10/22/2019
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`The Magic Number - Maintenance - Trucking Info
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`MAINTENANCE
`The Magic Number
`
`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.
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`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.
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`“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
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`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.
`
`“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.”
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`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 www.truckinginfo.com/underinflated.
`
`Trying lower pressures
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`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.
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`The Magic Number - Maintenance - Trucking Info
`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.
`
` Read more about
`
`Goodyear Michelin
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`Bridgestone Wheels
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`PSI
`
`Tires
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