Tire makers strive to fix their electric car problem

Electric cars are no longer the head turners they were just a few years ago. People are getting accustomed to the sight of them.

Globally, drivers registered 14 million electric cars—battery electric and plug-in hybrid—in 2023. These represent 18% of the cars sold and a 35% increase over the year before, according to the US Energy Information Administration. Some 40 million cars on roads around the world today are electric.

As more motorists familiarize themselves with their new electric vehicles (EVs), they are asking questions they didn’t raise with their internal combustion engine cars: Will I have to replace the battery? How many kilometers can I drive before my next charge? Will the range decrease in cold weather?

One question they may not be asking is whether the tires will wear out faster than the ones on their old car. After all, what connection could there be between electric motors, batteries, and tires?

It turns out that the connection is strong. Issues of weight, torque, and lack of coasting mean that tires on EVs are subject to more stress than the ones on gasoline-powered vehicles and wear out sooner. In response, tire makers and suppliers of polymers and other ingredients are developing new elastomers and introducing new materials so that tire wear doesn’t become a drawback to EV ownership.

One reason for wear is gravity. Because of those bulky batteries, EVs are heavier than conventional cars. For example, a gasoline-powered Toyota Camry weighs 1,500 kg; a Tesla Model 3 comes in at 1,800 kg.

Another is torque. As anyone who has seen videos of Teslas beating Lamborghinis in drag races knows, electric motors apply more force to the wheels than conventional cars do.

And finally, explains Dale Harrigle, chief engineer for consumer replacement tires at Bridgestone Americas, EVs don’t coast—or roll freely—like conventional cars do. Force is almost always being applied to the wheels, either through the car’s electric motors or its regenerative braking system.

“So there’s very little coasting that occurs in an electric vehicle, and that’s part of the reason why the wear life is reduced,” Harrigle says. Between the weight, torque, and lack of coasting, tires on an EV wear 20–30% faster than they would on a conventional car, he estimates.

Bridgestone and competitors like Michelin and Goodyear Tire & Rubber are already coming out with tires specifically designed for EVs. They offer improved wear resistance without sacrificing low rolling resistance, important for maximizing driving range.

The tires can’t compromise on grip either, which is critical to a car’s safety. Tire producers also have reduced noise, which is emerging as an important consideration for EV owners.

Tires are modern wonders made of natural and synthetic rubber, steel belting, fabric cord, and reinforcing additives, including silica and carbon black. Each component does its part to construct a product capable of carrying a 1½ metric ton vehicle the equivalent of two trips around the circumference of Earth.

Still, engineers are always improving tires. And when they do, they try to push out the corners of a guiding principle they call the magic triangle. It represents the three key properties—rolling resistance, wear resistance, and wet grip—that are critical to tire performance.

These three properties are interlinked, and engineers often have to negotiate trade-offs between them. One is between rolling resistance and wet grip.

According to Malte Wohlfahrt, director of R&D at the synthetic rubber maker Synthos, rolling resistance is determined by rubber’s tendency to dissipate energy when it is deformed in operation. When the car is rolling down the road, the driver wants that dissipation to be as low as possible to save fuel. But when the driver hits the brakes, dissipation needs to be high, especially when the road is cold and wet. The rubber has to operate in both modes.

Wear resistance can come at the expense of wet grip. For example, Harrigle says, more contact between the tire and the road results in lower wear. But performance in snow and rain requires grooves, which allow water to escape between the tire and road. “So there has to be an engineering trade-off, or a balance between all of these parameters,” he says.

In contrast, low rolling resistance and high wear resistance tend to go hand in hand, Wohlfahrt says. He likens it to a steel wheel on a train. Wear and rolling resistance are nil because, unlike rubber, steel doesn’t deform. But during braking, that steel wheel is prone to slide, and stopping takes time. “All the changes that you can do that improve rolling resistance also improve the tire wear, but at the same time go against the wet grip and the braking distance,” he says.

These rules of thumb aren’t absolute. For example, adding precipitated silica lowers rolling resistance, but its influence on wear performance might not be adequate for EV and other high-performance tires. This is one of the problems that tire producers aim to fix.

The addition of silica paired with coupling agents that help it adhere to the rubber has led to great decreases in rolling resistance in recent years and helped silica replace some of the traditional carbon black reinforcement, according to Stephen Moller, product manager for rubber fillers at the silica maker Evonik Industries. Evonik says green tires, which incorporate silica at up to 30% in the tire tread, can improve a car’s fuel economy by 5–8%.

“The silica-silane network is able to deal with a lot more stresses because it’s not just filling the rubber, it’s actually interacting with the rubber,” Moller says.

