View Single Post
Old 09-05-2012, 03:00 AM   #172
stoneage's Avatar
Join Date: Apr 2010
Location: Stockholm, Sweden
Posts: 243

Originally Posted by travlerajm View Post
The MgR/I formula works for swingweights in the 300 range to 400 range.
Do you mean that you should fix the swingweight or not? If you aim for a certain swingweight when doing the calculation my argument still holds, it doesn't matter if the target is 320 or 380. If you are saying the swingweight could become anything between 300 and 400, then you are saying something else than in you last post.

The physics of a pendulum are relevant to a tennis stroke even in the absence of gravity, because the centrifugal force is always present.
The physics, Yes. The specific solution, No. They are both controlled by forces, weight, moment of inertia etc. MgR/I is a solution to one situation/configuration, but it is not the solution to other problems, and definitely not the double pendulum.

I have found that a groundstroke can be reduced to the swing of a double pendulum, with the frequency of the upper pendulum (the arm and torso) almost independent of the racquet specs.
You, or Rod Cross? Because that is what he has been saying in a couple of papers. Except that he is talking about swing speed, which is something different.

Adding a kg to the butt of racquet would obviously affect the speed of the upper (arm+torso) pendulum, thereby reducing the optimum MgR/I value for the lower (racquet) pendulum. Thus the formula does not "break down" as you suggest - rather, it is the assumption that the upper pendulum is unaffected by changes to the racquet mass distribution that breaks down. The principles of the formula still hold.
If adding a weight changes behavior of the system it does. You can't say that it only affects one part and the other can be treated as if nothing happened. Apart from that MgR/I doesn't describe the lower pendulum.

An easy way to see see the significance of the MgR/I formula is to start the racquet high overhead with your forehand grip, and allow the racquet to sweep through a high-to-low-to-high stroke in a mostly vertical plane (much like a golf stroke). If you keep the wrist relaxed through the stroke, the racquet will exert a moment on your hand as it passes through the hitting zone. If MgR/I is > 21.0, the moment from the racquet handle will pull forward on your index finger. If MgR/I is < 21.0, the moment from the handle will push back on base knuckle of your index finger. If MgR/I is tuned properly, there will be virtually zero moment.

Zero moment acting on your wrist joint is highly desirable, because it means you do not need to apply any forces from the wrist during your swing to keep the racquetface aimed toward the target. This increases your ability to control the ball significantly.
I would love to see a proof of this, or at least an indication how this has been calculated.

Just a note here. I am not out on a crusade against you concept, I was intriuged when I first saw it and really tried to understand relevance of it. But so far I haven't seen anything that supports it. You have obviously studied a lot of racquets and found that certain values for swingweight and balance gives great racquets. That is very interesting and commendable. I just think you confuse those findings by combining them into a new parameter with doubtful mechanical relevance.

stoneage is offline   Reply With Quote