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Sports Testing 202: Vertical Ball Rebound Requirements and Considerations


June 30, 2016 0 comments Technical

ncaa vs nbaNote: This post was first published on LinkedIn™ . I chose to write this article before completing the introductory series on sport surface testing because ball rebound or dead-spots are the number one reason ASET Services conducts field testing on indoor courts. While I’m trying to keep these posts direct and brief this is a complicated topic, and I am afraid it  requires a relatively long discussion. Once again, I am going to focus this article around basketball rebound levels, but some of the points will cross over into any inflated ball, such as soccer. I hope that publishing this will help everyone understand the requirements of the DIN/EN/ASTM/FIBA/MFMA performance standards as well as their limitations.

Requirements: Three of the standards have essentially identical requirements for vertical basketball rebound. They are DIN 18032-2 (2001 issue), EN 14904 (2008 issue) ASTM F2772 (2011 issue). Each of these international standards requires that the ball rebound level meet or exceed 90% of the rebound height of concrete. Each of these standards requires that all points be within +/-3% from the average value. There are slight difference is that DIN 18032-2 (issued in 2001) will not allow any point to have a rebound level below 90% while the other two simply require the average to be above 90%. This means that it is possible for a floor that meets EN 14904 or ASTM F2772 to have rebound levels below 90% at isolated points. The North American market is unique in that some architects still specify performance using an old version of DIN 18032-2 issued in 1991. This legacy version requires the average value to meet or exceed 90%, but does not place any uniformity requirements. As a note: The use of DIN 18032-2 (1991 or 2001) ceased completely in Europe in 2006 when EN 14904 was issued. While many architects still use this standard in North America, it is no longer used in Europe.

Two associations (FIBA and MFMA) feel that today’s athletes and style of basketball require higher rebound levels than the minimum values established in the international standards. These two associations require ball rebound to meet or exceed 93% of the rebound produced on concrete. As of 2014, FIBA requires that the vertical rebound at all test points meet or exceed 93%, and it is our understanding that the MFMA requires the average to exceed 93%. Both groups also require that the rebound at all test points be within +/-3% from the average. Surprisingly, neither the NBA or NCAA have issued any performance requirements or suggestions for game courts or practice courts.

Considerations: I believe that this test is imperfect and that when owners and specifiers reference this method they need to consider why the results may not match their expectations.

First and foremost, I think people’s expectation of what this test does exceeds the information it provides. At ASET Services, I get several calls about dead-spots and many folks seem to think that their perception of dead-spots will be validated by lower rebound heights. During my career I have found that rebound height measured by this standard has a poor correlation with the ‘perception’ of a dead spot.

I realize now that I’ve mentioned dead-spots a few times and that there are many ways that the term can be defined. Clients routinely describe them as points where they feel the ball rebound is lower than other points, so for the purposes of this discussion allow me to use that definition. Based on experience, a combination of factors effect the perception of dead-spots. When the ball impacts the surface there are multiple sensory inputs created. Some of these include vibration levels of the sport surface, sound intensity, sound quality, and the rebound height. I believe that when most people perceive differences in any of these, they equate that difference with rebound height even if height was not effected. While I have not found rebound height to correlate well with the perception of dead-spots, I have found that the frequency or tone of the sound generated during the impact correlates far better with dead-spot perception (click here to a link to an abstract dealing with this topic).

Staying on this topic a bit further, the only standards that exist that define acceptable ball-surface response levels focus entirely on rebound height and do so at a precise inflation pressure. While different sounds and different vibration levels may cause a user to identify a dead-spot, there is no standard defining what is acceptable with regard to these properties (sound, or vibration). Field test evaluations to identify dead-spots are left with only one option and that is to consider the differences in rebound height between test points.

Secondly, I think the style of play and the speed associated with transitioning down the court has changed and that the change may have created a need for rebound heights that are greater than they were in the 1980’s when this standard was developed. One can not ignore the fact that on average basketball players are taller today than they were in the 1980’s. While a rebound height of 1.05 m might have been justified in 1980, today’s taller players and the speed of the modern game of basketball may desire, or even require, the rebound to be somewhat higher, which if the drop height were left the same would require a greater inflation pressure.

I’ve mentioned the pressure in the ball several times because I feel that is the most important item to consider with regard to this standard. The inflation pressure of an NCAA  game ball to achieve the required 1.05 m rebound is roughly 5.5 psi. Most basketball players I encounter think that is low and prefer the pressure to be closer to 8 psi. The pressure at which the rebound testing is done is very significant because it greatly effects the results of this test. Increasing the pressure in the ball increases both the rebound height on concrete and on the sports surface, but the concrete height increases faster than that the floor. This means that increasing the inflation pressure REDUCES the ball rebound of the floor as a percentage of that produced on concrete.  A point that produces 94% ball rebound at 5.5 psi may only produce 89% ball rebound at 8 psi. Conversely, you could also say that increasing pressure creates a wider rebound height range (low to high).

