Long development journey pays off for Tweel, Michelin

AKRON—For the record, Michelin's Steve Cron isn't a big fan of the name given to the product that has defined much of his 30-year career in the tire industry.

That product is the Tweel, the tire maker's easily recognizable product that combines a non-pneumatic tire with a polyurethane spoked wheel, and Cron was co-inventor of the technology with friend and colleague Tim Rhyne, who retired from Michelin last year.

"It's one of those things where someone just blurted out one day, 'It's like a tire and a wheel. It's a Tweel,' " Cron recalled. "It wasn't very popular, but once something like that sticks, it's hard to change it."

The senior principal product research engineer at Michelin North America Inc. has lived full time with the Tweel for the last 17-18 years, most of that with Rhyne. The two followed the Tweel from concept, through years of research, trial and error, and a premature showing in 2005 at the North American International Auto Show. That was followed, though, with applications in a wide range of non-automotive uses and Michelin's opening of a production facility in South Carolina dedicated to manufacturing the Tweel.

For his role in the development of this well-recognized Michelin product and his continued research into the next generation of non-pneumatic tire technology, Cron was presented with the 2018 Harold Herzlich Distinguished Technology award during the International Tire Exhibition & Conference last month in Akron.

Rubber & Plastics News presents the award in recognition of Harold Herzlich, who served as chairman of ITEC's technical program from the event's founding in 1994 until 2016 and was RPN's technical editor for 25 years before retiring at the end of 2017.

Long R&D journey

Cron earned a bachelor's degree in engineering from the University of Missouri and a master's in aeronautical engineering at the Air Force Institute of Technology at Wright Patterson Air Force base near Dayton, Ohio. That was followed by a six-year stint in the Air Force, including time working on intercontinental ballistic missiles.

He joined Michelin in Greenville in 1990, initially focusing on tire dynamics and vibrations and developed finite element code for simulating damped modal analysis of pneumatic tires.

Around 1997, he and Rhyne started musing about run-flat tires, which was an area of intense focus of a number tire producers during the 1990s. That included Michelin's Pax system, one of the many run-flat technologies that were high on hope but typically ended in failure.

Cron described he and Rhyne as a couple of tire mechanic nerds who were sort of thinking through how things work and how things might be approached in a better manner.

"You always looked at run-flats, and from a structural engineer's point of view, they never looked very happy," Cron said during an interview at ITEC. "Being squished, the rubber down there at the bottom and at the sidewalls was really working hard. It just made you think you could come up with a better way to do that, something a little more clever."

Cron and Rhyne weren't directly involved in run-flats at the time, but many of their friends and colleagues were. He said Rhyne really had taken it upon himself to think of a better way to do it.

At the same time, they were involved in a working group with marketing, sales and communications staff, where they were trying to cast a vision for the future of the tire industry. One of Cron's tasks was to describe to these non-technical types exactly how tires worked. He said it was a good exercise, being forced to explain to others what engineers like him took for granted.

"One of the things I was in the process of explaining to them is that your tire doesn't carry the load from the bottom," Cron said. "The ground is not pushing directly to the bottom of your wheel. In the automobile, you should think of the tire as hanging. The bead wires are supporting the wheels below, and the tire is hanging from the top."

With that in mind, it made it clear what was wrong with run-flats. "You've kind of messed up the pneumatic tire," he said. "You've taken the tire—a differential tension top-loaded structure when it's pneumatic—to being a bottom-loading direct from the ground to the rim.

"They don't work like the pneumatic tire in that they don't carry their load from the top," Cron said. "That's why they're hard to make work, because you have this little chunk of tire from the ground to the rim doing all the work."

Cron and Rhyne knew at this point that it was imperative to look at the problem differently, trying to develop a structure out in the tread band that could support the load being hung from it. They decided to take some of material used in Michelin's typical ZP run-flat tire and put it onto the summit of the design and see what could be done.

That's where simulation tools came in handy, and they used unimaginable amounts of CPU time running tests. "You need to build some kind of prototype to see if your thinking is realistic," Cron said. "You can do that with a finite element method and numeric simulation."

