There are no prescribed wet traction standards for commercial vehicle tires, but tire makers are more than aware of customer concerns.
TORONTO, Ont. — Funny thing about traction, it’s hard to tell how much you need, but it’s easy to tell when you don’t have enough. That’s one of my favorite quotes from Bridgestone’s former director of engineering for commercial tires, Guy Walenga. He’s retired now, and we sure miss his colorful observations about truck tires.
Tire traction is a galvanizing topic. Drivers fret about traction all the time. They get pretty excited even if they just perceive that a certain tread type doesn’t look grippy enough. But true to Walenga’s point, traction, or lack of it, is probably more of a worry than a real threat. It’s true that a rib drive tire may not have the grip in snow or mud that an open-shoulder drive tire has, but will the next round of greenhouse gas (GHG) emission cuts and their demands for tires with lower rolling resistance cause traction problems or compromise braking performance?
This issue came up last summer when I spent a couple of days on a test track with Meritor putting various combinations of drum- and disc-brake-equipped tractors and trailers through their paces in some stopping distance tests. It was a warm, sunny summer day and the concrete pavement was hot and dry. With all the combinations we tested, we consistently stopped from 95 km/h in a shorter distance than the Reduced Stopping Distance (RSD) braking requirements demanded. We had trailer brakes, too, and the trailers used in the configurations for RSD tests do not. Some of the stops we made were within 200 feet. That’s 50 feet less than the RSD requirements call for, so we were well under the wire.
We did have some ABS activity on the steer axle (of drum- and disc-equipped trucks), which suggests those tires broke traction under the hard braking. That’s easy to understand when you have 100 psi or more actuating the brakes. But the ABS did its job and the trucks stopped perfectly straight.
But for the 2021 round of GHG reductions, tire makers will need to lower the rolling resistance even further than the tires we’re now using.
“The brakes will continue to produce the same amount of torque as they do now, and they do not care what kind of tire is making contact with the pavement,” says Mark Ugo, senior test engineer for brakes at Meritor. “If the tire breaks traction, ABS will step in. That’s what it’s designed to do.”
Bendix is keeping an eye on this, but doesn’t expect any significant issues to arise.
“It depends on tire construction, rubber compounds and things of that nature, but there could be some minor impact on traction,” says Keith McComsey, director of marketing and customer solutions for wheel ends at Bendix Spicer Foundation Brake. “With air disc brakes, Bendix exceeds the [Reduced Stopping Distance] requirements by 20%, so we’re down in the 200-foot range. We are about 10% under on the drum side. The OEMs want that margin because they know there’s configuration differences in the vehicle that impact braking as well, whether it’s tire combinations or wheel base combinations.”
Winter driving poses its own problems, and GHG Phase 2 tires will still need to perform in snow.
Options for rolling resistance
The tire manufacturers have several options for reducing rolling resistance without necessarily compromising traction, including changes to the rubber compounding, tire shape, tread pattern, tire construction, and weight.
“The key to developing a successful product is coming up with the right combination of all these factors to get what the consumer needs,” says Phillip Mosier, manager of commercial tire development for Cooper Tire. “When developing low-rolling-resistance tires, we must take into account the regulatory requirements and consumer’s needs to come up with a successful product. So the proper balance of rolling resistance, treadwear, and traction must be maintained. It is possible to meet the rolling resistance requirements of GHG [Phase 2 standards] and the stopping distance requirements from the Reduced Stopping Distance mandate with low rolling resistance tires.”
Tire manufacturers work to optimize what’s known as the “performance triangle” of rolling resistance, traction, and distance to removal. In the past, when one performance benefit like low rolling resistance was optimized, the remaining benefits of the performance triangle sometimes took a back seat. Goodyear says that dynamic is much less prevalent these days.
“We are constantly developing technologies and designs to minimize tire performance trade-offs so fleets can enjoy optimal, all-around tire performance,” says Goodyear’s commercial technology director, Mahesh Kavaturu. “That said, as fuel efficiency regulations tighten, the need for new truck tires and even retreads that offer lower levels of rolling resistance will continue to grow, which is why we are investing even more resources to develop tires that help reduce fuel consumption, while at the same time helping to ensure that our products provide the traction that fleets also require.”
The major tire companies are all concerned with traction to a certain degree, and many of the changes required to meet rolling resistance through compounding have already been made, one tire expert who preferred not to be quoted told us. “Moving that bar even lower to where it’s going with GHG 2, you’ll see changes in tire construction and changes in the rubber compounds used within the casing — not necessarily in the compounds that hit the road,” he says. “The easy fruit, per se, was the tread compounding.”
He says the work that’s underway right now at the tire makers is more about maintaining wet traction while pushing rolling resistance lower. Wet traction is not part of the reduced stopping distance requirements.
“That rule had nothing to do with cold, wet, grip,” he says, “which is when it’s raining, when it’s cold outside, and you slam on your brakes. That’s when you see trucks sliding all over the place. That’s what the tire companies are concerned about, not dry road traction and stopping distance.”
There is actually no standard now that speaks to traction in terms of a measurable degree of pavement adhesion just as when cornering. There is language in FMVSS 136 rules (Electronic Stability Control Systems for Heavy Vehicles) that speaks to vehicle handling in various maneuvers including cornering on various pavement surfaces with various tires, but there is no standard for determining how much “traction” is required in a given situation.
“But with regard to reduced stopping distance, no, I don’t think the low-rolling resistance tire requirement called for by GHG 2 is necessarily going to affect dry stopping distances,” our shy expert says.
Obviously, the tire makers are not going to share detailed plans for meeting GHG Phase 2 at this point, but traction will be front and center, including wet traction.
While traction concerns are high among drivers, many don’t seem to notice that worn tires offer significantly less traction than even the most fuel-efficient low-rolling-resistance tires.
The challenge for tire designers
“The GHG 2 push for increasingly lower tire rolling resistance is certainly a challenge to tire designers, especially while balancing the other performances, like wear and traction, that are important to customers,” says Sharon Cowart, business-to-business product marketing director, Michelin North America. “Tire designers continue to strive to break those performance compromises through rubber formulation and tread design.”
One possible offshoot of the push for lower rolling resistance might be more application-specific tires, with traction considerations designed for the service the tire will see.
“While keeping the performance triangle in mind, it’s also important to consider the question, ‘What is this tire being asked to do?’” says Kavaturu. “A mixed-service tire that has been designed to roll across severe terrain might not be the best choice for a longhaul truck, and a longhaul tire that has been engineered for low rolling resistance is probably not the ideal choice for a truck that primarily travels off road, across debris-strewn surfaces. Some tires are better-suited to certain applications than others.”
Goodyear plans to release two application-specific tires toward the end of 2019, the Fuel Max RTD and the Ultra Max RTD. Goodyear says the Fuel Max RTD balances traction with fuel efficiency, achieved through inclusion of a new low rolling resistance tread compound, and long miles to removal, while the Ultra Grip RTD is designed for regional trucks that run in severe weather conditions and is engineered for exceptional severe-weather traction.
The debate over traction in fuel efficient tires isn’t new. GHG 2 will push tire makers to comply while at the same time maintaining the traction and tire wear attributes fleets demand. They have risen to the challenge in the past. Our bet is they will do it again.