Manually Measuring Swingweight

Irvin

Talk Tennis Guru
Thanks, I used the 1st 2 formulas. I am getting a higher sw calculation 9n my racquets using the 1st method vs the 2nd. No idea why.
If you weight 2 points you can identify balance but you can not determine inertia. Both balance and inertia identify a distribution but the are different. A barbell would have exactly the same weight any balance if 2 equal weights were placed at the ends or in the center. But the inertia would be very different.
 
If you weight 2 points you can identify balance but you can not determine inertia. Both balance and inertia identify a distribution but the are different. A barbell would have exactly the same weight any balance if 2 equal weights were placed at the ends or in the center. But the inertia would be very different.
Which of the 2 calculations in your view is more accurate?
 

Irvin

Talk Tennis Guru
Which of the 2 calculations in your view is more accurate?
More accurate? Both, I guess, but if your weighing 2 points on a racket you can only determine balance, if your scales are accurate and the 2 points where you measure the weight are at the same level. Weighing 2 points to determine balance is a poor way to measure balance, but you can’t measure inertia that way.

EDIT: If you could determine inertia from weight and balance there would be no reason to ever measure inertia. Why not just use weight and balance. Whoever thought up the SW calculator on the Stringway site has no glue what physics is About.
 
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bfroxen

Rookie
@Irvin is correct. Stringway should take that Swingweight Calculator down. It may provide results that are in the ballpark for a "typical" racquet, but it's misleading and completely useless for customization. Consider a racquet with 5 grams added at each the tip and butt. Then, remove those and place the 10 grams at the balance point. The balance point, head weight, butt weight, and racquet length (the inputs to the calculators) will be unchanged, so the calculator result will be unchanged. But, the swingweight will actually be drastically different.
 

mad dog1

G.O.A.T.
I don't know, yet. I have been talking with someone about testing an order to Canada, so I'm working towards it...

I also have started shipping the device and calibration rod together in one box (actually, three boxes combined into one).
Keep fighting the good fight! (y)
 

derick232

Rookie
@bfroxen Just curious, what 3D printer(s) are you using? What materials? On the physics related side, did you design this to use a specific spring with a set spring force, sprint constant etc? I've been looking at building one of my own that I can use with the swingweight tool but didn't know what kind of considerations I might need to take with the spring. Also, do you take into any of friction of the rotating shaft and the added moment of inertia of the racket mount? I know that since they are relatively small and are relatively close to the axis of rotation they wouldn't make much of a difference, again just not sure what sort of those things I would need to take into account on my own build.
 
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jmnk

Hall of Fame
Just curious, what 3D printer(s) are you using? What materials? On the physics related side, did you design this to use a specific spring with a set spring force, sprint constant etc? I've been looking at building one of my own that I can use with the swingweight tool but didn't know what kind of considerations I might need to take with the spring. Also, do you take into any of friction of the rotating shaft and the added moment of inertia of the racket mount? I know that since they are relatively small and are relatively close to the axis of rotation they wouldn't make much of a difference, again just not sure what sort of those things I would need to take into account on my own build.
All the points you have brought up are very valid ones. Theoretically it is almost impossible to take all of those into account. Fortunately you do not have to - because that's what calibration takes care of. For all practical purposes once the device is built with a given spring, friction, its own weight, etc you calibrate it with an object of known swing weight (actually you need at least two such objects) - and that allows you to figure out all the unknows empirically. If yu are interested you may want to look over https://tt.tennis-warehouse.com/ind...lly-working-one-total-cost-roughly-10.675711/
 

Irvin

Talk Tennis Guru
@Irvin is correct. Stringway should take that Swingweight Calculator down. It may provide results that are in the ballpark for a "typical" racquet, but it's misleading and completely useless for customization. Consider a racquet with 5 grams added at each the tip and butt. Then, remove those and place the 10 grams at the balance point. The balance point, head weight, butt weight, and racquet length (the inputs to the calculators) will be unchanged, so the calculator result will be unchanged. But, the swingweight will actually be drastically different.
For an idea of how the SW will change assume you have a racket with an even balance and you add 10 grams at the balance I = mr^2 = .010*0*0 = 0. if you split the weight and put 5 g @ tip and 5 g @ butt I = mr^2 = .010*34.3*34.3 = 11.8 SW difference but weight and balance never change. Technatic (Fred T) has been told this before but refuses to believe it.
 

