Something I don't understand about 'swingweight'!

[djones]

I thought swingweight ment, how heavy the racquet actually feels when 'swinging'/hitting the ball.
But I was just comparing these next two racquets about it's swingweight:
Wilson Hyper Hammer 6.2, 278 grams, swingweight: 328.
Head I Radical MP 297 grams, swingweight: 297.
I know their is some difference between those 2 in balance, but....
I have hit a few balls with the original Wilson 6.2 (terrible racquet btw), and by far, it doesn't feel as heavy as the Head I Radical.
My Head I Radical is about 325 grams, but how could the swingweight of the Wilson 6.2 be higher than the Radical???

 

[Mies]

Well there's one thing to consider. Swingweight is basically a measure for the moment of inertia of a racket, with respect to a rotation. However, you always determine a rotational moment of inertia with respect to some fixed location. Now, if I understood correctly, while measuring the "swingweight", the axis of rotation is fixed at the handle of the racket. So "swingweight" tells you something about how difficult it is to move the racket with respect to you lower arm (effectively, the wrist is your point of rotation).

This is only part of "how heavy the racket feels" when you swing it, because there is another axis of rotation, that being your shoulder (your elbow could be considered one as well, even further complicating the picture, so let's ignore the elbow joint for now). And this is not taken into account by "swingweight"

So specifically, in your case, lemme try and explain why the radical still feels "heavier to swing":

The I.Radical (according to TW stats):
balance point : 34 cm
Weight: 306 gr (You posted 297, and I think this is why the difference in "swingfeel" is larger than you expect, but I'll explain later).

The Wilson HH 6.2
balance point: 37 cm
Weight: 278 grams

Now assuming your arm is 65 cm long and you hold the racket in a straight line from your arm (of course, this is not how you would actually hold it etc., but for ease of calculation let's assume).

The moment of inertia for the Radical would be (ignoring that of your arm, which is constant for both cases, and since the moments are additive of the racket and arm are additive, we are allowed to ignore it when comparing rackets) :
0.306 * (0.65+0.34)^2=0.299 kg*m^2

For the HH 6.2 :
0.278 * (0.65+0.37)^2 =0.289 kg*m^2

You see: the moment of inertia of the radical with respect to your shoulder is HIGHER than that of the HH 6.2. In other words, it feels heavier to swing (around your shoulder).

If we calculate the moment of inertia around the handle:

Radical: 0.306*0.34^2=0.0354 kg*m^2
HH 6.2: 0.278*0.37^2=0.0381 kg*m^2

And this is what is reflected by "swingweight": the moment of inertia of the HH 6.2 around the handle is higher.


In this case, apparently the effect moment of inertia with respect to the shoulder is more important than the moment of inertia around your wrist (which is indicated by swingweight). So the Radical is easier to "whip" around with your wrist, but more difficult to get into motion as a whole.

Which of these effects is more determining to the "swingfeel" is probably a subjective thing, depending on strength etc.... It also depends on the how big the differences in total weight and in "swingweight" are. For equally heavy rackets, of course, swingweight will be the determining factor in which one feels more heavy. For equally "swingweighted'" rackets, the overal weight (and the balance point) will be determining, and all situations in between can occur.......

I hope this rambling post gives you some insight into the whole thing. Oh btw, I am by no means claiming to have presented a complete model here ( I also ignored the elbow etc. etc.) but I think this illustrates that swingweight alone doesn't tell the whole story, and that this gave some insight into why that is so......

I hope most of you didnt quit reading halfway ..........

Hmmmmmmmmmmmm after reading my post again, my self-esteem got quite a battering..........I'm a tennisplaying nerd......quite a harsh thing to realise..........

Anyways,
Regards

 

[monologuist]

wow...very impressive explanation!...I've always wondered about this too!...are you a physicist, or did you just happen to have those formulas off the top of your head?!

 

[Steve H.]

Good explanation, Mies.

Another factor in making the Hammer feel lighter to swing from the shoulder is that more of its mass is in the head, and the forearm naturally rotates backwards (supinates) at the beginning of the stroke and catches up later as the arm pronates. So actually when you start to swing forward, the swingweight added by the head gets cancelled out and is added back later in the stroke.

This is the principle of the Hammer and other head-heavy racquets -- the handle is light and easy to get moving, and then the heavier head whips through the ball, giving you more power (and less control!)

 

[Gaines Hillix]

I know their is some difference between those 2 in balance, but....
I have hit a few balls with the original Wilson 6.2 (terrible racquet btw), and by far, it doesn't feel as heavy as the Head I Radical.
My Head I Radical is about 325 grams, but how could the swingweight of the Wilson 6.2 be higher than the Radical???

You were on the right track. The difference in swingweight is caused by the head heavy balance of the HH 6.2 . There is more weight in the head of the racquet and less in the handle as compared to the I Rad.

For another good explanation, take a look here;

www.racquetresearch.com

and click on the link to The Effect of Mass and Swingweight.

 

[backhand]

M - A little physical explanation stirred in makes the "Which racquet should I choose?" more enjoyable. Excellent!