This has implications in the field as well. Dead-spots are relatively rare at 5.5 psi when the standardized methods are used, however they are more frequent when onsite balls with higher pressures are used. All of the standards above use the same pressure (concrete rebound height) levels. Use of a non-standard pressure would be informative and would be relatively meaningless since the when performance is specified one of the above standards is used.

Lastly, consider the design of the facility and the types of activities that it will support when selecting standards and performance levels for specifications. Out of the above standards, the FIBA requirements are the only ones developed with competitive play in mind. The DIN, EN and ASTM standards have been developed, for the most part, with multi-sport or multi-use facilities in mind. Supporting a wider array of activities may require some concession in ball rebound performance.

Field Testing Considerations: When I conduct a field test for ASET Services I take the time to educate our clients and to determine which standard, if any, was specified. We also take time to determine why they want the tests performed. This information will determine how many points we select and how those points are selected. I also take time to discuss challenges they may have with regard to sprung floors. Generally, sports floor manufacturers and sup piers in North America consider that they fulfill the specification when they submit their suitability or lab test report. I have experienced very few performance specifications where the performance requirements were clearly required to apply to the installation (here is a link to one such specification simply scroll to the bottom of the page). To date, I’m unaware of a legal precedent that clearly shows that specifications commonly provided to and used by architects apply to the actual installed surface. In some instances manufacturers have simply taken the position that it does not apply to the installation, and that they fulfilled the requirements by meeting them in the lab on a test sample.

Most of the standards require a rebound level to be +/-3% from the average value. This means that all standards with a uniformity requirement have established a 6% window of acceptable performance in the lab. It is my position that field testing can not be held to a tighter standard than that. When I conduct field tests with the intention of identifying dead spots, I create a 6% window from the the highest and lowest rebound levels to establish the areas or points that meet the standard. The reason ASET doesn’t use the average is that we do not feel that we can accurately determine the ‘true average’ of a sport surface in the field. This is due to the inability to accurately locate critical structural locations in a system after it has been installed. Also without extensive mapping it is impossible to determine what percentage of a floor responds the same as individual points tested.  As a licensed engineer and someone with more than 20 years experience in sports surfacing, it is my opinion that the statistical limitations of field testing justifies this minor change in evaluating performance. I will report when points fail to fall within the +/-3% from the average but I consider a simple 6% window from the maximum, or minimum, rebound level to be more appropriate. Certainly points that exceed the 6% window would fall outside of the performance allowed in the lab setting. 

Dead-Spot Perception: As I said, the most common request for field testing centers around perceived dead-spots. Many sprung floors are designed such that they produce notably different sounds and that sound difference is often perceived as a ‘dead-spot.’ The most extreme example is a sleeper only floor that uses a 2×3 solid wood sleeper 8″, 10″ or 12″ on center. The area between the sleepers in any system can sound, and even feel, quite different than over a sleeper. Additionally, if the floor uses a relatively hard pad (producing a Force Reduction of 35% or less), the points directly over the pad have significantly different sound and vibration levels.

Many times the effect of one key variable is neglected, and that is the construction of the ball itself and the variation of rebound levels produced by the ball. The referenced standards all require that the ball be dropped with the inflation valve pointing up. This ensures that the same general area of the ball is always the first to contact the surface. If the ball is allowed to randomly impact the surface, the results will bring the construction of the ball and the lack of uniformity in the construction of the ball into the perception of dead-spots.

Conclusion: This article is not intended to make the reader an expert on ball rebound testing, rather it and the other articles on LINKEDIN are of an introductory nature. If you are looking for more detailed information on floor performance tests visit the library at ASET Services.

As always if you have questions, please contact ASET Services, Inc. We conduct testing on courts (indoor, outdoor, wood and synthetic), artificial turf, playground safety surfacing as well as indoor and outdoor tracks. Reach us via email info@asetservices.com, phone 1.812.528.2743, or visit our website www.asetservices.com.

Note:The worst case I ever experienced was a school that had their basketballs inflated to 17 psi (almost two times the suggested maximum). We conducted testing at the 5.5-6.0 psi range required by the recognized standards and found relatively few problem points. When we conducted tests with the balls at 17 psi some rebound levels fell to 80%. We were able to explain why they ‘felt’ more dead-spots than we measured. Now, we tell every school we test for that the lower you can have your ball pressure the more uniform your performance will be. 

 

 

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