Swing Weight and MOI Explained

The Background
Measuring the Moment of Inertia (MOI) of an object is extremely scientific and involves a lot of advanced math. It includes measuring the mass, balance point, and mass distribution in relation to the object’s pivot point. While reading through most of that material can be pretty daunting, I’m going to keep things on the lighter side.
When choosing a bat, it’s important to select the proper length (EX. 33 inches) & actual weight (EX. 30 oz) for the player’s height / weight and skill level. What’s just as important, but often ignored, is choosing a bat with the proper swing weight & MOI. Unfortunately, bat manufacturers have decided not to release actual MOI data for consumers to compare for various reasons. Easton has attempted to address this by adding their Swing Weight Index, which is a good first step, but not exactly the same. This leaves players with one option for judging swing weight: picking every bat up, giving it a little wiggle and getting a few hacks in if it’s an option. With most bat sales taking place online, this is an unrealistic task for most players. So, many players are left buying blind without any idea of how the bat will feel in their hands or play in their particular swing. We need to do better than this.
Why it Matters
The swing weight / MOI of a bat can change depending on the mass distribution. If there is more mass towards the end cap away from the pivot point (located 6 inches up from the knob of a bat for measuring purposes), this will result in a higher MOI and heavier swing weight. If there is more mass closer to the knob and pivot point, this will result in a lower MOI and lower more balanced swing weight. If needed, manufacturers will adjust MOI to their specific targets by adding what they call casting to the inside of the end cap or the knob. Casting starts out in a liquid form poured into the designated area and cures into a solid.
While bat speed is directly related to performance results, so is MOI. The MOI has a direct effect on swing speed and final distance of the impacted ball’s flight. If you have 2 bats, one with a high MOI and one with a low MOI, identical swings and all other factors remaining constant, the bat with the higher MOI will always result in more distance.
Dr. Alan Nathan of the University of Illinois Physics department and Dr. Dan Russell of Penn State’s Physics and Acoustics program are some of the leaders in baseball bat physics world. They have performed countless hours of research and have helped establish some of the bat testing criteria being used today. I have read through a good number of their papers & publications and have been extremely impressed with the work. If you’re into the science aspect of baseball & bats, their publications are a must.  Here are links to their respective sites below:

Dr. Alan Nathan
Dr. Dan Russell


Photo credit: Dr. Dan Russell from the BESR days.

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