PDA

View Full Version : Measuring swingweight at different points


stoneage
01-24-2012, 08:37 AM
There seems to be some uncertainty whether hanging the racquet by strings as described by TWU (http://twu.tennis-warehouse.com/learning_center/howto_swingweight.php) is really OK. Some argue that hanging the racquet at the handle must better since the swing weight is calculated there. Theoretically you have nothing gain from hanging it at the handle. But I thought it might be interesting to see whether there is any difference in practice.

I therefore made 40 test of a racquet (Wilson kTour 95), hanging the racquet from two different locations at the handle and five at the strings. I measured the swing weight with the swingTool app (new version). The results can be seen below.

http://appmaker.se/img/swingtest.jpg

There are two types of variations.
1. Difference between the hanging points. This probably due to errors when measuring the distances to the hang point and the balance point (center of gravity). But there is no systematic difference between the points.

2. Difference between tests at the same hang point. These are probably due to errors in measuring the swing time and differences in the swinging angle.

However, all these variations are very small! The average of the test is 330 kg cm^2 and all measurements fall within +-0.5% which is even better than I expected.

So my conclusion is that it is OK to hang the racquet from the strings and you don't have build any fancy devices to hang it from the handle. A word of caution though, the lengths (to the hang point and balance point) are squared in the calculations so you have measure them carefully. I don't think the TWU "push over the edge" concept is accurate enough. The swing time must of course also be measured with some accuracy. I checked swinTool against a timing gate several times and the result was within +-0.002 s.

/Sten

_________________________________________________
racquetTune (http://racquetTune.com), swingTool (http://www.appmaker.se/swingTool) and fitnessMeter (http://www.appmaker.se/fitnessMeter), tennis apps for the iPhone.

corners
01-24-2012, 12:04 PM
Thanks for taking the time to clear that up Sten.

stoneage
01-27-2012, 02:20 AM
To complement to the above measurements I tested 3 different racquets of the same model (the same as in the first figure). I measured the balance in two different ways (same result), hang them at the same string and ran 6 measurements each. The result showed that there definitely are individual differences between racquets.

http://appmaker.se/img/swingtest2.jpg

jmnk
03-30-2012, 10:45 AM
@stoneage.
first of all thanks for your tennis realted ipod apps. I'm a very happy user.

I have a question regarding swingweight tool. Have you ever had a chance to verify the accurance of the tool against a bar (or whatever) that has a known swingweight? I mean it seems to me that the tool is very accurate (since I'm getting very similar results on each of n tries), but sometimes the results are not what the published specs are. I'm fairly certain that your tool is more accurate than those published specs, but I would love to know for sure. I have no access to anything with a known, certified swingweight, so I was thinking whether you do.

stoneage
03-31-2012, 01:56 AM
I have a question regarding swingweight tool. Have you ever had a chance to verify the accurance of the tool against a bar (or whatever) that has a known swingweight? I mean it seems to me that the tool is very accurate (since I'm getting very similar results on each of n tries), but sometimes the results are not what the published specs are. I'm fairly certain that your tool is more accurate than those published specs, but I would love to know for sure. I have no access to anything with a known, certified swingweight, so I was thinking whether you do.

The method that swingTool is based on is theoretically sound (I didn't invent it) and provided the measured data are exact, the result will be exact. So it is down to measurement errors. swingTool is really a time measurement device and a calculator. As I mentioned in the first post I have compared the the timing against a timing gate and the result was that the swing time usually was within +-0.002s. Since the swing time usually is 1-2 s this means that the timing error will give errors in swing weight that is less than 1 kg cm^2. I think this acceptable and certainly better that what you would get by manual timing.

The other errors are introduced by you :) As mentioned the lengths to the balance point and pivot point are squared in the calculation, so it is important to get them right (especially the balance point). You can use the TWU calculator (http://twu.tennis-warehouse.com/learning_center/swingweight_calc.php) to check the influence of different parameters. Enter some realistic values and then change one of them a little and see how it affects the swingweight.

Yes I have compared to a swinging rod and the results agree. However, since I can't buy a "reference swingweight rod" I still have to measure the weight and balance of the rod so I am introducing errors in the reference value as well. So the best I can do is to check that swingTool does what is it supposed to do, i.e. measure swing time accurately.

I have also noticed a difference between my swingTool measurements and the published values. It is difficult to comment on the accuracy of other measurements since I don't know how they are done. For example are the racquets strung or not? I have also noticed a difference in balance point compared to the specs. Maybe someone else has a good explanation?

