Altitude and ambient temperature affect pressure. For every 1,000 feet of elevation, pressure will vary 0.5 psi, which isn’t considered significant. Temperature variations are a different story. Every 10 degrees Fahrenheit change in temperature will result in an approximate 2-psi rise.
All tire manufacturers publish load/inflation tables and booklets with recommended inflation pressure by load. Another source is the Tire and Rim Association’s annual guidebook – visit www.us-tra.org to order.
How do you accurately check pressure?
The recommended method for checking inflation pressure is to use an accurate tire gauge. Keep it from getting banged around in your toolbox and losing its accuracy. Periodically have the gauge calibrated at a truck stop or tire shop, advises Doug Jones, a customer engineering support manager for Michelin.
Veteran operators may use a ball bat, billy club or tire iron to thump their tires, but you can’t reliably discern how much air is in a tire with that method, says Tim Miller, Goodyear’s marketing communications manager.
Kurt Grote, an owner-operator leased to John Christner Trucking, says he used a gauge daily before he switched to an automatic monitoring system three years ago. He was checking for 120 psi in his steers and 100 psi in his drives, regardless of the load. He says it took about five minutes each day. Today, even with the monitoring system, he still uses a gauge weekly to check pressures.
How often should you check?
Jones recommends checking pressure as part of your daily vehicle inspection. He notes that air pressure checks are part of pre- and post-trip inspections, as required by the Federal Motor Carrier Safety Administration. Also have pressures checked during preventive maintenance and other repair shop visits.
Checking pressure before you begin your route each day will give you a cold tire reading, which is preferred. Walenga says pressure will increase 10 percent to 15 percent once tires heat up from highway driving, a fact good to keep in mind if you’re in the habit of checking pressure when you stop for fueling.
“With CSA, I think that’s going to help to get people to check pressures and be more responsive to tire issues since tires are one of the things that will get the most [CSA] markups,” he says.
Should you adjust pressure for the load?
Adjusting pressure for every load variation would be ideal, but it’s not realistic, Walenga says. By inflating to your maximum loads, you will avoid the risk of being underinflated. All the same, he adds, “if you run lighter, you could go to a chart that shows different air pressures for different loads.”
Miller notes that “if you want tires to give the best performance, especially for wear and traction, you could adjust for every load.” He points to the many logging trucks outfitted with central inflation systems that adjust pressure to weight and terrain, whether off-road or on-highway.
Jones says to maximize tire design and engineering, you should match the pressure with the load. If they aren’t matched, tires can wear prematurely or fail, and fuel economy could fall. “If the load is too much for the air pressure, you’re going to have a problem,” he says. “If air pressure isn’t sufficient to carry a load, you’re also going to run into a problem.”
Operators who run less-than-truckload or long haul with multiple stops might consider inflating less than maximum pressures in all tires except the steers, but still stay above minimum recommended pressures, Stansbie says. Steers need maximum pressure to exert side force whenever the tire is turned. “If you have an underinflated steer tire, you need to turn the steering wheel a lot more on a curve than with a fully inflated tire,” he says.