Article

Tire Pressure, Fuel Economy and GHG Regs

November 2015, TruckingInfo.com - Department

by Jim Park, Equipment Editor - Also by this author

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Trailers that go unseen for months can benefit from automatic tire inflation. What about fleets that see their trailers every week? Photos: Jim Park
Trailers that go unseen for months can benefit from automatic tire inflation. What about fleets that see their trailers every week? Photos: Jim Park

Automatic tire inflation systems are being considered as a fuel-saving technology under the Greenhouse Gas Phase 2 rule. As the proposal currently stands, the Environmental Protection Agency and the National Highway Traffic Safety Administration are prepared to offer compliance credits to any original equipment maker that installs such systems.

Paul Crehan, director of product marketing for Michelin Americas Truck Tires, told HDT the agencies’ decision was based on evidence from a DOT study carried out between 2008 and 2010 with two carriers, Sheetz Petroleum and GFS Food Service.

“DOT found an average of 1.4% improvement in fuel economy due to tire inflation systems and wheel-mounted and valve-stem mounted TPMS (tire pressure monitoring systems) collectively,” Crehan says. “Taking this study as reference, [GHG] Phase 2 has proposed to account for a 1.5% factor for fuel economy if trailer ATIS is installed and 1% if tractor ATIS is installed.”

There’s little debate about the cost effectiveness of trailer ATIS when it comes to reducing the downtime costs associated with flat tires, but how much of a contribution can they realistically make to fuel economy?

Almost everyone is familiar with the adage that 10% under-inflation can cause a 1% degradation (or something to that effect) in fuel economy due to the increased rolling resistance of softer tires.

But what exactly does that mean? Are we talking about one tire out of 18, just the tires on the truck, just the tires on the trailer, or would all the tires have to 10% under-inflated to account for the 1% reduction in fuel economy?

We asked several tire makers that question, and the answer was universal: all the tires need to be 10% under-inflated before the truck sees a 1% drop in fuel economy. But it’s a little more complex than that.

Here’s how Terry Smouter, business development manager for Continental Commercial Vehicle Tires, describes it: “The overall fuel economy of the vehicle is dependent on the axle position of the under-inflated tire. The drive and trailer axles contribute more to the overall fuel economy of a truck than the steer position does.”

There have been multiple studies conducted both in Europe and here at home that show, in general, that tire pressure maintenance isn’t something most fleets are good at.

Smouter cited a European study that shows 12% of fleets are running tires that are under-inflated compared to the fleet’s inflation guidelines.

And Crehan cited a DOT study from 2003 that showed only 50% of tires were within 5 psi of recommended pressure, 1 out of 20 tires was more than 20 psi under-inflated and 20% of dual-tire assemblies have tire pressures that differ by more than 5 psi.

Recommended pressure

The benefits of rigorous tire pressure maintenance are no longer in question. So why do fleets generally do such a bad job of it?
The benefits of rigorous tire pressure maintenance are no longer in question. So why do fleets generally do such a bad job of it?

But here’s the rub. What exactly is recommended pressure, and how much is it costing fleets in fuel economy if they run tires at less than recommended pressure?

Try as you might, you will find only one official reference to tire pressure anywhere in this land; the tire makers’ load and inflation tables (also available through the Tire Industry Association). The L&IT speak only to the minimum pressure required to support a given load, which isn’t much to go on.

“The load and inflation tables are standards that are consistently maintained by all tire manufacturers and provide the guidelines to which tires can safely carry a given load,” explains Prosser Carnegie, product development manager for Continental Truck Tires, NAFTA. “The key concern as a tire manufacturer is when a tire is being run under the L&IT guideline, since this is approaching an unsafe operating condition. At the minimum pressure, the tire is essentially operating at the threshold of its load carrying capacity.”

It can be concluded than that any tire run below the minimum stated in the L&IT for a given load is technically under-inflated and at risk of related damage, such as tread separation due to high operating temperatures and increased flexing of the sidewalls, which can lead to zipper ruptures.

