Since 2012, EU tyre labels have rated the fuel economy, wet grip and noise level of all new tyres sold, providing a handy point of comparison for customers and spurring competition between new and established players. But in the CV world, where fuel costs have a huge impact on the bottom line, fleet operators and other customers are often the ones driving improvements in fuel economy from new tyre technology.
Reducing rolling resistance to improve fuel economy has been the major trend in truck tyre development for some years. The adoption of so-called ‘green tyre’ compounds in passenger cars, pioneered by Michelin in 1992, has led to the increasing replacement of carbon black with silica as the principal filler in car tyre compounds, leading to better wet grip and lower rolling resistance.
Silica is used to some degree in truck tyres, too, but where car tyres can use predominantly synthetic rubber-based silica compounds, the situation in trucks is complicated by their reliance on the extra strength and durability that natural rubber provides.
“Historically, low rolling resistance tyre tread compounds have tended to wear out faster and be more susceptible to tread cutting and chipping than their ‘standard’ counterparts,” confirms Gary Schroeder, Director of the truck and bus tyre business at Cooper Tyres. “Thus, when a tyre design achieves lower rolling resistance, it can result in undesirable side effects if the tyres are not carefully engineered.”
“Another typical difference is that low rolling resistance drive tyres tend to have shallower tread depths. The disadvantage can be fewer overall miles to removal, although many have been engineered to achieve a reduction in rolling resistance without compromising other areas of performance. Compared to passenger tyre treads, truck tyre treads are very lightly loaded with fillers of any kind, and we are unlikely to see compounds that resemble passenger tyre compounds in the near term.”
One of the leading scientists working to bridge the divide between natural rubber and silica is Dr Anke Blume, professor of elastomer technology and engineering at the University of Twente in the Netherlands. She begins by explaining how natural rubber offers the extra strength needed to make durable truck tyres capable of carrying heavy payloads.
“Natural rubber crystallises when it stretches, adding strength,” she says. “When the strain is released, the crystallization immediately disappears – it’s completely reversible and happens very fast.”
“You can try to use very pure, synthetic polyisoprene as a replacement for natural rubber, but you can’t simply add the broad variation of proteins required to make it behave in the same way. But while we cannot replace rubber [with] other synthetic polymers, we also know that the silica/silane system that is normally used with synthetic rubber in passenger car tyre treads cannot be transferred as-is to natural rubber. We have to find new ways of making it work with silica.”
Blume’s team is looking at ways to modify the natural rubber polymer in such a way that it interacts better with the silica during mixing, resulting in a strong, even mix rather than clots of one material or the other. Experiments using epoxy rubber have shown some promise but the temperature window where effective mixing occurs is very narrow, between 135-150°C. Another goal for future studies will be modifying the silica to find the right coupling mechanism to suit the structure of the natural rubber polymer.
“We have to consider the chemistry behind it, modify the polymer or the silica, and test it,” adds Blume. “It’s a lot of testing. There’s a lot more potential there and I think we can go much further, but it’ll be years of work.”
“If we can reduce the rolling resistance of truck tyres then the fleet owners will save a lot of money, but the abrasion resistance has to work well and this is always a weak point with silica inside. If we can find a way to maintain abrasion resistance while reducing rolling resistance by 10-20%, as it has happened in passenger car tyre treads, then there will be a big step forward.”
Blume points to weight reduction as a parallel stream of research being explored by the tyre industry to reduce rolling resistance, with Aramid or other fiber materials being lined up to replace some of the heavy steel cords within the tyre structure. This of course reflects a wider trend in the automotive and commercial vehicle industries for replacing parts that were previously metal with weight-saving plastic or composite alternatives. Key factors to consider are always that strength must be maintained and costs must be kept down.
Tyre aerodynamics can also play a part in saving fuel, according to Rick Shock, Senior Director of Aerodynamics at Exa, which has just added a rolling tire simulation function to its PowerFLOW computational fluid dynamics software.
The new Worldwide harmonised Light vehicles Test Procedure (WLTP) regulations mean that car and van manufacturers now have to declare the impact of individual wheel and tyre combinations on emissions performance and fuel economy. Car and LCV makers are therefore paying increasing attention to the aerodynamic impact of tyre designs, which can be up to half a percent of the total vehicle drag. WLTP doesn’t apply to heavy trucks, but the customers’ focus on fuel mileage means that Exa’s new tool is being applied there, too.
“European truck manufacturers are not beholden to WLTP but to something much stricter – their fleet operators,” says Shock. “It doesn’t matter to the operators what the OEM puts on the sticker or advertises as the drag value. They just want to know in actuality, as they’re operating their vehicle, what the consumption is going to be. Simulation gives the manufacturer the ability to know that a particular set of tyres can meet the on-road target that their fleet operators are aiming for, to accurately predict the on-road condition.”
Exa has worked with Class 8 commercial truck customers in North America to evaluate the impact that different treads can have on the aerodynamics of a heavy vehicle, with some surprising results.
“The aerodynamics are very complex as the air goes through the tread and around the shoulder and is then accelerated down toward the contact patch,” he explains. “You might have some tyres that are nobblier than others but that doesn’t necessarily mean you’ll have more drag, it might be less! The interaction of the airflow around the grooves, with the rest of the vehicle, is very non-intuitive.
“We’re finding that on the drive tyres, the impact of the rotating tread is not a huge contributor to linear drag – how much drag the truck experiences by the wind flowing over it,” Shock continues. “But it does impact the rotational drag. When you’re rotating the tyres there’s a torque component, a resistance to the air due to the rotation, that can manifest itself as equivalent drag. You have to give the vehicle more power to overcome that aerodynamic torque resistance.”
It’s thought that aerodynamic studies such as these will help to shape new, low-drag tyre designs to add to fleet operators’ fuel savings. In combination with other incremental changes such as weight reduction measures, plus the promise of a large fall in rolling resistance if the challenge of using more silica in tread compounds can be overcome, the future’s bright for fuel-efficient truck tyre technology.
