Mass moment of inertia calculation?

[glovemedic]

I am looking at the Crisco study (Med Sci Sports Excer 34:1675-1684) comparing wood and metal bats and trying to repeat their calculation for the "moment of inertia" without success. Please help me as I am not able to derive their numbers using the calculation MOI = mass (bat weight) * distance (bat length) [squared] .

Where did I fall off the truck?

Thanks!

[Jake Patterson]

I am looking at the Crisco study (Med Sci Sports Excer 34:1675-1684) comparing wood and metal bats and trying to repeat their calculation for the "moment of inertia" without success. Please help me as I am not able to derive their numbers using the calculation MOI = mass (bat weight) * distance (bat length) [squared] .

Where did I fall off the truck?

Thanks!

Chuck can you give us a link of the study?
Jake

[glovemedic]

Chuck can you give us a link of the study?
Jake

Thanks for the prompt reply. Follow this link, their data are presented in the table on this page.

http://www.kettering.edu/~drussell/b.../alumwood.html

[Breeves85]

I agree, that doesn't make sense about the formulation. I must be doing something wrong. Question though. That article was the MOI analysis of the aluminum bat, right? Am I wrong for thinking that the same principle of low MOI(lower MOI hollow aluminum bat doesn't help power through the impact area like a high MOI aluminum bat)doesn't apply as much to a solid wood bat? If it does, then why have the enlarged knob like Bonds? For example, can a 34/32 C243 actually have a higher batted ball speed than a 34/32 M110 or 73 swung at the same speed simply because it has a higher MOI? Since it seems that mass is the key component, why not just get a balanced bat with more weight? The added weight is going to be in the barrel despite the balance being the same. Could that help? I know you can't really modify the MOI of a wooden bat much, still something just doesn't seem to add up. I think when a pro chooses a wood bat model, they choose it based on how it feels rather than how easy they swing it at high speeds. Soriano swings a C243 but a bigger stronger A-rod swings a modified C271, Bonds swings a 73 and Mantle even swung a M110, the last 3 are balanced while the C243 is end-loaded. So why would they make their selection towards balance if the higher MOI is better? I'm so confused. Can anyone clarify?

Glovemedic, this is your fault. You're making me think too much.

[glovemedic]

I agree, that doesn't make sense about the formulation. I must be doing something wrong. Question though. That article was the MOI analysis of the aluminum bat, right? Am I wrong for thinking that the same principle of low MOI(lower MOI hollow aluminum bat doesn't help power through the impact area like a high MOI aluminum bat)doesn't apply as much to a solid wood bat? If it does, then why have the enlarged knob like Bonds? .... So why would they make their selection towards balance if the higher MOI is better? I'm so confused. Can anyone clarify?

I think the numbers in the table are wrong, if the formula for calculation is correct. I have googled MOI and found multiple sources that point to the same formula. From my reading of the articles, it appears that low MOI, or balance point closer to the knob, is desirable for its effect on swing speed. The big knob seems to help move the MOI/BP closer to the knob and allows for more mass in the barrel. There is a tradeoff between bat mass and balance point. You have to have enough mass to build momentum for crushing the ball, but at the same time have the MOI distributed closer to the handle to enhance acceleration. The big question then is why does Barry Bonds have a monster bat while A-Rod uses a more balanced bat and they both tons of hit home runs. The short answer is because the feel is better suited to their swing characteristics, and of course the obvious point is Barry is bigger than A-Rod, eventhough by our standards they are both big boys.

I am certainly not clear on all of this, but that is what seems intuitive to me so far. I am still looking for someone with more of a physics background than I to help me with the calculation, if indeed I am wrong.

Thanks for the help so far.