Following our article on the brief history of the airless tire, this article will present the advantages of the airless tire over the traditional tire and why we chose an airless tyre for our quieter and cleaner tyre prototype.
Though there have been countless innovations in pneumatic tyre technology, most have been small incremental steps towards optimisation in terms of road holding, safety, wear, cost and noise. For road vehicles tyres need to meet a balanced set of requirements in all of these aspects.
Considering this set of requirements there is wide agreement amongst experts that only small reductions of tyre noise emissions (less than 3 dB(A) compared to fleet average) are possible by optimising the current concept of a pneumatic tyre mounted on a rigid wheel.
Today’s low-noise pneumatic tyres feature optimised tread patterns. However even the use of the acoustically optimal tread pattern—the slick tyre without any pattern—will not enable the desired 6 dB(A) reduction in noise, and it would not be safe or provide enough traction, as already investigated in several previous projects (e.g. NordTyre, STEER).
Understanding of noise emissions from passenger car tyres has led to designs that have intrinsically lower noise emissions under standardised driving conditions. This has given rise to different tyre tread patterns, but also to more innovative solutions such as airless tyres. Such an airless tyre is a so-called composite wheel which has been demonstrated for a passenger car (C1 tyre), achieving a reduction in emitted noise by more than 4 dB(A) and a reduction in rolling resistance of 30% compared to best-in-class conventional tyres.
Leading tyre manufacturers have presented concepts and prototypes of their airless tyres: Michelin has shown the “Tweel”, and more recently demonstrated the Uptis which is said to be moving towards market introduction; Hankook presented the i-flex concept and similar concept tyres have also been also presented by Bridgestone, Goodyear and Kumho. At present none of these airless tyres have hit the market for road-going vehicles—though they are in use for off-highway, military vehicles and very special applications such as the “moon tyre” by Goodyear.
Optimisation of pneumatic truck tyres by altering the tread pattern and/or including a dampening foam element in the tyre/rim cavity has limited noise reduction potential.
Airless tyres (non-pneumatic tyres) do not rely on the use of a closed air volume to distribute weight. As such they can overcome some conceptual limitations of pneumatic tyres:
- Pneumatic tyres require grooves in the tread surface to transport water away from the contact patch to provide grip in wet conditions. Airless tyres can transport this water through the tread and do not require such groove
- Given that no air container is needed, some airless tyre designs allow more space on the inside of the tyre, which potentially gives more space for brakes or even for electric motors (see image at the begining of the article)
- The air cavity in a pneumatic tyre acts as a resonator, as well as acoustically corresponding to a “loudspeaker cabinet”
- Noise emissions of pneumatic tyres increase when the tyre is not inflated to the correct pressure
- Under and over inflation leads to tyre irregular wear patterns that increase tyre noise. These wear patterns should not occur on airless tyres
Though airless tyres have shown a great potential to reduce tyre noise, wear and rolling resistance, currently no airless tyres suitable for highway-going HGVs exist. Though a composite wheel for HGV use has been suggested there are no detailed designs and no such wheel has yet been demonstrated and experimentally evaluated. We will adapt the design of the composite wheel to meet the requirements of C3 category HGV tyres and will prototype and demonstrate two such tyre/wheel combinations—for both driven and non-driven axles. These prototypes will serve to illustrate the benefit of further exploring other proposed airless tyre designs and their applicability to HGVs
Demonstration of a low-noise tyre will show that the emitted noise from HGV tyres can be greatly reduced by tyre design, which can serve to enable the introduction of emitted noise requirements for HGV tyres.