DIY MOI (moment of inertia) Calculator for Baseball Bats

Cheat Sheet:

1- Measure Pendulum Period of Bat

2- Measure Length Between Balance Point and Knob

3- (If you have a kitchen scale, find exact weight of bat)

4- Input data points in calculator below

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Overview

Knowing the swing weight of a bats is an important piece of information when comparing, shopping and reviewing the thousands of bats available. The "swing weight" is often referred to as the MOI or moment of inertia. Although a bats true MOI is, arguably, the most important thing to consider when purchasing a bat, bat companies do NOT make that information available to the public.

The good news is, using the simple steps below you can calculate your bats MOI at home using some prety simple tools. You can use this data point to compare to bats you may be considering. Once you get the hang of it, calculating your bats MOI shouldn't take more that 5 minutes per bat.

Steps:

1- Measuring Pendulum Period
2- Measuring Length
3- Weighing the Bat

What You Need:

1- A Stop Watch (the one on your phone should work fine)

2- A Measuring Tape

3- A Bat

4- A Digital Kitchen Scale (not required, but good if you do have it).

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All the physicist/engineers/bat designers in the room need to put their practical hats on as I describe the steps. This is a duct tape DIY MOI bat calculations for the every day Joe. I have measured the MOI of bats very scientifically as well as while standing up in a sporting good store and, I can say with some confidence, this process works AT LEAST as well as we need it to. If you want an in depth look at MOI then here and here are some places to start.

Step 1: Measuring Pendulum Period.

This is the most difficult step.

We need to find out the time of your bat's "pendulum period". A "period" of the pendulum is how long it takes for the swinging of the pendulum to get from one side back to that SAME side. The picture below describes it best. A period is the cycle through the entire swing from one extreme (1) to the other (3) AND back again (5).

A full period on this simple pendulum is from image 1 to 5--where the 'bob' gets back to its original position.

We need to measure your bats "pendulum period."

This can be done very scientifically (lase timers, frictionless orings, etc.) or pretty much in the same chair you are sitting in right now.

One way to fake a pendulum swing.

First: Hold your baseball bat in a position that lets it swing like a pendulum with the knob at the origin of the swing. I do this by stabilizing my arm on my knee and pinky and ring finger on my desk. I then rest the knob of the bat on top of my index finger and thumb--which are in the shape of a C. I try and rest the bat so it has as little contact as possible with my finger and thumb.

Second: Take your stop watch in your other hand. Use that hand to push the bat to the side about 10 degrees or so. Let go of the bat and, if you are holding it as friction-less as possible with the other hand, the bat will begin to swing back and forth like, you might imagine, a pendulum! Do not let the bat run into your palm (or anything else) as it swings back and forth. Keep your hand the bat is in as still as possible--your knee and pinky/ring finger will help in this.

Third: Measure 10 periods (see above to make sure you understand a "pendulum period"). Start by pushing start on your timer at the apex of the swing at either side and saying "zero". As the bat comes back to the same place you started your timer say, "one". Let the bat swing back and forth, counting upwards as the bat keeps coming back to its original position you started your "zero" count. At exactly "ten" stop the timer. Be careful you don't start your timer and say "one" or you'll only measure nine cycles.

Repeat the third step AT LEAST 2 more times. I like to do it five times (for a total of 50 cycles). If you are doing it correctly, you will find each 10 cycle measurement is surprisingly consistent. Each one should not be off by more 1/4 of a second. If it is you are doing something wrong.

If someone else is available to repeat your steps then ask them to do so. This will verify the "pendulum period" isn't inconsistently incorrect because of something you are doing wrong.

Fourth: Once you've gathered the 10 cycle numbers (at least three of them) add them all up. Divide that number by the number of periods you've measured. That number is your "pendulum period."

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For example

2013 XL3 Easton 2 5/8 30 inches -9

1st ten cycles: 13.23
2nd ten cycles: 13.09
3rd ten cycles: 13.35
4th ten cycles: 13.31
5th ten cycles: 13.25

The outlier here is clearly the 2nd ten cycles (13.09). It seems the most off. I suspect I wasn't consistent in my steadiness or didn't pus the stop and start at the same point in the swing. I will delete that answer and sum the others.

13.23 + 13.35 + 13.31 + 13.25 = 53.14

53.14/40 cycles = 1.3285

The pendulum period for the XL3 2013 Eaton is 1.3285. Since my timer only had 2 significant variables, I round the number to 1.33. and input in the calculator below.

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Congrats! The hard part is over.

Step 2: Measuring the Bat

The Center of Mass on a 2 5/8 2014 DeMarrini Vexxum

Measure the distance between the balance point (or center of mass) and the end of the knob of the bat. There are lots of ways to do this but the most simple is to stick your finger out and move the bat along it until it balances. Try and get it as perfect as you can. Once you've found the spot, mark it and measure the distance between the balance point and the end of the KNOB. Get as close to 1/8 of an inch as you can. Write that number down.

Step 3: Weighing the Bat

See what I did there?

It is actually uncommon for bats to weigh exactly the stated weight on the bat. They are usually a good 1/2 ounce over if not 1 1/2 ounces over. I've heard that some bat manufacturers weigh the bat without the endcap, grip or even sometimes handle. This makes the total weight, after assembly, higher than the stated weight of the bat.

To calculate the MOI perfectly an exact weight will be more precise and the easiest way to do that is weigh it on a digital kitchen scale. (Or you can go to the local post office--but that seems like too much work for this).

If you don't have a kitchen scale handy then I guess you can fudge a bit on this one by assuming the stated weight on the bat is the actual weight (or add 1/2 an ounce if you want to have a better guess). Do know that your number will be off proportionally to your mistake.

DIY MOI Calculator

Now that we have our hillbilly MOI measurements (and, by the way, if you've done the above steps with care they are pretty good measurements) let's plug them in.

One last note to help make sense of the numbers below, the amount of power it takes to swing an object increases the further you try to rotate that object away from the balance point. A baseball swing, it can be argued, rotates almost as far away from the center of mass as possible: at the knob. Some prefer to the measure the MOI on the handle (6 inches from the knob). The numbers below show the MOI at the center of mass, at 6 inches from the handle and at the knob.

(Enter numbers as decimals. The calculator cannot deal with fractions)