Tires take one of the top spots on the list of Important Things. Yet they also rank pretty high on the list of Things You Never Think About Until They Break. Other items live in that overlap, like spleens, bridges and light bulbs, yet it really is a shame for tires to be there because it is not overstatement to call them crucial – you can get from New York to Los Angeles with a car missing a seat or even a cylinder. You will not make that drive on a car with four wheels and three tires.
It could be because tires are only the least bit interesting before they're put on your car. Once mounted they're essentially viewed as chunks of air-filled rubber, and only five-year-olds and racquetball players get excited about that. Even the tire companies know they can't play it straight when it comes to selling them, that's why Michelin has Bibendum (the proper name of The Michelin Man), Goodyear has its Blimp, and Pirelli, its calendar.
But even when you're not thinking about them, tires are where a healthy portion of your gas budget goes. "Tires are responsible for nearly 20 percent of the energy consumed to move a car powered by an internal combustion engine, and up to 30 percent for a full electric vehicle in urban use," said Doug Girvin, director of product marketing at Michelin Americas Small Tires.
Tires are your only connection to the road – so if they aren't happy, you probably won't be, either. To put them in their proper perspective, when you talk about taking a trip in your car, replace "I" with "My tires." Instead of "I made it here in three hours at 75 miles per hour," try, "My tires made it here doing 75 mph for three straight hours" and you could begin to get an idea of the work they do.
Better Fuel Efficiency
Now there is even more being asked of them. Low rolling resistance rubber has been getting press lately as tire makers continue to develop lines of tires specifically created to reduce the frictional losses and heat buildup that happen where the rubber meets the road. A tire that rolls more easily is a tire that will get you better gas mileage, with the ultimate aim being to do so without taking away performance.
The effort, however, isn't new.
"The radial tire was probably the biggest innovation in terms of fuel efficiency for its time," said Michelin's Girvin. "The way the carcass plies are arranged in a radial fashion makes it very efficient in comparison to the bias-ply tires."
But the inventor of radial ply tires, Englishman Arthur Savage, wasn't looking for better gas mileage. And although he invented them in 1915, they weren't picked up by a major manufacturer until Michelin did in 1946, and radials didn't become popular in America until the early seventies. It was events in the late seventies that get credit for providing the official push for lower rolling resistance tires.
"The movement toward low rolling resistance tires began about 30 years ago," said Kurt Berger, manager of consumer products and engineering at Bridgestone Group. "If we look at a typical tire from 1985 or so versus a current tire, we've seen probably about a 50 percent drop, or greater, in rolling resistance. A lot of that has been driven by tire manufacturers that supply original equipment tires. With CAFE requirements in the late '70s, automakers began requiring the tire manufacturers to make their contribution to up the fuel economy rating for the vehicle."
Fast forward to the early nineties, tire companies were producing and marketing low rolling resistance tires touting fuel economy benefits. But since this happened B.P.E. – Before the Prius Era – no one pays much attention to that now.
"Goodyear has been offering fuel-efficient and low rolling-resistance tires for several years," said Tim Richards, Goodyear tire engineer and project manager. "In fact, the first such recognized tire in the industry was Goodyear's Invicta GFE, for Greater Fuel Economy, in 1991."
Michelin wasn't far behind. "Low rolling resistance tire technology has been a focus for Michelin since 1992," said Girvin, "with the launch of the first Michelin Green X fuel-saving tire."
How They Work
When it comes to building a low rolling resistance tire, it's not just the tread that comes in for scrutiny.
"Each single component and material in a tire can contribute to the reduction of rolling resistance," said Alessandra Ferraris, director of R&D at Continental Tires. This is because "rolling resistance is a result of tire rotation and deformation by the road. It is the mechanical energy transformed into heat and it is caused mainly by material hysteresis. The low rolling resistance tire components are optimized to minimize hysteresis and deformation."
This is the sole trip we'll make on the Serious Science Express: hysteresis is the energy lost as the tire changes shape because of contact with the road.
