# Important Spec: Spin Index /Arm Safety Index

Discussion in 'Racquets' started by travlerajm, May 8, 2007.

1. ### travlerajmLegend

Joined:
Mar 14, 2006
Messages:
5,027
Location:
Seattle
Here is a useful formula to calculate how spin-friendly a frame is based on its weight distribution and stiffness. The index doubles as an arm-safety index, and it is much more accurate than the index used in racquetresearch.com (but derived similarly).

Spin Index = 10000*[ I - M(R-10)^2 + M(R-4)^2]/[3kM(R-4)^2]

I = swingweight about 10-cm axis in kg-cm^2.
M = mass in kg.
R = distance from butt to balance in cm.
k = RDC stiffness rating

Here are some guidelines for what these spin index values mean:

85-90: excellent spin - excellent arm safety.
80-85: decent spin - good arm safety.
75-80: so-so spin - so-so arm safety.
70-75: poor spin - tough on the elbow.

These values give a measure of the relative "bite" on the ball for a given swingpath, as well as the relative dwell time.
In general, any frame can be made more spin-friendly and more elbow-safe without sacrificing performance by polarizing the weight distribution (adding weight to both the tip and the butt).

Last edited: May 8, 2007
2. ### El DiabloGuest

Joined:
Feb 19, 2004
Messages:
3,979
What reason is there to think any of this has the least bit of validity? How was it derived? And aren't you the guy who told us how busy he's been writing books and then noted that he hadn't actually written a book?

3. ### jmverdugoHall of Fame

Joined:
Nov 22, 2006
Messages:
2,968
Location:
Houston, TX
Very Interesting .. acording to this de RDS001MP's index is 80,65 ... so basicaly more SW and less RDC more spin/arm friendly

4. ### sureshsBionic Poster

Joined:
Oct 1, 2005
Messages:
45,109
A table showing the numbers for representative racquets would have been useful.

5. ### haerdalisHall of Fame

Joined:
Sep 10, 2006
Messages:
1,597
Location:
Lund, Sweden
The good old POG OS (original specs) gets a value of 93.4 with this formula.

6. ### Bottle RocketHall of Fame

Joined:
Jan 12, 2006
Messages:
2,681
Location:
San Francisco, CA
Where does that formula come from?

Got a link to the page where the similar one is derived?

7. ### sureshsBionic Poster

Joined:
Oct 1, 2005
Messages:
45,109
In particular, I would like to know the numbers for: PS 85, K90, Ki5, PD, Volkl V1.

Ki5 and V1 are known for their comfort, PD for the opposite, and of course everyone would like to know the numbers for Sampras and Federer.

8. ### bumfluffSemi-Pro

Joined:
May 1, 2007
Messages:
520
Location:
South England/Scotland
Even my little knowledge of rackets tells me that there must be other factors aswell, such as string pattern, materials of the racket and even its shape.

9. ### sureshsBionic Poster

Joined:
Oct 1, 2005
Messages:
45,109
Material can be approximated by stiffness. That is why I was interested in Ki5 - it has granules in the head, but it has low stiffness too, if I remember right.

Shape - maybe the isometric shape of Yonex can make a difference, but by how much?

10. ### AmoneHall of Fame

Joined:
Dec 29, 2005
Messages:
1,864
Location:
Detroit Area, MI
String pattern, as per my knowledge, has little to do with spin potential. I got about the same spin on my topspin serve (the clearest example, to me, of spin that rarely changed much in my game) from the 18*20 DNX10 MP, and the 16*19 APC+.

Materials are not much of a factor by themselves; they effect measurable traits, there is no magic to it.

The only thing I might agree that shape effects is in headsize and (head)length. Generally, both result in more of a 'pocketing' effect, and more 'trampolining.'

---

That said, I don't know if this formula's good or not. I'd have to spend some time with it to say anything definitive.

11. ### vkartikvHall of Fame

Joined:
May 4, 2005
Messages:
3,277
Location:
The Gym
Can't just use some equation without knowing the basis of formulation.

