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Physics and Acoustics of Baseball & Softball Bats
Daniel A. Russell, Ph.D. Applied Physics, Kettering University, Flint, MI 48504-4898 The contents of this page are ©2007 Daniel A. Russell | 
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Physics and Acoustics of Baseball & Softball Bats
Daniel A. Russell, Ph.D. Applied Physics, Kettering University, Flint, MI 48504-4898 The contents of this page are ©2007 Daniel A. Russell |
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I have heard and read a lot of comments about how metal bats have changed the game of baseball, but until recently I had not seen any hard data to back up such claims. In 2004 I came across statistics published by the NCAA for Division I college baseball starting from the year 1970 through this year. The raw data includes yearly results for batting averages, home runs per game, runs scored per game, strikeouts per 9 innings, pitcher earned-run-averages, stolen bases, and fielding percentages.[1] A recent article about the 2005 College World Series in the sports section of USA Today[2] tabulated some of the same data along with a discussion of how hitting (home run production is way down) and pitching (pitching quality and strike outs are up) have changed in recent years.
I have plotted some of the data below and will discuss the trends, but there are three important dates I need to mention which will aid in interpreting the data. First, 1974 was the year aluminum bats were introduced to NCAA college baseball, and metal bats have been used almost exclusively since that year. Secondly, in 1986 the NCAA imposed a lower limit on the weight of a bat. Finally in 1999, after the 1998 season - during which a number of scoring records were broken - the NCAA implemented a performance standard to limit the performance of aluminum and composite bats. I discuss the details of the performance standard for baseball bats elsewhere, but it involves an upper limit on a quantity called the Ball-Exit-Speed-Ratio, a reduction of the maximum barrel diameter to 2.5-inches, the "minus-3" rule on weight which means that the difference between the weight of the bat (in ounces) and the length (in inches) can be nor no more than 3 integers ), and a lower limit on the bat's moment-of-inertia.
The plot at right shows the mean batting average for all NCAA Division I college baseball players as a function of year from 1970 through 2006. It is interesting to note that from 1970 through 1981 there appears to be an almost steady increase in batting average. However, it would be incorrect to extrapolate before 1970 to assume that batting averages have always been increasing from .000 at some point in the past. Instead it is more likely that the mean batting averages in the wood era prior to 1970 fluctuated around a value near 0.265 or so, though even this is speculation. The mean batting average spiked to 0.300 in 1981 and spiked even higher to 0.306 in 1985 before dropping and levelling our around 0.290 for most of the early 1990's. An increase from 0.265 to 0.290 (or higher) is quite significant, and suggests that players using aluminum bats make solid contact with the ball more often than former players did with wood bats. However, as we will see below, during the same time period that batting averages were quickly increasing, pitchers had increasing difficulty strikng batters out, with the number of strikeouts per 9-innings reaching an all-time low in 1981. Pitchers were still struggling in the early 1990's. So, it is difficult to conclude whether the higher batting averages are due primarily to aluminum bats, or poor pitching, or a combination of both. Two things are true. Aluminum bats typically have lower moments-of-inertia than metal bat and therefore may be swung more quickly. A batter who can swing the bat faster has a much higher chance of making contact with the ball since he can wait slightly longer to watch the ball before committing to a swing. Also, since metal bats don't break, many pitches that would have resulted in a broken bat dribbler for an out with a wood bat now result in base-hit singles from an aluminum bat. The drop in batting averages after 1985 may be due to the fact that the NCAA introduced the "minus 5" rule after that year. This rule stated that the numerical difference between the bat weight (in ounces) and the weight (in inches) could not be more than 5 units. A 34-inch bat manufactured in 1986 could weigh no less than 29 ounces. This change would have had an effect on bat swing speeds and might partly explain the immediate drop in batting averages starting in 1986.
But, bat performance didn't stay down. Starting in 1996 batting averages began increasing at a dramatic rate, reaching an record high during the 1998 season. Alarmed by this drastic increase in performance, the NCAA implemented a bat performance standard following the 1998 season. In 1999 NCAA introduced the BESR performance standard, the "minus-3" rule for bat length and weight, introduced a lower limit on a bat's moment-of-inertia, and reduced the maximum allowed barrel diameter. Now that bat performance is being controlled through laboratory testing and certification processes, batting averages appear to be remaining relatively constant near the values of the late 1970's and early 1990's. The NCAA is apparently happy with the current state of the game, and we will have to keep watching for the next several years to see if batting averages remain steady around 0.291 or continue to fall.
Before moving on to look at other data I should point out one other statistic I found which demonstrates that regardless of the bat, good hitters will find ways to make contact with the ball. In 1957 during the wood-only era the best college team batting average was 0.473. In 1994, when aluminum bats ruled the game, the best team college batting average was 0.474.[3] So, while one could well argue that aluminum bats have helped the average player to make solid contact with the ball more often, the best players in the game are able to hit about the same with wood or aluminum, at least when it comes to batting averages.
