# A tip for better accuracy when measuring swingweight

Discussion in 'Racquets' started by stoneage, Sep 13, 2012.

1. ### stoneageRookie

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The TWU way of measuring the swingweight by measuring the swing time of the racquet is a convenient and accurate method, provided that you get the data right. However, there is a small modification you can make to improve the TWU setup.

There are two lengths and the swing time that has to be measured. All three enter the equation squared, so care is needed. To measure the swing time accurately an app like swingTool comes in handy. The lenghts has to be measured with a ruler. In the TWU procedure it is recommended that you hang the the racquet from the top strings. But as I will show a lower point is probably better. You can hang it at any point you like, this is both theoretically sound and works in practice, as was shown in this thread. (It is actually the reason why hanging it from the strings works to begin with).

If you take the equation that calculate the swing weight from the swing time, keep the time constant and plot it for different hanging points you get a curve like this (the distance is measured from the balance point):

What is interesting is that the curve has a peak around 23 cm above the balance point and that the peak is rather flat. This means that around this peak the swing weight value changes very little when you change the distance. This in its turn means that an error when measuring the distance to the hang point will influence the resulting swing weight very little!

The curve will vary a little for different racquets, but you can generalize it by differentiating the expression that calculates the swing weight. That leads to the following expression:

Where h is the distance in m from the balance point, and T is the swing time. So if you have a racquet with a swing time of 1.3 s and a balance of 33 cm, you get h = 0.124*1.3^2 + 0.33 = 0.54 i.e the best place to hang the racquet is 54 cm from the end but. It doesn't mean you have to hit this point exactly, but if you hang it somewhere around there you will minimize the error from measuring the hang point.

Then it only remains to find the balance with enough accuracy

/Sten

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racquetTune, stringBed and swingTool racquet apps for the iPhone/iPad.

Last edited: Sep 13, 2012
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2. ### cornersLegend

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Great info. Thanks

3. ### esgee48Hall of Fame

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Or 22-24 cm above balance point of racquet. On an XL frame, I think I would prefer 22-24 cm above BP rather than 54 cm above butt cap.

4. ### Circa 1762Rookie

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Sten, this is very interesting. Just tried it out, and hanging the racket 23.5 cm above the balance point and 31.9 cm above the balance point (highest cross) gave me the same swingweight using the SW calculator. At 31.9 cm, adding 0.5 cm to the "distance from handle end to hang string" decreased swingweight by 3 kg*cm^2, while at 23.5 cm, adding 0.5 cm decreased swingweight less than 1 kg*cm^2 (no change registered). On the one hand, it's reassuring that hanging from the highest cross can give you accurate results if you measure distance carefully enough, but thanks for showing that if I use a lower cross, I no longer have to worry about fractions of millimeters!

5. ### stoneageRookie

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The formula had disappeared from the original post, but it is back now.
Sorry for the miss.

6. ### travlerajmHall of Fame

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Bravo. Well done.

7. ### stoneageRookie

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Thanks

You might know this, but if you hang the racquet on a string at twice the balance, i.e. at 2R you can get MgR/I directly from the swing time:

MgR/I=4*pi^2/T^2

Two problems though, it obviously only works for head light racquets. And there might not be a string exactly at 2R. But it might serve as a quick approximation.

8. ### taurussableProfessional

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fantastic tip! thanks stoneage!

9. ### RoarTTRookie

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But isn't the period T a function of the hangpoint itself? So when you say you have a racket with swingtime of 1.3s it all depends on where you hung the racket to get that time measure to begin with? I got one measure of 1.32 and one of 1.72 hanging the racket at two different strings, far appart. This gave two very differnt global maximum points when differentiating the two functions with respect to hangpoint (and keeping time constant).

10. ### stoneageRookie

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You are right that moving the hang point will change the swing time. So there is some oversimplifications in my reasoning.

The idea behind the curve is to look at the situation when you have measured the swing time and want to know much an error in the hang point will affect the calculated swingweight. Then it is correct to plot a curve with that time constant and see what happens if you move along the curve. If the curve is flat close to the hang point, a small change (error) when measuring the hang point gives little or no change in the swingweight. So far so good. The formula then calculates where the flat peak of that specific curve is. That is also correct.

But as Roar points out you can't use that formula and just put in any time. So the formula doesn't give you the optimal hang point directly. It tells you if the hang point you have used is close to the peak or not. If not, you have to move the hang point and measure again. In my experience is the optimal point a bit lower than usual "second string", but it might vary a little.

Also remember that all this is about fine tuning the set up, so it is no big deal if don't hit the peak exactly.

Last edited: Mar 30, 2016
11. ### RoarTTRookie

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Yes. The essense here being that you can't use this method to FIND your ideal hangpoint. Only to test how sensitive the hangpoint you have allready chosen is.