12. ### dacrymnProfessional

Joined:
Sep 3, 2006
Messages:
887
I'd assume the strings were the same? This seems counterintuitive. Everything has something to compromise. Why would you get the same spin with different string patterns? I don't know. I should try it though.
However, I do agree with head width. This generally helps spin, but only because i believe it to have more forgiveness when swinging up sharply.
________
volcano vaporizers

Last edited: Feb 3, 2011
13. ### haerdalisHall of Fame

Joined:
Sep 10, 2006
Messages:
1,597
Location:
Lund, Sweden
Spin friendlieness is a weird thing to work out. I got alot of spin from the POG OS no matter what tension and string I used. Not so with the NXG OS which is fairly similar. I do believe string pattern makes a difference. It seems only reasonable to assume so IMO.
The spinniest racquet I own is a modified PK ki 15 and it has a stiff wide frame which many say isnt good for spin. The same goes for the PD.

14. ### AmoneHall of Fame

Joined:
Dec 29, 2005
Messages:
1,864
Location:
Detroit Area, MI
Nope, actually the APC had softer strings... -_-;;

The basic Gamma equivalent of ALU, for the DNX

Prince Tourna Nylon for the APC+

15. ### dacrymnProfessional

Joined:
Sep 3, 2006
Messages:
887
Haha. That's interesting.
________
Edsel II

Last edited: Feb 3, 2011
16. ### tennis4losersSemi-Pro

Joined:
Jul 23, 2005
Messages:
661
I remember you saying you customized the prince o3 tour MID size. I have that in the 4 1/2 grip size. Where and how much lead tape will i have to place on this racket to reach maximum playability?

17. ### AlafterHall of Fame

Joined:
Oct 4, 2005
Messages:
2,447
What? But Trav said that NXG OS is the spinniest racquet ever! See where this is leading to?

18. ### dyloSemi-Pro

Joined:
Jun 22, 2006
Messages:
587
interesting formula, but i agree that string pattern will have an effect

19. ### haerdalisHall of Fame

Joined:
Sep 10, 2006
Messages:
1,597
Location:
Lund, Sweden
It is leading in the general direction of different strokes for different folks.
I had good spin in the NXG OS with kevlar mains and synt gut crosses but not with an all synt gut string job. With the POG OS the spin was great no matter what strings I had.

20. ### AlafterHall of Fame

Joined:
Oct 4, 2005
Messages:
2,447
so haer, spin friendly is not about formula is it then!

21. ### haerdalisHall of Fame

Joined:
Sep 10, 2006
Messages:
1,597
Location:
Lund, Sweden
Not if you ask me no.

22. ### jmverdugoHall of Fame

Joined:
Nov 22, 2006
Messages:
2,968
Location:
Houston, TX
I think this formula comes from a Deflection Moment analysis (I do not know if that is the correct english term in spanish is "analisis de momento flector". And eventhough (I think) some asumptions were made to get to the final result, i actually think this index is a pretty good indicator of the spin capabilities of the racket, not the shots a person can make with it. It has to do with how much energy can the racket absorbe and then release and how quick it is done.

To put the strings & strings pattern in the equation is another thing.

23. ### AmoneHall of Fame

Joined:
Dec 29, 2005
Messages:
1,864
Location:
Detroit Area, MI
So, would you say that it is a safe assertion that: "For a given frame, the index is a good measure of wether or not a change [only in weight] will make it more or less spin-friendly?"

24. ### sureshsBionic Poster

Joined:
Oct 1, 2005
Messages:
45,109
How can that be true when it is very well-known that open string patterns are spin friendly. See several TW reviews. I know that the TFight 315 did produce less spin.

25. ### sureshsBionic Poster

Joined:
Oct 1, 2005
Messages:
45,109
Someone PLEASE do the numbers for a few racquets. I am too lazy.

26. ### AmoneHall of Fame

Joined:
Dec 29, 2005
Messages:
1,864
Location:
Detroit Area, MI
I provided my experience on the matter back in an older post. I'm led to believe you read it.

You can choose not to believe me, but I know wether or not it's true. The fact is that reviews are biased by almost everyone -- those who are not biased, are also rarely highly visible. Basically, even the greatest of experts can be swayed by pre-conditioned ideas. For more information, see Malcolm Gladwell's Blink.

