Originally Posted by jmnk
@stoneage - yes, this is excellent explanation, and more importantly based on physics and not 'feel'. I never had any doubts you understand the nuances. I get all of that.
But what I do not understand is whether in MgR/I equation one shall use:
a) Ix (which is what is being published as 'swingweight')? or
b) Ix + Iz (which would take into account twistweight as well)?
Because in all previous threads and posts the only value ever mentioned or discussed was Ix.
Now, when JohnB points out that two rackets with identical mass, balance, and swingweight (Ix) - which obviously means that MgR/I is also identical - do feel differently, travlerajm says that this is expected since the twistweight (Iy) of those two rackets is different.
well, which is it than? Either 'I' in MgR/I is just Ix and you at least can claim you are using published specs to back up your claims. Or 'I' in MgR/I is 'Ix+Iy' and than the entire line of thinking backing up this MgR/I idea needs to be redone since:
- no one measures Ix+Iy unless specifically told to do so, and
- therefore people were tuning rackets to MgR/I=21.0 using incorrect value of I, and
- there's no way to compare that to pro's racket specs since they do not publish Iy of pros rackets.
To elaborate a little more on my previous answer and my view on this:
The twist weight is Iy, i.e. around the y-axis. The swing occurs mainly around the x and z axis, and is "controlled" by a combination of Ix and Iz, but since they are fairly similar the error by using only Ix, i.e. the swingweight, is small. So my first conclusion is: you can continue to use swingweight as before.
The twistweight will affect the feel of the racquet, but it doesn't influence the swing that much and can therefore be treated as a separate problem. Still I think twistweight is a neglected parameter that is worth keeping an eye on. Some more about twist weight here:
I am not going to comment on what to use in MgR/I since I don't know (I have yet to see a reasonable explanation how MgR/I influence a tennis swing).
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