31' International tire pressure concerns

[bdhdig]

I truly thank anyone for analyzing my trailer tire pressure situation.
I revisited a Cat Scale for up-to-date weights since installing PPP, Endurance tires and new axles over the past few years.
With full fuel tank (44 g), 60# propane, <10g H2O, empty holding tanks, the weights are as follows:
Steer Drive Trailer Total
TV 3660 4180 ----- 7840
w/ WD 3420 5200 6200 14820
w/o WD 3180 5500 6220 14800
Trailer tire weight would be 6200/4=1550# right?
Comparing my 31’ trailer with various AirForums members’ lead me to have 55-60 psi cold. Warm usually adds 5-10#

According to this chart the ST225/75R15 pressure should be 30 to 35 psi. Removing 20 psi would certainly soften the trailer ride but what about side wall heat build?
Does this sound acceptable to do?

[Action]

Assuming all four tires on a dual axle trailer are carrying an equal share of the trailer weight will get you in trouble with a tire that is over loaded.

Dual axle trailers typically have tires or wheel positions that are unequally loaded. Because the tires/wheels are concentrated in the center of the vehicle plus heavy items are typically not equally distributed and may not be close to the center of the trailer. Which means that heavy item has leverage in some distance from the tire that is supporting it.

Action

[richard5933]

There are two factors involved here - one is having enough pressure for the tires to carry the weight, and the other is having enough pressure for the trailer to handle properly going down the road.

When I ran the numbers on my trailer I had similar results to you. I opted to run them with 60psi and have been quite happy with that. It's more than needed for the load and provides good handling, but still low enough that the trailer doesn't shake apart.

You can exceed the recommendations in the chart up to the max level on the sidewall, but there is rarely a need to go that high.

[CapriRacer]

Originally Posted by bdhdig

I truly thank anyone for analyzing my trailer tire pressure situation.
I revisited a Cat Scale for up-to-date weights since installing PPP, Endurance tires and new axles over the past few years.
With full fuel tank (44 g), 60# propane, <10g H2O, empty holding tanks, the weights are as follows:
Steer Drive Trailer Total
TV 3660 4180 ----- 7840
w/ WD 3420 5200 6200 14820
w/o WD 3180 5500 6220 14800
Trailer tire weight would be 6200/4=1550# right?
Comparing my 31’ trailer with various AirForums members’ lead me to have 55-60 psi cold. Warm usually adds 5-10#

According to this chart the ST225/75R15 pressure should be 30 to 35 psi. Removing 20 psi would certainly soften the trailer ride but what about side wall heat build?
Does this sound acceptable to do?

First, those charts are MINIMUMS, not recommendations.

As mentioned above, you didn't account for uneven distribution of weight. I've estimated that you need to add 10%.

I've also recommended that you specify pressure at 115% of the rated load.

I explain the whole procedure here: https://barrystiretech.com/sttires.html

So adding 10%, then 15% to your 1550# = 1961 = 45 psi.

That still sounds low. My guess is that 60 psi is more like it.

So try the Pressure Buildup Test: https://barrystiretech.com/pressurebuildup.html

I'd start at a pressure you feel comfortable with before you start changing things. What were you using before?

[bdhdig]

"Comparing my 31’ trailer with various AirForums members’ lead me to have 55-60 psi cold. Warm usually adds 5-10#"

[richard5933]

Originally Posted by bdhdig

"Comparing my 31’ trailer with various AirForums members’ lead me to have 55-60 psi cold. Warm usually adds 5-10#"

No need to mention (or worry about) the pressure when warm. The manufacturers have that built into the equation.

[franklyfrank]

Originally Posted by bdhdig

I truly thank anyone for analyzing my trailer tire pressure situation.
I revisited a Cat Scale for up-to-date weights since installing PPP, Endurance tires and new axles over the past few years.
With full fuel tank (44 g), 60# propane, <10g H2O, empty holding tanks, the weights are as follows:
Steer Drive Trailer Total
TV 3660 4180 ----- 7840
w/ WD 3420 5200 6200 14820
w/o WD 3180 5500 6220 14800
Trailer tire weight would be 6200/4=1550# right?
Comparing my 31’ trailer with various AirForums members’ lead me to have 55-60 psi cold. Warm usually adds 5-10#

According to this chart the ST225/75R15 pressure should be 30 to 35 psi. Removing 20 psi would certainly soften the trailer ride but what about side wall heat build?
Does this sound acceptable to do?

Do not lower the recomended PSI to soften the ride of the trailer . I did that and paid for it dearly. Thread seperation blow out expensive damage to trailer.
Not to mention that I had to instal the spare in 90 degree weather on I 24 with massive traffic rolling by.

[4mer Paddler]

Originally Posted by bdhdig

I truly thank anyone for analyzing my trailer tire pressure situation.

Tires and tire pressures are part of a very complicated issue regarding towing safety, durability and comfort. And there are plenty of opinions presented on all of the various forums, enough to completely baffle just about anybody. My peace of mind was helped considerably by installing a TST 507 TPMS. It continually shows pressure and temperature, somewhat delayed, but good enough to get the gist of the tire information over the period of a longer drive. As an example, since Michelin don’t make a15” LT anymore, I air my Endurance tires to 55psi @ 50° to 60°F and watch it climb to about 62psi @ 80°F. I know it probably won’t indicate a disastrous blowout right away but should indicate a slow leak or temperature buildup. Happy so far (2 years, 18,000 miles). Good luck and drive safe.

