Physics in the On-Deck Circle

There is a lot of confusion regarding the on-deck circle. Most believe the next batter is required to be there. If so, it is not in the 2018 MLB Rules, which don’t even mention the circle. In fact, the words “on-deck” appear only twice; once in regard to the definition of a “save” and again in a comment on Rule 5.04(b)(4)(B) encouraging umpires to get the next batter to the plate “quickly.”

Even players are sometimes confused about the on-deck circle. Once, Adrian Beltre was in his favorite spot, warming up for his pending at-bat, when an ump told him to get in the batting circle. Entertainment ensued, as yyou can see below:

You might notice a good portion of the paraphernalia lying in the circle wound up strewn about as Beltre moved it. What is all that stuff anyway? We can get a hint from the comments of the Giants announcers as they describe the dangers faced by Buster Posey as he navigates the area to collect a pop-up.

Lead bat? Manny Mota Stick? Rosin bag? What is all that stuff, and what’s the point of it all? Well, there are two types of items. Some of them are designed to physically prepare for an at-bat, and the rest are intended to improve the batter’s grip on the bat. Let’s start with the grip-enhancers.

The Manny Mota Stick appears to be just a large cylinder of pine tar wrapped in cardboard so the gooey stuff can be applied just where it is wanted. Next time you are actually barbequing (not “outdoor cooking” with gas, you cretin) you might think about where charcoal comes from.

If you throw pine wood into a campfire, once the flame and smoke are gone, you’ll have smoldering charcoal. The stuff that was in the flame and smoke is pine tar. To collect the pine tar, you need to heat pine wood under high pressure in an environment where it isn’t allowed to escape. Then you get charcoal for your steaks and pine tar for your bat.

The rosin bag is, as you might guess, a bag of rosin, which is also a product created by heating pine. The difference between it and pine tar is the temperature is tightly controlled to allow only one component of pine tar to vaporize. This vapor then is cooled and powered. Note: be careful if you Google “rosin bag,” because there is now a far more common use for them than baseball.

The physics of the stickiness of rosin and pine tar must be extremely complicated, because the best explanation is “it is caused by the behavior of the atoms and molecules in them.” Of course, this is no explanation at all. The behavior of any material is caused by its atoms and molecules. That said, pine tar and rosin do illustrate two properties of materials – adhesion and cohesion.

A physicist would say rosin demonstrates adhesion while pine tar exhibits cohesion. Adhesion is the tendency of a material to stick to other materials. Cohesion is the tendency for materials to stick to themselves. 

Rosin doesn’t really stick to itself. That’s why it can remain a powder. Pine tar will stick to itself, so it doesn’t need to be kept in a bag. It can hold together in a stick. In addition to cohesion, pine tar exhibits adhesion as well. It sticks to bats, hands, and, in fact, anything it touches.

In addition to the traditional on-deck circle grip enhancers pine tar and rosin, there is a large variety of different sorts of spray tack – sticky stuff sprayed on from a can. After all, we’ve turned other products that are easy enough to put on from a stick into spray cans.

Now on to the physical preparation. Back when baseball was played in black-and-white (well, okay, before color cameras) ballplayers often would swing two or more bats as they waited for their ups. The prevailing theory at the time was that swinging the extra weight made the one bat used at the plate feel lighter and more maneuverable.

This is essentially the idea behind the lead bat. Instead of swinging multiple bats, just swing a single bat that is extra heavy. This mindset of swinging extra weight, while you wait, has generated many inventions such as the batting doughnut. Elston Howard, the twelve-time All-star and first African-American Yankee, is credited with its invention in 1955.

The doughnut as a substitute for a heavier bat has some specific physics-related issues. You probably have noticed is it much easier to swing a bat by holding the barrel end than by holding the handle. You haven’t noticed? Then try it. The point is that the “feel” of a bat depends not just on its total weight, but also on how that weight is distributed. 

Physicists call this the moment of inertia, or MOI. So, a bat held by the barrel has a smaller MOI than holding the same bat by the handle. The MOI is smaller because more of the bat’s weight is closer to your hands. So the MOI depends upon where the weight of the bat is in relation to your hands. That is, it depends upon the weight distribution. It also depends upon the total weight of the bat.

Adding a doughnut to a bat not only changes the total weight, it also changes the MOI of the bat in a very specific way, because all the extra weight is at one specific distance from the hands. That is, the bat is heavier, but it feels different than swinging a heavier bat of the same shape. Hence, the invention of the “batting wrap” or “bat sleeve.”

With a batting sleeve, the additional weight is distributed along the barrel much more uniformly than a batting doughnut. Thus, the weight distribution using a wrap is closer to the weight distribution in a regular bat than the weight distribution using a doughnut.

Which would you rather hear the above explanation from – a trained physicist or a ballplayer? Wait – don’t answer – it will just hurt my feeling. So, here is Harold Reynolds’ explanation of the doughnut and the wrap. Watch it while I salve my wounds.

The next step toward matching the original weight distribution would be a “lead bat” or a “weighted bat.” Louisville Slugger markets such a bat. It is 35 inches long and weighs 44 oz. Now, the weight distribution is just like that of a regular bat because the length and shape are about the same, but it weighs more. By the way, a bat with just about these specifications was often used in the batter’s box by Babe Ruth.

There is another type of warm-up bat that has become popular recently. It has a weight that can be moved closer or further from the hands. Here is a typical example. Adjustable-weight bats allow the player to change the weight distribution without changing the total weight. In a sense, this is the opposite of the leaded bat, where the weight distribution stays the same but the total weight is bigger.

At this point, you should be confused about which is the best bat to use when you warm up in the on-deck circle. There have been some limited studies by sports physiologists listed in the references below. None are definitive, or you would see every major-leaguer doing exactly the same thing in the on-deck circle.

In life, you can always tell when there is no ideal solution to a problem. The giveaway is when everyone has his or her own unique solution which is touted as being the only solution. This seems to be the case in the on-deck circle. There is no perfect way to prepare for an at-bat, so the pile of stuff in the circle just continues to grow. Be careful, Buster!

References & Resources

• Szymanski, David J, et al, “Effect of Various Warm-Up Devices on Bat Velocity of Intercollegiate Baseball Players” J. of Strength and Conditioning Research, February 2011 – Volume 25 – Issue 2.
• Liu, Chiang, et al, “Effects of Training With a Dynamic Moment of Inertia Bat on Swing Performance” J. of Strength and Conditioning Research, November 2011 – Volume 25 – Issue 11.