Combining weight, balance and swingweight

[0d1n]

Hi, just wanted to provide some feedback. I used your excel file to compare 2 of my racquets (same racquets actually, just different setups).

Old specs were roughly 350 grams, 32 cm, 340ish sw
New specs are roughly 335 grams, 32.9-ish cm (33 is a close aproximation), 340ish sw.

The result of the plot seems to be quite well matched with my experience...the curves are similar in shape, and converge towards the "around wrist" value.
I take it that means that on short swings there's not much difference but on longer swings the difference grows.
If my understanding is correct, that matches pretty well with my subjective experience of the differences...there's not much difference in maneuverability on volleys, but I feel the new spec more snappy on serves and forehands...and as a result it tires me out slightly less late in the 3rd set.

Pic : [IMG][/IMG]

[stoneage]

Originally Posted by kaiser

Hej stoneage, downloaded and extracted your Excelsheet, but can't open it. Only found a bunch of .xml files which open up as xml source code in IE8. What am I doing wrong?

I tried the latest version given above:
http://appmaker.se/racquetTune/relat...ingweight.xlsx
and it works fine for me.

I might be that your browser is set up to handle xlsx files in some specific way (i.e. not download them).

You might also try change the extension of the downloaded file to xlsx instead of whatever the browser has changed it to.

[Technatic]

This is a very interesting thread about an item that we (of the Dutch forum) also worked on.
We noticed that too many children play with a racquet that is either too heavy or too light for their age, type of play or stature.

So we created the Swingweight advisor which does the following:
This Excel sheet does 3 things:
1. It advises a Swingweight for a certain player based on type of forehand, age and build.
2. It calculates the SW of a racquet, when head- and throat- weight are entered. (Only have to weigh the weight at both ends of the racquet)
3. When the SW is too low it calculates how much weight should be added at a certain position.

There has been a lot of discussions about this on different forums and people built weighing systems to measure the SW as shown on this pictures.

Only one scale is ok but we found out that Head does the same measurements with 2 scales which is easier of course.

Maybe this is useful to you and when someone wants to have the Excelsheet just send me a pm.

[jmnk]

Originally Posted by Technatic

[...]
2. It calculates the SW of a racquet, when head- and throat- weight are entered. (Only have to weigh the weight at both ends of the racquet)
3. When the SW is too low it calculates how much weight should be added at a certain position.

There has been a lot of discussions about this on different forums and people built weighing systems to measure the SW as shown on this pictures.

Only one scale is ok but we found out that Head does the same measurements with 2 scales which is easier of course.

[..]

this method does not measure the swingweight. It measures the balance. You can than use the measured balance, weight, and the length to --approximate-- the swingweight. Depending on how the weight is distributed on a given racket the swingweight approximation will vary (sometimes it will be fairly accurate, sometimes not so much. For non-customized rackets it will be likely pretty good approximation).
But to be technically exact you cannot measure the swingweight that way

[Technatic]

This system certainly calculates the swing weigth.

The calculation goes as follows:
- From the head and throat weight the balance point is calculated.
- Then the moment of enertia is calculated from the balance point.
- Then the moment of enertia of is added for the displacement from the balance point to the rotation point that you want to choose. This rotation point lies on 10 cm from the end of the grip on most of the SW machines.

We compared the calculated values (Schwunggewicht berechnet) with the values measured on an Babolat RDC (Schwunggewicht gemessen) and as you can see these are quite close.



If you take into consideration that SW’s measured on different machines differ a lot these values do not differ too much.

[jmnk]

Originally Posted by Technatic

This system certainly calculates the swing weigth.

The calculation goes as follows:
- From the head and throat weight the balance point is calculated.
- Then the moment of enertia is calculated from the balance point.
- Then the moment of enertia of is added for the displacement from the balance point to the rotation point that you want to choose. This rotation point lies on 10 cm from the end of the grip on most of the SW machines.

