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Tire Inflation Article. Interesting. - Grizzly - 10-16-2017
The Magic Number
If you’re arbitrarily filling drive and trailer tires to 100 psi, you could be sacrificing tire life.
August 6, 2013 | by Jim Park
How do turtles mate? Why are women’s shirts buttoned on the left while men’s button on the right? How did they build the pyramids? How much air should I put in my tires?
Unlike the pyramids, tire inflation needn’t be one of life’s little mysteries.
A glance at any tire maker’s load and inflation tables has the answer, but many fleets inflate their drive and trailer tires arbitrarily to 95 or 100 psi – thus “overinflating” them and possibly sacrificing tire life.
Correct inflation pressure is dictated by weight, temperature and road speed – and the load and inflation tables.
Correct inflation pressure is dictated by weight, temperature and road speed – and the load and inflation tables.
I’ll let you in on a little secret: You only need between 75 and 80 psi in any drive or trailer tire when loaded to the maximum U.S. Interstate weight limit of 34,000 pounds per tandem axle group. Published load and inflation tables from Goodyear and Michelin indicate 75 psi (80 psi by Bridgestone’s table) is the minimum pressure required to support a tire load of 4,550 pounds. In a fully loaded 34,000-pound tandem, each tire in a dual assembly carries 4,250 pounds. That’s cold inflation pressure, by the way, the standard inflation pressure denominator.
Many fleets want to err on the side of caution by running 95 to 100 psi in single tires in dual assemblies? From a safety and compliance point of view, there’s nothing wrong with that.
“Operationally, it makes some sense to set tire pressures above the minimum to provide a margin of safety against running underinflated,” says Guy Walenga, Bridgestone’s director of engineering, commercial products and technologies. “Air leaks out of tires. It doesn’t leak in. All tires lose pressure over time, some faster than others for various reasons.”
Because drivers can’t be relied upon to check tires regularly, a little spare air isn’t going to hurt the cause.
Another oft-cited reason for running 100 psi in a drive or trailer tire is a potential reduction in rolling resistance.
“Yeah, there’s some small improvement there,” Walenga says. “Whether you’d be able to separate it from the rest of the noise in a fuel economy test, I’m not sure.”
Herman Miller never saw much of a difference. He’s the president of HJM Fleet Maintenance, and one of a rare breed who runs his tires close to the L&I table minimums – 110 psi in steer tires, 75 psi in drive tires and 80 psi in trailer tires.
“I believe that there may be some small increase in fuel economy, but I never could quantify it,” Miller says. “There are far more compelling reasons to run lower pressures than higher if you have a solid tire maintenance program.”
Miller’s fleet ran light loads, mostly general merchandise for retail chains. He seldom goes over 70,000 pounds GVW. But that’s not an excuse, that’s the reasoning.
“We experience much less tire wear and casing damage than I hear other fleets complaining about,” he says.
The key, Miller says, is the ability of the slightly softer tire to flex the way it was designed to.
“We had almost no impact damage in all those years, and we saw none of the irregular wear that a lot of fleets complain about,” Miller says. “Those problems come from tires that are too hard to function the way they are designed to function – that is, having enough give in the casing to absorb impacts like potholes and road debris, and having the optimum footprint or contact patch.”
Minimum, maximum, optimum
Tire inflation pressures are not arbitrary. Each tire maker establishes minimum pressures based on tire loading, and the construction of the tire establishes the maximum allowable pressure.
The L&I tables indicate the minimum, and the maximum is stamped on the sidewall. It’s worth noting, too, that wheel manufacturers establish maximum inflation pressures, stamped into the wheel, of usually 130 to 150 psi.
What’s lacking is discussion about optimum pressure.
Donn Kramer, director of product marketing innovation at Goodyear Commercial Tire Systems, says tire loads determine inflation pressure.
“The specific pressure for a given load is available from tire manufacturer’s load and inflation tables,” Kramer says. “Fleets also can use tables provided by the Tire & Rim Association, whose members set technical standards for manufacturing tires and wheels.”
Walenga says essentially the same thing. “The idea is to keep your inflation pressures in line with your tire loads, Always inflate the tires to carry the maximum load, even if you load light some of the time.”
For Miller, at around 70,000 pounds GVW, his tire loads are light for even 75 or 80 psi. And he’s well within the margins for a full 80,000 pounds GVW.
