Wilson's Stiffness Index vs RA - the greatest debate on TT at this moment

ryushen21

Legend
You can all thank @JGads for this thread existing. However, we've come to a point where this must be addressed.

Wilson seems rather insistent upon utilizing their own developed measurement called the Stiffness Index. It's supposedly how some of our favorite racquet lines were named (6.0, 6.1) etc. depending on which version of Wilson's own literature about this that you read, a frame with a higher Stiffness Index will be more flexible than one with a lower one.

On the other hand, RA is stiffness measurement that we get from most retailers and is measured at one point on the frame using an RDC machine. It does not account for hoop flex since it is not measured there.

So, how does one reconcile this difference between the stiffness measurements? We know that different iterations of Wilson 6.1 frames have had drastically different RA measurements despite never being relabeled by Wilson. This also makes establishing an approximate RA for the different Wilson SI rating quite difficult.

What are your thoughts TT experts?
 

tarkowski

Professional
I think trying to reconcile the two would be a difficult thing to do.

Within Wilson's offerings, I think the SI can be used as a broad brushstroke guide for communicating how a frame will feel and perform. I've been fortunate to have played nearly all of the iterations of the 6.1 95, in both patterns. Each rendition, though slightly different in RA, never had me thinking they were different enough to warrant new marketing or communication from Wilson. From an SI perspective, I'd rank them all the same. To go further, when playing the 'softest' nCode version, I would never confuse it with a classic Prestige MP (I think it was the Microgel that was out when the nCode was out?). It still felt very much like a 6.1 to me. The KFactor ('the stiffest') was brasher, a little more harsh at some points of contact, but, IMO, not qualitatively different than the nCode in its performance.

I believe that trying to use either the RA or Wilson's SI as anything more than broad brush strokes to get to the 'general area or class' of racket will prove disappointing at best and misleading at worst. One number just can't communicate everything going on when experiencing a frame in-play.
 

ryushen21

Legend
I think trying to reconcile the two would be a difficult thing to do.

Within Wilson's offerings, I think the SI can be used as a broad brushstroke guide for communicating how a frame will feel and perform. I've been fortunate to have played nearly all of the iterations of the 6.1 95, in both patterns. Each rendition, though slightly different in RA, never had me thinking they were different enough to warrant new marketing or communication from Wilson. From an SI perspective, I'd rank them all the same. To go further, when playing the 'softest' nCode version, I would never confuse it with a classic Prestige MP (I think it was the Microgel that was out when the nCode was out?). It still felt very much like a 6.1 to me. The KFactor ('the stiffest') was brasher, a little more harsh at some points of contact, but, IMO, not qualitatively different than the nCode in its performance.

I believe that trying to use either the RA or Wilson's SI as anything more than broad brush strokes to get to the 'general area or class' of racket will prove disappointing at best and misleading at worst. One number just can't communicate everything going on when experiencing a frame in-play.
What's interesting here is that another poster mentioned that Wilson also utilize SI to stand for swing index. And in that sense, I would absolutely agree with you. I've hit with several different iterations of the 6.1 95 (specifically the 18x20 pattern) and they all swing incredibly similar. There's just that distinct 6.1 swing feel. But in terms of actual stiffness, there are some clear differences. As you mentioned, there's no confusing an nCode with a Prestige, but I would say that the KFactor 6.1 was so stiff that it took substantial changes to string setup to get close to being playable for me.

This leads to the point that Wilson should reassess how it uses SI ratings. As a swing feel indicator, it's dead on. I would not question or change a thing at all about rating a 6.1 in terms of swing feel. But their stiffness index, lacks some tangible qualities that it should undoubtedly possess.
 

ryushen21

Legend
I believe that trying to use either the RA or Wilson's SI as anything more than broad brush strokes to get to the 'general area or class' of racket will prove disappointing at best and misleading at worst. One number just can't communicate everything going on when experiencing a frame in-play.
This leads to another question: why isn't there a more holistic way to assess stiffness across the entire racquet?

If you started with a racquet strung at mid tension and assessed from there, surely there is a mechanism to measure the stiffness in the throat and the hoop to provide a more accurate and overall impression of the frame's feel.
 

travlerajm

G.O.A.T.
Back in the 80s, all of Wilson’s racquets were spec’d with a Stiffness Index (SI). The SI gave the downward deflection of the tip in mm when a mass of 2kg? was suspended from the tip while the handle was clamped and the frame cantilevered horizontally.

At the stiffest end of the spectrum was the Wilson Profile 2.7, which only deflected 2.7mm (its RA was 84).

The classic 6.1 racquet deflected 6.1mm.

