Airless Tyres - Seminar Report

Airless Tyres
For more than 100 years, vehicles have been rolling along on cushions of air encased in rubber. The pneumatic tyre has served drivers and passengers well on road and off, but a new design by Michelin could change all that – the tweel airless tyre .The tweel  (a portmanteau of tyre and wheel) is an experimental tyre design developed by the French tyre company Michelin. The tyre uses no air, and therefore cannot burst or become flat. Instead, the Tweel's hub connects to flexible polyurethane spokes which are used to support an outer rim and assume the shock-absorbing role of a traditional tyre's pneumatic properties.

The increasing concerns over the green-house effect will in the near future require more attention to rolling resistance than ever before; in fact from an already high attention to a very high attention. 
The trend towards lower rolling resistance has been obvious for many years. Significant progress was reported in the recent Tyre Energy Efficiency Report in reducing rolling resistance, as measured for new passenger tyres, over the past 25 years. More tyre models today, when measured new, have rolling resistance coefficients below 0.009, and the most energy-efficient tyres have coefficients that are 20 to 30 percent lower than the most energy efficient radial models of 25 years ago. 
Another trend is the increased popularity of run-flat tyres; mostly having stiffer sidewalls or some material added that can avoid running a flat tyre on the rim. The above-mentioned Tyre Energy Efficiency Report concluded that run-flat tyres weigh more than conventional radial tyres — which increases their material and production cost — and they tend to exhibit higher rolling resis-tance. This author thinks that this may turn the trend back to more traditional designs, or turn the interest over into designs which have run-flat capabilities without increased rolling resistance. 
The increasing popularity and more frequent governmental support for hybrid or electric veh-icles will also require lower rolling resistance since this directly affects the distance one can run in the electric mode. 
Finally, it shall be mentioned that labeling of energy efficiency (in practice rolling resistance) of tyres is likely to happen in the near future. The intention is that consumers will use this informa-tion to their selection of replacement tyres; per-haps even vehicle manufacturers would use such information when deciding on OE tyres if this information will be available for the full range of tyre brands and dimensions and not only be determined by themselves for a few tyres. A conference organized by the IEA in November 2005 [IEA, 2005] indicated a rather universal support for the labeling of energy efficiency and also the Tyre Energy Efficiency Report suggested this.

Both rolling resistance and noise emission are expressions of energy losses in the rolling of tyres. It is not surprising that these characteristics are at large positively correlated; although exceptions exist. Nevertheless, it is this author's conclusion that exterior noise and rolling resistance will drive the tyre development to a large extent in the coming years [Sandberg, 2003]. Probably, the present focus on high-speed and high-power performance, which both are in some conflict with low noise and rolling resistance (and thus air pollution), will at last have to give in to the latter performances. 
Another present trend is the high priority put on the visual appearance of tyres, as a selling argument; in particular for "sporty" vehicles. The styling trend was heavily criticized recently as being in conflict with good technology by one of the foremost tyre experts in the world, Dr Joe Walter, in a column in Tire Technology Interna-tional [Walter, 2006]. It is likely that this trend will be broken when it is in conflict with the increasing environmental demands. 
Vehicle manufacturers will have to face the possible effects of this which may be uncom-fortable to some. 

Using traditional technology, the author suggests the following options as a few examples for reduction of noise: 
Adapting winter tyres for all-year use: The principles used in construction of winter tyres may be partly adapted to summer tyres; in order that summer tyres may obtain some of the favourable noise characteristics of winter tyres; yet having handling and wet friction properties acceptable for summer use. This may include using smaller tread elements, more frequent siping and softer rubber compounds. some compromises like these mentioned above are already seen in the all-weather designs being so popular in the USA. 
Can some winter tyres even be used the entire year? To answer this question, it is interesting that the author knows some tyre experts working for tyre companies who use "pure" winter tyres all the year. This is not to say that all or most winter tyres would be suitable also for summer use, but it suggests that at least some of them are so; probably with some sacrifices, for example wear. 
Reducing the air/rubber ratio in the tread pattern:
 In the SILENCE project one of the possibilities being explored is the reduction of the air/rubber ratio in the tread pattern; for example by reducing the width of channels in the tread pattern. It has been found that a combination of softer rubber and lower air/rubber ratio may influence tyre/road noise emission on an ISO surface by about 6 dB(A). If, today’s common ratio of 30 % is replaced with 20 % this would give a potential noise reduction of 3 dB(A). Work will continue; for example to see how a reduction from 30 to 20 % may be combined with acceptable hydroplaning characteristics (this may be difficult for high-performance cars). 
Using softer rubber compounds:
 Typically, winter tyres may have a Shore hardness of 55-60. It has been well demonstrated that softer rubber compounds result in lower noise emission, other things being equal. If tyres did not have to be produced for such high speed categories as today, softer compounds may be used. Softer tyre rubber compounds are already used in Japan and in USA, but in Europe they are considered less acceptable due to the high maximum speeds on certain motorways. If, for example, the green-house effect will force also Europe to introduce maximum speed limits on all motorways, the situ-ation might approach that in Japan and USA.