Why higher isn’t always better
Miller argues that inflating to the recommended L&I table pressures takes the tire back to where it was engineered to run.
Dual tires must be within 5 psi to maintain the same load on each tire, and to keep the circumference of the tires in a dual assembly the same.
Dual tires must be within 5 psi to maintain the same load on each tire, and to keep the circumference of the tires in a dual assembly the same.
“Inflating the tire to more than the recommended pressure for the load changes the shape of the tread, it stiffens the casing, and sets it up several wear and damage scenarios the tire wasn’t designed for,” he stresses. “I can only go by my experience, but I’ve had absolutely no problems in running at 75 or 80 psi in drive and trailer positions, where as I can point to excessive wear and damage from running 100 psi.”
Miller’s experience is backed up by tire experts.Al Cohn of Pressure Systems International (which makes the Meritor Tire Inflation System by PSI), explains that it’s all about the footprint of the tire.
“When you over-inflate, the footprint changes and it gets a little smaller,” he says. “But the biggest impact is – and this is what people don’t normally talk about – the loaded vs. unloaded condition, especially on trailers.”
You may spec that higher tire pressure based on the loaded worst-case scenario, but in reality, in many cases, that trailer is empty, or at least a lot lighter than fully loaded. “So when you’re empty…the tire is going to develop all kinds of issues like uneven wear because you’re bouncing up and down the highway.”
Several things happen to a tire that’s over-inflated for its load.
“The tread tends to crown, leaving the shoulders of the tire scrubbing along the road as it tries to keep pace with the larger circumference of the center of the tread,” explains Walenga.
Like Cohn, he says it’s the worst when running empty. “The tires just bounce along the road, scrubbing a little more rubber off the tread every time they hit the ground,” he says. It’s a cumulative effect, and it can be severe in fleets like fuel haulers that run empty half the time.
Safety margin
The standard arguments for running 95 to 100 psi at drive and trailer positions is to build a margin against “underinflation” for tires that aren’t checked regularly, and for improved fuel economy due to the lower rolling resistance of the stiffer tire.
Both are reasonable arguments, but they may not hold up well to scrutiny. The potential fuel savings, if any, would be small and difficult to quantify amidst all the other variables in any fuel economy test.
That leaves the safety margin. Few would argue that tire pressures – especially on trailer tires – are notoriously under-maintained. And we’ve all heard the warnings about what running underinflated tires does to fuel economy. But when we’re talking 10% underinflation having a 1% to 2% impact on fuel economy, we’re really saying all the tires on the vehicle (except steer tires) would have to be in the 65 psi range.
This could be made more complicated by lack of consistency in law enforcement over exactly what constitutes underinflated. Because of the Federal Motor Carrier Safety Administration’s new CSA enforcement program, the Commercial Vehicle Safety Alliance is currently exploring how to define underinflation for enforcement purposes. To learn more, go to http://www.truckinginfo.com/underinflated.
Trying lower pressures
More proactive tire maintenance could allow for running at lower pressures with minimal risk. Or you could take advantage of an automatic tire inflation system – set to 75 psi rather than 100 psi.
“No matter the minimum pressure selected by the carrier, automatic tire inflation systems can maintain that pressure,” says Bob Montgomery, vice president, Intelligent Transportation Systems, at Stemco LP, makers of the Aeris inflation system.
Here are few important caveats to consider if you are thinking of down-pressuring your tires.
• Most tire pressures are based on a road speed of 60 to 65 mph. If you run markedly faster, you need to compensate. Goodyear’s Kramer suggests fleets operating at 66 to 70 mph increase cold inflation pressure by 5 psi.
• Air pressure maintenance intervals are critical. The tire industry recommendation is to check tire pressure weekly, Kramer says.
• Fleets should maintain the proper minimum inflation for the load carried per the tire manufacturer’s recommended tables or the Tire & Rim Association’s table, and dual tires need to be kept within 5 psi of each other.
Keep in mind that like extending drain intervals, running tires closer to the minimum recommended pressure means closer attention to maintaining that pressure.
“You’re running closer to the edge, so you have to stay on top of your tire maintenance,” cautions Miller. “Fleets that see their equipment frequently can easily get away with this. Owner-operators certainly can, and fleets that run lighter loads, like we do, will certainly see benefits from the added diligence.”
Steer tires are different
This article mainly talks about drive and trailer tire pressure, but steer tires do require higher inflation pressures.
