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Amone 09-25-2006 05:48 PM

Racquet Mod Formulas: Matching Specs
Roughly two weeks ago, I posted a thread with an emberassingly simple system for attaining a given balance point, with given weights. Well, the original idea behind it's complete, and here it is:

Given a final weight, balance, swingweight, and one (1) added mass, it will spit out the location of the necessary added masses, as well as the added weight of the other concentrated mass. Checked with Steve H.'s original Racquet Mod excel sheet. All figures in metrics.

Now, without further ado, the spreadsheet.

The Spreadsheet has been updated to include some traits I thought important.

If you look a few replies down (the third, I believe) then you'll see that Travler suggested a few things I should add, and this is one: Instructions.

Begin by plugging in the values for the top three white squares, with the labelled amounts for the racquet you're using. For my DNX 10, I labeled them.


In that order. Do not add units! If you don't know off hand the specifications of your racquet, then check TW's info on it.
Next, in the next set of whited-out boxes, enter the intended specifications of your racquet, with all said and done. For instance, to go with the specs in the spreadsheet by default, I wanted to go with the 'Sampras setup' listed by TravlerAjm. That required a weight of 384 g, a 32 cm balance, and a swingweight of 367.
The fourth whited-out box in that section, labeled 'Weight 1 Mass,' you can enter just about any number lower than the total added mass in grams. If you're not sure how much mass you're adding the spreadsheet provides you with that piece of information. What this is, is the amount of weight to add at the first spot, or 'location 1.'

Afterwards, it will spit out a few numbers: The important ones are cells C23 and C24, though future variations on this spreadsheet will make use of the final recoilweight, polarization, and so forth, in a more central role.

A Note On The Effect Of Specifications:
Weight or Mass will change the static weight, and the punch on your volleys, but little else.
Balance will have a distinct effect on the feeling of your racquet, wether it feels like it plows through the ball or gets pushed around by it. This is measured in CM from the butt of the racquet.
Swingweight is a measure of the difficulty of swinging the racquet, but it is also a factor in the power level of the racquet: higher swingweight, higher power.
Recoilweight is a measure of how much the racquet yields to the incoming ball. More means less. A flat hit shouldn't need much of this, due to the 'plowing' nature of the shot, where a windshield-wiper forehand will probably need more considering that it pretty much absorbs the ball's incoming speed.
Effective Mass is a measure of how much mass is used when the racquet hits the ball. It's not intelligible, at this point, but it makes for a good reference point of how much your power will be changed.
Polarization Index is a measure of how spread out your weight is. The higher the polarization index, the spinnier your racquet will tend to be. (Explained in the link)

If either value for 'weight location' comes out as greater than the total length of your racquet, you need to modify your input 'Weight 1 Mass.'

travlerajm 09-26-2006 06:46 AM

This is a great addition to the board!

Amone 09-26-2006 11:10 AM

Thanks, Travlerajm. That's sound praise, coming from you.

travlerajm 09-26-2006 11:47 AM

There is a large contingent of posters who want to be able to match specs, but they're not sure how to do the math calculations. This will be a great service to those people.

With the addition of clear user instructions, definitions of the terms, and basic explanations for how each spec affects the way a racquet plays, I think this might deserve a sticky.

Amone 09-26-2006 11:57 AM

Oh, Jeez, that's all simple stuff I shoulda thought of! Finally, something to do with my day!

N-serve 09-26-2006 12:42 PM

I am a little confused about the "Weight 1 Mass". Will you explain this a little further? This is great work by the way.

Amone 09-26-2006 01:12 PM

Well, for instance, say you're adding 12 grams total. You might enter as mass 1 anywhere from '1' to '11' and that's how much weight you would add at the location specified for mass 1. It's really just a reference number, like that most classic of variables, x.

The choice of how much weight to add at location 1 or 2 is purely your choice. In the creation of this, I made a simpler calculator, using set masses, then added in variability afterwards.

Greg Raven 09-26-2006 05:28 PM

It doesn't make a huge difference, but your swingweight position factor of 10.16 should be 10. Swingweight is typically measured on machines that have a pivot point 10 cm up from the buttcap, not four inches up from the buttcap.

