John Hernlund
Nov 23 2011, 05:35 PM
I've recently begun to look into the details of PDGA's technical standards for approving discs in greater details. One of the first things that jumped out at me was the flexibility standard (this is actually called "flexural rigidity" by practitioners)...

Discs must...have a flexibility rating no greater than 27 lb. (12.25 kg)...Flexibility - The disc is held on its edge in a vertical position perpendicular to a scale with a precision of at least 2 oz. (56.7 g). The upper rim of the disc is then gradually pressed down within 5 seconds. The flexibility rating is determined at one of two points, depending on how the disc reacts to applied pressure. For discs that buckle, the flexibility rating corresponds to the point when the maximum weight is registered on the scale. For discs that do not buckle, the rating refers to the weight at the point when the inside rim-to-rim distance is at 50 percent of the disc’s diameter. The temperature of the disc is to be no higher than 25 degrees Celsius (77 F) when the test is performed. The ratings of three samples are determined, and the median score is used as the final rating. Discs that are unable to be bent to 50% of their diameters fail the flexibility test. Manufacturers are required to send samples of the most rigid discs they want considered for PDGA approval.

This has brought up some comments and questions, and I was hoping somebody from the PDGA Technical Standards Committee could respond:

1) For discs that do not buckle, the flexibility test weight is supposed to be registered at the point where the inner rim diameter is 50% of the outer diameter. This seems strange. The change in shape should instead be measured using the same diameter (inner vs. inner, or outer vs. outer), as it is a more physically meaningful measure of actual strain. The outside diameter is more objective than the inside. Measuring the outside diameter of the disc while bent under load relative to outside diameter prior to loading would seem like the obvious choice. For example, one could define the strain as...
where R_loaded is the outer radius (half the diameter) under load, and R_unloaded is the outer radius prior to loading.

2) This test is performed at a severe degree of strain. Except for the floppiest discs, this degree of strain is completely irrelevant to anything the disc would actually experience while being thrown in a PDGA event. The test could easily be performed at smaller strains. In fact, measuring rigidity at small strains is very simple. Measure the force applied as the strain (see above definition) is quickly (within a matter of specified seconds) increased from 0% to 1% (or some other suitably small value, but large enough to allow one to accurately register a force when the disc is near the maximum allowed rigidity) at a temperature of 25 celsius.

3) Unlike other measures (dimensions, mass), the current flexibility test seems certain to permanently damage many discs. This clearly inhibits verification and compliance. A rigidity test performed at small strains, as described above, won't damage the disc and could easily be performed on any disc prior to use.

4) In the wording, it should be made clear that the force registered is the maximum allowed up to the specified degree of strain where the standard is enforced. In other words, the force should never exceed the maximum threshold, even at strains smaller than the specified standard strain. This is simpler than using terms such as "buckling," which could be interpreted in a variety of different ways.

5) I have owned or handled numerous "PDGA Approved" discs that clearly do not pass the standard, particularly in premium plastic blends. In fact, I don't think most disc golfers would be surprised to learn that many production discs fail the flexibility test. Does the PDGA truly enforce this standard? If so, then I would imagine that the "Disc Golf Foundation" to which manufacturers must donate for violating standards has become very rich! If not, then why bother?

Nov 23 2011, 11:25 PM
Here's Jeff Homburg's response to the questions:

We could based the flex limit on the outside diameter, but I find that the inside diameter is simpler to measure. I agree that there are more sophisticated ways to measure the flex of the disc, but are they really worth it? I've checked the costs of paying outside labs to use more high tech methods, but these cost estimates far exceed the testing fees now charged to manufacturers. To me, it hardly seems worth it at this point.

At some point in the future manufacturers and the PDGA may need more high tech methods for disc testing, but we're not there yet. I'd be happy for someone to develop more effective, including cost-effective, methods to measure the flex. Perhaps John Hernlund would like to donate his time to do so?

I'd also be curious to see real data such as time-lapse photography that proves that discs do not double over (such as when a disc becomes "tacoed") when hitting objects such as trees and poles. It is my opinion that the methods we now use provide a reasonable and replicable way to define when a disc is too stiff. Our method provides a standardized way to measure and compare the flex of discs.

After performing these tests for the better part of two decades, my experience is that the flex test does not cause permanent damage. I defy others, based on blind tests, to even discern which discs have been tested versus ones that have not.

If you want to continue the dialog, I suggest contacting Jeff via the Tech Standards Committee link on the PDGA Contacts page.

John Hernlund
Nov 24 2011, 03:03 PM
Thanks Chuck! Will do. And yes, I'd be very willing to pitch in with any effort to upgrade/clarify the standards.