Combining weight, balance and swingweight

Discussion in 'Racquets' started by stoneage, Sep 1, 2012.

  1. Technatic

    Technatic Semi-Pro

    Joined:
    Oct 12, 2009
    Messages:
    474
    Wow is it my fault that I feel like I am in a court case being interrogated sometimes on this forum? Every word that is use seem to matter suddenly, while this is a technical discussion. Please take into consideration that I am Dutch and my English is limited.

    Hereby some answers:
    If our test approximate, the test machines approximate also because their accuracy is not better than with our system.
    IMO you should say both measure the SW, with their own accuracy.

    This is exactly the same as comparing SBS testers, these show a big difference between results of different tools, because the test (with the vibration) is too complicated, including a calculation with a square in it.

    There has been a different discussions on forums about this one of them on the GSS forum. We have different machines in our forum team (Prince and Babolat). It also seems that the calibration with these machines give different results with the same calibration bar.

    This is very basic, although it was not said before, just give a player with a late backswing a racquet with a high SW and you will see what happens. Give the same racquet to a “Söderling type” and that one will not have any problem.
    Newton has been right for many years already.
    New ideas can never be found in sources.

    As I said, I am not in court here, we did tests in the Netherlands and someone else in Austria.

    I do not think that the TW calculator also advises, we did not find an advise system.
    The basis for the calculation is the same only from a different point moved to the pivot point.

    This is the basis for the SW machines! So these machines have to use this forumula to get from swing-time to swingweight. And because the square is the first reason for the low accuracy.

    Could it also be that Head found out that this is quite a good way to test the SW of their racquet? Head would not be able to pay 2000 dollars for the Prince machine? Common!
     
  2. stoneage

    stoneage Rookie

    Joined:
    Apr 8, 2010
    Messages:
    243
    Location:
    Stockholm, Sweden
    You are hijacking a thread presenting a scheme with a rather sloppy foundation, you are telling people that their calculations are wrong without any evidence. And then you get sensitive because some errors and weaknesses are pointed out to you (in a rather civilized manner I would say)!!!

    There are a lot of people in these forums that don't have English as their native language, including me. And that is very seldom the problem in my experience. Personally I would prefer not to be treated differently because I am Swedish.
     
  3. Technatic

    Technatic Semi-Pro

    Joined:
    Oct 12, 2009
    Messages:
    474
    Hijacking? I think this is the same subject.

    I do not think that I said anything about calculations of others.
     
  4. kaiser

    kaiser Semi-Pro

    Joined:
    Feb 11, 2010
    Messages:
    703
    Location:
    Holland - Belgium
    I agree with Stoneage, you are effectively hijacking his thread with very long posts on your own tool. Although the subject is similar, Stoneage and you are presenting very different tools and have fundamentally opposing views on it. I would suggest you set up your own thread if you want to have this discussed on this forum. Perhaps others with insight in the physics of tennis rackets will then also chime in.
     
  5. Technatic

    Technatic Semi-Pro

    Joined:
    Oct 12, 2009
    Messages:
    474
  6. jmnk

    jmnk Professional

    Joined:
    Jul 1, 2009
    Messages:
    984
    to be fair I do not think Technatic is hijacking this thread. Nor he tries to 'push' any tool on anybody. Actually his premise of having a simple way of advising players on what swingweight they should use may have some merit.

    The discussion got a bit lengthy because he made some claims that are not 100% technically accurate. It would have not been a big deal if he just stated what the assumption are, clearly listed the goals, and perhaps provided some sources of his claims so one can verify for himself.

    Unfortunately for some reason he does not want to do that. oh well. Without proper reasoning and sources this is just another 'adjust the racket specs for the wristband usage' type of the thread (OK, I'm being facetious, this one is not as bad as the wristband thread).
     
