Reduced material use, noise levels and rolling resistance. Airless tyres could provide such benefits. A design breakthrough may be on the horizon within LEON-T, an EU research and development project that has 10 partners. VTI is participating through Research Leader Ulf Sandberg. He is responsible for work package 5, which is looking into airless tyres.
“This is a dream project for me even though I do not really need to work any more. I have strong feelings about the principle that I already started working with 34 years ago of which I have written a lot about”, says Ulf Sandberg.
It is no longer the engine that makes the most noise on vehicles, it is the tyres. The challenge of the work package is therefore to develop tyres for HGVs that reduce noise levels by at least 6 dB(A).
“The solution to achieve this may be airless tyres. A technically better term would be non-pneumatic tyres, but only technicians in the field would probably be able to understand this.”
For over 100 years, pneumatic tyres have been seen as the only option for motor vehicles. Recently, hopes have arisen for a better alternative – airless tyres. In addition to reduced noise, there may be many more benefits: rolling resistance may be reduced and thus energy consumption, there is a reduced risk of aquaplaning, there’s no need to check and adjust the air pressure, no risk of punctures, airless tyres are more environmentally friendly and fewer materials are needed, this means more space for other things in the vehicle, it is possible to produce the tyres through 3D printing and the designers will like that they provide a more futuristic look.
So what is it that replaces the air in the tyres? Ultimately, the air’s function is to sustain the weight of the vehicle within the tyre’s rubber ring. Nowadays, these tyres are incredibly complicated. Their sidewalls, belts, and treads can be made up of more than 100 different components or materials. In the airless version, the air and its containment are replaced with spokes. The spokes also act as suspension, which in a suitably flexible way transfers and dampens the force between the rim and the road surface.
“In airless tyre concepts these flexible spokes are usually made of composite materials, similar to the type found, for example, in modern recreational boats. It may sound simple, but in practice, there are complicated processes that have to work,” says Ulf Sandberg. Up until now, however, the disadvantages of airless tyres have outweighed the benefits – especially since the structures have not yet been able to withstand the extreme stresses to which vehicle tyres are subjected.
“All major tyre manufacturers have developed airless tyres as a concept at some point, but have not invested enough to make them work in traffic, except for Michelin, which is now testing a variant on smaller vehicles. A German doctoral student has developed an airless bicycle tyre, but it was not commercialised. Tyres filled with rubber instead of air are available for bicycles and in army vehicles, but are too heavy for general use.”
Hans-Erik Hansson, the inventor from Finspång who also developed the actual tyre construction, plays a key role in the work package. In the late 1980s, he had already developed an airless tyre to reduce rolling resistance for competitive cyclists and sulkies in trotting competitions. There, the composite material consisted of fibreglass and polyester laminate.
“Hans-Erik Hansson came to VTI to test his designs. But I saw a potential for passenger vehicles. With the help of funding from the Swedish Board for Technical Development, now Vinnova, Hansson developed a prototype for passenger vehicles as early as 1990. At that time, the project was only about testing the principle of noise reduction, which was successful.”
Research efforts persisted, but structural flaws eventually developed in the tyres, indicating that they were insufficiently robust. Better versions emerged in a major international project led by VTI in the late 2000s. Tests on normal road surfaces gave excellent results, but it turned out that the design was not adapted to high-speed driving when tested on one of Volvo’s test tracks, the so-called pothole track. It is an extreme test that all tyres must withstand, even if they are not exposed to such difficult conditions in practice.
The new innovative approach in LEON-T, which must work for truck tyres, is to replace the composite materials in the spokes with steel – which may be the solution to cope with the stresses that the wheels are subjected to. Fossil-free steel would also make the wheels more environmentally friendly.
“Here, the Stockholm-based company Lightness by Design made an important contribution in helping Hans-Erik Hansson calculate the strength of various steel structures. Our Spanish partner also participated by testing performance during movement with their models.”
Partnering in the work package is Applus+ IDIADA, a large Spanish vehicle and tyre testing institute that helps with simulations and modelling. The institute is also the coordinator of LEON-T. The steel for the spokes comes from Ovako Sweden AB, further processed by Lesjöfors Bruk AB and ID Modeller AB. Another project participant is the Chinese tyre company Linglong, which has contributed the rubber.
So far, the prototype has been tested in Linköping on an electric truck by driving at different speeds and observing how the tyre moves, the results of which have turned out well. Next on the agenda in the project is the testing of the tyre to check for potential problems and to make rolling resistance measurements on a steel drum in the Spanish test facility. Then there will be noise measurements.
In March, Ulf Sandberg will give a lecture on the design at a conference at the Tire Technology Expo 2024 in Hanover. The airless tyre has also aroused great interest in Japan, where Ulf Sandberg has held a seminar on the concept, which is now having a ripple effect there as well.
VTI is one of ten partners in the LEON-T project, which also deals with tyre issues other than developing airless tyres. Other Swedish partners include Hansson’s company Euroturbine and the University of Gothenburg. The project consortium won despite the strongest possible competition, it was namely in competition with ETRMA, the European Tyre & Rubber Manufacturers Association. The tyre companies considered that noise reduction for truck tyres with the requirement of 6 dB(A) is not possible.
“The commercialisation of the product will require substantial resources, which of course are not available in individual research projects of this kind, so that important work will have to be continued by the tyre industry,” says Ulf Sandberg.
“However, tyre companies have thus far not shown an interest in moving forward, as almost all of their factories and everything they are doing today would become obsolete, with the exception of the development of the treads. Eventually, however, tyre manufacturers must keep up with this development, as they are exposed to demands from the automotive industry,” he says.
Airless tyres
From the centre, the airless tyre consists of a central rim with steel spokes connecting to the belt and rubber tread. The flexibility of the spokes act like springs whose properties transmit the load forces to a belt of composite materials made of fibre-reinforced polyester. Around this is a rubber tread.
In LEON-T, the spokes are made of steel instead of the composite materials used in previous passenger vehicle designs. This will make the structure more durable for external stresses that result from acceleration, braking, speed, and the weight of heavy vehicles.
The project is funded by the EU’s Horizon 2020 research programme, grant agreement 955387.
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