Steer tire failure is suspected as the cause of the deadly collision between a truck and a motor...

Steer tire failure is suspected as the cause of the deadly collision between a truck and a motor coach in New Mexico just before Labor Day.

Screen grab via CBS Morning News

How much of a difference can 5 or 10 pounds of inflation pressure make to a tire? To a drive or trailer tire, it's probably insignificant. To a steer tire, it could be the difference between completing the trip safely or winding up on the national news in a messy and tragic wreck.

Last Thursday, just before the Labor Day weekend, there was a deadly crash between a Greyhound bus and a tractor-trailer on Interstate 40 in New Mexico near the Arizona border. Seven passengers on the bus lost their lives when the truck crossed into the opposite lanes and struck the coach more or less head on. Authorities believe a steer-tire blowout was the root cause of the crash, and news camera images of the suspect tire reveal the tell-tale signs of being run underinflated before it blew, allegedly causing the driver to lose control of the truck.

The National Transportation Safety Board will investigate this crash, so it's safe to assume that there will soon be a great deal of discussion about tire inflation and maintenance.

We want to stress that the cause of the crash has not been officially determined. Nevertheless, the shredded tire carcass hanging off the left-front wheel of the overturned truck will no doubt draw attention to the issue of blown steer tires. So we decided to share some of what we've learned in the course of our reporting on tire maintenance about the importance of proper steer-axle inflation pressure.

If you think 100 psi in a steer tire is okay, please keep reading.

Operating on the assumption that 100 psi is adequate inflation pressure for steer tires will...

Operating on the assumption that 100 psi is adequate inflation pressure for steer tires will leave you on the back side of the curve now that steer axle weights are routinely closer to 13,000 pounds and even higher.

Photo: Jim Park

The 100-psi Paradigm

It's pretty common to find fleets inflating drive and trailer tires to 100 psi. It's an easy number to remember, and it adds commonality to the maintenance process. But why inflate to 100 psi when the load and inflation tables for most North American tires call for something between 75 and 82 psi for a dual tire supporting a load of 4,250 pounds in a fully loaded tandem grouping of 34,000 pounds? The extra 20 or so pounds of pressure provide a margin of safety against a slow leak or tires that aren't checked regularly. It also stiffens the sidewall, which can help reduce rolling resistance. Other than that, there's no technical justification for adding an extra 30% to those tire's inflation pressure.

It's a completely different story for steer tires.

"Inflating tires on 12,000-pound steer axle to 100 psi may have been okay 20 years ago, but it's not enough anymore," Curtis Decker told us a few years ago when he was manager of product development at Continental Tire North America. "In most cases today a pair of 22.5 inch low-profile, load-range G tires on a steer axle will get you to only 11,600 pounds at 100 psi. Two tall '22.5-Gs' will give you about 11,900 pounds of load carrying capacity. That's still not enough for many of today's emissions-equipped, set-back axle tractors."

According to various manufacturers' load and inflation tables, a typical standard profile 11R22.5 load-range G tire needs 105 psi to get beyond 6,000 pounds load carrying capacity (12,000 per axle). For example, Michelin's tables indicate a pair of such tires would be good for 12,350 pounds (6,175 pounds each).

But beware, low-profile 22.5-inch. tires, such as a pair of 275/80R22.5 LRG, would need 110 psi (from Michelin's tables) to carry 12,350 pounds.

"Back in 2000, everyone considered 12,000 pounds as the standard steer axle weight," Decker reminded us. "But even with all the changes to trucks we've seen since then -- aerodynamic packages, emissions controls, set-back axles, etc. -- none of us looked as critically at the extra loads on steer tires as we looked at the issue of under-hood heat, for example."

This traditional way of thinking can cause problems, Decker said.

"When we're talking about inflation, people are using the same 95 to 100 psi target they used back in 2000 when the standard steer axle was 12,000 pounds," he says. "Today, we're going above 13,000 pounds without even thinking about it. Unfortunately, the tires don't respond well to underinflation."

According to Bridgestone, there are two ways to increase the load capacity of a tire: (1) Increase the volume of air in the tire (increase the tire's size) for a given air pressure, or (2) increase the pressure of the air in the tire for a given tire size.

Or, as Goodyear's Donn Kramer told us a few years ago before he retired as director of product marketing innovation, "For a 12,000-pound axle in sizes 11R22.5 and 295/75R22.5, fleets generally would use a load range G tire with single load carrying capacity of 6,175 pounds at 110 psi cold inflation. For a 13,000-pound axle in sizes 11R22.5 and 295/75R22.5, fleets would generally use a load range H tire with a single load-carrying capacity of 6,610 pounds at 120 psi cold inflation."

That's 13,220 pounds for a pair of tires on a steer axle.

