Tyres are a critical link in a vehicle's quest for efficiency. Of the multiple forces that a tyre encounters, rolling resistance is one of the most important challenges for engineers. In order to understand the phenomena of rolling resistance better and the ways to reduce it, we met Randy Clark, Vice-President, Norms & Regulations, Michelin Group for an interaction recently.
ROLLING RESISTANCE – CAUSE & SOLUTIONS
Clark told us that rolling resistance is one of the many drag forces acting on a tyre, which increase fuel consumption. Explaining its cause, Clark said that rolling resistance is caused due to the flexing of the tyre structure. Every time a tyre goes through a revolution, the weight on it causes the structure to flex and get back into shape once out of load, in a process known as Hysteresis. The energy absorbed by the tyre while regaining its shape is always lesser than that used to flexing it, leading to a loss of energy created by the engine. As a tyre gets hotter, the rolling resistance increases, leading to higher fuel consumption. The impact of hysteresis can be determined by the fact that of the energy required for rotating a tyre, anywhere between 0 % and 15 % is exerted by aerodynamic drag. The remaining is largely a result of the bending, compression and shearing of the tyre.
Rolling resistance's intensity, however, depends on multiple factors such as the rubber compound of a tyre. Tread patterns also make a significant difference, wherein shallow ribs can lower the rolling resistance, while deep lugs can increase it. The tyre's internal structure too makes a difference, be it radial or bias. A radial tyre's structure is such that it generates less heat and therefore low rolling resistance than a bias tyre.
Clark primarily talked about rolling resistance and its effects on commercial vehicles and mentioned that tyres alone account for about 1/3rd of the total drag on a truck. Every 10 % reduction in rolling resistance can lead to a fuel saving between three and four per cent. In India, owing to the factors mentioned above, rolling resistance can vary by a margin of over 30 %. Also, overloading significantly increases the rolling resistance, further lowering the fuel efficiency of commercial vehicles in India. Clark also told us that based on the discussed parameters rolling resistance can not only increase fuel efficiency but decrease emissions as well.
Citing the role that low-rolling resistance tyres can play in improving the operational efficiencies of truck fleets, Clark referred to a case study from the US. The Oak Ridge National Laboratory monitored 20 trucks over an actual fleet distance of 10 lakh km and found out that low-rolling resistance tyres alone lead to a fuel savings of about nine per cent.
BALANCING THE SCALE
As is the case with most technologies, pursuing higher efficiency in one operational parameter leads to inefficiency in another, forcing the engineers to accept such sacrifices at times. It's commonly thought that low-resistance tyres sacrifice grip in order to lower the rolling resistance, but Clark pointed that this isn't a mandatory requirement. He added that there are multiple choices available to a tyre maker to lower rolling resistance beyond lowering the grip. Apart from shifting from bias to radial, one could create a low-rolling resistance sidewall rubber. This would lower the resistance without affecting the grip, which is dependent on the tread rubber. In addition, various combinations of tread patterns and compounds can lower resistance without affecting grip.
Clark added that although rolling resistance cannot be eliminated, there is potential to lower it significantly. Low-rolling resistance tyres hence could play an important part for the growing automotive industry of India, which is expected to witness a seven per cent growth in vehicle kilometres in the next 10 years. Also, the country's trucks transiting from bias tyres to radial tyres should help improve efficiency. A focussed approach on low-resistance tyres for commercial vehicles then is a good way for India to enhance its transportation efficiency in the coming years, concluded Clark.
Text: Arpit Mahendra