15-01-2013, 04:48 PM
AIRLESS TYRES
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INTRODUCTION
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.
PROBLEM IN CONVENTOINAL TYRE
LOWER ROLLING RESISTANCE
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 [TRB 286, 2006].
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.
INCREASING CONCERN FOR LOW NOISE AND ROLLING RESISTANCE NECESSARY
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.
OPTIONS AVAILABLE WITH TRADITIONAL TECHNOLOGY
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 OPTIONS FOR LOWER ROLLING RESISTANCE
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.
THE QUIET TYRE WITH NO MARKET
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.
This author thinks that such neglect of quiet designs will be impossible in the future.
UNCONVENTIONAL TECHNOLOGY AND INNOVATIONS
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?
DESIGN OF TWEEL AIRLESS TYRES
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.
HOW IT WORKS
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
CONCLUSIONS
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.
[attachment=47061]
INTRODUCTION
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.
PROBLEM IN CONVENTOINAL TYRE
LOWER ROLLING RESISTANCE
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 [TRB 286, 2006].
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.
INCREASING CONCERN FOR LOW NOISE AND ROLLING RESISTANCE NECESSARY
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.
OPTIONS AVAILABLE WITH TRADITIONAL TECHNOLOGY
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 OPTIONS FOR LOWER ROLLING RESISTANCE
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.
THE QUIET TYRE WITH NO MARKET
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.
This author thinks that such neglect of quiet designs will be impossible in the future.
UNCONVENTIONAL TECHNOLOGY AND INNOVATIONS
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?
DESIGN OF TWEEL AIRLESS TYRES
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.
HOW IT WORKS
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
CONCLUSIONS
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.