After solid tyres, pneumatic tyres provided a much smoother ride. Now Michelin is proposing the first airless tyre in a century

History of tyres

The function of pneumatic tyres is to provide traction between the vehicle and the road while providing a flexible cushion that absorbs shock. With this potential advance by Michelin, the tyre industry will be going a full circle.

The first practical pneumatic tyre was made by Scottish inventor John Boyd Dunlop while working on his son’s bicycle in an effort to prevent the headaches his son had while riding on rough roads. Dunlop’s patent was later declared invalid because of prior art by fellow Scot Robert William Thomson, although Dunlop is credited with “realising rubber could withstand the wear and tear of being a tyre while retaining its resilience”. The development of this technology hinged on myriad engineering advances. Today, over 1 billion pneumatic tyres are produced annually in over 400 tyre factories

Pneumatic TiresA pneumatic, or air-filled, tyre is made of an airtight inner core filled with pressurized air. A tread, usually reinforced with steel belting or other materials, covers this inner core and provides the contact area with the road. The pressure of the air inside the tyre is greater than atmospheric air pressure, so the tyre remains inflated even with the weight of a vehicle resting on it. The tyre’s air pressure provides resistance against forces that try to deform the tire, but it gives to a certain degree a cushioning effect as the tyre hits bumps in the road. If you have ever been taken for a ride in an old-fashioned carriage with wooden wheels, you would understand what a difference a pneumatic tyre makes.

Pneumatic tires do have drawbacks, especially in high-performance or highly dangerous applications. The main problem, of course, is that a puncture of the tyre results in total failure. A blowout at high speeds can lead to a dangerous car accident. Military planners are concerned with tires getting blown out by gunfire or explosion shrapnel. A vehicle crew’s worst nightmare is getting trapped in a fire zone because their tyres are all flat. Obviously, an airless tyre cannot be disabled by a single puncture.

 Introducing the Tweel

Now Michelin has developed the first airless tyre in 120 years which its marketing department has christened as Tweels, an airless tyre which nevertheless still provides shock protection.

Consider all the benefits if tyres that cannot be punctured and their impact on existing industries:

• No more flats
• No more repair kits
• No more air compressors at filling stations and
• No more values

Michelin first announced the Tweel in 2005. The name is a combination of the words tyre and wheel because the Tweel does not use a traditional wheel hub assembly. A solid inner hub mounts to the axle. In turn that is surrounded by polyurethane spokes arrayed in a pattern of wedges. A shear band is stretched across the spokes, forming the outer edge of the tyre (the part that comes in contact with the road). The tension of the shear band on the spokes and the strength of the spokes themselves replace the air pressure of a traditional tire. The tread is then attached to the shear band. The Tweel looks sort of like a very large, futuristic bicycle wheel.

 Components of a Tweel Tyre

When the Tweel is put to the road, the spokes absorb road impacts the same way air pressure does in pneumatic tires. The tread and shear bands deform temporarily as the spokes bend, then quickly spring back into shape. Tweels can be made with different spoke tensions, allowing for different handling characteristics.

More pliant spokes result in a more comfortable ride with improved handling. The lateral stiffness of the Tweel is also adjustable. However, you cannot adjust a Tweel once it has been manufactured. To do that one has to select a Tweel with the desired characteristics.

 

For testing, Michelin equipped an Audi A4 with Tweels made with five times as much lateral stiffness as a pneumatic tire, resulting in “very responsive handling”. Michelin reported that “the Tweel prototype… is within five percent of the rolling resistance and mass levels of current pneumatic tires. That translates to mean within one percent of the fuel economy” of the tyres on your own car. Since the Tweel is very early in its development, Michelin could be expected to improve those numbers.

The Future of Airless Tires

What is the likelihood of purchasing a Tweel in the new future? First Michelin has to eliminate several flaws, the worst being vibration. Above 50 mph, the Tweel vibrates considerably. That in itself might not be a problem, but it causes two other concerns: noise and heat. A fast moving Tweel is unpleasantly loud.

Furthermore long-distance driving at high speeds generates more heat than Michelin engineers would like.

A final problem involves the manufacturing process itself. Making Tweels is quite a vastly different process from making a pneumatic tire. The sheer scale of the changes that would need to be made to numerous factories, not to mention tyre balancing and mounting equipment in thousands of auto repair shops, presents a significant (though not insurmountable) obstacle to the broad adoption of airless tires.

Because of these flaws, Michelin is not planning to roll out the Tweel to consumers any time soon. According to Michelin, “Radial tyre technology will continue as the standard for a long time to come.”

Nevertheless there are applications which are eminently suited to the use of Tweels viz low-speed applications, such as on construction vehicles. The Tweel is perfect for such use because the high-speed vibration problems will not be a factor in their acceptance. Furthermore the ruggedness of the airless design will be a major advantage on a construction site.

Michelin is also exploring military use of the Tweel. Currently the major shortcoming of wheeled armoured vehicles are their tyres. Even though the temporary solution has been to install run-flats, this is merely a symptomatic solution whereas the Tweels address the root cause of the problem.

Finally there is one major objection from my viewpoint which Michelin has to address: the name Tweel.