Curtennis

Semi-Pro
Congratulations you’re going to like it.
I hope so. As my game is improving im realizing swing weight is far more important than static weight or any other metric. I’ve been playing just one type of racquet lately so I’d like to get the three of them I own either matched, or spaced appropriate to have different racquets for different situations. For example I played in extreme wind yesterday. I should have ripped my lead tape right off, I could barely time anything right. Whereas today I played in perfect conditions against a ridiculously hard hitter so I could use his pace if I could absorb it with a heavy swing weight of my own…
After I get my racquets and my friends dialed in I’d probably sell the machine at a huge loss, but I might get addicted too :)
 

derick232

Rookie
Can someone explain what the difference between swing weight and moment of inertia are? I understand MOI but is SW just another word for it or is it something different?
 

bfroxen

Rookie
Can someone explain what the difference between swing weight and moment of inertia are? I understand MOI but is SW just another word for it or is it something different?
SW is the MOI about a specific axis. That being an axis 10 cm from the butt of the racquet, perpendicular to the handle and parallel to the string bed.
 

Strange hit

New User
I don't know, yet. I have been talking with someone about testing an order to Canada, so I'm working towards it...

I also have started shipping the device and calibration rod together in one box (actually, three boxes combined into one).
Please let us know when shipping to Europe will be available.
 

Aretium

Hall of Fame
What racket specs were you trying to match?
Sorry I didn't see that sw1 machine. Looks interesting.

I was trying to match all specs. I first matched balance and weight but felt very different.

Then I used a camera and swung the rackets and filmed it. Then used the swingweight calculator. But they still feel different so now I'm just gonna look for a pro to match it.
 

nyc

Hall of Fame
Got my SW1 yesterday and it's a pretty clever machine!

Set up was easy, went through calibration, and it is generating very consistent results. BUT it seems to me that while consistent, my measurements are about 5-6 pts too low.
My IG Youtek Prestige Pros come in at 315-316 SW, as opposed to 321-322. Made sure butt caps very firmly against back plate.

Looks to me I did something wrong in calibration? Got all check marks, but I didn't verify the actual rod length vs the printed tag on it. Is that something I need to do?
 

SonRK

Semi-Pro
My SW1 has been yielding results that match Tennis Warehouse's readings. Meaning, I have 3 frames that I bought from them that they measured and they are spot on with the SW1 (Blade 98, Pro Staff 100, Prince Ripstick)

Where I do differ is stuff specced out from ****. I have 5 racquets modified to be 302SW from ****, and my SW1 is reading 3-4 points heavier. Must be a difference in their machine's calibration, since it was consistently off for all 5 frames at the same 3-4 points.
 

jjs891

Semi-Pro
Got my SW1 yesterday and it's a pretty clever machine!

Set up was easy, went through calibration, and it is generating very consistent results. BUT it seems to me that while consistent, my measurements are about 5-6 pts too low.
My IG Youtek Prestige Pros come in at 315-316 SW, as opposed to 321-322. Made sure butt caps very firmly against back plate.

Looks to me I did something wrong in calibration? Got all check marks, but I didn't verify the actual rod length vs the printed tag on it. Is that something I need to do?
When you are calibrating, your numbers should match what’s written on calibration rod. In case you didn’t check
 

Power Player

Bionic Poster
Got my SW1 yesterday and it's a pretty clever machine!

Set up was easy, went through calibration, and it is generating very consistent results. BUT it seems to me that while consistent, my measurements are about 5-6 pts too low.
My IG Youtek Prestige Pros come in at 315-316 SW, as opposed to 321-322. Made sure butt caps very firmly against back plate.