For the typical long-haul steer tire found on the vast majority of over-the-road trucks, the minimum cold inflation pressure would be between 105 and 110 psi. Major tire makers’ load & inflation tables indicate that to support a 12,000-pound steer axle load (6,000 pounds per tire), a 22.5-inch low-profile load-range G tire should be inflated to at least 110 psi.
Each of the manufacturer’s tables indicates 110 psi is necessary for a load of 6,175 pounds. At 105 psi, the tire is rated for only 5,980 pounds. It may seem like we’re splitting hairs here, but if your steer axles are running at or close to 12,000 pounds (which many are today thanks to EPA ‘07, ‘10, etc.) at 105 psi, each tire would be 20 pounds over its weight rating for that pressure.
If you consistently load less than 12,000 pounds on your steers, you could get away with 105 psi, but Bridgestone’s Guy Walenga says operators should always inflate for the worst-case scenario.
“Running any pressure below the minimum required for the load is running underinflated,” he says.
More than a few trucks today are equipped with 13,200-pound steer axles, and loading gets perilously close to that on some occasions.
Even if you have the heavier axle, you’ll get into trouble if your tires aren’t rated for 6,175 pounds and/or you are running less than 110 psi in those tires.
RE: Tire Inflation Article. Interesting. - JMBT - 10-16-2017
Man, I’ve been practicing and preaching that for years, you put in however much air the manufacture says for what weight you’re hauling. Seems like a no brainer to me. But like the article said, “drivers can’t be relied upon to check their tire pressure regularly”. They must be talking about company drivers! Yet another reason allowing people to haul freight commercially on equipment they do not own, or at least have a vested interest in, or some accountability for, is a bad idea....
RE: Tire Inflation Article. Interesting. - uncleal13 - 10-16-2017
Yup. I run 85 lbs in my super-b trailer tires.
RE: Tire Inflation Article. Interesting. - Ragindirt - 10-16-2017
I will make changes to the way I run my pressures going forward.
RE: Tire Inflation Article. Interesting. - Waterloo - 10-16-2017
I dropped mine down, prior to this article. I haul light, and went to 100 from 110 on the drives, now I will more than likely go to 90 or 85 on those. And, I need to rotate the drives, front to back, along with some brake shoes on the rear axle, it is time. Steers, I need to look, 110 right now, three week old Michelin 16 plys, 12,000 lb axle, but always at 12,000 lbs.
RE: Tire Inflation Article. Interesting. - Rawze - 10-16-2017
Sorry, but I'll keep my drives at 120 psi cold. I broke them in at that pressure and they like it now that they have settled. They last a ridiculous average of 435,000 miles at those pressures and YES, we do see a few tenths losses in mpg when they get low. We also see a few tenths losses if a trailer has tires at only about 80 psi or so and its heavy. It is not unusual for us to put more air in them if we are taking the trailer half way or more across the country. -Keep in mind that we have every fuel reciept entered so that it can be tracked tracked state to state. The omputer shows us what is going on every time we enter a fuel receipt and how well or poor we did from the last fuel stop to the current one as compared to a million miles + of collected data of "from the last time you fuel in say Ohio, then Nebraska" for example. It is VERY accurate.
-- YES, we can see a few tenths easily like that because the computer also spits it out as a graph that at 0 is one mpg less, and at 100% on the scale, is only simply the peak of the last 100 fuel stops. I.E. -> We drop 0.5 mpg and it shows up as a 50% drop on the graph compared to the last 100 fuel stops. I did that on purpose a few years ago because I wanted a more accurate measurement of comparison.
Another thing it does is show us a list of the BEST "from this state to that state" averages and the worst from all 48 states to each other. That way we can work harder (slow down more) on the areas/regions/lanes we know cost us more fuel, and know we can speed back up a few mph in other areas to make up for it, raising our average overall both in optimized speed and in fuel efficiency.
We can also use that data to tell us what lanes we don't want to travel, and that sometimes taking a slightly longer route makes us more money. For example, running form Los Angeles to Chicago, there is the option of taking I-70 (2005 miles), I-80(2068 miles), or US-54 (2060 miles). Us-54 beats the pants off the other 2 routes in fuel mileage savings by quite a long shot. Remember, we use our total trip average speed of 48-50 mph (depending on load) to regulate how fast/slow we drive. We reset the GPS and YES the clock is still ticking, average speed steadily dropping when we are stopped (some GPS units will not do this, they stop the clock when you stop and I won't buy one of those models). If we fall behind because of stop-go traffic of the small towns along 54, then we go 63-65 through the areas where we know the best fuel mileage is and it all works out to always get us to the destination on time + maximum profits. <- We do haul UPS and fedex sh%it and YES we always deliver on time while only driving 55-58 mph, and rarely any faster.
