By Marc J. Elzenbeck
Washington State is eager to further cut carbon emissions. To that end, it’s anticipated to pass a law requiring residents to buy ultra-low rolling resistance tires when replacing original (OEM) equipment. Officials promise this will save hundreds per driver in fuel costs. This is true. For a given car or light truck in normal driving, these new “Low-Rollers” improve fuel economy by about 2.5 to 3.5%.
That’s the good news. Who doesn’t want to save some gas money?
The bad news? First, since the low-rollers are more expensive to produce, and priced accordingly, they probably won’t save much money, if any – even though Washington’s gasoline prices are the second-highest in the United States behind California’s. Second, the upside is tiny, because tires only account for about 20% of total vehicular friction. Third, tire production is a demanding material science where trade-offs are literally baked in and the risks of squeezing out the last possible mile per-gallon quickly and clearly outweigh the benefits.
The consequences to forcing wholesale adoption of these new products are predictable and will require increased caution from drivers accustomed to less caution. First, a quick spin through tire design. Spoiler: You can’t reduce rolling resistance without hurting traction.
For most of us, the simplest way to upgrade our vehicle’s safety and performance has been with tires, which have gotten better and better. Since the 1970s, general tire quality has progressed to something of a golden age, most happily in terms of grip in wet conditions, off-roading, and versatility. Other properties like compound, durability, sizes, noise, and fuel efficiency have incrementally improved, but the biggest, hands-down amazing strides have been made in wet grip.
Wet grip sounds sexy. Fun. And it definitely is. But why is that a good thing on a car? Think of rounding the blind, off-camber, decreasing-radius corner where Westside Highway SW turns east onto Cemetery Road. So many drivers have spun off in the same spot that the property owner on the north side has installed a big cedar log to protect her house from crashes. Because vulcanized rubber naturally sticks to wet pavement much less well than dry, even professional drivers can get surprised by slippery roads and lose control despite paying attention. Soles on shoes have treads so they stick, not slide.
Tires used to have a much wider gap between their practical dry versus wet cornering limits, how fast they could go around the same bend, but wet grip engineering closed this. If you barreled up to a blind corner in the rain 50 years ago, you would be well-advised to slow your initial entry speed to about half of what you might do in the dry. Margin for error was needed, because if you had to make a sudden correction or hit the brakes in the corner on a wet road, even mildly so by today’s standards, you could easily get sideways and not recover. Now, with great tire performance, we take wet or dry corners at the same speeds and can make big course corrections for granted, thanks to vastly improved wet grip.
Advances in tread patterns and grabby compounds squash and shoot water backwards, like Super Soakers, making for shorter stops and maintaining stable contact on soggy blacktops. These have greatly reduced accident risks in bad weather. Not long ago, a mere moment’s inattention or misjudging a patch of water could send you hydroplaning off into an unforgiving tree or ditch.
The industry’s technical progress over the past 50 years has been marvelous, and delivered peace of mind to steep and winding roads on dark and stormy nights. Selected sport models recently achieved a Holy Grail milestone, providing almost the same road-holding and stopping power on wet roads as on dry. This breakthrough is available to most any interested car owner willing to pay a few hundred extra bucks.
With experience taken from racetracks, we commonly use all-season tires to easily tackle the lighter snows and slushes the Pass is prone to. This has largely reduced the need for specialized snow tires, and these all-seasons often last for over 50,000 miles. As for off-roading, some highly fuel-inefficient 20-inch Trail Hogs trucked my whole family up 11,000 feet of steeply rutted wilderness tracks and back down without a scratch.
Tires are a compromise between the limitations of physics and chemistry, and now governments demand that manufacturers optimize for fuel efficiency. To comply, they have only one path: To reduce rolling resistance (i.e., the energy required to make them gain or maintain speed) by using a stiffer, harder rubber compound and shallower treads. Harder tire, easier roll.
But harder rubber means less grip. In most available models, that harder compound is enhanced with a silica-based additive that restores some traction without adding the friction back in. Ultra-low rolling resistance tires start out with a hard and slippery compound, then the science folks try to figure out how best to compensate for the inherent limitations. While they were able to tease a lot of stick-ability back into the rubber, testing has demonstrated that eco-class tires are a significant step backwards on every performance and safety measure. The traction just isn’t the same, and the drop-off is worst right where you’d expect it, but you least want it: in the all-important wet grip.
For example, it takes 20 feet longer to stop a car going 50 mph in the rain on Michelin E-Primacy tires ($300+ each at Tire Rack, a poorly rated 3.7 out of 5 on Tire Reviews) vs the same car using the regular Michelin Primacy. A 20-foot longer stopping distance (29.7 meters vs 36.2 meters, in a European test) also implies approximately 20% less straight-line traction. That’s an absolutely horrible result, and 20 feet longer is a huge difference when you stomp on the brakes to avoid hitting a deer jumping across a slippery ditch-lined country road. Or when you’re trying to not hit anything on any road.
While the specifics aren’t worked out, Olympia is already committed in concept to implementing something like California’s statewide Replacement Tire Efficiency Program for passenger cars and light-duty trucks. Their idea is to “ensure that replacement tires be at least as energy-efficient as the vehicle’s original equipment.” What trying to enforce that at a local Les Schwab or Costco will look like is a good question, but driving to Idaho or Oregon looks like a possible answer.
A tire, like some other things, rolls downhill. Fancy notions originating at the top of an organizational hierarchy become chain-of-command problems, with the heaviest responsibilities falling on the chain’s lowliest links. The implications of drivers suddenly needing 20% further to stop during emergency braking events are profound, but it seems that eking out up to one extra mile per gallon is the only metric Washington state’s legislature is focused on. Is sacrificing so much for low rolling really worth it?
