Moment of Inertia in Bats

Moment of what?!

If you’ve been around baseball or softball bats for a while, there is a chance that you may have heard someone mention the term “moment of inertia.” Most individuals hear that term, immediately get scared and decide that they already know enough about bats. However, a basic understanding of Moment of Inertia is a great asset to any ballplayer or parent! The bat experts at JustBats have put this post together so that you can become an expert in no time. Let's dive in.

What is the moment of inertia in baseball bats and softball bats?

Let's take you back to high school physics class and get down to the basics. Inertia is the measure of how difficult it is to change the velocity of an object by applying force. You can also measure Rotational inertia, which is the measure of how difficult it is to change the rotational velocity of an object that is rotating about a pivot point. Another term for rotational inertia is the Moment of Inertia (or MOI). 

One important thing to note is that when MOI is being used in reference to a baseball or softball bat, it can also be referred to as Swing Weight or Swing Weight Score. And when trying to understand swing weight or swing weight score, you are ultimately seeking to find the measure of how difficult it is to swing a bat. Confused yet? Moving on...

Moment Of Inertia... What is it NOT?

Moment of inertia is NOT the same thing as Listed Weight. The two are commonly confused. Listed weight will be the bat’s weight in ounces and is typically printed on the knob or the barrel of a bat. Two bats can have the same listed weight in ounces, but can be extremely different in regard to how difficult they are to swing. Now the question we've all been waiting for...

How do you measure MOI on bats? 

Incoming physics equation! To measure MOI, you’ll need the following numbers:

• Mass of the bat [m]
• Force of earth’s gravity [g]
• Distance between where the bat is gripped (pivot point) and balance point of the bat [d]
• The time it takes for the bat to make one pendulum swing [T] on an apparatus like the one pictured below:

*Note that this apparatus is not commonly available and makes correctly measuring MOI difficult for the standard player or parent.

With all of the above numbers, you can measure MOI by plugging them into the equation below:

The unit of measurement for Moment of Inertia is [oz*in^2]. If you were to get the required measurements (m | g | d | T) for a standard BBCOR baseball bat (31” - 34” BBCOR Bat) and put them into the above equation, the likely result would be a number ranging between 7000 [oz*in^2] and 12000 [oz*in^2].

Now how do you interpret these numbers? If you remember from earlier, we explained that MOI, Swing Weight, and Swing Weight Score are all the same thing. Let's say you had two BBCOR bats and were able to complete MOI measurements for both bats. One bat measured at 8500 [oz*in^2] and the other measured at 9500 [oz*in^2]. The bat that measured at 9500 [oz*in^2] would be the one with a higher Swing Weight Score. Thus, that bat would be the more difficult bat to swing between the two bats measured.

Let's also say you had a two-piece bat that you swung a few seasons back and you absolutely crushed balls with it! There is an extremely high probability that the reason you did so well with that bat is because the Swing Weight (i.e. MOI) was perfect for you. However, that perfect two-piece bat is no longer being made. Noooo! But don't worry, now you can scour the internet and see if you can find the Swing Weight Score for that older bat. If you can find it, perfect! You should be able to find a two-piece bat being currently made with nearly the same swing weight (as long as the swing weight scores are within 5% of each other; you should be golden). If that current two-piece bat gets solid performance reviews from trusted sources, then highly consider buying that bat. It will feel the same as your older bat and you should be crushing balls just like you did with that older bat.

We hope that some of you were able to make it through without too much confusion and are now more informed the next time that MOI, Swing Weight, or Swing Weight Score are a topic of conversation on the stands. If you ever have any questions, don’t hesitate to contact our bat experts via phone at 866-321-2287, email at experts@justbats.com, or live chat! Don't forget, we're here for you from Click To Hit!