In the last three months of the 2008 season, over 2,000 bats broke in Major League Baseball games - an average of about one bat per game. You’ve seen this happen: a big heavy shard of wood flying into the stands or toward the dugout. It’s dangerous to the fans and players. So the MLB decided enough was enough and commissioned a committee of engineers and scientists to provide recommendations for what should be done about all these shattering sluggers.

Before the early 1990s, almost all the bats in the majors were made of ash, a strong, dense hardwood that grows across a large part of the United States. That all changed when Barry Bonds broke the season home run record in 2001 with a maple bat. Suddenly half the major league players had followed suit. The MLB safety and health advisory committee determined that this rise in use of maple was a primary reason for the increase in broken bats. But why?

Wood is an anisotropic material. That means its engineering properties are directionally dependent; it’s strength and rigidity and toughness aren’t the same along every axis. This is because of how a tree grows - the majority of the cells that make up wood are vertical and elongated. You can picture wood as a collection of straws connected together along their edges. In material science, we would call this a uniaxial fiber composite. Like the oak shown here, ash is a ring porous wood, meaning it has vertical pores which concentrate along the annual growth rings. This creates a natural plane of weakness around each ring.

Dealing with anisotropy is one of the fundamental engineering challenges of working with natural materials like wood, but the makers of baseball bats have put a lot of thought into this. The manufacturers logo on ash baseball bats is always placed on the face grain, and players are instructed to swing with the logo up. This ensures that the baseball contacts the bat on the edge grain and the bending stresses aren’t concentrated in the weak axis of the annual growth rings. This is the equivalent of hitting on the edge of a deck of cards rather than the face.

For the best performance, it’s crucial that the grain runs as close to parallel with the axis of the bat as possible. The angle that the grain makes with the long axis is called slope of grain, and it has a huge impact on the overall strength. For a long time, the wood for baseball bats was riven or split to produce blanks. Milling lumber this way ensures that the grain is straight because the wood naturally separates along the fibers. But as mills grew, it became more economical to use saws. You can saw through wood in any direction your heart desires, so it became necessary to grade bat blanks to make sure the slope of grain was small. It is easy to grade ash because the planes of weakness are so visible, the annual growth rings create nice clear lines.

But wood has another natural plane of weakness radially from the center of the tree. Look at the end of any log that’s had a bit of time to dry and you can see this. Unlike the ring porous ash, maple is diffuse porous. That means it’s pores are spread throughout the wood rather than being concentrated along the growth rings. At first glance that seems like a good thing, since you no longer have planes of weakness along the rings, but what the committee found was exactly the opposite.

Without the porous growth rings, the weakest planes in maple are those that are radial from the center of the log. But these planes don’t have a strong visual cue like growth rings, so it is much more difficult to grade maple blanks for slope of grain. Bat manufacturers were used to keeping the growth rings straight along the bat, but many didn’t recognize the need to ensure that the grain was straight in the orthogonal direction as well. The MLB safety and health advisory committee determined that the majority of bats were failing due to slope of grain, and you can look through pictures of broken bats and see this oval-shaped plane that is characteristic for slope-of-grain failure. The committee issued a list of recommendations that was immediately adopted by the MLB to help reduce the number of broken bats, including two key changes to how bats are manufactured.

First, bat makers are required to rotate the logo placed on maple bats by 90 degrees. Since the weakest planes in maple are radial to the log and not along the growth rings, maple bats are stronger when the face grain is used as the hitting surface. Second, manufacturers must place a dot of ink on the face grain of all maple bats. This makes it easier to see the direction of grain orthogonal to the growth rings and grade the wood for slope of grain. If you look close, sometimes you can see this dot on the handle of some bats in major league games.

Did these changes make a difference? There have still been accidents with shattering maple bats - including one of the worst incidents to date when Tyler Colvin had his lung punctured and missed the rest of the 2010 season - but overall bat breaks have actually been reduced by half. These days on average you’ll only see about one break every two games which is quite an improvement.