Every tire you’ve ridden on is balancing act, the triangulation of qualities that negate each other. Great-handling tires don’t last. Durable tires are loud. Quiet tires can’t handle. The rubber wrapped around the wheels on every new car is a carefully crafted compromise that favors some traits over others, because you can’t have it all.
Electric vehicles complicate things further, because they demand more of everything. They lack the roar of an engine to drown out the brain-numbing drone of rubber on asphalt, so quiet matters. Range is crucial, so the tires must play their part in pulling every mile from every watt. The torquey performance demands rubber stout enough to keep up. And electrics are expensive enough without worrying about buying them new shoes every few thousand miles.
That’s why Teslas and Chevy Bolts share track time with Corvettes and Camaros (along with Accords, Camrys, Sentras, and plenty of others) at Michelin’s R&D center near Greenville, South Carolina. Engineers and test drivers put new designs through rigorous tests on a plethora of surfaces at sprawling test track shrouded in trees to ward off Peeping Pirellis.
These engineers juggle more than 200 variables---rubber compounds, construction methods, sidewall design, belt arrangement, tread design, groove width, and so on---to find the best combination for a given manufacturer and model.
As they focus on electrics, they’re looking to have it both ways. “Historically, the tire industry has created tires with either high performance or low rolling resistance, requiring customers to make the trade-off for range or grip,” a Tesla representative says. Harder rubber lasts longer, for instance, and extends range but limits the stickiness you want in performance tires. The upstart automaker wants tires that “break down this historic compromise” by offering minimal noise, plus maximum performance, range, and comfort. That’s Michelin’s job.
The company makes tires for 45 percent of EV’s built in the US, including Tesla's Model S and Model X, the Nissan Leaf, and Chevy’s Volt and upcoming Bolt EV. And it's the sole provider of tires for Formula E, the electric racing series. The lessons learned with one program---improvements in longevity, rolling-resistance, etc.---carry over to others.
For the Bolt, coming later this year, Michelin delivered what it calls its best tire ever, at least in terms of rolling resistance. That’s the tendency for a tire to deform, requiring more energy to keep it rolling. The less energy you expend there, the farther you go on a charge, making a stiffer tire optimal. The tradeoff, though, is comfort. A steel wheel, for example, offers negligible rolling resistance, but is no fun to ride on. Michelin declined to reveal exactly how good the Bolt tire's performance is.
To get there, the engineers created a customized compound and construction that stiffens the tire without degrading comfort. The variation of the Energy Saver All-Season tires going on the Bolt also are self-sealing in the event of a puncture, eliminating the need for a spare or even an inflation kit. That saves weight, which also helps boost range. Acoustics were a factor in the tire’s design---down to tweaking tread patterns, block sizes, and groove widths to minimize noise---but it wasn’t as much of a driving concern because the car’s relatively lower speeds won't generate quite as much noise as Tesla’s more performance- and luxury-oriented lineup.
Tesla has challenged Michelin in a variety of ways. “We’re pretty proud of the work we’ve done with them,” says tire engineer Ed Gliss. For Musk, Michelin created a compound that minimizes heat buildup, allowing the tire blocks to retain their rigidity and not bend or flex excessively while driving. That offers the best mix of rigidity and adhesion, minimizing rolling resistance while maximizing handling.
The Model S wears a bespoke variation of Michelin’s Pilot Sport 2 tires, which are ultra-high performance tires designed for higher-torque sports cars. “Smaller electric cars mostly care about rolling resistance to extend their range, but Tesla wants that plus good handling and low noise,” Gliss says. “But the Tesla is heavy, with all the batteries under the floor. So the weight-loading is a bit bizarre. You can’t just take a tire off the shelf and expect it to handle the Model S. We had to reconfigure it to be able to absorb all those forces.”
Handling that unconventional weight distribution came down to “carcass” design---the interior construction of the tire. Adjusting the number, size, and placement of the steel belts created a stronger product that could accommodate the heavy loads from both the car itself and as the assorted, more dynamic loads that come with cornering and accelerating. Tweaking the rubber compound selection to achieve the right combination of rigidity and noise attenuation sealed the deal.
Pushing the boundaries with future electric cars---straining that balance between performance and EV tranquility---will require plenty of new science, research, and many thousands more miles deep in the woods of South Carolina. It’s where the rubber truly meets the road.