/Sten

_________________________________________________
racquetTune (http://racquetTune.com), stringBed (http://www.appmaker.se/stringBed) and swingTool (http://www.appmaker.se/swingTool) racquet apps for the iPhone/iPad.

jmnk
03-31-2012, 03:19 PM
The method that swingTool is based on is theoretically sound (I didn't invent it) and provided the measured data are exact, the result will be exact. So it is down to measurement errors. swingTool is really a time measurement device and a calculator. As I mentioned in the first post I have compared the the timing against a timing gate and the result was that the swing time usually was within +-0.002s. Since the swing time usually is 1-2 s this means that the timing error will give errors in swing weight that is less than 1 kg cm^2. I think this acceptable and certainly better that what you would get by manual timing.

The other errors are introduced by you :) As mentioned the lengths to the balance point and pivot point are squared in the calculation, so it is important to get them right (especially the balance point). You can use the TWU calculator (http://twu.tennis-warehouse.com/learning_center/swingweight_calc.php) to check the influence of different parameters. Enter some realistic values and then change one of them a little and see how it affects the swingweight.

Yes I have compared to a swinging rod and the results agree. However, since I can't buy a "reference swingweight rod" I still have to measure the weight and balance of the rod so I am introducing errors in the reference value as well. So the best I can do is to check that swingTool does what is it supposed to do, i.e. measure swing time accurately.

I have also noticed a difference between my swingTool measurements and the published values. It is difficult to comment on the accuracy of other measurements since I don't know how they are done. For example are the racquets strung or not? I have also noticed a difference in balance point compared to the specs. Maybe someone else has a good explanation?

/Sten

_________________________________________________
racquetTune (http://racquetTune.com), stringBed (http://www.appmaker.se/stringBed) and swingTool (http://www.appmaker.se/swingTool) racquet apps for the iPhone/iPad.
many thanks for a detailed explanation. I have no doubts that the theory is sound. and if you have indeed checked with a uniform rod, and the results are as theoretically expected, I think that is good enough.
I just want to use my own practical results to challenge some of the theories that are posted here and there, and wanted to make sure your tool can be used as a reliable 'measuring stick'. In case someone wanted to question those experimental results.
I think that those posted specs are sometimes really, really approximation based on whatever assumptions one made, and that's why they vary so wildly, since the assumptions are not the same.
I'll stick with results I get from your tool....

now on somewhat related topic. While I get very consistent results across multiple tests with swingTool, the results with racquetTune (that check string tension) are nowhere near as consistent. I mean it seems that there are just many more variables, mainly how hard I hit the stringbed, the exact spot I hit the strings, etc. Would you agree? Or am I doing something wrong?

ccelis
04-06-2012, 12:33 PM
Hi, just curious how this app compaires to the Swing weight calculator on the USRSA website. I don't have a IPod or Iphone, so I am trying to figure out another weight to measure swing weight. Is this app more accurate?

Thanks

stoneage
04-07-2012, 04:04 AM
now on somewhat related topic. While I get very consistent results across multiple tests with swingTool, the results with racquetTune (that check string tension) are nowhere near as consistent. I mean it seems that there are just many more variables, mainly how hard I hit the stringbed, the exact spot I hit the strings, etc. Would you agree? Or am I doing something wrong?

I would say that racquetTune usually gives very consistent results. I have tested a racquet until the accuracy bar was "full", tapped reset, tested again to full accuracy and repeated that 10 times. The results usually are within +- 0.1 lb. You can also see a consistent behavior at:
tt.tennis-warehouse.com/showthread.php?t=398516 (http://tt.tennis-warehouse.com/showthread.php?t=398516)

I have also put a rather firm requirement in the app on which readings to accept so the problem is usually to get a reading rather at all than the readings being to varied. If you mail me privately I can tell you what result to send me so I can analyze them.

/sten

stoneage
04-07-2012, 04:05 AM
Hi, just curious how this app compaires to the Swing weight calculator on the USRSA website. I don't have a IPod or Iphone, so I am trying to figure out another weight to measure swing weight. Is this app more accurate?
Thanks

As mentioned above it uses the same method, but the iPhone will give you a more exact measurement of the swing time than what you get by using a stop watch. So the measured swingweight will be more consistent.

/Sten

syke
04-07-2012, 04:19 AM
Hi, just curious how this app compaires to the Swing weight calculator on the USRSA website. I don't have a IPod or Iphone, so I am trying to figure out another weight to measure swing weight. Is this app more accurate?

Thanks

racquetTune, swingTool & netHeight are only reasons you need to go out and buy a iPod Touch & iPhone. Lol...

I just need an app now to correctly measure balance.