# Swing speed vs Head weight

Discussion in 'General Forum' started by Cycril, Jun 14, 2015.

1. ### Cycril Regular Member

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What about applying equal force at the handle of the racquet. It will definitely make a change right? By this, which force of impact will be harder if given with the examples above?

#21
2. ### craigandy Regular Member

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Maybe he only swings at 40 mph no matter what racket he has got, I don't know.

Anyhoo I have got a little table of Swing weights from real rackets (the zf(a) has had weight added to handle for this) I used this http://twu.tennis-warehouse.com/learning_center/swingweight_calc.php Take it for what it's worth.

A higher swingweight has more power at the same swing speed

@Cycril as you can see from the table "more head weight = more power." is not true, if you were to swing all rackets at the same speed. You can see that the heaviest head @ 44.8g actually had the lowest swing weight.

I included the two rackets at the bottom of the table which shows that a relatively headlight but heavy racket can have a higher swing weight than a light racket which is head heavy. (hence more "power" from the same swing speed all else equal) Just to show that BP isn't the be all either.

Just to add I am aware that it is all pretty worthless with regards to actually playing badminton, before there is any backlash here

#22
Last edited: Jun 18, 2015
3. ### amleto Regular Member

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somethign has gone wrong with the calculations - the vtzf(a) and vt80 have near identical swing times, therefore the swing weight should also be very similar.

or at least the swing weight has not been moved to the correct axis.

#23
Last edited: Jun 18, 2015
4. ### craigandy Regular Member

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Yeah you could be right, I have left the link and all the figures are what I got. If you type in a higher or lower time it does not effect it "that" much so not sure how it is calculated or if it is correct. I certainly may have been a couple tenths of a second off as I just doing it by eye quickly with stopwatch but nothing that would greatly change the figures. It must take into account the wildly different BP's

#24
5. ### craigandy Regular Member

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What do you mean?

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6. ### visor Regular Member

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That method of measuring MOI depends on very accurate timing measurements. And imho hanging it by the stringbed may work with tennis rackets but doesn't work well for badminton rackets, which should be hung by the handle.

#26
7. ### craigandy Regular Member

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I am sure you are right but how so? If you change the time by even a whole second lower or higher for that vt80 and the swing weight only goes from 32 to 42 respectively which isn't that much.

Also how do you hang it from the handle? I could try it.

#27
Last edited: Jun 18, 2015
8. ### amleto Regular Member

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as far as I can make out, this calculator is measuring the moment of inertia about an axis that goes through the hang points. But what is needed is the moment of inertia about an axis that goes through the handle end somewhere.

#28
9. ### amleto Regular Member

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here's what the calculator is doing:

Code:
```function swingcalc() {
var wtunits=document.swingform.wtunits.value
var balunits=document.swingform.balunits.value
var hangunits=document.swingform.hangunits.value
var weight=document.swingform.weight.value;
var balance=document.swingform.balance.value;
var hang=document.swingform.hang.value;
var time=document.swingform.time.value/10;
if(wtunits=="oz"){
weight=weight*28.35;
}
if(balunits=="in"){
balance=balance*2.54;
}
if(hangunits=="in"){
hang=hang*2.54;
}
var hang_bp=hang-balance;
var term1=hang_bp*(Math.pow(time,2)/40.28);
var term2=(Math.pow(hang_bp,2))/1000;
var term3=(Math.pow((balance-10),2))/1000;
var swt=Math.round(weight*(term1-term2+term3));
document.getElementById("swingweight").innerHTML=swt;
return false;

}
```
the random '1000' would seem to be a g -> kg conversion (or maybe cm -> metre... shrug).
I haven't figured out what 40.28 is, although it is pretty close to 4 * Pi * Pi. It looks like the formula being used is a modification of

I = T^2 mgR / (4 pi ^2)

https://en.wikipedia.org/wiki/Moment_of_inertia#Simple_pendulum

In short I don't understand the calculation

#29
Last edited: Jun 18, 2015
10. ### amleto Regular Member

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For anyone that cares, pretty sure I've figured it out. I'll post if anyone is bothered

#30
11. ### visor Regular Member

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Sure, I'm interested... ☺

#31
12. ### amleto Regular Member

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So you set up your racket to swing on the strings and take some measurements. Using

I = T^2 mgd / (4 pi ^2)

You can calculate the m.o.i around the axis it's been pivoting on. This isn't what we want though - we want the moi about an axis through the handle somewhere. Enter the parallel axis theorem. With some basic algebra we can rejig the formula to obtain

I_r = I_m - md^2 + mr^2

where:
I_r is the m.o.i that we're interested in.
I_m is the measured m.o.i from the little experiment
d = distance between hanging point and bp
r = distance between bp and arbitrary 'desired axis point in the handle'.

The factor of 1000 is to convert the mass into kg to give 'nice' numbers.

#32
13. ### visor Regular Member

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Makes sense.

Which reminds me, there's a Swedish app called *SwingTool* for both Android and iOS that already takes this into account, ie MOI thru the handle. I've posted about this before in another thread a year ago.

Basically you plug in your four inputs of total wt, balance point, handle(swing) point, and hanging point. Then the app uses the smartphone's video camera to precisely count the time for X number of oscillations, and it spits out the swing weight.

But I still think it's somehow better to hang by the handle for badminton rackets...

[video]https://youtu.be/7llkEd_QAMs[/video]

#33
Last edited: Jun 19, 2015
14. ### amleto Regular Member

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Why? 1) it's not practical and 2) There will likely be more friction however you do it.

#34
15. ### craigandy Regular Member

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I see, thanks.

#35
16. ### craigandy Regular Member

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so what units do we use? for mass and distance in this one is it cm and kg?

#36
17. ### visor Regular Member

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Seems like it. But if you get too many decimal points, then it's just a matter of a factor of 1000 to correct.

For that app, the input is cm and g.

Have a look at the video, it eliminates user error by precisely measuring the time of oscillations for you.

#37
18. ### Ihasdapie Regular Member

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Ignoring the math, in-game it depends on the person. Some people generate more power with head-heavy, some with head-light. The differences in technique are the variables. Some people are inbetween; head-heavy light (<80gm) or head-light heavy ( >90gm).

#38
19. ### amleto Regular Member

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whatever units you want. Just be careful to be consistent. Eg if using cm then 'g' for gravity is not 9.81...

#39