Home Runs per GameThe second plot at right shows the number of home runs per game for NCAA Division I college baseball from 1970 through 2006. The data pretty much follows the same trends as the batting average data. However, while batting average indicates how often a player made solid contact with the ball resulting in at least a single or more, the home run data indicates that after aluminum bats were introduced more of the hit balls were going over the fence. There is a steady increase after aluminum bats were introduced, and after a brief surge in the mid 1980's the number of home runs per game dropped and settled to a fairly constant number during the early 1990's. I don't know yet what happened in the early 1980's to produce the surge in home runs, but 1985 was the year the very popular Easton Black Magic baseball bat was introduced.[4] Metal bat technology improved again after 1995 and the frequency of home runs reached an all-time record high during the 1998 season.* Then, as was the case for batting averages, the frequency of home runs sharply declined after the NCAA imposed performance limits on bats. Home run numbers now appear to be settling back close to values during the early 1990's or early 1980's, though they are still significantly higher than in the wood-only era prior to 1974.I have also made plots of Runs Scored per team per game and Earned-Run-Averages for pitchers, which follow the same general trends as batting average and home runs. * During the 1998 College World Series between the University of Southern California Trojans and Arizona State University Sun Devils (USC won 21-14) at least 35 out of 111 CWS records were broken and 17 more were tied.[5,6] Both teams used Louisville Slugger aluminum bats. Records broken during the championship game included runs scored (35), home runs (9), RBI's by one player (7). During the championship series USC also set records for team batting average (.378), runs scored (62), hits (88), and total bases (152). |
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An interesting data set is the number of strike outs per 9 innings for NCAA Division 1 college pitchers. During the same years that batting averages, home runs, and runs scored were increasing, pitchers were having a terrible time getting hitters to strike out. This may not be directly related to the introduction of aluminum bats, and bad pitching may also be part of the reason why batting averages and home runs increased in the years after aluminum bats were introduced to college baseball. Notice, for example, that the number of strike-outs per nine innings dropped by a full strike-out in the three years before aluminum bats were introduced. However, it is true that aluminum bats have had an impact not only on hitting, but also on pitching. One of the biggest impacts of aluminum bats on the pitching game is that they took away the effectiveness of inside pitches.[7] When a players were using wood bats, a pitcher could throw an inside pitch in attempt to jam the hitter. If the batter made contact with the ball it would likely be on the handle or the taper region of the bat. Balls hit in the handle or taper region of a wood bat don't go very far, and often result in a broken bat. As a result, pitchers could get a lot of players out by throwing inside pitches to batters swinging wood bats. However, aluminum bats don't break for hits on the handle. In fact, many hitters can still put the ball in play beyond the infield when the ball is hit from the handle or taper region of a metal bat. Some of the effects of aluminum bats on pitching can even be seen in the majors. Watch older players like Roger Clemens, or retired pitchers like Ryan Nolan, who grew up in the wood only era and learned to effectively use inside pitches to intimidate batters. They like to throw inside and often intentionally brush back (and sometimes hit) batters who crowd the plate. Many younger batters are used to being able to crowd the plate and get upset when a pitcher throws an inside pitch. And many younger pitchers aren't nearly as likely to throw inside pitches because they have learned through years of experience that inside pitches can be hit (by players using metal bats).
An interesting feature of the strike out data is that the number of strike outs began to increase in the late 1990's and reached an all-time peak in 1999, right around the same time that batting averages and home runs reached their peaks. This would suggest that most players were either hitting long balls for extra bases and home runs or striking out. It would be interesting to see the break down for base hits and doubles compared to home runs and strike outs for the same time period. I would expect to see a drop in base hits.
As a final note the data shows that during the last few years that the number of strike outs per 9 innings is right about the same as it was back in the wood-only era prior to 1974. College pitching seems to be catching up to metal bat technology.
One more set of statistics which illustrate the effect of aluminum bats on the level of play in college baseball comes from the Cape Cod Baseball League, the premier amateur summer baseball league for leading pro prospects. Prior to 1984 players in the CCBL were allowed to use aluminum bats. Starting with the 1985 season the league returned to wood bats only. The graph at right shows batting averages and number of home runs per game from the years 1981-1988. As the data shows, both the mean batting averages and the frequency of home runs per game dropped noticeably after the return to wood.[7] Most baseball players who successfully make the transition from using aluminum bats in college to playing with wood bats in the pros will agree that if you hit the ball just right with a wooden bat it will go about the same distance as a ball hit with aluminum. The trick is learning to hit the ball just right. With an aluminum bat you can hit the ball almost anywhere along the length of hte bat and still get the ball past the infield. With a wood bat the task is much more difficult.
An interesting 1994 study of Japanese High School baseball players[8] comparing the wood-only and metal bat eras in Japanese baseball found that after the introduction of metal bats, winning teams had a higher percentage of larger, stronger players. In the wood-only era winning teams won games by getting lots of men on base with well placed singles, moving players by stealing bases and sacrifice bunts, and applying squeeze plays. After metal bats were introduced, winning teams won mostly by relying on the long ball with larger players who had built up muscle mass through weight training and who could take advantage of the hotter metal bats to hit more multiple-base hits and home runs. Both the average player physique and the number of home runs per game increased substantially after the introduction of metal bats into Japanese high school baseball.
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