The examples above have potentially lower rolling resistance in common to the lower noise emission. However, the rubber compound is of extra importance here and additions such as silica mean progress to this performance parameter. 

An example of a successful noise reduction design was presented in [Saemann et al, 2001]. Dr Saemann and his colleagues had produced, by means of traditional measures, a truck tyre that was equally quiet as a slick tyre. However, al-though the tyre had fully acceptable properties in other respects than noise, it was found that this tyre was not desired or needed by the vehicle industry, partly due to its visual appearance, partly due to that there was no need for any quieter tyre by the vehicle industry. 

The pneumatic tyre provides a rolling performance in most important respects that is amazing. Only a minor defect may demonstrate that this performance is not a matter of course but a result of a sensitive design. But this does not go without saying that the pneumatic tyre is the only useful device that could provide a safe, quiet and economic rolling for a vehicle. If a mere fraction of all the resources spent on tyre development so far would be spent on, for example, development of the composite wheel or the so-called TWEEL (see below), what can one achieve then? 
An interesting editorial appeared in Tire Techno-logy International recently. It was written by the former Director of Research at Dunlop Tyres in the UK, Dr A. R. Williams.
What is standing behind the corner? Are there some tyre innovations or unconventional designs that may offer a breakthrough or at least a large step towards lower noise emission and rolling re-sistance? The following describes a few examples of such attempts currently being explored.

The Tweel consists of a cable-reinforced band of conventional "tyre" rubber with molded tread, a shear band just below the tread that creates a compliant contact patch, and a series of energy-absorbing polyurethane spokes. The rectangular spokes can be designed to have a range of stiffnesses, so engineers can control how the Tweel handles loads. The inner hub contains a matrix of deformable plastic structures that flex under load and return to their original shape. By varying the thickness and size of the spokes, Michelin can generate a wide array of ride and handling qualities. The tread can be as specialised as any of today's tyres and is replaceable when worn.

The Tweel doesn’t use a traditional wheel hub assembly. A solid inner hub mounts to the axle and 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. On it sits the tread, the part that comes in contact with the surface of the road. The cushion formed by the air trapped inside a conventional tire is replaced by the strength of the spokes, which receive the tension of the shear band. Placed on the shear band is the tread, the part that makes contact with the surface of the road. When the Tweel is running on the road, the spokes absorb road defects the same way air pressure does in the case of pneumatic tires. The flexible tread and shear bands deform temporarily as the spokes bend, then quickly go back to the initial shape. Different spoke tensions can be used, as required by the handling characteristics and lateral stiffness can also vary. However, once produced the Tweel’s spoke tensions and lateral stiffness cannot be adjusted

 One of the greatest advantages of this technology would be the fact that the tyre is service-free. No more air pressure check, no more flat tires and no more blow-outs mean a lot less to worry about when driving car. It is also conceived to last longer. Also, the balancing between traction and comfort could become a thing of the past. That’s because Michelin has found that it can tune Tweel performances independently of each other, which is a significant change from conventional tires. This means that vertical stiffness (which primarily affects ride comfort) and lateral stiffness (which affects handling and cornering) can both be optimised, pushing the performance envelope in these applications and enabling new performances not possible for current inflated tires.
It doesn’t require maintenance and it is risk-free, the Tweel tyre could be a good choice for special vehicles like those used in the army, in the construction business or even in the exploration of other planets. In 2009, Michelin has developed for NASA a Tweel-based tyre to be used in the latest generation of lunar rover vehicles. The Michelin Lunar Wheel maintains flexibility and constant ground pressure, allowing the vehicle to move through loose soil and craters. In addition, it combines low mass and high payload capacity, making it 3.3 times more efficient than the original Apollo Lunar Rover wheels. Its textile tread enables the rover to maintain traction at very low temperatures.
Tweel technology could also penetrate the personal mobility market. At the public demonstration of the Tweel, Michelin placed prototypes on the iBOT, a personal mobility device for physically impaired people, and the Segway Centaur, a four-wheeled ATV-type vehicle that uses Segway’s self-balancing technology. 