    Last edited: Jan 9, 2013
  7. kaiser

    kaiser Semi-Pro

    Joined:
    Feb 11, 2010
    Messages:
    703
    Location:
    Holland - Belgium
    Correction: Wet wristband thread... :)
     
  8. 0d1n

    0d1n Hall of Fame

    Joined:
    Jul 8, 2006
    Messages:
    3,693
    Location:
    Cluj-Napoca, Romania
    No worries, from my point of view your contribution to the thread was very useful. Any additional information with some logic and some solid reasoning behind is useful IMO.
    I can also totally understand your more pragmatic/practical approach when compared to the almost dogmatic responses you got from some people.
    Yes...your method won't be "mathematically exact" but for the purpose of matching and/or comparing racquets it will do. Just like the TW university is very useful, both their customization sections and their "information about sweetspot sizes" etc sections.
    I can also see the use of the OP's excel and his posts are valuable as well. Can't really see what all the fuss is about frankly...
     
  9. jmnk

    jmnk Professional

    Joined:
    Jul 1, 2009
    Messages:
    984
    I do not think anybody is being 'dogmatic' here. Just technically correct.
    The whole fuss is really about a single word. rather than saying:

    "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)"

    he should have said:

    "2. It approximates the SW of a racquet, when head- and throat- weight are entered. (Only have to weigh the weight at both ends of the racquet)"

    that's really the end of it. No big deal. Which is why I'm surprised he does not want to admit that the method is an approximation and instead he digs into defending the accuracy.
     
  10. Technatic

    Technatic Semi-Pro

    Joined:
    Oct 12, 2009
    Messages:
    474
    I have worked for 35 years in the development of measuring tools in the Dutch Nuclear Centre and I can assure that every measuring tool approximates.
    We developed tools that measured the movement of salt in a salt mine, and we were very very happy with an inaccuracy of +/- 10 %.

    We bought 2 exactly the same scales from Ikea, the difference at 80 kg was more than 1 kg, and this is a simple test.

    So if the inaccuracy of a complicated test as the Swingweight test is within 5 % of each other that is quite good.
     
  11. stoneage

    stoneage Rookie

    Joined:
    Apr 8, 2010
    Messages:
    243
    Location:
    Stockholm, Sweden
    No, it is actually the opposite. The purpose of this thread is to give a tool to analyze the combination of weight, balance AND swingweight. By substituting the real swingweight with a recalculated balance all relative swingweight curves becomes the same and the purpose of this thread meaningless.

    But you are of course free to make any contribution, so hijacking is probably a too strong word, sorry for that.

    Yes, you did that in post 93 by just saying that my calculations were wrong without any further comment. You have since edited that post (e.g. my quote in in post 94 doesn't exist any more) confirming my calculation and adding information about the way you calculate swingweight that was not in your earlier presentations of the model. I didn't see that until I went back before writing this post.
     
  12. stoneage

    stoneage Rookie

    Joined:
    Apr 8, 2010
    Messages:
    243
    Location:
    Stockholm, Sweden
  13. corners

    corners Legend

    Joined:
    Jul 31, 2008
    Messages:
    5,441
    I agree. This is not the first time a two-scale approach has appeared on this message board. I don't understand these efforts at all. For some reason the concept of swingweight seems to elude some people, and they then go about trying to establish an alternative measure to swingweight without realizing that what they are searching for is swingweight. I guess swingweight or moment of inertia falls under the "elusive obvious" category.
     
  14. Technatic

    Technatic Semi-Pro

    Joined:
    Oct 12, 2009
    Messages:
    474
    we try to make an advisor!

    I think that there might be a misunderstanding:
    We try to develop an SW - ADVISOR like we have A tension advisor also on our forum. The SW measuring system is just a part of that.

    We want to achieve that players can choose the right SW for their type of tennis-technic and especially for children!

    If there are other threads that tell more about a SW advisor I would be glad to know them.
    We are just trying to make our system as perfect as possible.
     
  15. corners

    corners Legend

    Joined:
    Jul 31, 2008
    Messages:
    5,441
    I must not understand what you are trying to accomplish. If you are trying to advise players on what swingweight is appropriate for them, why are you measuring something other than swingweight? You can't determine swingweight using two scales, but you can determine swingweight using two pencils and a stopwatch. I'm just confused as to why you are trying to reinvent the wheel, so to speak, when the wheel we have (the concept of swingweight and the methods for its measurement) is perfectly adequate.
     
  16. jmnk

    jmnk Professional

    Joined:
    Jul 1, 2009
    Messages:
    984
    @Technatic - one more question.