Mark Smale, executive director of advanced polymer science at Bridgestone, explains that fillers can contribute to rolling resistance by forming networks that break and remake as the tire goes through its cycles. “Silica does that less than carbon black,” he says.

Defying the traditional trade-offs, silica improves wet grip while also lowering rolling resistance. The likely explanation is that, being a polar material, silica penetrates the water on the road surface more easily than hydrophobic materials like carbon black and rubber do, Moller says.

For EVs, tire makers know that improvements in wear cannot come at the expense of rolling resistance. Brad Heim, Goodyear’s vice president of product development in the Americas, notes that vehicle range is a big selling point for EVs. “Therefore, the rolling resistance of the tire becomes a very important element,” he says.

But Heim is realistic about the new technology in Goodyear’s EV tire, the ElectricDrive2. “It is not necessarily a revolutionary change in the tire,” Heim says. “The tire is still round, obviously, and has a very similar shape. It is more evolutionary.” For example, he says Goodyear adds more layers of ply to the tires so that they can withstand the higher weight.

For synthetic rubbers, the key to improved tire performance has been functionalization. Polymers such as styrene- butadiene and polybutadiene rubbers are made using two main processes. The emulsion-based process is the workhorse for styrene-butadiene rubber, but it doesn’t allow for much catalysis or control over polymerization, Wohlfahrt says.

Solution polymerization, in contrast, uses catalysts that can control molecular weight, pinpoint polymerization, and allow for the addition of functional groups to the polymer.

For Bridgestone’s new Turanza EV tire, chemists developed a polymer that the company calls PeakLife. Smale says the product was in development for just 2 years before it was produced commercially in the tire maker’s synthetic rubber plant in Louisiana.

Using a new catalyst and solution polymerization, Bridgestone fine-tuned the microstructure of the polymer, in this case a cis- polybutadiene rubber, while adding functional groups that react with the surface of the silica to disperse it securely in the polymer matrix. “We can get the benefits of the controlled microstructure and the adhesion to the filler, which then lets us get both wear and rolling resistance simultaneously,” Smale says.

This sounds plausible to Li Jia, a professor of polymer science and chemistry at the University of Akron. He explains that silica-filled rubber is susceptible to a problem called chipping.

“When the silica is not adequately dispersed, you have these aggregates that initiate cracks,” he says. “Chunks of rubber can be lost quickly and easily.” But when the silica is dispersed better, the tire doesn’t have the big particle aggregates and is less prone to wear.

“Efficient functionalization can play a significant role in the polymers that we are making because it helps to interact better with the fillers in the tire,” Synthos’s Wohlfahrt says. “With the silica in the tire compound, it provides better mechanical strength so it becomes less prone to tire wear and also helps with rolling resistance.”

Carbon black makers are adapting their product to tackle a similar problem. Earlier this year, Cabot launched Propel E8 carbon black globally, after a regional launch in China, the nation that leads the world in EV adoption. “We quickly saw this interest in trying to balance wear with rolling resistance,” says Katie Tuttle, a marketing director for Cabot.

Tuttle says some grades of carbon black are best suited for low rolling resistance and others for wear. Propel E8 was able to capture both properties. The company isn’t disclosing how it was able to achieve this, other than to say that the innovation is related to particle structure and aggregate size and distribution. Tire makers have already adopted the product, she notes.

EVs aside, wear is becoming a focus for the tire industry overall, Tuttle points out. The European Union is seeking to regulate microparticle emissions from tires, one of the leading sources of microplastics in the environment. “Tire abrasion is a major contributor,” she says.

Beyond the magic triangle, another important attribute for EV tires is noise control. All tires are susceptible to air-cavity resonance, which propagates through the structure of the vehicle and into the cabin, Goodyear’s Heim says. Drivers of conventional vehicles hardly notice it over the engine noise. But EV engines are nearly silent, and the vibrations that rise from the road are more conspicuous.

“So minimizing that noise becomes a big deal,” Heim says. Goodyear applied an open-cell polyurethane foam to the interior surface of the ElectricDrive2 to dampen the vibrations, an approach it calls Sound Comfort technology. “It can get up to 50% reduction of that noise that is created,” Heim says.

Bridgestone takes a different approach to the noise problem. Its QuietTrack technology consists of features molded into the grooves of the tire that break up the resonant frequency of the sound. Harrigle says the feature works over a wider range of frequencies than acoustic foam and doesn’t involve adding another layer inside the tire.

More fuel-efficient tires, longer-lasting tires, and now tires that run with less noise. Cars are changing drastically, whereas tires are evolving more gradually. But unlike the gasoline engine, tires are definitely here to stay. And with the help of chemists, they will be continually improving.