The Effects of Bowling Ball Hardness: What is Special About the Purple Hammer Bowling Ball?

With all that is currently going on surrounding bowling ball hardness, Creating the Difference decided to run some tests in an effort to help explain some things about hardness and help the bowling community have a better understanding of this topic. Though not an exhaustive study, we wanted to collect some information and see if we could understand more about this topic.  With the help of Mikey Pinel, we sought to answer some of the following questions:  

• Does changing the hardness of a ball have a direct impact on both footprint and/or performance? 
• Is there a way to tie hardness to performance? 
• Why do Pros throw Purple Hammer so much that it is the dominant ball on the PBA Tour?

A big controversy in bowling is centered around the use of a Durometer. Let's start there with what is a durometer? This picture shows what a durometer looks like and how it is used:

A durometer is a device which is used to indent an object (like a bowling ball) to measure the resistance to indentation.  This resistance reflects the hardness of an object. Since objects can be very soft or hard, there are multiple Durometer scales used to determine hardness. In bowling we use the Shore D Scale.  

How does it work?

The indenter - in this case, a needle - penetrates the sample and measures how far the indenter travels, up to 0.100 microinches.  The Shore D scale used for bowling balls goes from 0 to 100 microinches. There are other Shore scales; however, for bowling balls, Shore D is the best scale because it covers the full range that bowling balls fall within. The Shore D scale indenter has a spring force of 10 lbs.  According to the USBC Specifications Manual: “for an approved bowling ball the minimum hardness changed from 72D to 73D on Dec. 31, 2020. Production of models previously approved below 73D must cease after July 31, 2022. The average surface hardness from 10 readings around the bowling ball shall not be less than 73 durometer D when measured in compliance with ASTM D2240-15 at 70-77 degrees Fahrenheit. NOTE: Minimum hardness”

To put this in perspective here are some examples of common materials and where they fall on and various Shore Scales:

For our testing, we brought in David, who is an expert on checking bowling ball hardness.  We discussed what we were going to check and test.  Using the Standard Operating Procedure from USBC, we checked the hardness of each of our bowling balls, the results of the averaged data are in the figure below.

We learned some things from this testing.

1. Bowling balls can have a very different hardness in different spots.
2. Ball color can impact hardness Yellow was harder than Purple, for example.
3. Phasing can impact hardness.
4. Sanding a bowling ball didn't impact hardness during our testing.
5. None of the Brunswick made Purple Hammers we checked were soft.
6. Multiple Ebonite made Purple Hammers were soft.
7. Multiple Storm Spectres were soft.
8. Temperature can impact hardness.

Our testing is only reflective of us following the USBC's SOP on hardness testing. This isn't to say the results would be the same if this test were done by USBC.

Based on our results, we felt there was a wide enough range in hardness with our initial testing to go to the next step in determining what these numbers mean.  Our testing parameters for the next step were developed with the help of Neil, who is an expert in bowling ball testing. Our process was as follows:

1. Narrowed down the balls
2. Using a ramp, we rolled each ball multiple times from a set height over a piece of carbon receipt paper
3. Collected the imprint on the paper left by the ball
1. This is called the footprint of the bowling ball. 
4. Using a micrometer, we measured each footprint in inches.

The results can be found in the figure below:

From this data, you can see a loose correlation between hardness and footprint size. In general, the softer the bowling ball, the larger the footprint. Next, we wanted to see if we could get any performance information from rolling these balls down the ramp. We wanted to see if there was any correlation between ball Speed Loss (Delta Speed), Footprint and Hardness.  We re-oiled our lane using the Red Square Flat Pattern. Using our ramp, we placed each ball with the pin at 12 o'clock and the CG at 6 o'clock parallel to the lane.  This positioning was meant to remove core dynamics from the test. We rolled each ball down the ramp multiple times and measured ball speed change down the lane using Specto.

The results of our test can be found in the figure below:

Note: this test simulates rolling friction which is different from sliding friction.  Rolling friction happens when a ball is rotating in the same direction it is traveling.  A bowling ball only does this, if ever, on the very backend of the lane. This is typically called rollout. More commonly, a bowling ball experiences sliding friction when the rev rate increases while the ball speed decreases over time. 

Watch this video to see a bowling ball increase rev rate over time. In the video, the ball never rolls out. This is noted by the increase in rev rate all the way to the end of the video. This indicates that our testing method doesn't fully cover real life ball motion. However, this testing method is repeatable and consistent.