We compared the calculated values (Schwunggewicht berechnet) with the values measured on an Babolat RDC (Schwunggewicht gemessen) and as you can see these are quite close.



If you take into consideration that SW’s measured on different machines differ a lot these values do not differ too much.

OK, could you please kindly provide a formula that does this:

"Then the moment of inertia is calculated from the balance point"
?

I'm truly interested.

[Technatic]

OK, could you please kindly provide a formula that does this:

"Then the moment of inertia is calculated from the balance point"

The formulas for the SW are:
Balance point:
Bp = W head * L/(W head + W throat)*10^3

Swingweight:
SW = Tot Weight * (Bp – Oset)^2 + Tot Weight *( 3*1,5^2*L^2)/12000

Bp = distance balance point to end of the grip. (cm)
W = weight (gram)
Tot weight = W head + W throat (gram)
Oset = the offset of the pivot point
1,5 = the approximate radius of the cross section of the racquet (=relatively unimportant).

[stoneage]

Originally Posted by Technatic

The formulas for the SW are:
Balance point:
Bp = W head * L/(W head + W throat)*10^3

Swingweight:
SW = Tot Weight * (Bp – Oset)^2 + Tot Weight *( 3*1,5^2*L^2)/12000

You can't calculate swing weight/moment of inertia from weight and balance (as JMNK already pointed out). The formula you give might be used to approximate the swing weight for some racquets. But it assumes a certain weight distribution in the racquet so you can never know whether it is correct or not. And in contrast to the real swing weight it doesn't add any new information about the mechanical properties of the racquet.

To give an example why this doesn't work lets consider three racquets, all 70 cm long and with a mass of 250 gram evenly distributed. Then:
to A: Add 50 g evenly distributed
to B: Add 50 g to the midpoint
to C: Add 25 g each to the top and bottom

You end up with three racquets that have that same mass (300 g) and the same balance point. So if you try to calculate the swing weight from these two values you will end up with the same swing weight, independent of which formula you use. With your method all three will be identical.

If you instead calculate the actual swing weight for these racquets you will get:
A: 310 kg cm^2
B: 290 kg cm^2
C: 350 kg cm^2

Which clearly is a significant difference.

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

[kaiser]

Originally Posted by stoneage

I tried the latest version given above:
http://appmaker.se/racquetTune/relat...ingweight.xlsx
and it works fine for me.

I might be that your browser is set up to handle xlsx files in some specific way (i.e. not download them).

You might also try change the extension of the downloaded file to xlsx instead of whatever the browser has changed it to.

Strange, when I open your link I get WinZip opens a zip file called eqv_mass_norm.zip, which contains three folders: _rels, docProps and xl, and a file called [Content_Types].xml. These folders contain a number of mainly .xml docs, but no .xlsx files which I regularly use for my work. In xl, there is a file called workbook.xml that sounds like it could actually be an Excel file.

[ChicagoJack]

Hi Technatic - What is that 8 sided racquet? Is from late 80's? Any info would be appreciated! Thanks - Jack

[stoneage]

Originally Posted by kaiser

Strange, when I open your link I get WinZip opens a zip file called eqv_mass_norm.zip.

I have ziped the files and uploaded them. Try this link:
http://appmaker.se/racquetTune/relative_swingweight.zip

[kaiser]

Thanks a bunch, that did the trick (although I am at my wife's laptop now...)!

[Technatic]

To give an example why this doesn't work lets consider three racquets, all 70 cm long and with a mass of 250 gram evenly distributed. Then:
to A: Add 50 g evenly distributed
to B: Add 50 g to the midpoint
to C: Add 25 g each to the top and bottom

You end up with three racquets that have that same mass (300 g) and the same balance point. So if you try to calculate the swing weight from these two values you will end up with the same swing weight, independent of which formula you use. With your method all three will be identical.

