Reinventing the Wheel (and the Tire, Too)
By NORMAN MAYERSOHN

Published: January 3, 2005

GREENVILLE, S.C.


The Tweel, an experimental tire and wheel combination developed by Michelin, is designed to replace today's air-filled tires. Flexible polyurethane spokes deflect over obstacles.

THE first automobile to use air-filled tires was a racecar built by André and Édouard Michelin in the early 1890's. More than a century later, the French company founded by the Michelin brothers is so identified with pneumatic tires that its mascot, Bibendum, is a man made of little else.

Now, after decades spent persuading the world to ride on air, the company has begun work on an innovation that could render the pneumatic tire obsolete. Engineers at Michelin's American technology center here envision a future in which vehicles would ride on what they call the Tweel, a combined tire and wheel that could never go flat because it contains no air.

Arriving at a conference room recently to explain the development project, a research engineer, Bart Thompson, used the Segway Human Transporter that he rode to the meeting to illustrate his points. Aboard this high-tech visual aid - one of those self-balancing electric scooters best remembered for the optimistic claim that it would reinvent personal transportation - Mr. Thompson whizzed down the hallway and out to the lobby, pirouetting among the benches and planters to demonstrate the flexibility of the Tweel.

To be sure, the Segway would be a very small market for Michelin, the world's leading tiremaker, but it is an apt demonstration vehicle for the Tweel. The first commercial use of the integrated tire and wheel assembly will be on the stair-climbing iBOT wheelchair, another product developed by Dean Kamen, the Segway's inventor; Michelin said it would announce another application at the Detroit auto show next week.

The tiremaker has high expectations for the Tweel project. The concept of a single-piece tire and wheel assembly is one the company expects to spread to passenger cars and, eventually, to construction equipment and aircraft.

The Tweel offers a number of benefits beyond the obvious attraction of being impervious to nails in the road. The tread will last two to three times as long as today's radial tires, Michelin says, and when it does wear thin it can be retreaded.

For manufacturers, the Tweel offers an opportunity to reduce the number of parts, eliminating most of the 23 components of a typical new tire as well as the costly air-pressure monitors that will soon be required on new vehicles in the United States.

In recent years, manufacturers have devoted an increasing amount of attention to tires that let motorists continue driving after a puncture, for 100 miles or more, at a reduced speed. Several such "run flat" designs are now available, providing convenience and peace of mind for travelers as well as freeing automakers to eliminate the weight and cost of spare tires.

Michelin, which markets run-flat tires under the Pax name, took a different approach in developing the Tweel. Its goal: a replacement for traditional tires that is designed to function without air in the first place.

Mounted on a car, the Tweel is a single unit, though it actually begins as an assembly of four pieces bonded together: the hub, a polyurethane spoke section, a "shear band" surrounding the spokes, and the tread band - the rubber layer that wraps around the circumference and touches the pavement.

While the Tweel's hub functions as it would in a normal wheel - a rigid attachment point to the axle - the polyurethane spokes are flexible to help absorb road impacts. The shear band surrounding the spokes effectively takes the place of the air pressure, distributing the load. The tread is similar in appearance to a conventional tire.

One of the basic shortcomings of a tire filled with air is that the inflation pressure is distributed equally around the tire, both up and down (vertically) as well as side-to side (laterally). That property keeps the tire round, but it also means that raising the pressure to improve cornering - increasing lateral stiffness - also adds up-down stiffness, making the ride harsher.

With the Tweel's injection-molded spokes, those characteristics are no longer linked - a point of particular excitement to an engineer like Mr. Thompson because of the potential it holds for improving handling response. The spokes can be engineered to give the Tweel five times as much lateral stiffness as current pneumatic tires without any loss of ride comfort.

The Tweel auto project is in its infancy - "Version 1.0," Mr. Thompson said - and only a single set of car Tweels exist. A test drive in a Tweel-equipped Audi A4 sedan on roads around Michelin's research center proved to be far less exotic than the construction method or appearance would suggest. The prototype Tweels are noisy, as Mr. Thompson warned they would be, a problem traced to vibration in the spokes.

The Tweels also transmit more of the feel of a coarse road surface than customers would tolerate in a production tire, but the level is understandable considering the early stage of development. More important, the steering's response as the driver begins a turn is excellent, and large bumps were swallowed up easily by the Tweels and the Audi's unmodified suspension.

There are other negatives: the flexibility, at this stage, contributes to greater friction, though it is within 5 percent of that generated by a conventional radial tire. And so far, the Tweel is no lighter than the tire and wheel it replaces.

Almost everything else about the Tweel is undetermined at this early stage of development, including serious matters like cost and frivolous questions like the possibilities of chrome-plating.

Logical uses - military vehicles, for example - would come years before automobiles, but Michelin's business projections accommodate the possibility that the Tweel may not be an overnight success. This would be nothing new for Michelin: the radial tire it invented in 1946 was not widely accepted in the United States until the 1970's.

The Tweel's see-through hub and spoke design make it look like a black rubber wagon wheel.