Keeping Up the Tire Pressure
TORONTO — What's a few psi between friends? A thousand dollars, maybe more, if you're unfortunate enough to suffer a tire failure somewhere off the beaten track. Inadequate inflation pressure is the leading cause of blowouts, surpassing even road strikes and curbing incidents.
"You don't go from properly inflated to a blowout instantaneously unless you hit something on the highway," says Curtis Decker, manager of product development at Continental Tire. "We estimate that about 80 percent of the roadside tire failures are a direct result of creeping air loss."
In other words, 80 percent of blowouts could be prevented if tires were kept properly inflated.
There is a well-founded expectation that tires will lose two percent 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 says it's uncommon to find a perfectly sealed tire/wheel assembly, so the actual rate of seepage could be as high as two percent per week, or two percent 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 warns. "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."
Of course, you still have to determine what constitutes proper inflation, and that's a chore unto itself. Tire manufacturers make inflation pressure recommendations based on the size and construction of the tire and on the load the tire carries. It's worth noting that it's not the tire itself that supports the load, but the volume of air inside the tire. In most cases, a larger tire can support heavier loads at the same pressure.
If you check Bridgestone's load and inflation tables, for example, you'll notice that the same tire in 22.5-in. and 24.5-in. sizes (both load range G) have different weight ratings at the same pressures. An 11R22.5 single at 100 psi is good for 5,950 pounds, while an 11R24.5 can carry 6,350 pounds. That's because the 24.5-in. tire contains a larger volume of air to support the load.
Fleets can run into problems here because there's an inclination to accept "traditional" or standard pressures for certain wheel positions, regardless of the load, application or tire construction. Historical thinking puts steer axles at 12,000 lbs, for example, but many steer axles today are running at 13,000 lbs or higher, thanks to emissions hardware and other factors.
As Goodyear's director of product marketing innovation, Donn Kramer, puts it, "For a 12,000-lb. 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 lbs. at 110 psi cold inflation. For a 13,000-lb. axle in sizes 11R22.5 and 295/75R22.5, fleets should use a load range H tire with a single load-carrying capacity of 6,610 lbs. at 120 psi cold inflation."
The load range rating of the tire ensures the tire 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.
In the tire makers' eyes, any tire that is run at a higher load than a prescribed pressure allowed is considered overloaded. In other words, on a steer axle rated for 12,000 lbs, each tire has to be capable of carrying a 6,000-lb load. If, based on Michelin's inflation tables, a typical 11R22.5 load range G tire was inflated to 100 psi, it would carry only 11,900 lb. at 12,000 lb it would be overloaded. What if time and inattention had let that pressure dwindle to 90 psi? It would be overloaded by almost 1,000 lb.
The same principles apply to drive and trailer tires, but the margins for error are a little wider. Typically, a drive or steer tire (295/75R22.5) inflated to 100 psi is good for around 5,300 lb (depending on the manufacturer) in a dual setup, or 43,000 lb in a tandem grouping. Even with Canada's higher weights, there's still a margin of up to 800 lb per tire.
Running in a fully loaded 17,000-kg tandem grouping, that tire would still be okay with as little as about 85 psi before being considered under inflated. On an 18,000-kg grouping, you'd have to maintain at least 95 psi. That margin disappears fast as axle weights increase. The tire people say tires should always be inflated to the maximum load you expect to haul, so if you're running heavy in Ontario and Quebec, air pressure maintenance is more critical than it would be in the western provinces or Atlantic Canada.
Paul Crehan, director of Product Marketing at Michelin Americas Truck Tires, advises users to check the load and inflation tables for recommended inflation pressures for particular tires, and not to assume "100 psi" is good enough in all cases.
"Our Data Book contains inflation charts for all our truck tires," he says. "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.
Even with all the technology we can throw at tires today, the most basic yet the most important is maintaining adequate inflation.
"If a fleet has nothing else but a good air-pressure maintenance program, it will reap substantial benefits over having no program at all," says Doug Jones, customer engineering support manager, Michelin Americas Truck Tires.
It needn't be elaborate, but it has to be consistent, Jones says. On the upper end of the spectrum, there are tire pressure monitoring systems on the market that self-report and upload tire pressure data regularly and automatically. Some contain full reports from mileage, age, wheel position, temperature and pressure; some report only exceptions. On the other end of the spectrum, pencil and paper will work just as well for smaller fleets.
Here are Doug Jones' top five steps to good tire inflation management:
1) The fleet tire-management program should be written, communicated, monitored and enforced. Appoint someone to check the tire pressures.
2) Establish target pressures and maintain them with calibrated air pressure gauges and trained employees willing to diligently check the pressures.
3) Conduct regular yard checks or tire pressure audits, document the results and take appropriate action.
4) Establish a routine for tire maintenance and inspections, including tire rotation, vehicle alignment and wheel and valve cap service.
5) Consider outsourcing tire management. If you don't have the time or resources to set-up and run a maintenance program, there are many reliable outlets that can help.