Originally Posted By: AstroTurf
Thanks CapriRacer
Jim
PS Could you please add some clarification for those who might be listening in about the 35, 44, and the 51 psi differences for passenger tires? Thanks Again, Jim
Be glad to.
One of the things that the tire standardizing organizations do is set the relationship between inflation pressure and load carrying capacity. Normmlly the standard is stated in the form a table delineated by tire size. This table has increasing load capacity as the inflation pressure increases - up to a maximum inflation pressure.
Part of the standard are supplemental notes that are used to describe different situations that would chhnge this relationship.
For example, mining haul trucks may operate at different speeds depending on the configuration of the haul road. Some are twisty, so the speeds are slow, while others may be downhill and generate high speeds. The tire standards offer guidance by way of these supplemental notes - at slower speeds, tire can carry more weight.
Further, some of these tires are used on cranes that operate on paved surfaces - as opposed to unpaved haul roads. In that case not only would the tire standard suggest that using more inflation pressure (above what is listed as a max in the table), but other alterations to the load vs pressure relationship.
In the case of passenger car tires, the standard is predicated on a set of conditions normal for passenger cars - paved surfaced, highway speeds, etc. - and the load table reflects that.
But if a tire is operated at high speeds, the standard offers guidance in how to deal with that - higher inflation presures, and/or reduced loads.
But what is printed on the sidewall of a passenger car tire doesn't give a lot of detail about all of that. It is a simple statement: Max Load, Max pressure - without all the detail as to when these apply.
For normal operation, the inflation pressure is maxed out at 35 psi. This would apply for S and T speed rated tires. But if the vehicle is going to operate at higher speeds, then higher pressures would be called for.
For H speed rated tires, the test pressure is 44 psi - so an H speed rated tire has to have a sidewall pressure of at least 44 psi.
For V and higher speed rated tires, the test pressure is 51 psi.
So the tire standard reflects these 3 situations. The problem here is that this is an over-simplification - and doesn't address the situations in between. The tire standard is general enough to accommodate those.
Some tire manufacturers take a very strict view of this and will only show 35 psi on S and T tires, 44 on H, and 51 on V and higher. Others take a different view on this and will put 44 (or even 51) on S and T rated tires, and 51 on H's. Some of this is due to pressures from the marketing department - who care little about the technical considerations.
So you will find a variety of pressures listed on the sidewall of tires even though the constructions are very similar. And since all the tire manufacturers point to the vehicle tire placard, what is listed on the sidewall almost has no meaning - and certainly not some of the meanings sometimes subscribed to it.
I know this is a long post, but I felt I needed to talk a bit about the depth of the background behind tire standards.