    Where does the above formula come from? I think it cannot be correct, perhaps you have just made a mistake typing it in.

    The swingweight is expressed in kg*cm^2.
    with the above formula you would get:
    SW = Tot Weight * (Bp – Oset)^2 + Tot Weight *( 3*1,5^2*L^2)/12000
    SW = kg * cm ^ 2 + kg * (unitless1 * cm^2 * cm^2)/unitless2 = kg*cm^2 + kg*cm^4

    (unitless1 =3, unitless2=12000, and 1,5 is in cm since as per your explanation it is a radius of the cross section)

    so the result is not in kg*cm^2 - which means it cannot possibly be correct.

    lastly, in the table you have posted in post #85 - what does Waage1 mean/represent? What does Auflagepunkt mean/represent?
     
  17. Technatic

    Technatic Semi-Pro

    Joined:
    Oct 12, 2009
    Messages:
    474
    I think that I tried to explain already that there are more ways to calculate and measure the SW. The USRSA way is really not the only way, there are basic mechanics.
    And besides we want to put it all in one excel sheet which will be the basis for an online advisor as we have for the Tension also, just click on the red beam in the middle www.stringforum.net/nl.

    It is our goal to create an SW advisor and up to now nobody came up with another SW advisor, so this is certainly not the matter of the wheel.


    Excuse me typing error; 3*1,5^2+L^2 insteady of 3*1,5^2*L^2

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

    The basic formulas which are used are:

    1. For a cylinder around the axis x or vertical axios:

    [​IMG]

    These 2 formulas are the same.
    Because the radius is very small in relation to the length the radius hardly has influence.

    2. For moving the pivot point to any other point:
    I = m * d^2
    d = de displacement in cm.

    This formula is also used in the USRSA calculation to move the pivot point from the top of the racquet to the handle.

    Waage 1 means Scale 1.
     
  18. jmnk

    jmnk Professional

    Joined:
    Jul 1, 2009
    Messages:
    984
    Ok, for the last time, as this indeed turns into 'adjust racket specs when using (wet) wristband' thread. Kind of sad, your first post in this thread sounded actually interesting.
    you are confusing 'calculate' and 'measure'. It is pretty much impossible to 'calculate' the SW (Moment Of Inertia) of an arbitrary object when you know only the mass, length, and balance. The formulas have been derived for many simple objects - as you can see here:http://en.wikipedia.org/wiki/List_of_moments_of_inertia
    But a tennis racket can hardly be approximated by any of such simple objects.

    Which is why method to 'measure' the SW has been devised. It is actually interesting from the physics perspective that SW is --easier-- to measure than to calculate.

    I'm not sure what you mean by 'USRSA way'. The method of measuring used in tennis tuning machines, or Stoneage's app is based on a so-called compound pendulum setup. That method, if performed carefully, will result in an exact SW value for any object, uniform or not.
    (actually, now that I think of it, I'm not aware of any other industry standard method of measuring Moment of Inertia. Could you please provide link(s) to a description of such other methods?)

    This is a nice goal. Two problems.
    1. you are using inaccurate value for the Swingweight of the racket - which is what we are debating here. See more explanation later on.
    2. You are assuming, I'm not sure on what basis, that there's is some 'ideal' swingweight for a player, depending on said player technique, arm length, and build. Is there any medical/biomechanical/other proof supporting such thesis? Why not take into consideration the height? Or the shoe size? How how much a player can bench press? Or 100s other criteria?


    ok, now at least the numbers add up.

    let's start with the correct part. The second bullet point is neither USRSA, my, or your invention. It is a well-know parallel axis theorem. Of course it is used when calculating Moment of Inertia around an arbitrary axis if MOI around an axis going through the balance point is known.

    But the first bullet point - oh boy.

    You are assuming that an arbitrary racket has --the exact same-- Swingweight as a uniform, solid cylinder with the same mass, length, and cross-section.

    Don't you see that it is a far, far stretch to assume that? At the very least you could have used a formula for a thick-walled cylindrical tube since the racket is actually hollow inside. Even better - there are approximation derived specifically for the tennis rackets, just search for works of Brody, Cross, etc.