Looks to me I did something wrong in calibration? Got all check marks, but I didn't verify the actual rod length vs the printed tag on it. Is that something I need to do?
Are you 100% sure your Prestige Pros should actually be 321-322?
 

nyc

Hall of Fame
Are you 100% sure your Prestige Pros should actually be 321-322?
According to TW specs and from empirical experience comparing to other sticks with similar specs, they should be 320-325. So I do think calibration must be off somehow.

I'll recalibrate and double check the specs on the rod. If they don't match do you override those numbers?
 

Power Player

Bionic Poster
According to TW specs and from empirical experience comparing to other sticks with similar specs, they should be 320-325. So I do think calibration must be off somehow.

I'll recalibrate and double check the specs on the rod. If they don't match do you override those numbers?
Honestly I suspect that the SW is correct if the rod is correct. I don't override the numbers,I just check the SW on the Rod now and then. If it is dead on the, numbers are the numbers. TW is updating its SW all the time plus who knows what strings they used compared to what string you are using.
 

nyc

Hall of Fame
There was another person that had a similar issue and @bfroxen was able to find the problem. Not sure if this is the same thing, but you might check with him.
he was very quick to reach out, yes. Top notch customer service! I wanted to check here first before emailing him, and he beat me to it!
 

PrinceYonex

Rookie
There was another person that had a similar issue and @bfroxen was able to find the problem. Not sure if this is the same thing, but you might check with him.
That was me. In my case there was an issue with some measurements related to the calibration rod, and @bfroxen resolved it quickly. I would only add that you shouldn’t go by the TW listed specs. Those are based on a small sample size, and actual racquets tend to show a good deal of variance when it comes to swingweight, even with Yonex (which most consider to be the best of the big brands at quality control). So it’s quite possible that your racquets real swing weight might be quite a bit lower than the listed spec.
In my case, the issue started because I had measured some of my racquets on a friends SW machine, so I had a solid base of comparison to start with.
 

Crazydoc

New User
Weighing 2 points to determine balance is a poor way to measure balance,
Sorry Irvin, but I've rarely read such rubbish. How do you come up with this statement, because it is fundamentally wrong?
With the wrong measuring equipment and too little knowledge, I accept that it is more susceptible to errors, but that is already the only disadvantage...

regards
 

Irvin

Talk Tennis Guru
Sorry Irvin, but I've rarely read such rubbish. How do you come up with this statement, because it is fundamentally wrong?
With the wrong measuring equipment and too little knowledge, I accept that it is more susceptible to errors, but that is already the only disadvantage...

regards
If you are leasuring the weight of the tip and the butt at 27” and 0” it is an excellent way to determine balance. If you think it is possible for you to measure the at those points laying a racket down on 2 scales that’s foolish thinking. So before you start determining the balance you must determine the exact points when the racket sits on the scales.

All you’re measuring is the center of mass between 2 unknown points.
 

Irvin

Talk Tennis Guru
Got my SW1 yesterday and it's a pretty clever machine!

Set up was easy, went through calibration, and it is generating very consistent results. BUT it seems to me that while consistent, my measurements are about 5-6 pts too low.
My IG Youtek Prestige Pros come in at 315-316 SW, as opposed to 321-322. Made sure butt caps very firmly against back plate.

Looks to me I did something wrong in calibration? Got all check marks, but I didn't verify the actual rod length vs the printed tag on it. Is that something I need to do?
Don’t assume all rackets will match TW specs. Murphy’s law says if anything can go wrong it will. As I see it there are 2 possible problems here. Either your calibration rod is off by 6 points or your SW is off by 6 points. It’s easy to check the calibration rod. Weigh the rod and check the length inertia of a rod is ml^2/12. That gives you the inertia as the center of the rod now to get the SW you must convert the inertia to a point 10 cm from the end. Calculate the radius from the rod’s center to the 10 cm pivot. SW = I + mr^2 that should tell you pretty close exactly what the lower SW on the rod is. If the SW labeled on the rod is a couple points off you need to contact @bfroxen.

