Published by Kyle H. Lead Product Engineer
MOI or Moment of Inertia: What Is It?
The prevalence of the term “MOI”, an abbreviation for “Moment of Inertia”, has become much more common in today’s world of baseball and golf specifically. But what exactly does this term mean, and how does it relate to your baseball bat?
MOI is an engineering metric used to quantify the rotational inertia of an object – or the object’s tendency to resist angular rotation. In this sense, it is similar to the concept of torque, and is measured in respect to a single “pivot” point. In the case of bats, the pivot point is the hitter’s hands, and thus, all MOI measurements are calculated from a single point (6.0” from the tip of the knob) that approximates the central axis of an average hitter’s hands during the swing.
Moment of inertia is important to measure in bats because it results in a metric that quantifies “swing weight”. Rather than simply denoting a bat as “endloaded” or “balanced”, MOI can label any single bat with an exact measurement of how heavy the bat may feel to a player. These MOI values will be as low as 3,000 (oz/in2) for a 27”-17oz youth model, and as large as 11,000 for a 34” BBCOR model.
The MOI of all bat models is heavily scrutinized throughout the design process, and in some cases can be easily manipulated to achieve the correct swing weight or “feel” desired for a certain model or category. In its essence, MOI is measuring the amount of weight or mass away from the pivot point (the hands) - and thus, the further from the pivot point you go (down the barrel of the bat, toward the endcap), the more extremely any addition of mass will affect MOI. This is why technologies like the Axe HyperWhip endcap are so impactful – as any weight (as small as a single gram!) saved in the endcap (the furthest possible distance from the pivot), will have the most drastic effect on the MOI of the bat. This concept can be seen and felt most plainly in the Axe Bat Speed Trainers.
In the case of the Barrel Loaded and Handle Loaded speed trainer models, both bats are 36.0oz. However, the bats are loaded with this weight in opposite ends of the bat construction. Despite both bats being 36.0oz, the difference in “swing weight” can be felt immediately by anyone holding or swinging these bats – the barrel loaded model feels far heavier than the handle loaded model, which despite being 36.0oz, feels quite similar to a game bat. This feeling is easily quantified by the moment of inertia:
- 33” Axe Bat Speed Trainer Barrel Load: 13,500 oz/in2
- 33” Axe Bat Speed Trainer Handle Load: 10,250 oz/in2
The Speed Trainers make MOI a very recognizable feeling, with two bats differing by approximately 3,250 points. MOI has similar value in BBCOR, youth, and softball bats, but the margins are much smaller. A visual depiction of Axe Bat’s 2023 BBCOR line is below. The chart separates the BBCOR line by length (31”-34”), and shows the moment of inertia for each bat. The total length of the bar for each bat length approximates the heaviest and lightest swinging bats you might find in the BBCOR category in each length. As advertised, the Strato swings as light as nearly any bat in the industry, while the Avenge Pro Hybrid offers a more mid-loaded feel. For reference, we typically expect a high-level player to be able to notice a change in MOI when bats differ by more than 100 points.
Specific call-outs of “endloaded” and “balanced” models can also be commonly found in the slowpitch industry. A visual depiction of Axe Bat’s 2023 Avenge Pro USSSA slowpitch MOI’s can be seen below (in the 26oz-28oz models).
Armed with the information above, you can likely make a more educated and confident decision when deciding which bat is optimal for you or your child.
Have further questions regarding this topic? Don’t hesitate to reach out via chat or email to an Axe Bat customer service representative.