Glazier Trucking earns its keep by moving big loads long distances in minimum time. And not unlike typical owner/operators, this Tulsa, Oklahoma carrier pulls its 53-footers behind 260-in. wheelbase sleepers powered by 500-hp diesels.
Glazier maintains its 295/75R22.5 steers at 105psi and drives and trails at 100psi. That combination has proven best for long tire tread mileage and retreadability.
Compare that line haul solution with Murphy Construction, of Appleton, Wisconsin, with its 11-yd front discharge mixers sporting steer axle loads of more than 20,000lbs. Murphy runs L load range 445/75R22.5s up front and airs them from 115 to 120psi. The rest of their mixers' 73,000 gcw is split up between eight smaller drive tires and two others on tag axels, so they don't need as much inflation pressure. And these mixers don't begin to log the kind of road mileage or sustained speeds as the Glazier semis do.
Meanwhile Chicagoland's Pepsi Cola runs full service delivery trucks in cities' stop-and-go traffic with 80psi in their LT235/85R16 light truck radials. In this case, six tires together support loads up to 16,000lbs.
And Dayton Freight Lines, an Ohio-based LTL carrier, runs 100psi in its steers and 90 to 95psi in its drives while using 295/75R22.5s, the same size that Glazier uses. The difference: Glazier handles maximum truckloads while Dayton specializes in LTL freight and its semis rarely approach 80,000gcw.
Different axle loads, different speed requirements, different service conditions -- in determining proper air inflation pressure, all kinds of factors enter into the mix. A semi hauling 50,000 gcw at 65 mph doesn't need the same tire inflation as a 65-mph vehicle approaching 80,000lbs.
Likewise, an 80,000lb logging truck crawling along a stone-lined gravel trail can get by with 60psi or less nflation pressure until it reaches improved roads and resumes normal speeds. (Goodyear tests have proven you can safely reduce inflation pressures for brief periods if speeds are kept under 35mph. Don't try that in your linehaul semi at full highway speeds.)
In general, running trucks at normal speeds for prolonged periods with inflation pressures just 20 percent under what is recommended can reduce tire mileage by 16 percent and fuel mileage by 2.5 percent. If you err in inflation pressures, experts will tell you it's better to be overinflated than underinflated. Extreme overinflation can also lead to other problems though, such as irregular wear patterns and reduced puncture resistance.
Excessive casing heat resulting from underinflation is the culprit. When you run underinflated tires for long periods, the centrifugal load and speed forces build extra heat as the casing flexes, leading to tire fatigue and the ultimate break down in tire structure.
Checking (and correcting psi) in the middle of a hot day or after driving for several hours is a common way to shortchange your tires. During normal service, for example, inflation pressures can increase 10 to 15psi due to increased casing heat. If you bleed out this overage, your tires will be 10 to 15psi underinflated after they cool.
Ambient air temperatures also affect tire air inflation. For instance, your tires will gain 2psi for each 10° F increase in ambient air temperature over 60 degrees. Bleeding out this overage midday with temperatures over 90 degrees could also be a mistake.Another sure way to reduce tire mileage to removal is to increase your highway speeds without compensating by reducing truckload and increasing tire inflation pressures. The penalty for running 75mph instead of 55 without compensating can be 20 percent in reduced tire mileage.
The benchmark is reduce tire load by 4 percent and add 5psi to your tires when increasing speed from 55mph to 65, 12 percent tire load and 5psi when increasing speed from 55 to 75mph. At the higher speeds, tires flex more and make longer footprints, which can lead to cupping and fast shoulder wear.
All other factors being equal, you can determine the proper air inflation pressure for your tire loads by referring to tables. Determine the maximum load your tire (single or dual) is likely to encounter. Then, using your tire size/ply rating, find the correct load in the table that is close but slightly more than the maximum you expect. The inflation pressure list at the top of the column is your minimum pressure.
Note that singles and duals have different allowable loads for the same inflation pressure. The reason is to provide a cushion for the surviving dual if its axle partner should fail. Within the same service application, it's relatively easy to figure out the proper inflation pressures for your tires based on their axle loads. To maximize tire life, however, you should also bring other considerations to the table. If one shoe doesn't fit all, nor does one tire... or inflation pressure.
INFLATION TIPS
- Inflate for maximum load your tire must carry
- Check psi when tire is cold
- Use flow-through metal valve caps
- Use air dryers in your system to avoid introducing moisture
This article originally appeared in a slightly different form in
COMMERCIAL TIRE MANAGEMENT and is used with permission. |
