Hand Position and Moment of Inertia (MOI) in Hitting

By Andrew Cresci – Hitting Intern

Revisiting a previous study on hitters’ grip positions run by Driveline Hitting Coordinator, Tanner Stokey, let’s look at these effects through the lens of bat fitting to understand how hand position impacts swing metrics via bat physics.

The place where a hitter places their hands on the bat will have a direct impact on the MOI (Moment of Inertia), due to the alternation of the pivot point. The pivot point is the area around which an object rotates, determining how much force will be required to change its rotational velocity.

The industry standard for measuring a bat’s pivot point is 6 inches from the knob, but to get a more accurate number, the measurement should extend from the knob to where the hitter’s grip is located on their handle.

Looking at our hitter in the example above, altering the hand position will change how the weight of the bat is dispersed across the implement. While the standard measurements stay the same, the distribution and force required to move the bat through space will change as the pivot point does. 

This is calculated through a conversion equation, with MOI_(x) being our updated point:

MOI_(X) = MOI₍₆₎ – W_(OZ) * (L_(IN) – 6)² + W_(OZ) * (L_(IN) – X_(IN))²

With this altered measurement, our bat’s MOI will increase or decrease for the hitter in response to the adjusted point.

This means only utilizing the 6 inch industry standard in the bat fitting process is not an accurate figure, because it does not take into account an individual hitter’s hand size and grip placement. This compounds further when you take into account a player’s overall sensitivity to MOI alterations in general—as bigger, stronger hitters will be less impacted by smaller changes. 

In this way, the same exact bat will offer differing MOI values depending on the individual hitter. 

There actually is science behind your little league coach telling you to “choke up.”

How much did these grip alterations affect the MOI?

Utilizing our specialty fitting equipment, we’ll take a standard 34 in – 31 oz bat and measure at the 6 inch standard. Once we have all of our data in place, we will take the true length, weight and standard MOI measurements and run them through our equation with our custom pivot point.

• Standard 6 Inches: 10,952.77 oz*in²
• Normal (6.81 in): 9,540.75 oz*in²
• Pinky-off (6.10 in): 10,772.88 oz*in²
• Choked-up (8.82 in): 6,225.80 oz*in²
• Split grip (8.82 in): 6,225.80 oz*in² 

As you can see, moving the pivot point has a big impact on the measurements of MOI. This measurement is also relative to hand size, as we see the numbers deviate considerably from the 6 inch industry standard pivot point based on the manipulation of grip placement, depending on the individual.

Recapping the impacts of MOI

Lower MOI bats will see an increase in bat speed as well as adjustability because the weight distribution is placed towards the handle and, ultimately, the hitter’s body. These benefits will come with diminishing returns, though, as the lower the MOI, the less mass you’ll have present in the barrel.

Conversely, higher MOI bats will offer greater barrel mass, which will transfer force from bat to ball at a greater rate (i.e. higher exit velocities), but can lower bat speed while also limiting adjustability.

As we are not changing the selected bat, but rather the position of our pivot point, effective length may also impact the metrics as well. When changing the actual bat, differences in length will impact the position on the sweet spot (6 inches from the end of the barrel) for every inch we add or remove from the original length of the bat. In this instance, changes to grip position will not alter the position of the bat’s sweet spot, but rather the hitter’s feel for the sweet spot location. This is why we utilize several different hitting implements here at Driveline, using proprioception training methods to build an athlete’s awareness of the barrel in space.

Taking this knowledge into account, it provides context from a bat physics point of view for the findings in Tanner’s original study.

What Does This Say About the Data?

Bat Speed: The speed of the bat at the point of contact.

Lower MOI, higher bat speed potential right? 

Yes, but the length of the bat we are swinging will impact this as well. While we lowered the MOI of our bat by choking up, we’ve effectively removed a portion of our implement. Normal and pinky-off grips produced the fastest swing speeds because of the presence of a “longer lever.” Increasing the length of a lever (our bat, in this case) while keeping applied force constant, will result in an increase in torque. 

Torque (T) = Length of Lever (L) * Force applied (F)

Torque in rotational motion is equivalent to force in linear motion (ie. speed). 

This is why most bat fitting processes that don’t look at the whole picture, usually end up recommending the longest test bat because it produces the highest average swing speeds, despite its potential negative impact on efficiency and quality of bat-to-ball collisions. As the length of the bat changes, so does a hitter’s capacity for adjustability and barrel control, while creating the potential for higher volumes of “mis-hits.”

Incorporating this trade off can be measured on each batted ball event by utilizing Driveline’s Smash Factor metric (1.2 being perfect flush contact):

Smash Factor = 1 + (Exit Velocity – Bat Speed)/(Pitch Speed + Bat Speed)

Rotational Acceleration & Time to Contact: How quickly the bat accelerates into the swing plane and the time from first move to contact.

The lower MOI grips and the creation of the shorter implement is the easiest to accelerate. With a lower MOI, less force is required to rotate the object around the pivot point. This pairs with time to contact, as the lower MOI provides adjustability and less force to rotate the bat, accelerating into contact quicker. 

*While the split grip lowers the experienced MOI, the nature of the grip’s spacing slows hand speed as well as other acceleration numbers you would normally expect to inflate. The trade off for this difference is with a flatter bat path (attack angle of 11.8 degrees).

Connection at Impact: Relationship between the body’s tilt and the vertical bat angle at contact. (90 degrees is ideal)

The split and choke-up grips provide a lower MOI variance, along with a shorter effective length that lets a hitter manipulate the bat more easily. As the pivot point moves further down the implement, more force will be required to rotate it, which will make it harder to control.

Takeaway and Potential Uses

MOI variance can have a big impact on swing metrics. To an extent, you can manipulate MOI by simply altering hand position and therefore changing a given bat’s pivot point and MOI. The grip study did not consider the impact on batted ball data, which presents another huge factor in potential changes when MOI and swing metrics are altered.

The fact of diminishing returns on either end of the fitting spectrum remains, as the trade-offs for increased bat speed vs increased force transfer can be calculated via mathematical equations, but we will continue to research how they directly impact individual hitters.

Looking at work by leaders in this field of research, Dr. Nathan Allan highlighted the importance of a bat’s MOI over all other implement measurements in its impact on batted ball performance. In contrast, other contributors within the bat performance space have looked to other combinations and in-house metrics to solve this question.

At the highest levels of the game, these changes can be the difference between a double and a homer, or even the wOBAcon increase that wins a player’s arbitration case.

For more information on how our fitting process can make sure your bat is maximizing your performance, check out my previous blog on the subject. 

You can also catch up on the physics behind choosing a bat in Hitting Trainer Richard Prigatano’s Bat Fitting research as well.

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