Here is the article mentioned on our website.  Feel free to contact me with questions you may have.

The Grading of Wood for Baseball Bats

Most bat manufacturers rely on sawmills to process their wood.  Processing involves harvesting logs, sawing logs to produce billets, and kiln drying billets to an appropriate moisture content level.  Most sawmills do a very good job of processing billets according to “lumber” standards (no knots, checks, splits, etc.).  However, bat manufacturers always request baseball bat billets that have exceptionally straight grain, which is stricter than most “lumber” standards.

The importance of straight grain is critical, because the wood property that has an overwhelming effect on the strength of the final baseball bat is slope-of-grain.  Slope-of-grain is how close to parallel a piece of wood is cut with respect to the longitudinal axis of wood cells in the tree.   When a piece of wood is cut perfectly parallel to the grain direction of the tree, it will have the highest strength.   When wood is cut at an angle to the grain direction of the tree, the strength quickly diminishes.

There are two types of slope-of-grain:

The angle made by these two lines is the RADIAL Slope of Grain.

Radial slope-of-grain – this is the angle between the annual growth rings of the tree, and the straight line down the middle of the wood.    In the photo below, the red line represents the centerline of the round wood billet.   The pencil line is drawn along the growth rings.

This EDGE GRAIN  (i.e. RADIAL Slope of Grain) is the number one visible wood characteristic used by bat manufacturers to grade their wood.

The straighter, the better.
Tangential slope-of-grain – this is the angle of the wood grain on the flat-grain face.  Tangential Slope of Grain is the culprit in most multiple-piece failure broken bats…

Tangential slope of grain is the angle that the wood fiber makes with the centerline of the wood.   In Hard Maple, this fiber is not easily visible.

The ink dot on the flat-grain face (i.e. Tangential face) is shown, along with a red line, which represents the centerline of the round wood billet.

Tangential slope-of-grain – this is the angle of the wood grain on the flat-grain face.  Tangential Slope of Grain is the culprit in most multiple-piece failure broken bats…

Tangential slope of grain is the angle that the wood fiber makes with the centerline of the wood.   In Hard Maple, this fiber is not easily visible.

This is the reason for the new 2009 requirement that all maple wood baseball bat manufacturers must now place an ink dot on the handle of the wood bat.   When an ink dot is placed on the raw wood surface, the ink bleeds ALONG the wood grain.    This bleeding highlights the direction of the wood grain.

These are 2 very important facts to understand…  (1) a billet can have absolutely perfect straight grain on the edge-grain face, and yet, have a very bad fiber angle on the flat-grain face, and (2) nearly all broken bat failures that are due to slope of grain are due to a bad angle on the flat-grain face (i.e. due to tangential slope-of-grain).

The frequency of broken wood bats has grown since the introduction of hard maple wood bats, primarily because manufacturers were not aware of the above characteristic.   Basically, it is difficult to see the tangential slope of grain with the naked eye…  Hence, the ink spot.

Furthermore, the likely reason why wood bats break more often today, compared to years past is…   the method in which wood is processed from the log.   Years ago, labor intensive methods were used to split logs, and then turn those triangular staves into round billets.  This resulted in wood baseball bats that had straight-grained wood.   Nowadays, with higher-production sawing equipment, and increased demands for yields and efficiency… a large percentage of wood billets are SAWN from logs.   If wood is not SPLIT, then it is most likely being sawn across the radial and/or tangential slope of grain… resulting in round billets like the one shown above (with tangential slope of grain).

Some sawmills still use the split-log method to produce billets for baseball bats… however, this is more common for ash, because it splits more easily than maple.

Wood Baseball Bat Strength and this property called “Slope of Grain”:

How do baseball Bats break?:

Typically, bats can be categorized into four types of failure: (1) rupture-type failure, (2) slope-of-grain failure, (3) brash failure, and (4) annual ring separation.  The two more common failures are Rupture and Slope-of-Grain.

Rupture-type failure in the handle of a baseball bat is the type of failure that occurs when a straight-grained piece of wood is bent until it breaks.  The classic “beam failure” is observed, where there is tension failure on one side and usually shear cracks along the length of the handle. Bats that fail this way are a strong indicator that the bat contained straight-grained wood, and the stress created from hitting a baseball simply exceeded the strength of the wood.    This is considered a “good” failure, because the bat remains intact, and does not fly apart.


This is a Rupture failure, which is typically a Single-Piece Failure

Slope-of-grain failure in the handle of a baseball bat is a very different type of failure.   Because the strength of the wood quickly decreases when slope-of-grain increases, failure occurs along the slope-of-grain plane.    The noticeable characteristic in a baseball bat that has failed due to large slope-of-grain is the oval-shaped failure plane (seen in the handle portion below).


This is a Slope of Grain failure, and typically fails into Multiple Pieces
*the most dangerous to the players and fans.


For bats with excessive slope-of-grain, how much is strength reduced?

The effect of slope-of-grain on the impact bending strength of wood

It is well documented that when slope-of-grain increases in wood, strength properties decrease.  The relationship between slope-of-grain and strength loss is described in the literature by a formula called the Hankinson’s equation.  As the slope-of-grain increases (i.e. as the angle between fiber direction and longitudinal axis of the piece of wood increases), the strength properties of wood decrease.  In the USDA Forest Service Wood Handbook, this effect is represented by the following plot:

The different graphs represent different properties.  For IMPACT BENDING STRENGTH, which most closely relates to the strength of a wood baseball bat in use… the bottom graph is used.

