10/22/2019
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`Understanding truck tires and air pressure
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`Understanding truck tires and air pressure
`
`Published on September 08, 2011 12:00 AM
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`Every fleet determines its recommended tire pressure based on the worst vehicle load. Since loads vary by wheel
`position, using the load/inflation tables published by any tire manufacturer will determine the proper tire
`pressure based on the specific load. That is why many fleets have a different air pressure specification for steer
`versus drive versus trailer tires. Those load/inflation tables are all based on an ambient temperature of 70
`degrees F.
`
`You check a tire at 8 a.m. and the shop temperature is 70 degrees F. The load/inflation table shows that you
`should be running 100 PSI on the 295/75R22.5 load range G steer tire based on the worst case load of 5,780
`lbs./tire. Using a calibrated pressure gauge, you measure exactly 100 PSI. The fully loaded tractor-trailer leaves
`the yard and begins running on the interstate at 65 MPH. Within five minutes, the driver leaves the cab and
`checks that same steer tire and finds 107 PSI. He or she gets back on the road and drives an additional 20
`minutes, then rechecks the same steer tire and measures 115 PSI. Thirty minutes later the tire pressure is still 115
`PSI while the outside temperature has not risen from the 70 degrees F.
`
`Time for a five minute coffee break. The driver stops the vehicle, finishes a cup of coffee, and finds the tire
`pressure is now up to 118 PSI. Why? The sun is shining brightly, causing the increase in tire pressure. The driver
`now gets back on the highway and runs for another hour before checking the tire pressure again; pressure is still
`118 PSI. The tire has reached a steady state condition and will stay at this pressure assuming all operating
`conditions remain the same, including the outside temperature.
`
`But, of course, that is not going to happen. The road surface will have an impact on the tire pressure, as well. For
`instance, there is a difference between running on concrete and asphalt. The surface temperature of asphalt is
`usually hotter than concrete and may increase the tire pressure another one or two PSI. Now the tire pressure
`may reach 120 PSI due to this. Vehicle speed can increase the tire pressure even further. Running at 75 MPH
`versus 65 MPH will generate more heat and the tire pressure may reach 125 PSI.
`
`Should you be concerned that the tire is 25% “overinflated” at 125 PSI? The answer is simple. NO! Tires are
`designed to take these load, speed and temperature variables into account when a commercial radial truck tire is
`designed, developed and tested. Welcome to the real world. This is why tire companies clearly state in their
`literature to never check a hot tire for pressure because you will think that the tire is overinflated when it is
`exactly where it should be when it comes to the tire pressure.
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`https://www.fleetequipmentmag.com/understanding-truck-tires-and-air-pressure/
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`Sleep Number Corp.
`EXHIBIT 2064
`IPR2019-00500
`Page 1
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`10/22/2019
`Understanding truck tires and air pressure
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`Don’t take air out of a hot tire! A truck tire can take several hours to revert back to its original pressure. And you
`simply do not know where in the cycle you are checking the tire pressure. It is recommended to check vehicle
`tire pressures first thing in the morning after the tires have cooled down overnight.
`
`Having said that, you need to be aware that cold weather can create a different challenge. As an example, a tire
`that has cooled down after running all day and measures 100 PSI at 70 degrees F will lose pressure if it sits out
`overnight and the temperature drops to 20 degrees F. When the tire is checked, you will find only 90 PSI in the
`tire because when the temperature drops, so does the tire pressure. Every loss of 10 degrees F equals a loss of
`two PSI. You may think that the tire is underinflated but is not. As soon as it warms back up to 70 degrees F, the
`tire pressure will be back to the specified 100 PSI.
`
`We always say that heat is a tire’s worst enemy. And it is when a tire running underinflated generates excessive
`internal heat due to increased sidewall flexing and a longer tire footprint (more rubber on the road).
`
`An underinflated tire is always much more serious than a tire being a few PSI overinflated. Excessive heat will
`eventually lead to tire failure.
`
`Copyright © 2019 Babcox Media, Inc. All Rights Reserved
`
`https://www.fleetequipmentmag.com/understanding-truck-tires-and-air-pressure/
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`2/2
`
`Sleep Number Corp.
`EXHIBIT 2064
`IPR2019-00500
`Page 2
`
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