27. ### jmverdugoHall of Fame

Joined:
Nov 22, 2006
Messages:
2,968
Location:
Houston, TX
Using this formula I dont see a change on the weight will have a big effect on the index. You should do the numbers, I am just saying this because the weight is in both parts of the fraction.

28. ### TennisandMusicGuest

So the more spin that is produced the safer the racquet is to play with? How is it that the two attributes are intertwined in such a way? Where is this derived from?

29. ### AmoneHall of Fame

Joined:
Dec 29, 2005
Messages:
1,864
Location:
Detroit Area, MI
Weight can (and almost unavoidably will) change both the balance point, as well as the swingweight, both of which are also in the formula; Swingweight is the only one unique to one part, though.

I believe that assertion comes from the belief that a more stable, more flexible frame is better for the elbow. As I understood it, the formula Travler was seeking seems to be more closesly related to the spin, and the fact that it happened to match that relationship was a coincidence.

30. ### TennisandMusicGuest

There is still no basis for these numbers. Was this made up by traveler or found somewhere?

31. ### AmoneHall of Fame

Joined:
Dec 29, 2005
Messages:
1,864
Location:
Detroit Area, MI
*shrug* I would like an answer too, but for my own purposes.

32. ### jmverdugoHall of Fame

Joined:
Nov 22, 2006
Messages:
2,968
Location:
Houston, TX
I think you are right, I thought about it too but since you can find really heavy rackets with low sw and also you can find rackets with the same weight and differents balances I thought that you was thinking about this to when you proposed to change just the weight.

33. ### AmoneHall of Fame

Joined:
Dec 29, 2005
Messages:
1,864
Location:
Detroit Area, MI
Suffice to say that there are ways to add lots of weight and get little-to-no change in either balance point or swingweight, but there are no ways to get no change in [both].

34. ### travlerajmLegend

Joined:
Mar 14, 2006
Messages:
5,027
Location:
Seattle
In case it is not apparent from the formula, spin index is increased by doing any of the following:

1. Increasing swingweight while holding mass and balance constant.
2. Making the frame more headlight while keeping swingweight and mass constant.
3. Decreasing mass while keeping swingweight and balance constant.
4. Decreasing stiffness while keeping the mass distribution constant.

BTW, typical modern pro specs like Safin's (365 SW, 32.3cm, 353g, 62 RA) come in at about 90 on the spin index scale.

35. ### cyberwing88New User

Joined:
May 4, 2007
Messages:
29
haer, i remember you have nfury which has spin factor 106! does it make sense to you if you compare it with POG OS?

36. ### haerdalisHall of Fame

Joined:
Sep 10, 2006
Messages:
1,597
Location:
Lund, Sweden
No not really. My modified nFury gets a value of 123 with this formula. I would say it is true that the modified one lets me hit with more spin and is better for my arm then the unmodified one. Probably as arm safe as you can get really.
Spin production however comes easier with the POG OS or mid.
Personally the 2 spinniest stock racquets I have tried are the PK ki 15 pse and Yonex pro rd-70 long. The Pk ki15 pse gets a value of 73 using specs from tennis.com.

37. ### cyberwing88New User

Joined:
May 4, 2007
Messages:
29
ya, the formula should take string pattern and racket length into account. Yonex RDX500MP has a very low score 80 too. The formula is too dependant on the flex.

38. ### Big BorisRookie

Joined:
Nov 2, 2006
Messages:
151
Really nice piece of work

Really nice piece of work again, Travlerajm!

I think there is a huge difference in the spin you get from a open and a dence pattern. When playing with 16/18 Wilson PSC 6.1 family frames I feel the ball kicking off the stringbed with spin (or at least I experience this from the result of the strokes) and I can sort of direct the ball to go down even though sending it of with a high outgoing trajectory.

But with the 18/20 family I don't feel/get that kick/spin and have to send the ball closer to the net to get it to land where i want, or hit radiquously much down and up. Further the 18/20s are harsher on my arm I.

So it would be very interesting to have some kind of stringbed pattern and tension approximations/adjustments aside of, or built into, the spin/arm friendlyness equation.

So, Travler, if you have the time, interest and compassion....