How, then, do we reconcile a tire running at 90 psi when the L&IT says a minimum of, say, 80 psi is required for the load? Is this under-inflated? 

“That would not be considered an under-inflated tire in the strictest terms when it comes to carrying a given load,” Carnegie says. “But consider a fleet whose standard pressure for a drive tire is 100 psi. The 10% under-inflation is not in relation to the L&IT standards, but 10% under normal operating inflation pressures.”

That might clear up the matter of what defines an under-inflated tire, but there’s still another question to grapple with. Are fleets that choose to run 90 psi in their tires, rather than 100, giving up 1-1.5% in fuel economy?

Possibly, but weighed against other operational concerns involving tire inflation pressure, it might seem like a fair trade-off.

“There are factors outside of pure load carrying capacity that can be optimized with higher inflation pressures,” Carnegie says. “This is something that is tuned based on each individual fleet’s application and tire performance needs.”

Fleets that consistently run lighter loads, for instance, may see some tire wear benefit from running lower inflation pressures, say 85-90 psi rather than 100. The softer tire’s contact patch may be more even, thus contributing to longer wearing tires with less irregular wear.

But under-inflation can create a number of problems as well.

“Under-inflation causes tires to flex more as they roll down the highway and can hurt a truck’s overall fuel efficiency, due to the simple fact that under-inflated tires force a vehicle’s engine to work harder,” says Brian Buckham, general manager for product marketing at Goodyear Commercial Tire Systems.

Peggy Fisher, CEO of TireStamp, a provider of tire pressure monitoring systems, points to an experience she had during her early days at Roadway when radial tires were still relatively new.

“We were getting some very uneven wear on those tires,” Fisher recalls. “Our supplier at the time recommended that we drop pressure from 100 to 85 psi. The only thing that did was noticeably increase fuel consumption. Needless to say, we went back to 100 psi very quickly.”

With all that rubber on the road, under-inflated tires can be an expensive proposition. 
With all that rubber on the road, under-inflated tires can be an expensive proposition.

Is more better?

It’s accepted that running 100 psi rather than the L&IT minimum of 75-80 psi will improve fuel economy by reducing rolling resistance. So why stop there? Why not inflate to 110 or even more?

According to Continental’s Smouter, running 100-110 psi on drive and trailer tires would lead to better fuel efficiency than 75-80 at the maximum legal load, but there are consequences at the high end, too.

“As the inflation pressure increases, there is a potential for more irregular wear and more impact breaks,” he says. “Each fleet needs to find their own ideal operating inflation pressures based on their individual application and needs.”

And that brings us right back to “proper” inflation pressure. The answer to the inflation question really is whatever works for your fleet.

“To gain the benefits of fuel efficiency, tires need to be inflated to the proper pressure for the specific load to get the maximum benefit,” says Michelin’s Crehan. “Tire inflation contributes to more than fuel efficiency. Proper tire pressure contributes to maximum total performance including wear, retreadability, casing endurance and traction.” 

With that said, should fleets worry if their 100-psi tires are found to be at only 90 psi?

“That would indicate something is wrong with the tire,” Fisher says. “It’s either leaking or it hasn’t been maintained for a very long time. If all the tires are 90 psi, they probably have not been serviced in months.”

Expanding on that point, Crehan says tires found to be at 20% less than the recommended inflation target — that is, the fleet’s chosen pressure provided it’s within the minimums prescribed by the L&I tables — must be considered as run flat and removed.

To answer the question of the effectiveness of automatic tire inflation systems, or even tire pressure monitoring systems, if eventually included in the final GHG Phase 2 rule: They will probably help, but they come with issues of their own, including additional maintenance in some cases.

“ATIS will make a difference to fleets that don’t see their trailers very often to maintain them or who just don’t maintain them,” says Fisher.

Given fleet track records with pressure maintenance, maybe EPA and NHTSA will be handing us a benefit we won’t even recognize until we get it.

Comments

  1. 1. João Reis Simões [ December 04, 2015 @ 04:18AM ]

    If tires' pressure is excessive it jeopardizes brake efficiency.

