Charles Goodyear Medalists tell story of Tweel's journey

WARRENSVILLE HEIGHTS, Ohio—Tim Rhyne and Steve Cron have a hard time picturing themselves as the type of researchers who would receive the Charles Goodyear Medal.

The two retired Michelin North America engineers were honored with the highest honor of the ACS Rubber Division during the association's recent Spring Technical Meeting in Warrensville Heights for their work in developing the Tweel, the tire maker's airless tire that included a non-pneumatic tire with a polyurethane-spoked wheel.

Rhyne called the news that they had been selected as co-medalists—the first time the Rubber Division has had joint winners—as "totally unexpected, probably undeserved." When asked why it would not be deserved, he said the two were not chemists, as a majority of the medalists are.

Cron, in fact, said when he perused the list of past winners, the only other person he saw that had been honored for developing an end product was another Michelin alum, Jean-Marie Massoubre, a researcher associated with the early development of the radial tire.

"It's hard when you've been down in the guts of something like this," Cron said. "It's hard to see this as being significant. It's just what we did. Everybody else is doing good work too. It doesn't seem like we did anything other than just doing our jobs."

The two said they felt a bit more comfortable when the Tire Society last year bestowed them with the group's Distinguished Achievement Award, explaining that they'd both been members of the society and served as associate editors of the group's Tire Science and Technology journal.

Though humble about receiving the Charles Goodyear Medal, the work the two did to help take the Tweel from an idea to a prototype and then to a tire that has proved successful in a wide range of non-automotive uses, was not lost on those who select the recipients of the Rubber Division's Science & Technology Awards.

The Tweel has its own dedicated production facility in South Carolina and accounts for more than $25 million in annual revenue for Michelin.

"There were many challenging technical issues that were overcome in order to bring airless tire technology to a sufficiently high level of performance to compete in the marketplace," said one of the letters nominating them for the medal. "Drs. Rhyne and Cron have worked tirelessly and with unrivaled creativity to resolve the outstanding technical issues, and their work has clearly resulted in unprecedented success."

The two collectively spent 70 years at Michelin, with Rhyne working there from 1978 to 2017, when he retired as a senior research fellow. Cron's career at the tire maker began in 1991 and he retired in 2021 as a senior principal product research engineer.

Rhyne, in fact, resisted Michelin's advances for quite some time before joining the Greenville, S.C.-based tire maker. He received both bachelor's and doctorates in mechanical engineering from North Carolina State University.

While in graduate school, one of his friends who worked for Michelin passed on Rhyne's name to Michelin recruiters, but Rhyne said he wasn't interested. "I was working for a small machine shop and I intended to stay with them to see if we could grow the business after I graduated," he told sister publication Rubber News.

The tire maker came calling again a year later, and they invited him down for an interview. He said they put him in a room with someone on both sides of him. One would hand him a math problem to work out, while the other would write up another equation for him to tackle immediately after.

This went on for three hours, and had he known that was the process, he said he never would have come to the interview.

But he did well, as he had just passed the doctoral math preliminary exam at N.C. State, the same test given to math majors. "For that, I had studied nothing but math for six months to get ready, because they like to flunk the engineers," Rhyne said. "So for that brief shining moment I knew as much math as those two guys. They couldn't write a problem I couldn't solve."

Michelin called a few days later and made an offer. Rhyne turned them down, giving them "some big number" that he said he would not work for less than. The official on the phone, Rhyne said, was mad and slammed the phone down.

"About two weeks later they called me back and said, 'We'll meet your number,' " he said. "And I thought, 'Oh God, what am I going to do now? They called my bluff.' And that's how I ended up at Michelin."

Rhyne started in machine design, and ended up running that part of the business. "They let me design and build a machine shop for the research facility," he said. "That was the best project I ever worked on, except for maybe the Tweel."

After a time, a superior asked him to go into tire design, but he resisted. He felt tire design was "just too weird," and besides, he liked metal.

But the next year at review time, he was told he was going in tire design. It turned out to be something Rhyne enjoyed, as he designed tires for 10 years.

"You want to be terrified, I had never designed a tire in my life. I had a small team and we got this tire line done," he recalled. "You went up to the factory next door, and there's our tire streaming down these conveyors by the thousands and thousands. I thought, 'Oh my goodness. I'm sure we made a mistake somewhere.' "

It was the first tire at Michelin that was fully designed with the use of finite element analysis. It was also the first 80,000-mile tire, and Rhyne said Michelin produced about 100 million tires from the line in various versions—but basically the same design—over about 10 years.