For those who are paying attention: there is an additional lesson here. A bowling ball either slides OR it rolls. Here is a graphic to help illustrate the point:

With the collected data, we can see there is a loose correlation between hardness, footprint and delta speed. The data indicates softer balls lose more speed.

Next, we wanted to find out if this had any relevance to actual use in the bowling world. For this test, we decided to bowl and collect more data. We took a ball that was 73.8D and softened it to 64.9D. Don’t ask how we did it. No, it was not using a solvent like MEK or Acetone. We bowled on our flat pattern while the ball was at 73.8D. We treated the ball to soften it; then, bowled again.  We used Specto to collect all data both before and after treating the ball.  

This testing created the following interesting observations:

1. When our ball was 73.8D it behaved erratically on the lanes: 
1. It was sensitive to speed
2. It was sensitive to any sort of change to the bowler input
3. There was essentially no room for error
4. It was extremely difficult to hit the pocket consistently
2. When our ball was 64.9D we observed less erratic behavior
1. The ball slowed down 1 MPH more than when it was at 73.8D
2. The ball hooked from places it never did before softening
3. We had about 5 boards of area to hit and still hit the pocket 
4. The ball was much less sensitive to speed changes

The following video shows a comparison of the same ball being thrown at 73.8D vs 64.9D:

It seems that as of late, the majority of PBA Shows feature someone or multiple people throwing Purple Hammers. Click here for an example or just go watch any PBA show. After looking at the data and discussing the results with the team, we came up with the following conclusions:

Pros who bowl in an environment that is typically flat front to back, like the PBA sport patterns, can benefit from using bowling balls that are softer.  A softer bowling ball slows down more than a harder one and has a larger footprint.  This allows it to generate friction earlier in the pattern. This is a similar effect to when the lane pattern has a taper in it from front to back. That is customary in a typical house pattern. The Purple Hammer can make a lane pattern “play” easier because it is more forgiving than harder bowling balls or reactive bowling balls.  A softer Purple Hammer allows the pattern to play more blended. This is good for a bowler for many reasons; but, in particular, it makes it easier to stay in the pocket and adjustments are easier to predict. 

Our experiment exaggerated this effect because of the softness of the ball. Our tester had a rev rate of 350 RPM’s.  A top level pro is around 500 RPMs. This would indicate that a pro should be able to see the impact of softness at a higher hardness level similar to what we saw at 64.9D. This theory is also backed up by the fact that you see pros using the Purple Hammer a lot on TV. There have been entire shows where that was the only ball used. Yet you rarely see a Black Hammer being used on tour even though it has been around longer. The Black Hammer is the same core and base coverstock materials as the Purple Hammer but it is 78D-79D hardness. Player rev rate also matters because you see a lot less PWBA women throw that ball on tour.  Most of the women tend to have a lower rev rate compared to the men.

One of the most glaring points indicating the Purple Hammer has a competitive advantage over the competition, is that there is a lot of data that suggests that Purple Hammers get softer with use and over time. Now you can begin to understand why it is such a dominant ball. If there were ever a perfect ball, which there isn’t, it would be one that got “better” with use. As these Purple Hammers get softer, they become even more useful (like the 65D one we treated).  The pros know some Purple Hammers are more usable, which helps explain why many of them have older Purple Hammers. The PBA has created a rule to address this.  If you are interested click here to learn more.

All of this is not to say that you should run out and go buy a Purple Hammer. Remember the  pros we enjoy watching on TV are better than most of us amateurs. They are bowling in an environment we don’t get to see very often, if ever. In addition, they are bowling with people who throw it like they do.  This means the transition they see on tour is very different than what the average bowler sees in leagues or tournaments.  

House Shot vs PBA (Sport) Shots

When bowling on a typical house pattern, the "tapered lane pattern" advantage created by a softer urethane bowling ball can be neutralized by the fact that a taper is built into the lane pattern already. Thus, a reactive ball can take better advantage of that type of lane condition for the vast majority of bowlers. Though urethane has its place, it's not for everyone or every lane condition.

If you are curious about how the Purple Hammer is different from other balls that are called "urethane" watch this video.

Thank you David, Mikey, and Neil for your input and help getting this initial test done.  Though our study was not complete, it is a start.  The PBA and USBC are working to make changes in an attempt to curb potentially illegal products in the playing environment in ways each organization sees fit. Though not perfect, much like our study, it is a start.  We are optimistic that we will all learn and take some positives away from the current situation; and that bowling will be better for it in the end.