differential by only 2.2 lbs. between brands. Far too many athletes rely on the flex number as the basis for evaluating a pole and depend on the flex number as a magic number. Let me help you throw that myth out the door with this statement.
The flex number of a 12' 2" PVC plastic tube is the same as 12'-125 lbs test pole but you would not vault with it would you?
A flex number is only a relative stiffness number and can only compare stiffness with each length. A flex number is relative to the brand and the experience the pole maker has and only allows comparison of resistance to what was made by that maker previously. All these properties and the load it will hold is determined by the pole pattern and the type of glass used. So flex numbers are a guide and cannot be related or used to calculate the load a pole holds only competitive loads. They are only a relative stiffness of one pole brand to the same pole brand and within that same length only, all things being equal. One brand pole may be made a different way, one may have a longer sail or shorter sail, and it may have more or less wraps of the body wrap. A method of flexing the pole to measure more of the pole is not going to derive the same or a relative flex number of a system that measures only the sail of the pole.
It is strongly recommended that you forget about flex numbers and use weight ratings until you are building a series of poles, or until you have at least 4 to 6 poles in your series.