Wednesday, July 06, 2011

A New Can of Balls

UPDATE: The other tests I referred to in this post were flawed. I dropped balls from a lower height and assumed, incorrectly, that the coefficient of restitution would be the same as if I'd dropped them from the 100 inch ITF specification. Turns out the COR decreases as velocity increases. So the prior tests over-estimated the 100 inch rebound.

I just opened a new can of Penn Championship High Altitude tennis balls.  The balls had been sitting at 68 F degrees for a week.  They averaged about 57 to 58 inches of rebound when dropped from 100 inches (68 F degrees).  That's within the sea level specification of 53 to 58 inches.  Nice.

But I've tested other balls that rebounded 62 inches.  Well, that makes sense.  The specification range for high altitude balls is 48 to 53 inches at sea level.  So that means my range at 5,300 ft is roughly 57 to 63 inches.  So if I happen to get a high altitude ball that's at the low end of the specified range, I'll have a ball in Denver that is within the sea level specification.

That's just luck, of course.  We're still stuck with balls that, on average, exceed the mid-range of the sea-level specifications by about 8%.  Since the balls slow down less up here once hit, that's a big problem.

I've tested roughly 9-month-old balls and they're at the bottom end of the sea level spec for rebound.  Nobody seems to like the sound and feel of those balls up here.  Maybe nobody likes balls at the bottom of the spec at sea level either.  I don't know.  But up here, balls that meet the rebound spec, still leave us with a game that's about 16% faster through the air.

Lighter balls slow down more, but they feel odd, too.  Balls with fluffier naps may be the answer.

The Prince Tournament ball fluffed up a lot.  Maybe I'll have to track down more of those.

In the meantime, I still think the Stage 1 "green" ball is a better compromise than the current high-altitude ball.  We used them for an "elite" junior group today and the play was more fun, I thought.  Then I hit with some on an artificial grass court and again the play was fun.  It's nice to have time to set up and it's nice to be able to take a cut at the ball.  Both are not too common at this altitude.

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