I misunderstood the matter at first:
If you add weights in one point you havd to add the Moment of inertia of that weight separately to get the right total SW.

meaning:
In case of adding 25 gram at both ends: a
The added SW with pivot point at 10 cm= 25*(10^2 + (Length - 10)^2 gr >>> 25 * Length^2
In case of the 50 gram in the center
The added SW = 50 * (bal-10)^2

If the balance point is at 35 cm and the length is 70 cm the added SW's are 90 and 31250 kgcm^2.
So this is the same difference as you mention between 350 and 290 kg cm

This is what the last section of the SW advisor calculates when you add weight.



We are testing this system and comparing it with actual SW tests on for quite some time now and we think that it is quite accurate.

Hi Technatic - What is that 8 sided racquet? Is from late 80's? Any info would be appreciated! Thanks - Jack

It was a sample from a manufacturer, more than 20 years old, I don't think that it was ever produced.

[stoneage]

Originally Posted by Technatic

The system certainly calculates different Swingweights for these different cases.

It certainly does not since the W head and W throat in your formula are the same for all three cases! (You can try it)

The "innovation" of calculating swingweight from balance and weight appears on Talk Tennis at least once a year, so you are not alone in this misconception. The problem is that it works as long as the weight is evenly distributed over racquet, but you can never tell when it doesn't. It also makes the concept of swingweight meaningless since it just a rearrangement of weight and balance. If you don't want to measure swingweight (which is not that difficult) it is better just to talk about weight and balance as such.

Finally, if you look at the definitions of swingweight (J) and balance (R) for and object of length L and weight M:



You can see that both depend on the distribution of mass m(x). But since the integrals are different, you can never use R to calculate J without knowing m(x).
(I have used the one dimensional case to simplify)

[smith1212]

Hello,

I have a little problem off topics but didn't found a thread: Because there wouldn't be an android version in the near future, I got my hand on an Ipod touch 2G with ios firmware 4.2.1. I bought your racquettune app via itunes-appstore. But when I try to install the app, I get the message, that the app isn't compatiblle with my ipod touch. I then saw, that the newest version 4.2.1 of your app requires ios 4.3 or later, which doesn't seem to exit for the touch 2g MC-model. As I noticed, the version 4.1 was made for ios 4.1 or later, which would fit. So where can I "downgrade" from raquettune 4.2.1 to 4.1? Or is there another way to get the 4.2.1. version on an ipod touch 2g?

[jmnk]

Originally Posted by stoneage

It certainly does not since the W head and W throat in your formula are the same for all three cases! (You can try it)

The "innovation" of calculating swingweight from balance and weight appears on Talk Tennis at least once a year, so you are not alone in this misconception. The problem is that it works as long as the weight is evenly distributed over racquet, but you can never tell when it doesn't. It also makes the concept of swingweight meaningless since it just a rearrangement of weight and balance. If you don't want to measure swingweight (which is not that difficult) it is better just to talk about weight and balance as such.

Finally, if you look at the definitions of swingweight (J) and balance (R) for and object of length L and weight M:



You can see that both depend on the distribution of mass m(x). But since the integrals are different, you can never use R to calculate J without knowing m(x).
(I have used the one dimensional case to simplify)

completely agree.

for those that are physics/math challenged Imagine you --could-- in fact accurately and absolutely calculate the swingweight by measuring just the weight, the balance, and the length of an object (or in this case a racket).
Why would Babolat or Prince tuning machines have a special function for measuring a swingweight? Why wouldn't they just measure the weight, the balance, and the length of an object and immediately give you an accurate swingweight value based on some match formula? Or why would Stoneage develop his excellent swingweight measuring app that does require a bit more involved procedure than just weighting a racket ?

[Technatic]

Hi guys,
I think that it might be good to discuss our basic intention:

* I certainly understand that people with knowledge about physics will say that it is impossible to calculate the Swingweight of a racquet very accurately.

But the major question is: Do we want a system that makes it possible to compare racquets or do we need very accurate values of the SW?

