Wednesday, June 29, 2011

Ball Test

UPDATE 10/4/2012: The tests below failed to take into account the effect of changing coefficient of restitution (COR) with respect to velocity. Therefore, these tests, conducted with a drop height of less than 100 inches, do not reflect the rebound characteristics required by the ITF specifications. My simplifying assumption, that I could drop the balls from lower heights, was wrong.

Science is about experiments.  Enough theorizing.  I went to the BRG Lab and tested some tennis balls.

I didn't have the skill or the inclination to set up a standard testing station, so instead I picked a convenient height -- the BRG water heater -- and measured it.  57.5 inches.  My thermometer said it was 68 F degrees, exactly as specified by the ITF.  According to the ITF specifications (click), a ball dropped from 100 inches should rebound between 53 and 58 inches at sea level.  I'm at about 5,300 ft, but I want to find a ball that rebounds 53 to 58 inches up here to replicate what sea level players experience in terms of the ball's bounce off the ground.  A rebound height of roughly 30.5 to 33.5 inches from my lower drop height approximates the sea level specs (I did some algebra to find that range).

First I dropped a two-day old Penn Championship High Altitude ball.  The ball rebounded roughly 36 inches in three drops.  That's about 12% above the spec range midpoint.  In line with my previous predictions.

Next I dropped an old Penn Championship High Altitude ball.  I opened this ball's can in October of 2010 (it's 6/29/2011 today).  I dropped this ball twice and it rebounded about 31 inches and 32 inches.  Nicely in the spec.

Then I dropped a year old (at least) Tretorn FunLite ball.  This is a "green" transition ball made for U10 tennis.  It is supposedly a 25% reduced bounce and flight ball.  This ball rebounded 32 inches.  Nicely in the spec.

Then I dropped a Penn Control+ green dot ball.  Like the Tretorn FunLite, this is a 25% reduced bounce and flight ball.  This ball rebounded roughly 32.5 inches.  Within the spec range.

Geez, no wonder my low-altitude friends say those transition balls seem "less weird" to them than our normal high-altitude balls.

So which ball should be the preferred ball for tennis at my altitude?

I tested another ball today, the Wilson Green Dot reduced speed, transition ball.  Like the Penn Control+ and the Tretorn FunLite, this ball is marketed as a 25% reduced speed ball.  The first time I tested this ball, it really hopped, coming in at 62.5 inches, dropped from 100 inches.  That's way too high.  But the ball was really hot.  After being out of its can for about six hours, and out of the heat for about four hours, the ball rebounded 54.5 inches.  Nicely in the 53 to 58 inch spec range.  I'll test it again tomorrow after it's been at 68 F degrees for 24 hours [UPDATE:  No change from night before, i.e. roughly 54.5 inches].  This ball has a mass of 49 grams, so it should slow down nicely as it moves through our thin air.   Good possibility for replicating low-altitude play at my altitude.

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