How Temperature Affects Ball Compression
Mark McDowell, Ph.D. and Ron Noebe, Ph.D.

There have been heated debates over the past few years about how the bats have gotten out of control and are ruining the game of slo-pitch softball. There has also been an unprecedented amount of returned bats due to severe denting and cracking and the bat manufactures are constantly trying to explain or find a reason for these returns. One main reason is that the temperature of the ball has more to do with denting than players care to notice or understand. We will try to shed some light on how temperature affects the compression of a softball and what this does to the high-tech bats that we spend hundreds of dollars for each year.

While there are limits placed on bats (BPF) and limits place on ball COR values (.40, .44, .47 and .50 COR), there is absolutely no limit placed on ball compression. Ball compression can be blamed for just about every case of severe denting and cracking of your bat. The ASTM standard for measuring BPF uses a ball with a compression value between 350 and 375 lb./0.25". There are very few polyurethane core balls today that meet this standard. Instead, ball compression values are more like 500lb./0.25"or higher and this is definitely a problem. Using high-compression balls translates into more home runs, higher batted ball speeds, decreased reaction time and most of all, severe bat denting.

We decided to look into the issue of ball compression by running a series of experiments on ball compression at the recommend room temperature (70-72 degrees F), at 45 degrees F and at 15 degrees F in order to show our readers how the compression values is affected. We chose to use 4 different polyurethane core balls for our study. What we found was totally shocking!!! There is a valid reason for bat manufacturers putting warning labels on the bats to not use them under 60 degrees F. This is the point where the properties of the ball changes and in most cases, the ball is now getting harder than a golf ball and in other cases, the ball is actually as solid or hard as a rock!!!

By looking at the data in Figure 1 and Table 1, we can clearly see how the compression values are effected by temperature. We have always wondered why there were so many more home runs hit in Fall softball in our area than in the summer. We now have a reason for this.

Your softball bat is a thin-walled flexible tube, designed to return energy as efficiently as possible. When a ball hits the bat, you get deflection of the thin wall, which is referred to as the "trampoline" effect and you may get deflection or bending in the handle, which is referred to as the "springboard" effect. You are returning more energy to the ball by making the bat-ball collision more efficient. The harder the softball, the more elastic deflection you will get in the bat, increasing the "trampoline" and "springboard" affects and thus, returning more energy or speed to the ball.

As you increase the hardness of the ball, your bat starts responding more to the bat-ball impact by deforming elastically and returning that elastic energy back to the ball. A harder ball will tend to come off of a bat faster and travel farther. There is a downside to this and that is if the ball is too hard, the bat will also deform inelastically, which leads to denting. Bat manufactures are constantly trying to make a durable bat the will perform and last an entire season.

What happens when the Temperature gets below 60 degrees???

The answer is quite simple, your bat will begin to dent!!! We get several requests for high-tech bats that will perform in 50 degree weather. Our immediate response is that there are NO bats on the market that are designed to meet that request. Players obsessed with hitting more home runs enjoy the higher compression balls and this is a big problem. Players today need to put away the high-tech bats when the temperature gets below 60 degrees and use a bat with thick walls or a bat that he/she is willing to sacrifice. It is an easy solution but it will not happen. With the confidence that multi-wall bats bring, not many players are willing to put these bats down and switch to a bat that they know will not perform like a multi-wall bat. When the temperature falls below 60 degrees, just about every swing has the potential to dent, crack or cave in the bat.

What's the Solution???

The solution is easy to recommend but will be very difficult to implement. There needs to be serious limits placed on ball compression in the near future. Here at Bomani Sports Research, Inc., we recommend a ball compression that does not exceed 450 lbs./0.025". We feel that this is a safe value that will provide good reaction time and keep home run hitters hitting home runs and singles hitters hitting singles. This means that there has to be two types of softballs used in leagues, a ball for the spring/summer season and a ball for the fall season in order to compensate for the lower temperatures.

A better solution is to have the BPF combined with ball COR and compression in order to have a more realistic performance factor. For example, if the ball is stamped .47COR-375 lbs./0.025", then we would know that the ball meets the BPF 1.2. However what we would typically see if the high compression balls of today were used would be more like .47COR-525 lbs./0.025", which does not meet the BPF 1.2 requirement for USSSA.


Bring back safety and sanity to softball by requiring that BPF, ball COR and ball compression be included in all regulations. Once you put these values under control, the game will return to its original purpose, to be safe and enjoyable for all ages.

Ball compression increases approximately 60lbs/0.25" for every 10oF decrease in temperature!!!

Figure 1. Compression Plot

Table 1 - Compression Values
Ball Tested
Average Ball Weight
at 70oF
at 42oF
at 15oF
6.42 +/- 0.12oz.
564 +/- 14 lbs/0.25"
713 +/- 6.2 lbs/0.25"
888 +/- 18 lbs/0.25"
6.53 +/- 0.06oz.
490 +/- 5 lbs/0.25"
706 +/- 42 lbs/0.25"
875 +/- 9 lbs/0.25"
6.38 +/- 0.05oz.
574 +/- 11 lbs/0.25"
730 +/- 16 lbs/0.25"
886 +/- 32 lbs/0.25"
6.76 +/- 0.09oz.
539 +/- 17 lbs/0.25"
686 +/- 23 lbs/0.25"
836 +/- 17 lbs/0.25"
There was no appreciable difference in weight between the as-received balls and the balls weighed immediately after aging and testing. This is an indication that the compression affects observed were due to temperature differences only and not due to absorbed moisture.

According to ANOVA analysis, the differences in the mean values among the treatment groups are not great enough to exclude the possibility that the difference is due to random sampling variability; there is not a statistically significant difference in the mean values for the as-received balls and the balls tested without a cover. This means that the core properties completely dominate the compression behavior or hardness of the ball.
Copyright © 2010 Bomani Sports Research, Inc.