Cron's journey to Michelin didn't have quite the twists and turns as Rhyne's. He received a bachelor's in mechanical engineering from the University of Missouri, and a master's in aeronautical engineering from the U.S. Air Force Institute of Technology.

During his military service, he was sent out to California to work on intercontinental ballistic missiles, an assignment he didn't care for because it was very bureaucratic. He got the chance to teach at the academy in 1988, which he did enjoy.

"It really sort of helped solidify my understanding of (the) basics of mechanics," Cron said. "Just the simplest things, because I had to teach it five times. Every two days, teach it over and over. Things you learn in school but you don't really quite have them internalized."

But he wasn't planning on being a military lifer, so he went to a recruiting conference with Michelin, and started at Michelin Americas Research Center in 1991.

Cron said the theme for much of his career was simulation. He also did tire design like Rhyne, and led a team there for awhile.

"I always seemed to have this pull to come back to do more simulation and design work," he said.

At some point they both ended up in the research side of Michelin, and started having lunch together, though neither quite remembers how that started.

In 1997, the lunch discussions turned toward successful implementation of various structural concepts for non-pneumatic tires. It wasn't anything formal, just working in their spare time. Cron said he began using a fair amount of simulation time, scheduling every computer on campus to run simulations from 8 p.m. to 7 a.m. each day, so as not to interfere with the work of others.

In early 1998, Rhyne said they developed a picture of what they thought would work. That led to building the first prototypes by late 1998, and an invitation to demonstrate the technology to Francois Michelin.

He went to France with the non-pneumatic tires, which still had sidewalls at this point, and drove Francois and Edouard Michelin around the firm's test track.

"It was quiet, comfortable and very nice. It was also heavy, had high rolling resistance and was very costly to make. But besides that, it was good," Rhyne quipped.

Francois, however, wasn't convinced.

He said Rhyne was letting the tire warm up and then it would inflate. Rhyne had to show him there was no valve stem and no air in the tire.

Up to that time, the two said they had to "beg, borrow and steal to get a mold." Rhyne said they then got some molds from France, and they refined that concept to the point where the two equipped a Corvette with Tweels and drove it successfully from South Carolina to California and back.

One key element was using the bead from the Pax run-flat tire that Michelin had unsuccessfully tried to perfect at that time. The bead from Pax worked well with the Tweel concept.

Sometime around 2000, after working under the radar for three years, Cron and Rhyne got the freedom and support to work on the project full time. Cron said from that point until about 2009, they worked of a wide variety of potential applications. From the Segway to IBOT wheel chairs, the Tweel was getting a lot of notice.

Rhyne said one hangup, however, was something referenced in the "Innovator's Dilemma," a book by Clayton Christensen. One aspect is that new technology really needs to start from the bottom and work its way up. "Of course what the company wants to do is start at the top because that's where the money is," Rhyne said.

But the unveilings in 2005 at the Detroit and Paris Auto Show were too early, according to Cron. "We could drive around in a car, but we were nowhere close to being ready to go to market with that," he said.

On the other hand, the Tweel was excelling in the mundane applications, such as lawn mowers, skid loaders, hand trucks and wheelchairs. In other words, much like Christensen's work suggested a product such as the Tweel should progress.

"In the end, that is how we entered the market. On turf and construction equipment, and today that's what we sell still is off-road, low-speed unregulated products," Cron said.

By about 2011-12, Cron said it became clear that the Tweel technology was not going to be successful on automobiles. So Michelin constructed a factory to build the Tweel and assemble a team to build on the applications that were successful.

Rhyne and Cron, though, weren't part of that team. They were to stay and work on passenger tire technology.

Michelin told Rubber News it was determined that cars provide a new set of challenges, particularly in providing handling close to pneumatic tires and driving at higher speed. That evolved into what Michelin now calls its Uptis design, which features rubber spokes reinforced with resin-embedded fiberglass.

Having a good part of their careers focused on one technology is a double-edge sword, the Charles Goodyear Medalists said.

Cron said it's good to be able to concentrate on one thing to keep making it better, but then there is a sense of disconnect to other parts of Michelin.

Said Rhyne: "It was very enjoyable, but we had deep roots in the pneumatic tire part of the business ... and that's the main business of the company."

While the Tweel never took over the car tire market as hoped, its inventors are proud of what they did accomplish and the applications where it does shine.

Cron, though, still wishes there had been Tweels made for smaller uses. He pointed out a cart nearby equipped with wheels.

"That thing is a disaster," he said. "If you go over a bump, whatever is on the table will jump off. Plates will break. I know how to make a Tweel for that thing that is so good. Spectacularly good."

Spoken like a deserving recipient of the Charles Goodyear Medal.