At first, the duo's work wasn't really classified as an official project. Most of it was done on "spare time, with spare money and spare molds," he said. Management was OK with letting them go ahead with it, as long as it wasn't interfering with their "real work."

That went on for some time, and at one point they found themselves doing demonstrations for then-Michelin CEO Francois Michelin. At this point, their work was still constructed to look like a tire, including a sidewall but no air. Cron said they actually had to put slots in the tire to prove to Francois Michelin there was no air pressure building up on the inside of the tire.

Lightbulb moment

At this point Cron and Rhyne were trying to think of other ways to create a connection between the ring on the outside and the hub. They began by simply using a tire sidewall, but modeling and math showed that was not the best way to make the connection. "If you connect yourself all the way across the width of the ring down to the hub, then it would be a much more efficient way to get the load between the ring and the hub," Cron said.

They did equip a Chevy Corvette with a set of the NPX tires—as they were named—and took it on a cross-country road trip to Las Vegas. They said the tires had a comfortable, quiet ride with decent handling and high-speed management, but the tires' weight and inability to properly adjust to cornering made them unfit for the market.

"In a straight line, we could run for a long, long time," Cron said, "but any kind of cornering and we would have eventual carcass compression fatigue."

They knew they needed a different design to make it the concept work. About the same time, Michelin was making Pax support rings out of cast urethane. "We said let's just make a lightweight spoke type of structure to go between the reinforced rubber ring and the hub," he said.

The concept they came up with comprises a rigid hub connected to a shear band by means of flexible, deformable urethane spokes and a tread band, all functioning as a single unit. And thus the Tweel was born.

There was a period of about two years beginning in 2000 where work went forward on the Tweel and the NPX designs until the latter was phased out. Cron describes the 2000-09 time period as the time of the "Tweel tire sand box," during which the team developed a whole assortment of applications for the Tweel.

It was at the 2005 Detroit auto show that Michelin unveiled a Tweel concept tire for automobiles. "At that point we were a little bit ahead of ourselves," he said. "When we built that first automotive Tweel, that really made it clear it would be smarter to go work on low-speed applications first, and then work your way up to the higher-speed applications."

Measuring progress over such a long-term project could be tricky. This was accomplished primarily by validating the simulations or mathematics with a prototype.

"So you go build a physical, experimental tire and go test it," he said. "Load it. Roll it. Do whatever you have to do to test it and find out how good your predictions were. If they're good, that's a leaping off point for the next step."

Making progress

The first successful application was a Tweel for skid-steer loaders. "It was so good compared to the pneumatic tire," Cron said. "People just loved it whenever we would do a demo, either internally or externally for potential customers. I would say that was the first real success in terms of performance."

He doesn't classify that as the biggest commercial success, however, because skid-steer loaders have other solutions, such as solid tires and track, that some customers still favor.

So that distinction goes to the Tweel made for the zero-turn radius mower, according to the Michelin engineer. "On mowers, everything's on pneumatic tires," Cron said. "And if you run a commercial lawn mowing operation and you have a flat tire on one of those big mowers, it's a bad day. You better have a spare with you and equipment to change it."

Besides turf and skid-steer loaders, the third main product area Michelin does business in is utility vehicles, which includes golf carts and some ATVs.

Another milestone came in 2014 when Michelin invested $50 million to construct and equip a 135,000-sq.-ft. factory in Piedmont, S.C., to manufacture Tweels. Cron saw that as an indication the Michelin sees a future in the technology. "Anytime you have a new technology and they're building a plant to produce it, that's pretty exciting, for the community as well as the company," he said.

As Cron remains focused on non-pneumatic tire technology for Michelin, he has moved on from the commercial product side to focusing on the next generation of the product. But he still sees bigger things in store for the Tweel, a product he has lived with for the last two decades.

"The Tweel I definitely consider a technical success," he said. "It still needs some work to become a really great commercial success. Technically, though, it's successful."