    I have now serious doubts that your approximation is good enough to 5% accuracy. I have several rackets, I'm going to do measurements on my own to check how good such approximation is. (since we have already established that you are not willing to back up any of your claims with sources or actual empirical numbers).
     
  19. Technatic

    Technatic Semi-Pro

    Joined:
    Oct 12, 2009
    Messages:
    474
    Too theoretical against too pragmatic

    I think that it is clear that we have completely different approaches:
    The difference between us might be a case of too pragmatic against too theoretical.

    In development engineering it is not very important that the theory is completely right it is important that the developed tool is working well.

    And there is a saying in developing: “The test is always right”.
    Meaning that if the tool works well there is no need to have any doubts.

    Theoretically you are right with the thin walled cylinder.
    But in this case the influence of the cross section is so small in relation to the influence of the length, that it will not make a difference.
    And it means that you have to enter more info (like wall thickness) that you do not know anyway.

    We have 2 major objectives:
    - We want a complete advise – measure – tune tool in one system (an Excel sheet at this moment). When there is an online tool later the coach can do the hole trick on court with his cell phone.
    - We want a low cost, easy to transport, test system ( a little scale ) so that it is easy for coaches and players to use.
    In our opinion weighing is more accurate and faster than measuring swingtime.
     
  20. jmnk

    jmnk Professional

    Joined:
    Jul 1, 2009
    Messages:
    984
    Actually - let me ask one more thing. Given the formula for Moment Of Inertia of a solid cylinder (which is correct on your picture) - where does the 1/12000 term in your equation come from? Shouldn't it be just 1/12 and all the lengths be expressed in [m] meters?
     
  21. jmnk

    jmnk Professional

    Joined:
    Jul 1, 2009
    Messages:
    984
    I have no words anymore. Let's just hope that the Dutch Nuclear Centre does not come up with some w e a p o n guidance system - who knows where it would end up when fired :) (sorry, I couldn't resist)

    Let me finish on a joke (told long time ago by one of my teachers). In this lab they wanted to find out how a fly hears sounds. So they put a fly on a table, clapped hands - a fly flew away. They plucked out one wing. Clapped again - the fly flew away again - but not as swiftly. they plucked away the remaining wing, clapped hands - but nothing happened. the fly remained on a table. the conclusion - a fly hears by its wings. Because you know, to quote you, " as they say in developing: “The test is always right”. " :)
     
  22. 0d1n

    0d1n Hall of Fame

    Joined:
    Jul 8, 2006
    Messages:
    3,693
    Location:
    Cluj-Napoca, Romania
    You are...of course..."technically correct". That does not mean you are also "right" in this debate.
    The man has admitted that his purpose is not to be "exactly precise" he has explained his premise more than once, and I understood it.
    I think it makes sense and anyone can make a personal decision on whether to use his "adviser" or not.
    I understand his SW number will not be "correct" as well, I'm an Automation and Computer Science graduate from a 5 year Technical University and as a result I've studied plenty of mathematics and enough physics both in high-school and during university. I don't use those much anymore, and may have forgot most formulas or problem solving methods, but the conceptual understanding is still there.
    Even with that in mind I'm still not too bothered about his approach. I think it CAN add value to many people if used correctly.

    Edit.
    P.S. How about going back to the original subject...the combined "weight / balance / swingweight" issue ? I also think that one can be very useful and can hopefully (with the help of the graphic part) help some people on this forum understand that "adding weight to the handle" does not make a racquet more maneuverable, not in the real world anyway.

    Cheers.
     
    Last edited: Jan 11, 2013
  23. JackB1

    JackB1 G.O.A.T.

    Joined:
    Jul 23, 2009
    Messages:
    14,044
    Location:
    Atlanta GA
    Hey Stoneage....a little off topic, but does your racquettune app measure
    tension in the new 16x15 Steam 99s accurately? I just measured mine that came off the stringer, strung at 58lbs and it measured 43! How could that be?
     
  24. Shroud

    Shroud Hall of Fame

    Joined:
    Apr 17, 2013
    Messages:
    3,457
    Location:
    Bay Area

Share This Page