A decade later, around the turn of the century, Wilson’s marketing department decided to change the definition of SI. They re-defined it as a Swing Index. From then on, it ceased to have any publicly transparent physical meaning. The Swing Index was supposed to denote how fast a swing the racquet is designed for. A high Swing Index racquet was for “advanced players” with long swings. A low Swing Index racquet was marketed to beginners or for players with short bunty swings.
 

haqq777

Legend
You can all thank @JGads for this thread existing. However, we've come to a point where this must be addressed.

Wilson seems rather insistent upon utilizing their own developed measurement called the Stiffness Index. It's supposedly how some of our favorite racquet lines were named (6.0, 6.1) etc. depending on which version of Wilson's own literature about this that you read, a frame with a higher Stiffness Index will be more flexible than one with a lower one.

On the other hand, RA is stiffness measurement that we get from most retailers and is measured at one point on the frame using an RDC machine. It does not account for hoop flex since it is not measured there.

So, how does one reconcile this difference between the stiffness measurements? We know that different iterations of Wilson 6.1 frames have had drastically different RA measurements despite never being relabeled by Wilson. This also makes establishing an approximate RA for the different Wilson SI rating quite difficult.

What are your thoughts TT experts?
Well, in my humble opinion it is prety simple, in theory atleast. Gather all the racquet RA and SI numbers in a spreadsheet. Plot a scatter plot. Check to see for a linear correlation between the two.

I can look for racquet RA, no problem. Very easy to find those. The issue probably will be digging out all the SI values. Did a quick Google and it requires some more time consuming research it seems. Lots of Wilson racquets that even Wilson hasn't advertised SI of.
 

Yamin

Professional
I thought they're measured at different points? This would make them directly incomparable.

I don't like that Wilson is using SI now supposedly. I feel like it's a way to hide their QC issues especially when neither number is published. While RA doesn't necessarily define softness it's the standard measurement at the moment.

Wilson uses RA for their "tolerances" so the statement that it's inaccurate so they don't use it is kind of BS. Seems like another way to justify not publishing RA and further confuse people so they can't complain.
 

jmacdaununder2

Hall of Fame
Then there's Head with their own 'S.I.'; S12 to S1 and L1 to L7 or whatever - i.e. Swing Index with 'S' for short and 'L' for long swings.
 

Zoolander

Hall of Fame
This leads to another question: why isn't there a more holistic way to assess stiffness across the entire racquet?
I remember reading this post by the prince tennis official, if this is sought of what you mean.... they measure stiffnes in 4 zones as well as torsional stiffness.

This is certainly and interesting conversation. RA is often discussed as a measure of stiffness and one racquet is compared to another in terms of its RA but I can design two racquets with the same frame shape, string pattern, weight, balance, swingweight and RA, that can play quite differently. RA is just a measurement of the overall racquet deflection under load, with the less overall deflection at the tip producing a higher RA. However much like we all accept that there can be two racquets of 300g with massively different mass distribution (balance, swingweight, PMOI etc.) we can also have very different stiffness profiles for the same RA. In fact we measure the localized deformation under load at various different stiffness zones along the length of the racquet, for example the Phantom 100P and Phantom 100X 305 have the same RA, however the stiffness profiles are different:

Phantom 100P
ZoneDeflection
I70
II63
III63
IV49
TORSION148

Phantom 100X
ZoneDeflection
I82
II69
III52
IV61
TORSION183

Note here that these numbers are deflection, so the higher the number the more flexible it is in that zone (opposite to RA). Zone I is close to the tip, zone II is middle of hoop, Zone III is lower hoop and top of shaft, zone IV is lower shaft and top of handle. So in this example the Phantom 100X is actually stiffer in the lower hoop and shaft than the Phantom 100P but softer higher in the hoop, however they still have the same overall RA. Then we still have torsional stiffness values and in plane "twitch" stiffness which measure the stiffness of the hoop (imagine squeezing the frame between 3 and 9 or between 12 and 6 as examples, although we measure these in plane stiffness profiles across more zones also). These in plane stiffness also can make a dramatic difference to the response and feel of the frame.

All of this can be controlled through layup design, material choices, fiber angles etc. and we do tailor different racquets to have different stiffness profiles by design based on what we are trying to achieve in terms of racquet performance and feel.

I hope this helps, but I'm happy to elaborate on this if needed.

Tim
 

HitMoreBHs

Semi-Pro
Then there's Head with their own 'S.I.'; S12 to S1 and L1 to L7 or whatever - i.e. Swing Index with 'S' for short and 'L' for long swings.
HEAD stopped using their “Swing Index” ages ago, then came out with “Control Power Index”. They never bothered to state any equivalence between both indices nor indicate whether they were directly comparable.

Never put much stock in either, but I suppose it’s a reasonable attempt at putting their range on some control-power scale.

 

Hansen

Semi-Pro
fun fact:
head doesn´t have a single 16 mains frame in the control categorys 100-300 with a ra below 65.
 
Top