If we want to compare racquets we need a “measuring system” , with an accuracy that is as high as possible.

Another fact is that test results from different SW machines differ quite a lot, which proves that this is not an easy measurement to do.
So these machines are impressive and expensive but not accurate.

So if there is a much cheaper way to do a test, with about the same accuracy, there is a good reason to choose this test method. And a scale or even 2 scales are much cheaper than one SW machine.

What is our intentions with the SW advisor?
The idea of the SW came up because we advise the Dutch Tennis *** about material for young children.
- It should avoid that players play with completely the wrong SW and especially young children.
- We want to make it very easy for coaches to advise the right frame for a certain player.

The system does 3 things:
1. It advises a Swingweight for a certain player based on type of forehand, age and build.
2. It calculates the SW of a racquet, when head- and throat- weight are entered. (Only have to weigh the weight at both ends of the racquet)
3. When the SW is too low it calculates how much weight should be added at a certain position to get the desired SW.

So for our purpose we need an advise system, a measuring (calculating system) and a way to customize racquets. This is what the Excel sheet (and the online version later) does.

And of course we wanted to know how accurate our “measuring” system is compared to the official machines.

Our conclusion out of these tests is that the difference between our system and official machines is not bigger than the difference between the different SW machines.

So the inaccuracy of our system is not worse than that of the official machines. And a difference of less than 5% of the measured value is not bad at all for a complicated test.

You will understand that we are not interested in theoretical solutions without practical possibilities.

Why would Babolat or Prince tuning machines have a special function for measuring a swingweight? Why wouldn't they just measure the weight, the balance, and the length of an object and immediately give you an accurate swingweight value based on some match formula?

There are different reasons for this:
* A swing weight test based on measuring the swing time of an object is a very logic choice. The SW can be calculated with one formula from the swing time
T = 6,28 *rt (SW/w*L)
* Calculating from the Head and throat weight is much more complicated.
* Prince and Babolat sell these machines so it generates income.
* Prince and Babolat may do it with their machine Head seems to do it with the weighing system:
http://www.youtube.com/watch?feature...Y&noredirect=1

At 4.10 min you can see that they do the same kind of test.

Hopes this explains our intentions.

[Technatic]

I would like to explain the thoughts behind the upper part of the advisor, which advises the right SW:

The advise is based on the fact that a player needs more force when he has to accelerate the racquet more.

I.O.W.:
Someone who makes his back swing very early can move his racquet backwards slowly because he has a lot of time to do that.
Someone makes his backswing very late (after the bounce) needs much more force to accelerate his racquet because he has less time to bring it around.

So the first player can easily play with a higher SW without needing much force, it will cost the second player a lot of power to play with a high SW.
Compare Söderling with Nadal or even better (for the oldies) with Steffie Graf.

It is logic that stronger and older players can play with higher SW’s easier than tiny built youngsters. That is why you can enter a build and age of the player.

[jmnk]

Originally Posted by Technatic

Hi guys,
I think that it might be good to discuss our basic intention:

* I certainly understand that people with knowledge about physics will say that it is impossible to calculate the Swingweight of a racquet very accurately.

But the major question is: Do we want a system that makes it possible to compare racquets or do we need very accurate values of the SW?

completely agree here. In many, many applications an approximation is good enough. no one argues that. the only point of contention is where you stated in the earlier post:
"This system [e.g. spreadsheet where you enter weights at the ends, and length of the racket] certainly calculates the swing weigth."

it does not do that. it --approximates-- the swingweight, assuming that all rackets have the mass distributed the same way (i.e according to the same function).

Originally Posted by Technatic

If we want to compare racquets we need a “measuring system” , with an accuracy that is as high as possible.

Another fact is that test results from different SW machines differ quite a lot, which proves that this is not an easy measurement to do.
So these machines are impressive and expensive but not accurate.

Honestly I never heard that before. Do you have a source for that claim?