Greg Raven 09-26-2006 05:39 PM

Oops, you also use this same 10.16 factor in your big equation, too, but it still doesn't make a huge difference.

AJK1 09-26-2006 06:09 PM

There's a lot of holes in these posts my friends, be careful using these recommendations, it's not all correct. The 10cm SW point being left out was a glaring mistake.

Amone 09-26-2006 06:23 PM

Ohhh? I used The Physics and Technology of Tennis as my reference. It refers to the swingweight being measured from four inches. So if you have a problem, take it up with Messr's Brody, Lindsay, and Cross. I do, but I just worked around it. (specifically, their change-in-swingweight formula was HORRIBLY inaccurate, but for the simplest of reasons.)

Now, AJK1, that sounds like a pretty agressive statement. Back it up. Give me exacts. I want my output to be statistically perfect, so give me numbers. Give me examples.


If my formula's wrong in theory, then I challenge you to provide me a better one. It'll probably take you about a day. If only in technicality, then where is my fault? I worked a lot harder on my product than you did on your trolling, so you cite your facts.

Amone 09-26-2006 06:33 PM

And if I come off as offended, it's only because AJK1 decides to come in, and instead of questioning my choice of any specific thing, like Greg did, I can't dissent because he hasn't said anything, he's just slandered my work without proof. I apologize if I came off as rude, or prudish, or whatever.

Punisha 09-26-2006 06:49 PM

great job thats an awsome job! Thnx heaps for posting. and stfu trollers!

Amone 09-26-2006 06:51 PM

Thanks, Punisha. As much as I don't like being randomly trolled, I rarely expect to agree with AJK, and I enjoy the challenge, so no problems there. However, I was a little sheepish about sharing, so I'm glad that most folks are so supportive.

Punisha 09-26-2006 06:55 PM

btw its not idiot proof yet... how do i work out balance. Is it distance in cm from butt?

Amone 09-26-2006 06:57 PM

Yes, sir. I had thought I addressed that point, but on reconcideration (read: looking back at the OP) I did not. The issue has been addressed.

Punisha 09-26-2006 07:16 PM

Ok and what do the wieght locations mean. I got one @ 12.59151523
and another @ 143.9539859

and finally what the hell do the recoil weight weight and effective mass mean lol :P thnx...

ps im doing this for all those idiots out there that dont know what these things are

Amone 09-26-2006 07:26 PM

Okay, you DEFINITELY can't do it with your specs, then. I'd change the mass for 'Weight 1 Mass' until I got an answer in which both numbers are below the total length of the racquet. ;)

Effective mass is, basically, how much weight the racquet uses when hitting the ball. At this point, the equation's totally buggered, but consistent, so I'd use it more for comparison purposes than anything else.

Recoilweight is how much the racquet recoils. Theoretically, you want less of this if you're a flat-hitter (the transfer of motion is direct; the way you swing the racquet plows without the need for recoilweight) and more if you're spinny (exact opposite).

AJK1 09-26-2006 08:27 PM

Amone, i don't have to justify my statements to you or anyone else, i am not trolling, i'm stating what i know based on my close working partnership with my racquet technician. I am entitled to post comments here as much as you are, just because you don't like it, is not my problem. I don't want other members here taking what so-called "experts" you and travlerajm post as gospel, since my tech says a lot of it is nonsense. A lot of it has holes in it as stated by me and other readers, and if i see a problem, i'll make an objection. I'd much rather consult Greg Raven on racquet issues, as he obviously knows what he is talking about. It's his business! Not a bunch of high-school science kids sitting in their bedrooms on the computer.

travlerajm 09-26-2006 10:38 PM

Instead of "effective mass", I like to use the "hitting weight." Interestingly, the simple definition of hitting weight that I use correlates well with Cross's results for "rebound power," even though Cross never seems to make the connection in his papers.

The hitting weight HW = M*(R - 4), with M = static weight in kg, and R = distance to balance point in cm (the 4cm offset is the location where the racquet handle intersects with the extension of the forearm). The hitting weight gives you a very accurate measure of a racquet's power level due to the weight distribution.

If you take any 2 racquets with the same hitting weight, same stiffness, and same tension/stringlength ratio, they will have approximately the same coefficient of restitution.

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