The load range rating of the tire ensures it is capable of carrying the weight at a given pressure while maintaining the same footprint and amount of sidewall flex. Tire manufacturers may take different approaches to the design and construction of their tires when it comes to load range, but all are designed to meet certain government and industry standards.

So, what's the correct load range and inflation pressure for your steer axles? There's no single correct answer to that question.

Have your drivers axle-weigh each load over a period of time to get an idea what your steer axle loads are, and don't forget to account for the amount of fuel onboard. If the tanks are half full, there could be few hundred pounds or more not appearing on the scale ticket.

Once you have determined your steer axle loads, discuss the axle loads with your tire supplier. But be prepared to increase the inflation pressure in the steer tires. A load range G tire at or close to 12,000 pounds needs 110 psi, while a load range H at anything over 12,500 pounds is probably going to need 110 to 115 psi cold pressure.

Michelin's Paul Crehan, formerly the director of Product Marketing at Michelin Truck Tires, noted a few years ago that his product line is designed with specific dimensions, load ratings and sizes for a particular loads and applications.

"Our Data Book contains inflation charts for truck tires. A customer locates the tire’s size on the sidewall and then can utilize the table for proper inflation. These charts are broken down by wheel diameter and the specific PSI for singles and duals. The maximum load and pressure on sidewalls are also listed."

The same applies to the other premium brands. It can be a confusing process, but tire suppliers are always willing to help. Make use of the resource.

Steer tires support more weight than drive or trailer tires and there's little margin for...

Steer tires support more weight than drive or trailer tires and there's little margin for underinflation or excessive speed.

Photo: Jim Park

Seepage, Slow Leaks & Punctures

"You don't go from properly inflated to a blowout instantaneously unless you hit something on the highway," said Decker. "We estimate that about 80% of the roadside tire failures are a direct result of creeping air loss."

In other words, 80% of blowouts could be prevented if tires were kept properly inflated.

There is a well-founded expectation that tires will lose 2% of their inflation pressure, by volume, over about 30 days even when the casing, the valve stem and the tire bead/rim flange contact area are in perfect condition. The problem with that line of thinking is that people are inclined to say, "I guess I only need to check my tires about once a month." Wrong.

Decker said it's uncommon to find a perfectly sealed tire/wheel assembly, so the actual rate of seepage could be as high as 2% per week, or 2% per day if there are other irregularities, such as puncture wounds from nails, a contaminated rim flange or bad valve stem.

"If you build your tire maintenance practices around what you're told is normal air loss, you're going to get caught on the back side of the curve," he warned. "At best, you'll see irregular wear related to inflation, poorer fuel mileage, etc. At worst, the tire will blow out because it has been run flat and damaged by excessive sidewall flex and deterioration of the rubber compounds."

Remember, steer tires do not have that 30% margin we build into drive and trailer tire inflation pressure. On a 13,220-pound axle, you'll need a minimum of 120 psi cold inflation pressure. Minimum. If you have a slow leak, you'll be below minimum before you even cross the state line. And then there's the question of speed.

Road Speed and Inflation Pressure

Hardly anyone gives this a second thought anymore, but road speed does have an impact on inflation pressure. You won't see many on-highway tires from the top-tier manufacturers rated for less than 75 mph today. However, some of the off-shore brands and less expensive tires on the market may still have 65 mph speed ratings. As well, if your tires happen to be underinflated through neglect or mismanagement you could inadvertently be exceeding speed or load capacity of the tire.

When a tire manufacturer rates a tire for 75 mph, it assumes the tire is properly inflated. Steer tires carrying a 12,000-pound axle load (6,000 pounds per tire) must be inflated to a pressure that allows for at least that much load on the tire. This is hardly an issue with drive and trailer tires at 100 psi as they are inflated to well above the minimums for normal axle loads and road speeds. Steer tires, however, run much closer to the margins where proper inflation pressure becomes critical.   

Also, don't overlook the potential for running tires in unintended applications. The wrong tire, an urban or regional tire, can easily enough wind up on a highway truck with potentially devastating consequences.

With steer tires, 5 to 10 psi can make a heck of a difference. Make sure you have the correct size and load-range of tire on your steer axles — and be extra careful if you're running a 13,000-pound axle. Inflation pressure is critical at those weights.

Edited 9/5/2018 to further emphasize that the cause of the crash has not been officially determined and correct which tire was in question. Edited 9/11/2018 to remove information on inflation pressure incorrectly attributed to a tire maker and to clarify when a quotation was obtained.

About the author
Jim Park

Jim Park

Equipment Editor

A truck driver and owner-operator for 20 years before becoming a trucking journalist, Jim Park maintains his commercial driver’s license and brings a real-world perspective to Test Drives, as well as to features about equipment spec’ing and trends, maintenance and drivers. His On the Spot videos bring a new dimension to his trucking reporting. And he's the primary host of the HDT Talks Trucking videocast/podcast.

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