  2. 2. Susan Nelson [ December 04, 2015 @ 08:09AM ]

    A nice discussion of the factors to be considered in order to determine appropriate tire pressure. Reference is made frequently to the study of commercial vehicle tire inflation pressures released in 2003 by FMCSA. Maybe it's time to take a look at the current state of inflation pressures by repeating the study in the field.

  3. 3. Art Meyer [ December 04, 2015 @ 12:08PM ]

    Before it can be a part of anything the vehicle and all the various tire companies need to get on the same page. I can go to any of them and each one will give you a different answer as to the tire pressure you should be running., also what pressure is run in Phoenix with the roads and temps their vs. the roads and temps in lets say Chicago can cause a difference in required pressure to get the best wear out of the tire.

  4. 4. bill [ December 10, 2015 @ 12:04PM ]

    If a Fleet manager Hasn't been seriously looking at Tire Inflation systems for All of his Fleet (Trucks and Trailers ) He Isnt much of a manager . The Brazilian style On Board Tire Inflation systems (like SUPER-AIR OBTIS , Nicolini , Vagia ,and others) Save Huge Money and Vastly Improve safety for All who are on the Road .
    We have been adding OBTIS to All of our Fleet (straight trucks , tractor's ,and Trailers ) and even our light duty trucks and delivery Vans .

  5. 5. Bob Rutherford [ December 11, 2015 @ 08:37AM ]

    I lean towards the Australian systems like http://www.aircti.com/. One issue that the trucking industry seems to be missing is the relationship between over inflation and jackknifing. Here is a blog about it:

    https://www.linkedin.com/pulse/jackknife-jackknifethat-question-bob-rutherford?trk=mp-reader-card

  6. 6. chet cline [ December 13, 2015 @ 01:27PM ]

    Sorry folks, the above information is not correct. Load to inflation tables recommendations are not the minimum pressure. No where do they day 'minimum'. No where.... These are the optimal pressures provided by the manufacturer which is the expert. Michelin goes further, showing loss of tire and casing life if the tire is run over or under these optimal pressures. My 20 years experience owning and developing AIR CTI, every truck can obtain at least 30% more tire life, better handling, shorter stopping distances, reduced maintenance, reduced infrastructure damage, and reduced environmental impact. This idea that 100 psi is good, is simply wrong. Steer tires are usually under inflated, while drive and trailer tires are 25% over inflated. As to Chicago and Arizona, it's the temperature that matters. Every tire manufacturer supplies temperature charts too.

    Tire pressure maintenance systems are great for trucks that always run the same load, if set at the correct hot pressure. But, if the truck load changes regularly, you need Central Tire Inflation. It is a great investment in your future. AIR CTI is the best CTI, bar none.

  7. 7. Bob Rutherford [ December 17, 2015 @ 10:14AM ]

    Tire Footprint Management Part One

    We need to explain the concept of the load and inflation tables (L&IT) by using a simpler concept of tire footprint management. Properly managing anything is to optimize and harmonize various factors that must be considered. Management is understanding a system, not just the components of that system, many of which are in conflict.

    As an example, because of the EPAs focus only on one factor in tire footprint management being rolling resistance, the smaller the footprint the less rolling resistance. However, the tire needs to have a certain amount of resistance for braking. The conflict to be managed is which do we choose a big footprint for braking or small one for fuel economy? Is the optimized size in the middle? Do you see this problem expressed in the L&IT table?

    While we are optimizing and harmonizing the tire footprint system we also have to deal with the load, the road conditions and lest not forget the speed factor. The tire engineers have tired to show the complexity of the tire footprint in a L&IT that the trucking industry seems to have trouble understanding, and they do it for each tire design.

    I bet some scholar could prove that improper footprint management, due to EPAs meddling, has saved millions of dollars in fuel but wasted billons of dollars in bridge and highway repairs. What about scrapped tire cost due to irregular wear from over inflation.

    As an industry we have got to get serious about tire footprint management.

 

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