Swingweight Estimation Equation

[travlerajm]

Here is a handy and very accurate equation to estimate your swingweight simply by measuring the mass and balance of your racquet accurately. It will generally be accurate to within +/-10 kg-cm^2. The uncertainty is due to the unknown degree of polarization of your racquet. Be sure to measure the mass to nearest gram and balance accurately to nearest half millimeter!

Here it is:

SW at 10 cm from the butt =

1.5MR^2 + 20M(5 - R)

M = mass in kg
R = balance in cm

You may notice that this is simply an application of the parallel axis theorem - i.e., the swingweight about the butt end is approximately 1.5MR^2.

 

[Amone]

Wow, Travler! Your equation isn't the equation for an infinitely thin rod! 2 kudos for you! (And that's my backhanded rant for the day... bloody Physics and Technology of Tennis...)

 

[Islandtennis]

Travler, I am little confused. R is the balance. That is the distance from the butt to the balance point isn't it? If that is the case, 5 - R will always be negative. Is that correct?

 

[jace112]

post deleted...

 

[sypl]

Well it better be negative, else your swingweight will be something like 700!

But I think the results I'm getting with this are somewhat dodgy. I plug in the numbers and get excessive swingweights.

Take the figures from TW and see. Here's the flexpoint prestige MP: http://www.tennis-warehouse.com/descpageRCHEAD-FXPP.html

Mass: 0.346kg
Balance: 32.0675cm

According to the formula, the swingweight is:
1.5*0.346*32.0675^2 + 20*0.346*(5-32.0675) = 346.39334469375

Quite a bit over the 323 TW claims. The formula consistently overestimates no matter what racquet I try it on.

 

[Polaris]

Works pretty well for my Redondo 98.

M = 0.343 kg
R = 31.115 cm (10 pts HL)
SW by formula = 318.96 (TW spec is 324)

Here are the approximate numbers for the PK Copper Ace

M = 0.34 kg
R = 30.7975 cm (11 pts HL)
SW by formula = 308.30 (don't know of a reported spec, but would have guessed it to be about 315)

Travlerajm and sypl, do you guys think that the point at which the approximation tips from underestimating the SW to overestimating the SW occurs around 8-9 points HL balance? I mean, the racquets I examined have 10 pts HL or more and I got a lower SW, while the racquet that sypl used, has 7 pts HL and that gave a higher SW. Since the SW varies as R^2, the errors ought to depend more heavily on R.

 

[sypl]

I plugged in the numbers for about 20 different frames. Conclusions:

• Formula rarely ever underestimates. It did for the following racquets though
  • Redondo (93): -0.3%
  • RDS 001: -1.25%
  • Avery M3 Control: -1.66%
  Them's some craaaaazy headlight players racquets.
• Seems that the heavier the racquet is, the more headlight it needs to be in order to fit the equation.
• Doesn't work AT ALL for granny racquets. Tried it on the n1 Force, Metallix 10 and IPEX 2.0 and got differentials between 18 and 26%

Wish I could attach the spreadsheet for all to see.

 

[WhiteSox05CA]

Great, this will definately come in handy.

Have you tested it out on stock frames to see if it matches the company's listed SW??

 

[travlerajm]

The formula works best on racquets that don't have extreme variation in density over the length of the frame. The granny sticks sometimes have certain parts of the racquet with almost no weight.

THe formula is quite accurate for pro-style specs, because adding lead to the hoop of a player's racquet tends to even out the density distribution.

 

[300Gkid]

POLARIS, i've finally found someone with a copper ace, since there are no more PM's, can you tell me the specs for the copper ace, i couldn't find anyone that knew.

 

[Polaris]

300Gkid, Here is what I found from the Racquet forums and from what I can read on my racquet:

Head Size: slightly less than 90 sq. in
Weight: 340 g (no lead)
Swingweight: Approx 315 kg-cm^2 (no lead)
Composition: 40% graphite, 60% fiberglass
Balance: 11 pt HL
String Pattern: 16 x 19
Length: 27 in
Stiffness: Couldn't find it exactly, but should be around 55, similar to my Redondo
Grip: Wilson Pro white overgrip on top of originial Cowhide Leather grip
Grip Size: 4 1/2

BTW, if you are considering buying a copper ace remember that the following variants are available:
1. Original Copper Ace, with copper colored graphics on brown racquet
2. Revised Copper Ace, with copper colored graphics on steel-gray/blue racquet
3. Turquoise blue colored racquet with 100 percent graphite
4. PK Copper 110 sq. in racquet

#1 and #2 above are identical in most respects, at least from the pictures that I have been able to examine. My racquet (#2) has a grommet strip. #1 and #2 are the advanced racquets, and have a cover that simply says Kennex Copper Ace.

#3 is definitely a Walmart style racquet with a different (black/blue) cover, and funky lettering that says "Copper Ace".

I haven't seen #4 around.

 

[Amone]

300Gkid, Here is what I found from the Racquet forums and from what I can read on my racquet:

Head Size: slightly less than 90 sq. in
Weight: 340 g (no lead)
Swingweight: Approx 315 kg-cm^2 (no lead)
Composition: 40% graphite, 60% fiberglass
Balance: 11 pt HL
String Pattern: 16 x 19
Length: 27 in
Stiffness: Couldn't find it exactly, but should be around 55, similar to my Redondo
Grip: Wilson Pro white overgrip on top of originial Cowhide Leather grip
Grip Size: 4 1/2

BTW, if you are considering buying a copper ace remember that the following variants are available:
1. Original Copper Ace, with copper colored graphics on brown racquet
2. Revised Copper Ace, with copper colored graphics on steel-gray/blue racquet
3. Turquoise blue colored racquet with 100 percent graphite
4. PK Copper 110 sq. in racquet

#1 and #2 above are identical in most respects, at least from the pictures that I have been able to examine. My racquet (#2) has a grommet strip. #1 and #2 are the advanced racquets, and have a cover that simply says Kennex Copper Ace.

#3 is definitely a Walmart style racquet with a different (black/blue) cover, and funky lettering that says "Copper Ace".

I haven't seen #4 around.

Must be really flexy, actually... 60% fiberglass, and fiberglass holds stiffness not a whole lot better than wood.. (read: better, but not on the level of graphite)

 

[mctennis]

I just have someone do that for me at a true full service racquet company. It's easier and a lot more scientific than just guessing. IMHO.

 

[NoBadMojo]

this formula is actually worse than if someone who is sensitive to swingweight would guess at the swingweight by swinging the frame along with a frame of known swingweight. it doesnt consider how weight is concentrated along the length axis of the frame. it may happen to work as a rough estimate for frames of a similar weight concentration by coincidence..ie, so called 'players frames'

 

[sypl]

How does the babolat machine work anyway? Does it just grip the handle at 10cm and then see how long it takes to swing a certain distance?

 

[psp2]

How does the babolat machine work anyway? Does it just grip the handle at 10cm and then see how long it takes to swing a certain distance?

Basically, you're correct. You insert the racquet's handle into the clamping device. The axis of rotation is fixed. You push the correct button for swingweight measurement and start the swing manually (like a pendulum). Once finished, the machine spits out a #.