Originally Posted by gmw photos
First of all, I'm not trying to pick a fight here. Second, I have no dog in this fight anyway because I don't buy ST tires. Having thrown those disclaimers out there, I think the key word in the above quote is "confusing".
Here is the quote from the Goodyear product service bulletin issued to dealers:
"Goodyear Marathon Special Trailer tires, with the “ST” size designation, are speed rated at 65 MPH (105 km/h) under normal inflation and load conditions."
....it then goes on to say:
"If Goodyear tires, with the ST designation, are used at speeds between 66 and 75 mph (106 km/h and 120 km/h), we recommend the cold inflation pressure be increased by 10 psi (70 kPa) above the recommended pressure based on the trailer placard for normal inflation and load conditions."
Now then, to me, the confusing part of this is that the "implication" is the consumer could increase the inflation to 75psi. I suspect that is NOT what Goodyear is trying to say however. What they "are" saying is the inflation can be increased 10 psi "above the recommended pressure based on the trailer placard for normal inflation and load conditions".
In other words, they may be saying you could literally increase the inflation pressure 10 psi above what the load/inflation table states is the acceptable minimum.
So for instance, if the load/inflation table says that you could run 55 psi for the given specific load, then you could run 65 mph at 55 psi. To increase your speed to 75 mph, you could increase the pressure to 65 psi. Further, you cannot assume an increase in load capacity by this pressure increase, only a speed capacity increase.
Hopefully Roger or Barry will weigh in on this, but I personally have not seen a tire manufacturer that "allows" for tires to be inflated beyond the psi stated on the tire sidewall. I will stand corrected on that however if one of the tire engineers say that I am in error on that.
Allow me to start in a different spot.
The load carrying capacity of a tire is affected by a great many things. We're all familiar that tire size (physical dimensions) and inflation pressure have an effect. Those are delineated in the tables.
But there are other things - and generally offered in the form of a different table - or a modification of the table. These separate tables or modifications to existing tables are for conditions DIFFERENT than delineated by the table heading.
For example, a P type tire table is for typical highway speed on relative smooth paved surfaces on softly sprung vehicles. The modification for light trucks is a derating of 10% for more stiffly sprung vehicles on less smooth surfaces at slightly slower speeds (what a pickup truck is normally used for). For the sake of simplicity, trailer application is included in that.
In the case of the ST tire table, the changes for speed could have been done in a couple of different ways: An increase in inflation pressure or a reduction in load - which from a tire engineer's perspective is the same thing - deflection. Higher speeds reduce the load carrying capacity - or stated from the other direction, higher loads result in less speed capability at a given inflation pressure. That is what the notes on the Goodyear load table are about.
BTW, those notes are taken directly from the Tire and Rim Association (TRA) Yearbook load tables - and here is where Roger and I disagree - they would apply regardless of who manufacturers the ST tire. The reason Roger and I disagree is that some manufacturers may not want their tires operated in any condition other than what THEY publish (I presume for legal reasons) - where I am coming at this from the theoretical point of view about how tires behave.
Now let me address the issue of increasing the inflation pressure. If you look carefully, you will see that the the sidewalls of tires state a maximum load - and either a maximum inflation pressure - OR - a pressure where this max load occurs - like this:
Max Load XXXX, Max pressure YY
- OR -
Max Load XXXX at YY pressure.
Note that the first way doesn't say what the relationship is, and the second way doesn't say what the max pressure is.
Side note: Occasionally, some manufacturers will use this form: Max Load XXXX at Max pressure YY. I think this is incorrectly stated, but I am willing to accept they are trying to prevent people from overinflating the tire.
I can explain why it is OK for Goodyear (and the TRA) to specify an inflation pressure increase beyond the max stated in the table, but the explanation involves some engineering concepts that are just too long for a post like this one. (After having written that, I think I will do another post on this subject AFTER folks have digested this one!)
Soooooo, the Goodyear ST tables have included some notes if it is desired to operate at a speed higher than 65 mph that the table specifies.
So, you may say, why is this so complicated. In the case of automobiles, the car manufacturers have done a good job of sizing the tires and defining what is needed for their vehicles - in the form of the vehicle tire placard on the driver's doorpost.
In the case of pickup trucks and SUV's, manufacturers of those vehicles haven't done as good of a job in the past, but as of 2008 have addressed the issue to the same level as the car manufacturers.
But trailer manufacturers are behind. In the past, their tire specifications (size and inflation pressure) were marginal - and in some cases worse than that. Trailer manufacturers do not have the extensive feedback system that car and pickup truck manufacturers have (dealers and the warranty system), so in most cases, they don't know there is an issue. To complicate matters, they slough off tire issues to the tire manufacturer - and the local tire dealers frequently don't handle off brands, so tire sizing issues don't get reported to where changes would be made.
That is why it is important to report tire failures to NHTSA. They are the government agency responsible for safety. They are NOT going to address issues they don't know about. Whether it is the trailer manufacturer under specifying or the tire manufacturer underdesigning, is for NHTSA to investigate.