Goodyear 215/75R17.5 proper inflation pressure for our trailer weight

[Wayne]

@CapriRacer I'm hoping you can give us a best practice on my inflation question.

This is the inflation chart from Goodyear

Our 5th wheel trailer weighs about 20,000 pounds when loaded. The pin weight is right at 4,000 pound, so each axle is carrying about 8,000 pounds. As best I can tell, each tire is carrying about 4,000 pounds.

I have been keeping the cold pressure right at 120 psi. On a hot summer day, the TPMS sensor on the end of the valve stem will report 135-140 psi and around 135°F when the ambient temperature is 90°F. This seems like a lot of heat to me, hence running the tires at 120 psi cold.

The chart says to run the tires at 100 psi for 4,000 pounds. This seem counter intuitive with the tires generating so much heat. Am I better off at 120 psi or should I be running at 100 psi per the chart?

Thanks!

 

[MSCH]

Wayne 135F is not hot.. in mechanical terms
the pavement can be 150 or more.. and a tire sitting in the sun can exceed that..

anyway, you can leave it to Capri racer for the rest

 

[CapriRacer]

@CapriRacer I'm hoping you can give us a best practice on my inflation question.

This is the inflation chart from Goodyear
View attachment 1931

Our 5th wheel trailer weighs about 20,000 pounds when loaded. The pin weight is right at 4,000 pound, so each axle is carrying about 8,000 pounds. As best I can tell, each tire is carrying about 4,000 pounds.

I have been keeping the cold pressure right at 120 psi. On a hot summer day, the TPMS sensor on the end of the valve stem will report 135-140 psi and around 135°F when the ambient temperature is 90°F. This seems like a lot of heat to me, hence running the tires at 120 psi cold.

The chart says to run the tires at 100 psi for 4,000 pounds. This seem counter intuitive with the tires generating so much heat. Am I better off at 120 psi or should I be running at 100 psi per the chart?

Thanks!

Barry's Tire Tech: ST Tires

On the above webpage I have a procedure that covers this situation. While the webpage is devoted to ST tires, the procedure is OK for any trailer. regardless of what kind of tire is on the trailer.

So let's do the math:

The OP says his trailer has 16,000# on the tires.

The procedure says that if you only know the weight on the tires, that weights is to be divided by the number of tires (4,000#), then add 10% more (= 4400#), then add another 15% (= 5,060#)

So the OP's table says to use ..... Opps! The max load 4805# (at 125 psi!

OP reports pressure buildup's of +15 to 20 psi (13% to 16%) That seems too high, but is consistent with the calculations.

My gut says that this just might be one of those cases where the trailer manufacturer was using the old way of sizing tires - that is marginally! This is common for commercial tires where tire failures are kind of expected, but operating costs are more important! More importantly, the tire manufacturer is expected to step up and make a more robust tire to deal with the situation.

 

[Wayne]

So the OP's table says to use ..... Opps! The max load 4805# (at 125 psi!)

You have a great website!

So my tires:

1) Are overloaded
2) Should be run at 125 psi cold
3) When I replace them, I should look for trailer tires that have more load carrying capacity.

If my trailer were 2' longer it would have had triple axles. New Horizons should have built my trailer with triple axles.

What is the absolute burst pressure of these tires? Would it hurt anything to run them at 130 psi cold?

Do you know of any tires in my size that can carry more load?

 

[CapriRacer]

You have a great website!

Thanks. It seems to be my legacy.

So my tires:

1) Are overloaded

Not exactly, but from an engineering perspective, things could be better.

2) Should be run at 125 psi cold

That would reduce the risk.

3) When I replace them, I should look for trailer tires that have more load carrying capacity.

Good idea!

If my trailer were 2' longer it would have had triple axles. New Horizons should have built my trailer with triple axles.

What is the absolute burst pressure of these tires? Would it hurt anything to run them at 130 psi cold?

Burst pressure is several times the max pressure. Yes, you could run 130 psi.

Do you know of any tires in my size that can carry more load?

I did a quick Google search and found several Load Range J tires. They were off brands.

 

[Azjeff]

Not what you wanted to hear Wayne but good to know so you can correct it.

Barry, you answered one question I had, burst pressure is 2x max pressure.

2nd question is: there must be a safety margin on the max load ratings of tires, say a tire has a 2000 lb rating @ max psi. What would the engineered max rating be? It seems like the +10%, +15% (+27%) safety factor you mention would be stacked on top of that in reality right?

In the build spec sheet for our trailer I remember side to side weight numbers and they were pretty close, maybe 200# different. If I understand your ST page I need to add 7% + 15% to the heavier number since I know side to side. Not much difference from the 10% + 15% if you don't know the side to side.

 

[Wayne]

I did a quick Google search and found several Load Range J tires

Thanks - I didn't think about the next load range up

 

[CapriRacer]

Barry, you answered one question I had, burst pressure is 2x max pressure.

Ah, not exactly. More like 3 or 4 times.

2nd question is: there must be a safety margin on the max load ratings of tires, say a tire has a 2000 lb rating @ max psi. What would the engineered max rating be? It seems like the +10%, +15% (+27%) safety factor you mention would be stacked on top of that in reality right?

I've been working on a new webpage trying to cover this topic. It's difficult to explain and I need to figure out a way to do that. So, here goes:

Tires are all about fatigue. The trick is to have the tire wear out before a fatigue failure takes place.

In engineering, we delineate fatigue using an SN curve: S = stress, N = number of cycles to failure. In a passenger car tire, the N is in excess of 80 million cycles. You can't operate a test for fatigue with 80 million cycles. It would take months.

So we employ a trick - a step load test. It's a standard industry test where the test is run at 50 mph, each step is 2 hours long (=100 miles). At that speed, the heat buildup isn't that large, so we are testing just the effect load has on the tire (more or less). The first step is 85% of the rated load. If the tire passes that step, we add 5% to the load and test for another 2 hours - and we keep adding 5% until the tire fails.

This test became the basis for the government test. They used 100% load as passing.

But that changed because of the Firestone / Ford situation - they added a low pressure step after the 100% step, and a bunch of other changes . I explain in more detail in my webpage on DOT testing: Barry's Tire Tech: DOT Testing

I suppose you could figure out where on the SN curve a particular test result put you, but most of the time we just go with the more miles is better! That means I don't have a good handle on how much more load capacity a particular tire has - if I even knew what those step load results were for today's tires.

The way we did this is with tire returns. As we made improvements based on the step load test results, we got fewer and fewer endurance failure returns. And just to let everyone know, we needed to look at the tires to not only make sure it was a failure not caused by external factors, but also to see where the failure occurred.

And just to be clear, the 7% and 10% additions I mention is for side-to-side and front-to-rear weight variation. It's based on data others have published. Not every vehicle has that much variation - I'm just taking a worst case.

But the 15% is based on how vehicle manufacturers who have a lot of experience in setting tire pressure specs. It's just good engineering to apply a reserve capacity when specifying things. (BTW, we no longer use the term "Safety Factor" when describing over capacity. In tires, we use "reserve capacity".)