EDIT: A point or 2 off on the lower inertia because of the rods length will be magnified to a much larger difference when you hang a weight out on the end. So 1 or 2 points on the low spec is 5 or 6 on the high spec.

EDIT: If your rod is labeled wrong your calibration will maladjust the calibration during calibration. Measure the inertia with no racket. It should read very close to zero. Just checked mine and it was 0.06 kgcm^2
 
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Irvin

Talk Tennis Guru
Yes of course that is a prerequisite!
That’s why IMO measuring balance using weight is a poor method. You must first make 2 leasurements at the head and butt before doing a calculation to determine balance, and any errors you make are compounded.

EDIT: Not to mention your racket must be level and both scales must be calibrated. Too many ifs in that scenario. In theory it’s great just not practical.
 

bfroxen

Rookie
In theory it’s great just not practical.
It's not practical to do without some sort of fixturing, but with fixturing, it's great. It looks to be very well implemented on the Yonex Precision Scan.

Ideally, the measurement points should not be at the butt and tip; they should be closer to the center of mass. That improves measurement precision. Here's a contraption I use to measure balance when I need a precise measurement. I usually use a simple balance board now, as it gives me sufficient precision for matching with the SW1.
 

Irvin

Talk Tennis Guru
@bfroxen I agree, here is a similar set of fixtures I used a long time ago for finding balance. I mistakenly thought back then I could use that weight distribution for also finding inertia.
 

Irvin

Talk Tennis Guru
@bfroxen why not use a single scale in the center near the COM With rests above and Below the COM? When the scale reads the max value you have the COM directly above it.
 

bfroxen

Rookie
@bfroxen why not use a single scale in the center near the COM With rests above and Below the COM? When the scale reads the max value you have the COM directly above it.
If I understand correctly, you’d need to move the racquet to find that point. That would be difficult to do with high precision.
 

Irvin

Talk Tennis Guru
If I understand correctly, you’d need to move the racquet to find that point. That would be difficult to do with high precision.
True but I could easily move a racket on a balance and measure the tip weight to determine how much weight is being supported by the tip. If the butt is resting on the balance board @ 31.2 cm and comes off the balance board at .17 g is measure on the scale at tip for 2 rackets can’t I assume they both have the same balance? Unless you have a flat spot on your rod you’re using for balance, or the rod diameter is too big you’re not going to be far off.
 

Irvin

Talk Tennis Guru
It's not practical to do without some sort of fixturing, but with fixturing, it's great. It looks to be very well implemented on the Yonex Precision Scan.

Ideally, the measurement points should not be at the butt and tip; they should be closer to the center of mass. That improves measurement precision. Here's a contraption I use to measure balance when I need a precise measurement. I usually use a simple balance board now, as it gives me sufficient precision for matching with the SW1.
When using that fixture do you take scales before placing racket on the fixture?
 

EggSalad

Rookie
I’m curious, In general if you add weight to a racquet but do so only in the handle, does it have less of an impact on swingweight than if you were to add that weight into the hoop?
 

Irvin

Talk Tennis Guru
I’m curious, In general if you add weight to a racquet but do so only in the handle, does it have less of an impact on swingweight than if you were to add that weight into the hoop?
Yes, but adding weight in the handle increases the inertia of the racket while at the same time lowering the balance point.
 

EggSalad

Rookie
Yes, but adding weight in the handle increases the inertia of the racket while at the same time lowering the balance point.
Excellent. Thank you.

I’m playing around with my Head Speed Mp 360+ before I sell them to see if I can get them to play More the way I want.

I’m probably going to end up around 337g (strung, dampener, leather + 2 OGs and a little putty in the handle). I’m trying to make it more headlight and not make too much is a change to SW.
 

bfroxen

Rookie
When using that fixture do you take scales before placing racket on the fixture?
Yes, I tare the scales before placing the racquet. It’s probably not clear from the photo, but there are sharp-ish edges where the fixture rests in the scales.
 
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