NOTE that the graph above decreases the most rapidly when the angle of grain increases:

As an example of how to use the above graph…  the slope-of-grain failure in the bat below has approximately a 1:6 slope-of-grain failure plane.   “1:6” is pronounced “1-in-6”, and it means that 1-inch of grain deviation per 6-inches of length along the bat.


Transforming “1:6” to an angle, this is approximately a 9.5-degree fiber angle, compared to a line along the length of the bat.

A roughly 10-degree angle means that the wood only has about 40% of the strength of a bat with straight-grained wood.
Bats break into multiple pieces more easily when the grain is not straight.

A survey of broken bat failures


With a trained eye, an observer will notice that a very large percentage of broken wood bats that fly apart and into the field of play, are predominantly due to slope-of-grain failure.  The photos below show the noticeable oval-shaped failure plane.

When you see this oval-shaped failure plane, it is due to SLOPE OF GRAIN.

Important points to understand all the changes in MLB bat rules:

1.  A wood billet can be graded out to have perfectly straight annual rings (i.e. it looks perfect), which means that it has straight grain in the radial face.    But it can have severe slope of grain in the tangential surface (like in the photo shown earlier).      This hidden defect is more common in hard maple because of its fine-grained nature, and because maple logs are not typically split …   basically, the grain is difficult to see on the flat-grain face.

2.  Wood bat manufacturers are doing a pretty good job of sorting out straight-grained wood in the radial surface, but the tangential slope-of-grain is often overlooked.  This is why manufacturers are now required to place an ink dot on the flat-grain face of maple bats…  so that inspectors can come in and see the grain.

The ink will bleed along the grain of the wood, and this will highlight the “slope of grain” on the tangential face.

3.  Because past research data has shown that wood is stronger when contact is made on the flat-grain face, manufacturers of diffuse-porous wood baseball bats now have to orient their logos 90-degrees different than white ash – to promote flat-grain contact.

This is the important part that even most manufacturers still do not understand…

If you have perfectly straight-grained wood that has been split with a wedge and turned into a baseball bat… there is a slight difference in strength between flat-grain contact and edge-grain contact (flat-grain contact is higher).   BUT… most wood baseball bats are not made from perfectly straight-grained wood that has been split with a wedge (anymore).   When bats DO have some slope of grain (which is nearly all of them) then making contact on the flat-grain face is NOTICEABLY stronger than wood that is contacted on the edge-grain face.    To improve the strength of THE HANDLE of wood baseball bats as much as possible, the change to flat-grain contact was made for all diffuse-porous species.    Past research data shows that this strength increase can be as high as 15% to 30% for impact bending strength.

Other reasons bats are breaking?

There are many (strong) opinions as to why bats are breaking in baseball.   These include:

1.      Bat shapes are too severe (big barrel, small handle)

2.      Weight drops need to be increased (from -3.5 to -2)

3.      Wood is being over-dried

4.      and many others.

It should be understood that there are MANY factors that influence the strength of the bat… and all must be taken into consideration.   HOWEVER, some properties have stronger influence that others… and this is why you have only read about changes relating to “slope-of-grain”.

MLB decided to make changes because Slope of Grain had the largest IMMEDIATE influence on increasing the strength in bats.   And NO other people’s opinion did not get ignored.

For example, here is (roughly) the influence for each of the above properties:

(1)  Regarding Bat Shapes….  If you take a bat shape that has a 0.93″ handle diameter, and increase it to 1.00″, this would be a (1.00/0.93)3 increase in strength… that’s a 24% increase.    Or stated another way…   a 0.93″ handle diameter is only 80% of the strength of a 1.00″ handle diameter.

(2) Regarding Weight Drops…  If you increase weight drops from -3.5 to -2, that means you are using a slightly denser piece of wood… which does offer higher strength.   However, to make a -2 bat only requires a billet that is approximately 4% to 5% heavier than the billet used to make a -3.5 bat.  Looking at a density vs. strength relationships for wood, this 4% to 5% heavier wood will only translate into a small percentage increase in strength (maybe 5%?)

(3) Regarding Over-Dried Wood…   Over-dried wood would typically break in a noticeable manner… brash, for example.  However, for the most part…  when a bat is observed to break into multiple pieces, it is typically due to slope of grain.    When more bats are observed to fail by brash, and/or when straight-grain bats begin to explode… it is those type of failures that can be suspected to be due to over-dried wood.

Why target Slope of Grain first (before the other properties)?

Many broken bats, like the ones shown in the photos on this page (and still seen today in games)…  are found to have slope-of-grain angles as large as 10 to 15 degrees.    Using the Hankinson chart above…  this is saying that bats are entering our games that have strength that is approximately 40% to 25%, respectively, of straight-grained wood (e.g.  1/3rd to 1/4th the strength of straight-grained wood). If the slope of grain requirement is made stricter, that means that bats should only have a slope-of-grain angle of approximately 3 degrees…   that means that the weakest bats ALLOWED in MLB should be over 280% stronger than the bats we’ve seen break with 1:4 and 1:6 slope-of-grain angles.

If you address bat shape only…  and make bats 24% stronger…  it won’t matter, because bats with bad slope-of-grain have wood that has 1/4th the strength.

If you address weight drops only… and make bats 5% stronger… it won’t matter, because bats with bad slope-of-grain have wood that has 1/4th the strength.

If you address slope-of-grain… and establish a minimum slope of grain level, you can make the weakest bats in the game be approximately 3 times stronger than the weakest bats we have been observing these past few years.