Air pressure jumping in front tires
I have a Freightliner Tag with 22.5 tires. I set rear tires at 85-88lbs and fronts at 117lbs. After driving rears jump up to 98-105. The fronts will hit 143-147. The fronts are within 1-3 lbs of ea...
Forum Discussion
This is from a Large Fleet Truck company on advising their drivers on Tire pressures and temps versus the Tractor and the trailer. Doug
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.
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.
