Paul Ridenour, who was a close associate of the late Mo Pinel, worked for USBC in Equipment Specifications from 2006-10 in the time when the organization moved from the Milwaukee area to Arlington, Texas.

Ridenour was part of the team that worked on USBC’s ball motion study that, ironically, studied pretty much everything but hardness, which at the time had become a non-issue and would remain that way until the PURPLE HAMMER and PITCH BLACK made it an explosive issue again in bowling in the past few years.

Ridenour, who decided not to move to Texas to stay with USBC after his father died and his mother broke her leg a week later and needed his assistance, is a program manager for Design and Technical Solutions and has an MBA from the University of Wisconsin. He also remains a competitive bowler.

He has been an occasional source for 11thFrame.com when technical expertise has been needed, and as we chatted about USBC’s new urethane rules and reactions on social media, I suggested he write a guest column.

Here it is:

Understanding urethane bowling ball hardness

By PAUL RIDENOUR

There’s been a lot of debate and speculation in the bowling community about urethane bowling balls and their hardness. I’d like to share some science-based insight to help clear up a few misconceptions.

First, let’s look at the difference between urethane balls and reactive resin balls. Urethane bowling balls are made by combining two key components: polyols (the building blocks) and isocyanates (which connect the polyol chains). These chains can be long or short, hard or soft, and the mix of them determines the ball’s properties, including hardness.

To measure hardness, the United States Bowling Congress (USBC) and manufacturers use an ASTM method called D2240. This method sets strict limits for temperature and humidity during testing. While it’s widely accepted and non-destructive, it’s also sensitive and sometimes inconsistent. That’s why on-the-spot tests are more of a quick check than a precise measurement. Other hardness tests exist, like ultrasonic testing (lab-based and non-destructive) or destructive methods, but these aren’t practical for everyday ball checks.

A common belief is that urethane balls “test” softer over time, but don’t truly soften. In reality, they can. When urethane materials are exposed to moisture and heat, their internal chemical structure can change. This is similar to what pro shop operators see if plug material sits out too long and begins to foam. Over time, moisture in the air and the heat generated as balls travel down the lane can be absorbed, leading to structural changes that make balls test softer.

Research supports this possibility. A study by Xu and Liu in China showed that urethane materials can indeed soften under these conditions. To prove this in bowling, one could measure the “glass transition temperature” of brand-new versus used urethane balls using a tool called a Differential Scanning Calorimeter (DSC). My hypothesis is that older, softer-testing urethanes would show a lower glass transition temperature compared to new, unused ones of the same model.

This type of testing could confirm whether balls like the 2017/2018 Purple Hammers or older urethanes are chemically softer than new versions.

I’ll close with one of my favorite sayings from Mo Pinel: “Everyone is smarter than I am, don’t worry—they’ll all tell you they are.”

Xu, D.-H., Liu, F., Pan, G., Zhao, Z.-G., Yang, X., Shi, H.-C., & Luan, S.-F. (2021). Softening and hardening of thermal plastic polyurethane blends by water absorbed. Polymer, 228, 123926. https://doi.org/10.1016/j.polymer.2021.123926