Published by Pressure Systems International, the manufacturer of the Meritor Tire Inflation System by PSI, the Commercial Fleet Tire Digest (www.psitiredigest.com) is a comprehensive guide to reducing commercial vehicle tire costs. The guide is authored by Al Cohn, a recognized industry tire expert who joined PSI in 2006 after spending 28 years with a leading commercial tire manufacturer. Cohn’s extensive experience addressing tire issues is used to inform fleets of best practices. Excerpts from articles in two recent issues, addressing proper tire pressures and tire balancing, include:
What pressure should I be running?
The most common question being asked by fleets today is: “What pressure should I be running my tires?” It sounds pretty simple but the choice for tire pressure can have significant impact on a fleet’s tire budget.
Commercial truck tires are designed so that the optimum tire footprint is the key to long tread life and good fuel economy. However, the tire footprint changes once the tire is put into operation depending on the actual tire load. Air is what carries the load and as a result the recommended tire pressure must be based on the worst-case load that the tire will actually carry. The important piece of the puzzle is that a fleet must determine the actual worst-case load per tire, not vehicle, for steer, drive and trailer positions.
As an example, let’s say you are running a typical 18-wheeler with a maximum load of 80,000 lbs. GVW. You will need to run over a scale to determine how the 80,000 lbs. is distributed across each axle. Then you’ll need to divide by the number of tires on each axle to determine the actual tire load. A typical distribution for an 80,000-lb. GVW load is:
• Steer 12,000 lbs. or 12,000/2 tires = 6,000 lbs./tire
• Drive 34,000 lbs. or 34,000/8 tires = 4,250 lbs./tire
• Trailer 34,000 lbs. or 34,000/8 tires = 4,250 lbs./tire
In this scenario, assuming all tires are the same size (295/75R22.5), the steer tires will have a different pressure specification than the drives and the trailer tires. There still are many fleets that want to spec a single pressure for all their tires because it is easy. However, this is not the recommended solution since it won’t give you the best value for your tire dollars.
Once you have the lbs./tire calculation, the next step is to look up the recommended pressure in the load-inflation tables that are published on the websites of all the tire manufacturers. To use these tables, you must know the worst-case load and the tire size. There also are different pressures depending on whether the tire is being run as a single (usually steer) or a dual (usually drive and trailer). You just need to be careful in reading the chart.
In our example, the steer tire maximum load is 6,000 lbs. When you look at the “single tire line” on the chart you would see that 105 PSI would be too low for these tires since the max load is only 5,980 lbs. The correct pressure is 110 PSI because that pressure will carry up to 6,175 lbs. For the drive and trailer tires, the worst-case tire load is the same for all tires on those axles—4,250 lbs. each. You need to look at the dual tire line to determine the correct pressure. Surprisingly, all you would need is 75 PSI in each tire to carry the load. Many fleets run as high as 105 PSI in drive and trailer tires. But you can see from the load/inflation table that 105 PSI is not required to carry the load, and would not give you the best results. The tire footprint becomes shorter at the higher pressures so the tires’ shoulders will not be in full contact with the ground, leading to fast shoulder wear and early tire removals.
Understanding load/inflation tables will help you make the correct decision in choosing the optimum tire pressure for your fleet.
Tire/Wheel End Balance and Run out
Fleets have different perspectives on tire/wheel balancing. Assemblies that are out of balance can lead to vibration and irregular tire wear, which lead to early tire removals and driver discomfort. Understanding balance and tire run out can be confusing.
An out of balance rotating assembly exists when there is an unequal distribution of weight about the rotating axis. A tire/wheel assembly can be static or dynamic out of balance. If there is a static out of balance, the assembly will bounce. Appropriate-sized weights will bring the assembly into static balance. The standard practice is to put half the weight on one side of the wheel and half the weight on the other side. Just because the assembly is statically balanced does not mean that it is dynamically balanced.
Dynamic imbalance causes the assembly to wobble. Dynamic balancing systems tell you how much weight to put on each side of the wheel and what location to place it. If an assembly is dynamically balanced, it is also in static balance. Dynamic balancing considers assembly balance in two planes while static balance takes into account only a single plane.
Run out is an up/down or side-to-side visible movement when the assembly is rotated. It can be measured by using a run out gauge that has a needle. There are two kinds of run out, radial and lateral. If, when you place the run out gauge a given distance from the centerline of the tread and rotate the assembly the distance between the needle, and the tread remains constant, that assembly has zero run out. If there is a radial run out, its effect is to raise and lower the vehicle as it moves along and the result is a vertical hop or bounce.
Lateral run out can be measured by placing the run out gauge near the tire sidewall. When the assembly is rotated and the distance between the needle and the sidewall is constant, the lateral run out is zero. When there is lateral run out, the result is a side-to-side movement of the rotating assembly, which leads to the vehicle traveling alternatively right and left. The driver would feel a shimmy or wobble.
Fleets are typically encouraged to check their tire/wheel assembly balance on the steering axle. In most cases, if a driver reports a vehicle vibration issue, getting the steer tires in balance will usually solve the problem. It is also recommended to balance the drive tires if the driver’s seat vibrates. Keeping your tire/wheel assemblies in balance will help maximize tire removal miles and keep your drivers happy.
For information on the latest tire pressure monitoring and automatic inflation systems, click here.