Or, as explained by Michelin's Girvin, "Rubber isn't perfectly elastic, and hysteresis is the property that consumes energy. You take two different types of rubber, drop one and it bounces almost back to the same level from where you dropped it. In that case there wasn't a lot of lost energy. Take the other and it almost dies as soon as it hits the ground. It's absorbing more energy. That absorption of energy is the generation of heat."
To put some numbers to it, "The tread area which has direct contact with the road is the most significant contributor to rolling resistance," said Bridgestone's Berger. "It's about 40-50 percent of the rolling resistance value. The sidewall and the casing contribute about 20-30 percent each, and even the bead area that mounts up against the wheel contributes."
What, then, does a manufacturer do to lower the rolling resistance of a tire? They could make solid hoops of super hard rubber, since a metal tire – like the first tires used in the 19th century – has the lowest rolling resistance. Those metal tires and the solid rubber tires that followed them, however, had other problems like horrible traction and an atrocious ride.
So today's tire makers came up with something different: They formulate different rubber compounds and add various natural and synthetic materials that will minimize heat in a particular area without giving up performance.
Rubber Is Just The First Ingredient
One thing you might not realize about tires: a single tire isn't made of a single kind of rubber. In fact, there can be up to 20 different kinds of specifically formulated rubber compounds in just one tire. On top of that there is a dictionary's worth of natural and synthetic ingredients perfected to help the rubber do its job: silica fibers, carbon black, volcanic sand, functionalized polymers, sunflower oil, corn starch-derived polymer fillers, carbon fiber, kevlar, Dynamic Temperature Distribution, Nano-Pro Technology.
Michelin recently announced the use of sunflower oil in its Primacy MXM4. "Vegetable oils have been used as extender oils in the rubber industry, not only tires, for several years," said Girvin. "We were faced with the technical challenge of developing a high performance tire for modern luxury performance vehicles that could still deliver superior cold weather winter grip on wet and snowy roads. Sunflower oil provides unique properties to cured rubber that allow us to overcome one of the traditional balance-of-performance issues when trying to design high-performance all-season tires."
The corn starch-derived polymer filler in Goodyear's BioTRED "improves reinforcement of rubber compounds on a molecular level to reduce hysteresis and rolling resistance," said Richards. They "create better dispersion of their reinforcing links within the rubber compound, thereby improving treadwear performance." Volcanic sand is "used in the 'Ice Zone' of the Assurance TripleTred to enhance winter traction," and carbon fiber and Kevlar are used to reinforce the sidewall.
Continental's Extreme DW and DWS tires "have lowered rolling resistance technology called Dynamic Temperature Distribution," said Ferraris. "It reduces distortion for and improves tread-life."
Bridestone told us that they are looking at 300 aromatic oils, such as sunflower oil, and they've also developed what they call NanoPro Technology (NPT), "an umbrella of micro technologies."
"Carbon black is a major component of tread rubber," said Berger, "and if you look at a conventional rubber compound there's a lot of contact between the individual carbon black molecules. It's very random, and that contact actually creates a lot of friction between the molecules, and heat buildup. With NPT we've been able to insert polymers that attach the to carbon black molecules and actually cause them to space themselves somewhat homogeneously. They're not touching each other, so you get a much cooler running temperature, which results in a significant reduction in rolling resistance. It also improves the traction."
It might not be NASA, but it's looking more like it every day. The key point here is that traction is important. It's great if a tire rolls more easily, but not if it gives up its grip on the road, or if it becomes unduly harsh or noisy. So even though compounding is king, the development of a tire always comes, shall we say, full circle. Gains in one area must be balanced by their affects on every other area – a new heat resisting compound might need a new tread block design in order to reign in the decibels and extract the best performance and tread life.
Furthermore, while we're talking about low rolling resistance tires specifically, the technologies cited are migrating to the mainstream and performance oriented lines for every manufacturer we spoke to. It's true that the Ferrari 458 Italia driver cares more about grip than the gas pump, but the Bridgestone Potenzas or Michelin Pilot Sport 2s that his car wears will be leaner and greener because of advancements gained by Bridestone Ecopias and Michelin Energy Savers.