It is not the perfect tire. At least not yet. One of its biggest flaws is vibration. Above 50 mph, the Tweel vibrates considerably, thus generating noise and heat. A fast moving Tweel is reportedly unpleasantly loud. Long distance driving at high speeds generates more heat than Michelin engineers would like. That’s why, for the moment, the first applications of the Tweel are in low-speed vehicles, such as construction vehicles. The Tweel is perfect for such use because the ruggedness of the airless design will be a major advantage on a construction site. Michelin is also exploring military use of the Tweel, which would be ideal in combat situations, where conventional tyres are an easy target. 
Another big obstacle in the Tweel’s way is the tire industry itself. Making Tweels is quite a different process than making a pneumatic tire. The retooling of the many tire factories, plus the equipment necessary to service the new tire around the world represents also an important obstacle to the broad adoption of airless tires. Because of these drawbacks, Michelin is not planning to roll out the Tweel to consumers any time soon.
Last but not least, another challenge for the Tweel could be the drivers themselves who would see their beloved radial tires and rims replaced by a not so good looking Tweel. Of course, Michelin could place some covers to hide the spokes, but the psychological impact on the consumer should not be neglected. It might be the inventor of the Tweel, but another company is working on a similar project. Resilient Technologies is developing their own airless tire, known as the NPT (non-pneumatic tire). That company is using a more aggressive development and marketing strategy aimed at military use. The NPT is based on a different configuration of spokes, but the general idea is the same as Tweel's.

Given the high speed problems with the Tweel, the first commercial applications will be in lower-speed, lower-weight vehicles such as wheelchairs, scooters, and other such devices. The iBOT mobility device and Segway's Concept Centaur were both introduced with Tweels. Michelin also has additional projects for Tweel on small construction equipment, such as skid steer loaders, for which it seems well-suited.
The first large-scale applications may be in the military where a flat-proof tyre would be advantageous. Military testing has indicated that the Tweel deflects mine blasts away from the vehicle better than standard tyres and that the Tweel remains mobile even with some of the spokes are damaged or missing.
NASA has contracted Michelin to develop a wheel for the next generation Lunar Rover based on the Tweel. This has resulted in the Lunar Rover Initiative AB Scarab wheels.
The first large-scale applications may be in the military where a flat-proof tyre would be advantageous. Military testing has indicated that the Tweel deflects mine blasts away from the vehicle better than standard tyres and that the Tweel remains mobile even with some of the spokes are damaged or missing.
NASA has contracted Michelin to develop a wheel for the next generation Lunar Rover based on the Tweel. This has resulted in the Lunar Rover Initiative AB Scarab wheels.
Future of Tweel Technology:
For Michelin, Tweel is a long-term vision that represents the next step in a long path of industry-changing innovations. Fifty years ago, Michelin invented the radial tyre and there is no question that radial tyre technology will continue as the standard for a long time to come. Michelin continues to advance the performance of the radial tyre in areas such as rolling resistance, wear life and grip.
In the short-term, the lessons learned from Tweel research are being applied to improve those conventional tyre performances. In the future, Tweel may reinvent the way that vehicles move. Checking tyre pressure, fixing flats, highway blow-outs and balancing between traction and comfort could all fade into memory.

It is concluded that tyres featuring low noise and low rolling resistance will be required in the near future and that the interest in and need for im-proved characteristics in this respect will receive much more attention and priority in the tyres of the next 10 years than for present market tyres. 
If the climate changes will force a sudden and dramatic change in transportation and vehicle emissions policies, which is not an unlikely scenario, the tyre and vehicle manufacturer who fails to consider unconventional solutions may suddenly find itself in an inferior position to the one who can see and actually explore the possibilities of new technologies. 
There are possibilities to reduce noise and rolling resistance further than today by traditional tyre design measures; in particular if the extreme high-speed demands (speeds in excess of 200 km/h) can be abandoned. 
It is further concluded that there are several possi-bilities for a breakthrough in tyre design for low noise and low rolling resistance within the next 10 years or so, provided sufficient resources are spent on developing the concepts presented above.

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