Question: Do you or Steve H think that the typically measured swingweight is really a better indicator of wrist stress (since it's the point of rotation), while calculating arm segment swingweights better characterizes elbow or shoulder stress? This is something racquetresearch.com never broke apart, beyond showing that higher swingweights produce lower stresses overall. Might be interesting for those of us who have to factor specific joints into racquet choice...

 

[Mies]

With respect to Monolinguists question on whether I'm a physicist: no ............a chemical engineer .........So I do have a technical background.....I didn't recall the moment of Inertia formula's by heart, but I know where to look (google or the physics book behind me .......Also I started looking into this cuz up and untill a few months ago, I was hitting with a stiff, 265gr (unstrung) , 35 cm balance point and 102 sq. inch bat. I felt like I had to bash every shot with all the strength I have to get any decent pace and spin on the ball. It also felt as if I was hitting with a wooden board. No feel at all....then I started looking into why that was and whether a heavier racket would help me....Now I play with a 95 sq. inch heavy (I don't know it's weight but I put 12 grams of lead in the top and it was already heavy, i guess it's now around 350-360 grams) and I'm hitting heavier balls easier........but anyways, who cares about my personal tennis experiences.

With respect to Backhand's question: I am not sure. There's a few effects I can imagine determining whether you have more or less stress on your wrist (and other joints) with respect to swingweight and weight.

First of all: If you swing a high swingweighted racket your wrist, more or less, get's "pushed in the right position" due to the higher moment of inertia with respect to your wrist i.o.w. you bend it towards your lower arm (more or less what Steve pointed out as well). Upon ball impact, this is the position where your wrist is most stable and thus you experience less stress (at least that's my assumption). Also, a heavier swingweighted racket will "plow through the ball more", i.o.w. the shock on your arm(both wrist and elbow) will be less at ball impact because you hit the ball closer to the rackets center of gravitiy. The racket is then most resistant to being moved in any other direction that it was already moving in, resulting in less shock.

However, I can imagine starting to swing a higher swingweighted racket causes more stress (on your wrist), because it's more difficult to get it into motion. And I am not sure what causes ppl's wrist injuries: Ball impact or the onset of starting to swing a higher swingweighted racket? Perhaps someone can comment on this?

The same goes for other joints as well I suppose, except you should look more at total weight instead of swingweight, as I explained in my previous post, because this affects the shoulder (and to a lesser degree, the elbow) more. Once you get a heavy racket moving, it is more stable upon ball impact due to its higher moment of inertia and impact thus results in less stress.....However, getting the heavier racket into motion, needs more strength and I can imagine the stresses on your muscles and tendons being larger when getting the racket into motion. I am not sure which one of these two exactly causes the injuries.......

Btw, I think the elbow is a special case, since the muscles that move your wrist are attached to your elbow (and of course, your hand/wrist) by tendons and ppl often have injuries there. So this might be due to forces on the elbow joint as well as the wrist joint, at least, I can imagine they do.

Again, I am no racketscientist nor fysiotherapist and my knowlegde of the human body is rather limited (well, my knowledge of the interior that is .......... ) . These are just my musings so please correct me if I am wrong........or add something to my story.

Regards

 

[streamline]

Babolat's RDC swingweight ratings simply fix "R" - the radius measurement at 10cm from the end of the handle. Swingweight is a relative, not absolute measurement - as long as everyone uses the same radius (in Babolat's case, 10cm), the results can be compared. Theoretically, it's "how heavy" the racket feels statically in your hand. It's not a perfect measurement, but it's simple to measure, and gives you a starting point in comparing rackets without having to make more complicated measurements.

The "head heaviness" is just a measure of how far from the geometric center of the overall racket it balances on a fulcrum. If the balance point is more toward the handle, the racket is "head light". If its' more toward the head of the racket, it's "head heavy". An 11 oz racket that is head heavy will have a higher swingweight than an 11 oz racket that is head light - so swingweight DOES take into account balance point.

 

[b.]

It would be nice if it could be added some 15'' extension for measuring "elbow" SW as well. I guess it would be simple to do. Only one more number to consider, and not unimportant one.

 

[Mies]

Streamline,

I might have misunderstood you, but I think you said that I didn't take into account balance point. That is not true, look at my calculations, its present in both the "wrist" and "shoulder" calculations. All I was saying was that, when calculating moments of Inertia for the different joints, the difference in balance point between two rackets (who's weight differs significantly from each other!!!!) is less important for the shoulder joint.

Calculations show that (for this example) the difference in total weight is more important than the difference in balance point when calculating the moment of inertia with respect to the shoulder. Of course, if you have two equally weighted rackets with very different balance points, the balance point will be the determining factor for the difference in all moments. The other extreme is 2 equally balanced rackets with a totally different weight In this case, the difference in total weight will be the determining factor for all moments.

If both the weight and the balance point differ (as in this example), you will have to calculate the moments for each point of rotation to show whether balance point or total weight is most responsible for the difference in moments for both rackets.
This may vary for each point of rotation, as shown in calculations for this example.

Regards

 

[raftermania]

I think my head exploded reading mies explanation... well done