- Jun 5, 2014
A look at low-rolling resistance tires
Do they really save fuel, and at what sacrifice of other important tire attributes?
The occasion was a small media show-and-tell put on by Korean tire-maker Kumho to tout its latest high-performance tires, which were indeed very good on Mid-Ohio's rolling, twisting road course and in autocross tests in an adjacent parking lot.
But the Kumho folks also briefly mentioned their latest low-rolling resistance (LRR) tire, which we were not there to test that day. Not designed for driving fun, LRR tires are instead optimized (as their name implies) to minimize energy used as they flex and compress while your vehicle rolls down the road.
Not designed for driving fun, LRR tires are instead optimized to minimize energy.
That Mid-Ohio day has been in the back of my mind for quite a while, but until now I regretfully haven't gotten around to looking into LRR tires, though I do have some experience with them from my 1990s years as test and development manager for GM Advanced Technology Vehicles, primarily on the production EV1.
Clive Roberts, my lead development engineer at the time, worked exhaustively with tire suppliers before settling on the best at the time, from Michelin. It was tall, narrow and hard, but Clive and the Michelin team made it work amazingly well despite its ride-challenging 50-psi pressure.
Every tire's balance of attributes – dry cornering and braking, wet cornering and braking, noise, wear, rolling resistance, etc. – can be graphically expressed on a "spider" chart. Each characteristic is plotted along one spoke of the web, its performance represented by its distance from the center, so an attribute-balanced tire's chart is a near-circle.
But the tire engineer's hair-pulling challenge is that optimizing one attribute often diminishes others, so it's very difficult to get all of any tire's key characteristics to the outside of the web. And in our pursuit of the lowest possible rolling resistance – critically important to an electric car that begins each day with the energy equivalent of a half-gallon of gas on board – as we pushed that attribute out, we found that wet traction and wear were adversely affected.
Somewhere between five and 15 percent of the fuel consumed by a typical vehicle goes to overcome the hysteresis of the tire material as it rolls, a force represented by the tires' rolling resistance coefficient (RRC). Construction and materials are most important, but tire pressure, vehicle weight and speed all play a role. Most modern passenger car tires have an RRC between 0.007 and 0.014, with a lower coefficient meaning they will use less energy.
"Fortunately, many years of developing technology for original-equipment business, where fuel efficiency has long been a high priority, has helped establish Continental as one of the leaders in low rolling resistance technology," contends Continental Tire product planning manager Sam Dollyhigh. "It is true that a tire is a complex balance of performance attributes, and typically wet performance, wear and rolling resistance conflict with each other. However, through advancements in technology, lower rolling resistance does not mean other attributes such as mileage or wet traction have to be sacrificed.
"Lower rolling resistance does not mean other attributes such as mileage or wet traction have to be sacrificed" – Sam Dollyhigh
"Through significant investments in R&D, Continental has been able to grow the spider chart. We still focus heavily on wet braking and wet traction because our customer's safety is our top priority, and we also understand that long mileage is a top criterion for consumer satisfaction. So we do not use strategies to improve rolling resistance that would lead us away from either wet traction or mileage."
Continental labels its latest advancements in polymer technology EcoPlus, "which combines our technical expertise in rolling resistance with excellent mileage and class-leading wet braking," Dollyhigh asserts. Continental's CrossContact LX20 (for Crossover, SUVs and light trucks), PureContact (for luxury performance vehicles) and TrueContact premium all-season tires (for passenger cars and minivans) use EcoPlus technology. For the latter, Continental claims not only long wear and best-in-class wet braking but also that it can save as much as $550 in fuel cost over its life. "We are currently researching the next generation of EcoPlus Technology," Dollyhigh adds, "which will concentrate on improving the hysteretic properties of various internal tire components to further improve rolling resistance without any impact to the other important tire performance characteristics." And while LRR tires can be more expensive, "We don't ask the consumer to pay more for low rolling resistance," he says. "We compete on price with the competitive set."
All tiremakers have made significant reductions in rolling resistance in recent years, and many market tires (both OE and replacement) specifically developed to minimize it. Which ones are worn by some of today's most energy-efficient EVs and hybrids?
The Nissan Leaf EV uses Bridgestone Ecopia all-season tires on S and SV grades and Michelin Energy Savers on top SL trims. Chevrolet's Spark EV uses specially-developed Bridgestones, while the Volt gets Goodyear Fuel Max LRR all-seasons. "Tires are a balance of ride, handling, braking, noise, traction and efficiency," says Volt chief engineer Andrew Farah. "With Volt and Spark, efficiency was given more weight than usual in the balance."
Ford's Focus EV and C-Max Hybrid use Michelin Energy Saver A/S, which was selected, Ford says, "because it offers some of the industry's lowest rolling resistance while still allowing us to meet our vehicle dynamics standards and best-in-class NVH." The Fiat 500e uses Firestone Firehawk GT all-season tires. "Our rationale behind that," explains 500e chief engineer Brett Giem, "is that they are one of the two 15-inch tires already in the Fiat 500 assembly plant, they meet the weight ratings we need, and an industry search confirms that they have the lowest rolling resistance rating available worldwide."
I can't advise which replacement LRR tires to slap onto your EV, hybrid or conventional vehicle when the time comes. We all would like to maximize energy savings, but don't pay an unreasonable price for what will turn out to be a small-percentage improvement, or sacrifice other important tire attributes for the lowest available RRC. Do some research and competitive comparisons, then make an intelligent, balanced decision based on your specific priorities.