Posted by: Dan (daniel.miechels@gmail.com) on Thu Jun 17 14:51:51 2010

> Dan -
>
> Cross's paper explains how what is observed in the BB swing is different from a passive/grvity driven double pendulum. Here is a model you can play around with. If you turn on the yellow tracer to see the path of the distal tip of the second pendulum, you can see how the path shortens/quickens when the pendulum had a passive running start which is similar to intentional THT at launch and prealaunch in the swing Jack describes.
>
> http://www.maths.tcd.ie/~plynch/SwingingSpring/doublependulum.html
>
> Another rule of the double pendulum (also knwn as rule of the flail) is that maximum tip speed will be when the two pendula line up. In the passive model, the second/distal pendulum just keeps on turning, but in the BB swing, the bat and lead forearm stay lined up.
>
> These are the 2 things Cross emphasizes:
>
> 1- early postive couple/THT
>
> I would go further to say that this is not only a feature of the BB swing, but is the most important control mechanism of the swing/plane matching/adjuctment,etc
>
> 2- late "negative couple" which is perhaps best described in the TGM (the golf maching/Homer Kelley) as "extensor action".

Hi Tom,

I agree.

I have always believed the basic double pendulum was a poor model of the full baseball swing, since it is only passive and the torques injecting energy into the system are caused by gravity and are a function of pendulum angle quite different than a baseball swing. In my post I was just stating that the same priciple causes most of the bat speed. What the batter does at the start when the bat's momentum is low enough to control seems to be the most important and least scientifically studied aspect.

Studies like Cross's are valuable in that they show the real forces that must be acting on a real baseball bat. Unfortunately it is very unlikely that the batter studied by Cross was a good example of the sort of swing we want to study. (For those who haven't read the paper, it was done at an Australian University in Sydney, I went to university in Sydney and know the level of baseball. All of our good players go to the US, there is no money in baseball in Australia - yet).

You stated:
"In the passive model, the second/distal pendulum just keeps on turning, but in the BB swing, the bat and lead forearm stay lined up."

I would say this happens because after impact:
a. the bottom wrist can't let the bat move further until it rolls (after which the bat can continue to turn)
b. the trailing arm straightens, absorbing the bats energy.