Originally Posted by Technatic

So if there is a much cheaper way to do a test, with about the same accuracy, there is a good reason to choose this test method. And a scale or even 2 scales are much cheaper than one SW machine.

sure, as long as you call it what it is - an approximation.

Originally Posted by Technatic

What is our intentions with the SW advisor?
The idea of the SW came up because we advise the Dutch Tennis *** about material for young children.
- It should avoid that players play with completely the wrong SW and especially young children.
- We want to make it very easy for coaches to advise the right frame for a certain player.

The system does 3 things:
1. It advises a Swingweight for a certain player based on type of forehand, age and build.

I can't comment on this. I do not know if there's a biomechanical, medical, or other reason why a given swingweight would be 'the best' for a given player. Perhaps there is. what is the source of that claim?

Originally Posted by Technatic

2. It calculates the SW of a racquet, when head- and throat- weight are entered. (Only have to weigh the weight at both ends of the racquet)
3. When the SW is too low it calculates how much weight should be added at a certain position to get the desired SW.

So for our purpose we need an advise system, a measuring (calculating system) and a way to customize racquets. This is what the Excel sheet (and the online version later) does.

so it is essentially the same thing that TW's own swingweight calculator does, right?

Originally Posted by Technatic

And of course we wanted to know how accurate our “measuring” system is compared to the official machines.

Our conclusion out of these tests is that the difference between our system and official machines is not bigger than the difference between the different SW machines.

So the inaccuracy of our system is not worse than that of the official machines. And a difference of less than 5% of the measured value is not bad at all for a complicated test.


You will understand that we are not interested in theoretical solutions without practical possibilities.

could you provide some data on this claim based on 20-30 rackets, with varying characteristics: hammer type rackets, head light rackets, etc?

Originally Posted by Technatic

There are different reasons for this:
* A swing weight test based on measuring the swing time of an object is a very logic choice. The SW can be calculated with one formula from the swing time
T = 6,28 *rt (SW/w*L)

this is not merely 'a logical choice'. this formula is based on physics laws. You can easily look it up why the formula is the way it is. It is --not-- an approximation, it gives you a swingweight value as per physics definition of what the swingweight actually is.

Originally Posted by Technatic

* Calculating from the Head and throat weight is much more complicated.

calculating may be (due to the complexity of the formula) - but that is not the point. The point is that 'measuring' the oscillation time is way more complicated that merely measuring the weight. thus overall the method approximating the SW based on weight is way easier (but not exactly accurate).

Originally Posted by Technatic

* Prince and Babolat sell these machines so it generates income.

yes. And these machines provide the functions that the weight-based formula does not. Is it worth thousands of dollars - definitely not to me. I go with Stoneage's app that gives me the exact measurement for $2.00

Originally Posted by Technatic

* Prince and Babolat may do it with their machine Head seems to do it with the weighing system:
http://www.youtube.com/watch?feature...Y&noredirect=1

At 4.10 min you can see that they do the same kind of test.

Hopes this explains our intentions.

that video does not show anything that can be used in this discussion. It shows someone weighting a racket and than we see few numbers. I think we can safely assume those SW numbers are --NOT-- an approximation, but the results of actual measurements. if HEAD cannot afford Prince tuning machine i think they can at least get Stoneage's app.....

but again, for what you are doing the approximation is likely quite enough.

[stoneage]

Originally Posted by Technatic

It is logic that stronger and older players can play with higher SW’s easier than tiny built youngsters. That is why you can enter a build and age of the player.

jmnk has given good and relevant replies in his last post so I have nothing to add there.

I just want to say that I have not against you scheme of classifying racquets for different players, it might be useful. But you could skip the swingweight in the scheme. Just use the balance and weight and say that player "so and so" should have a light, top heavy racquet. That is much easier to understand for most players.

Since all racquets with the same weight and balance will get the same swingweight with your formula anyhow it doesn't add anything and it will be easier and clearer just to omit it.