Sweet spots on a baseball bat are the locations best suited for hitting pitched baseballs. At these points, the collision between the bat and the ball produces a minimal amount of vibrational sensation (sting) in the batter's hands and/or a maximum speed for the batted ball (and, thus, the maximum amount of energy transferred to the ball to make it travel further). On any given bat, the point of maximum performance and the point of minimal sting may be different. In addition, there are variations in their locations between bats, mostly depending on the type of bat and the specific manufacturer. Generally, there is a 1.57-2.0-in (3.8-5.1 cm) variation in the location of the sweet spot between different bat types. On average, the sweet spot occurs between 5 and 7 in (12.7 and 17.8 cm) from the barrel end of the bat.
The sweet spot's location for maximizing how far the batted ball travels after being hit can be calculated scientifically. When a batter hits a ball, the bat will rebound from the force of the collision. If the ball is hit closer to the handle end, a translational (straight-line) force will occur at the pivot point. If the ball is hit nearer to the barrel end, a rotational force will occur at the handle end near its center-of-mass—causing the handle to move away from the batter. This rotating motion causes a force in the opposite direction at the pivot point. However, impacts at the sweet spot results in these two opposite forces being balanced, causing a net force of zero—something that can be measured by scientists.
The tampering of baseball bats to modify the bat with the intention of enhancing its performance is an illegal activity. In many cases, it involves modifying a bat by drilling a hole about 6-9 in (15.2-22.9 cm) into the barrel end. The hole is then left empty or is filled with some type of lightweight material such as cork, cork balls, or rubber. A solid wooden plug, which matches the bat's color and grain, is then placed over the hole. The plug is sanded and varnished in order to hide the modification. Such modified bats are often called aberrant or corked bats. The modified bat differs from the original bat by its lighter weight and smaller moment of inertia (resistance to drag). Bats drilled out in this way are not allowed to be used within the various member organizations of the National Association of Professional Baseball Leagues (of which the National and American Leagues are members) under the Official Baseball Rules.
Other illegal ways to tamper with bats have been used in the past. Wet pine tar (a sticky substance) has been used to improve a batter's grip on the bat. Nails were sometimes secretively driven into bats to provide a harder surface for the collision with the baseball.
In the early 2000s, engineers at the University of Massachusetts Baseball Research Center in Lowell, Massachusetts, tested illegal cork bats and legal wooden bats. The test is viewed within the sporting industry as one of the most exacting scientific laboratory studies ever performed on bat efficiency. A mechanical hitting machine was used to compare the bats swung at exactly 66 mph (106 km/h). The measurement was taken at the "sweet spot," which is about 6 in (15.2 cm) from the tip of the bat. A mechanical ball-throwing machine pitched the balls at exactly 70 mph (113 km/h). The center's director, James Sherwood, stated the test results showed that baseballs traveled farther when hit by corked bats because they deformed to a higher degree and this increased the energy of impact with the pitched ball (maximizing the energy changes as a result of the hit and increasing the distance the ball could travel). However, the corked bats also showed obvious stress (such as cracking) after only a few collisions. Within the study, Sherwood stated that a corked bat produced about a 1% increase in the speed of the batted ball, which related to about a 2% increase in distance traveled. For instance, a 400-ft (122 m) fly ball hit with a legal bat would produce a 408-ft (124 m) fly ball hit with a corked bat. Thus, a corked bat allowed a batter to swing the bat faster and the ball to rebound quicker.
On the other hand, a corked bat, which is about 1.5 oz (42.5 gm) lighter than a solid wood bat, has less mass. A less-massive bat means that it will have a less-effective collision with the ball. With this tradeoff—faster swing speed and springier impacted ball versus smaller mass—many experts state that the difference between the two bats is negligible. However, other experts note that such differences are why the bat continues to be illegal to use. Some studies describe a psychological benefit when using corked bats. Batters "think" they are able to hit the ball further and, therefore, are more efficient at hitting greater numbers of balls. Whether such physical and psychological differences warrant that the corked bat remain illegal has been difficult to determine.