When it comes to my #1 operating expense, I am not playing around. I track it like a hawk and anything that effects it, I am aware of it right away. Seems lately most guys take that last 0.1 mpg for granted, but when I started out with my own truck,.. fuel was peaking $4.50 a gallon in places.
RE: Tire Inflation Article. Interesting. - ZeroNthedark - 10-16-2017
(10-16-2017 )Rawze Wrote: They last a ridiculous average of 435,000 miles at those pressures
Geez, I got a whopping 73,xxx miles out of my last set of drives on my tri-axle dump. And that's not hauling ass either, but they were at 110 psi. Must be these great PA roads!
RE: Tire Inflation Article. Interesting. - Hammerhead - 10-17-2017
(10-16-2017 )Grizzly Wrote: The Magic Number
If you’re arbitrarily filling drive and trailer tires to 100 psi, you could be sacrificing tire life.
August 6, 2013 | by Jim Park
Grizzly, I know you are not Jim Park, this is directed at the article.
Jim Park, point blank, you are a shill to anyone who will pay you one single green dollar bill to say what they want you to say.
Where is your research verifying the verbal diarrhea you spew?
I see a lot of "quotes" and "talk" from tire manufacturers, pst...the guys who want to sell you tires to make money! about running lower pressure, but where is their research data?
There is one factual point in the article, but it is not the one they focus on. That fact, plain and simple, one tire pressure number is NOT the optimum for every load. I cannot and will not dispute a fact. Every single load (unless the weight never varies, impossible) will have a slightly different optimum pressure value based on weight, temperature, road surface substrate, speed, climate, etc.
If you want to find that optimum it's not that difficult...get a thermal imager and get a temperature reading across the surface of the tread and adjust pressure until there is no variance...ok, so in all reality is actually quite difficult and you'll never have it "right", but by all means go ahead and try.
Under inflated tires actually do provide a benefit.
Yes I really did say that.
But you will never see that benefit in a class 8 vehicle. Laws of physics and the Law of the Land will not allow it to be realized.
An under inflated tire with enough speed, which you cannot achieve, will actually "grow" because of centrifugal force and the speed gains and added mechanical efficiency to be an effective cost offset. They simply are not built to withstand the forces required to see these advantages. And conversely if you could see sufficient speed to gain these advantages, you will then begin to experience center high point wear.
A lower inflation pressure results in an increased "footprint". The first thought that most people think when they hear this is better weight distribution which should be better right? Yes, but here's why it's actually wrong; the incresead footprint then also results in a longer "dwell" time that the contact area of the tire is physically supporting the load. This increased dwell time will result in increased wear, resulting in lower overall tread-wear life. This increased wear will give the appearance of a smoother flatter wear seducing you into thinking your tires are wearing better. And they are. They are wearing flatter, and better, but they are also wearing faster ultimately resulting in lower overall treadwear mileage life.
Here is the choice;
Tire A: worn perfectly flat, the tread depth measurement is 100% equal across the face as well as the entire circumference of the tire
Tier B: worn with light patterns showing, and depth is lower along the center than the edges and is not truly consistently worn
Most people say they want Tire A. I'll take Tire B. It doesn't appear as pretty at end of life cycle, but it lasted 400k miles to Tire A's 300k miles. That's 100k free tire expense miles, and I'll take free expense miles any day I can, and twice on Sunday.
And I haven't even started with the rolling resistance part of the discussion yet. I've had a couple of beers and a shitty day so I'm not gonna start on that right now, but holy sh!t anybody that believe there isn't that much difference has zero understanding of basic physics, and has no right writing an article giving advise on the subject. Wow.
Here, let me tell you about biochemistry. I mean I must know how it works, because the 'roid freak from the local gym told what this crap he injects himself with really does...
RE: Tire Inflation Article. Interesting. - Brock - 10-17-2017
I tired out Rawzes 120 psi deal, it did not work for me but that is because I run 50% empty miles I'm sure. I wore out the centre strip in no time flat! If I were running loaded 90% of my miles then I would do it again.