11-15-2017, 10:59 AM
The tire companies have designed their tires to safely and reliably carry a load and to ride reasonably well. The air pressure in the tire is what supports the load going down the road. The combination of PSIG tire pressure and the square inches of road contact determine the load carrying of the tire. For instance, on a tire supporting 6000 pounds, at 100psig there would need to be 60 square inches of road contact. (60 in squared X 100 lbs/in squared = 6000 lbs)
Tires are round and must flatten in order to produce the needed contact surface to carry the load. Flattening requires flexing of the tires and flexing generates heat. Tires are designed so that the flexing needed for developing the weight supporting contact surface will not generate heat injurious to the tire. Increasing the air pressure, for a given weight supported, reduces the contact surface, reduces heat generation, reduces ride softness, and reduces maximum braking capability. Reducing tire pressure , for a given supported weight, increases tire temperature, increases steering squirreliness, increases ride softness, and increases tire wear. Finding the right balance is the job of the tire designer and then, having made their decisions they can tell us how much air pressure is needed to produce the contact surface for supporting the weight.
As users all that above is the easy part; for a given tire, so much weight requires so much pressure. Now how do we take into account the effects of ambient temperature. We are directed to measure the pressure only when the tires have been resting for some time, usually an hour or two. Why? The manufacturer knows they will heat up as we drive, and they know about how much they will heat up above ambient temperature. Their tables are designed to create the designed surface area while driving, when the tires have heated as much as they would expect above the ambient temperature. Their expectation is that you will set the pressure using their tables and with the tires cooled to ambient. When first driving the tires will be underinflated and generating extra heat, that is OK because that extra generation will dissipate as the tires approach the increased pressure that is expected. Then everything will be as intended.
We only need to concern ourselves with the 65 degree table above is if we are setting tire air pressure in a setting with a temperature significantly different from the one in which we will be driving, say, in a 65 degree garage and pulling out and driving in minus 20 degrees.
If we set the pressure in our tires according to the load tables and make any adjustments when the tires are at ambient temperature to where we will be driving we will be OK. Unless we start in Montana in February and drive to Palm Springs there is usually little reason to worry about changing pressures along the road. However, I consider it good personal practice to measure tire temps when I stop and occasionally back that up with pressure checks.
Tires are round and must flatten in order to produce the needed contact surface to carry the load. Flattening requires flexing of the tires and flexing generates heat. Tires are designed so that the flexing needed for developing the weight supporting contact surface will not generate heat injurious to the tire. Increasing the air pressure, for a given weight supported, reduces the contact surface, reduces heat generation, reduces ride softness, and reduces maximum braking capability. Reducing tire pressure , for a given supported weight, increases tire temperature, increases steering squirreliness, increases ride softness, and increases tire wear. Finding the right balance is the job of the tire designer and then, having made their decisions they can tell us how much air pressure is needed to produce the contact surface for supporting the weight.
As users all that above is the easy part; for a given tire, so much weight requires so much pressure. Now how do we take into account the effects of ambient temperature. We are directed to measure the pressure only when the tires have been resting for some time, usually an hour or two. Why? The manufacturer knows they will heat up as we drive, and they know about how much they will heat up above ambient temperature. Their tables are designed to create the designed surface area while driving, when the tires have heated as much as they would expect above the ambient temperature. Their expectation is that you will set the pressure using their tables and with the tires cooled to ambient. When first driving the tires will be underinflated and generating extra heat, that is OK because that extra generation will dissipate as the tires approach the increased pressure that is expected. Then everything will be as intended.
We only need to concern ourselves with the 65 degree table above is if we are setting tire air pressure in a setting with a temperature significantly different from the one in which we will be driving, say, in a 65 degree garage and pulling out and driving in minus 20 degrees.
If we set the pressure in our tires according to the load tables and make any adjustments when the tires are at ambient temperature to where we will be driving we will be OK. Unless we start in Montana in February and drive to Palm Springs there is usually little reason to worry about changing pressures along the road. However, I consider it good personal practice to measure tire temps when I stop and occasionally back that up with pressure checks.
Jon Kabbe
1993 coach 337 with Civic towed