[Andy543]

The Airstream placard calls for the maximum of 80 PSI. Most trailer manufacturers call for the maximum on ST tires. It is the vehicle manufacturer's, not the tire manufacturer's, responsibility to specify the pressure, as they have to take into account how the vehicle will be used and how it's going to ride and handle. I believe Airstream and other manufacturers are concerned about mitigating trailer sway, so they want the maximum.

[sfranklin]

I used to fill my 30' trailer tires to 76lbs, but realized with about a 10% increase based on heat, I'd wind up around 83psi and popped rivets galore. On a 4k trip I popped maybe 9-10 internal rivets.

My trailer has a GVWR of 8800lbs, and lets just assume I fill it and am at the full 8800lbs, which puts me at about 2200lbs per tire +/- given that the loads are uneven. The max rating at 80psi is 2830lbs, giving me approx 630lbs of reserve capacity. If I fill my tires to 69psi, I will have about 2600lbs of carrying capacity, or 400lbs of reserve capacity +/-. As the tires heat, I have measured they get to 76psi getting me around 2720lbs of carrying capacity or 520lbs of reserve capacity +/-.

Outside of weighing each tire when the trailer is loaded, the only option I know of would be to weigh the rig at a cat scale. So if I did this and loaded up my trailer was about 8400lbs, the numbers look something like this-

2100lbs +/- per tire in capacity at 8400lbs. 2600lbs @69psi with 500 reserver

2100lbs +/-, 2720 at 76psi (after a few miles on the tires) puts me to 620lbs per tire in reserve capacity close to if I had filled to 80psi cold, and I don't need more than 80.

I still found 3 popped rivets over 1000 miles of travel so far, 2 of which I am uncertain were from my travels. So my net difference after the tires heat up has been 7psi from filling at 69psi compared to filling at the full 80psi.

[bdhdig]

Originally Posted by 4mer Paddler

Tires and tire pressures are part of a very complicated issue regarding towing safety, durability and comfort. And there are plenty of opinions presented on all of the various forums, enough to completely baffle just about anybody. My peace of mind was helped considerably by installing a TST 507 TPMS. It continually shows pressure and temperature, somewhat delayed, but good enough to get the gist of the tire information over the period of a longer drive. As an example, since Michelin don’t make a15” LT anymore, I air my Endurance tires to 55psi @ 50° to 60°F and watch it climb to about 62psi @ 80°F. I know it probably won’t indicate a disastrous blowout right away but should indicate a slow leak or temperature buildup. Happy so far (2 years, 18,000 miles). Good luck and drive safe.

I too have the TPMS and have the pressure set at 55# which builds to a pretty constant 60#± on all four.
I read BarrysTireTech very informative website as well.
Hence, I will keep my Endurance tires at 55#

I sincerely thank everyone for their input.

[Action]

Any data pressure data after tires are no longer cold is not relevant.
Not relevant because there is no data from the tire manufactures on hot tire inflation pressures.
Only cold tire inflation is given.

The only thing one can use a hot tire pressure data is to determine if the tire is under pressured. That is because the hot tire pressure has increase be something greater than 10%.

A hot tire inflation pressure (which will be greater than cold inflation) does not change the load capacity of the tire. There is no data to support that. Because there is no tire data on hot tire pressures. Only cold inflation pressure.

Action

[richard5933]

Originally Posted by Andy543

The Airstream placard calls for the maximum of 80 PSI. Most trailer manufacturers call for the maximum on ST tires. It is the vehicle manufacturer's, not the tire manufacturer's, responsibility to specify the pressure, as they have to take into account how the vehicle will be used and how it's going to ride and handle. I believe Airstream and other manufacturers are concerned about mitigating trailer sway, so they want the maximum.

You have a point, to a point.

A couple of thoughts about this...

First, we don't know the reason Airstream sets the recommendation at 80psi. It could be to reduce sway like you mentioned. It could be to do everything possible to reduce the chance of sidewall problems from overheating. Or it could be a product of the legal team. Who knows.

Also, what do you do with a trailer like mine with a placard reading a 50psi recommendation? My trailer was built before the high-pressure ST tires were all using nowadays. They recommended 50psi, which was the max of the tires available at the time. Likely they would have recommended 80psi if the available tires could go that high. But the question remains, what pressure is necessary for proper handling? Proper braking?

Tire pressure affects more than just tire heating and blowout prevention. It also affects the contact patch size which has a direct relationship to braking, and it affects how much your trailer bounces or rides soft.

When I first installed the Goodyear Endurance tires on the trailer I filled them to 80psi. The trailer bounced and skitted around a lot. It just didn't feel right. When I lowered the pressure to 60-65psi there was a noticeable improvement in the trailer's tracking and road manners. 60psi is still far above the minimum for the weight the tires are carrying, so I'm not at all concerned there. And it's still above the OEM recommendation of 50psi.

So long story short, sure - go ahead and run 80psi if that's what makes your trailer handle the best. But it isn't going to make all trailers hit their optimal balance of all the various concerns.

[Gkiesel]

Run 60psi as Richard suggests.

[Wazbro]

Originally Posted by Andy543

The Airstream placard calls for the maximum of 80 PSI. Most trailer manufacturers call for the maximum on ST tires. It is the vehicle manufacturer's, not the tire manufacturer's, responsibility to specify the pressure, as they have to take into account how the vehicle will be used and how it's going to ride and handle. I believe Airstream and other manufacturers are concerned about mitigating trailer sway, so they want the maximum.

Definitely wrong for the OPs trailer, his 1977 Airstream tire placard probably says 50 psi, maybe but doubtful 65 psi. Definitely not 80 psi.

[Andy543]

Originally Posted by Wazbro

Definitely wrong for the OPs trailer, his 1977 Airstream tire placard probably says 50 psi, maybe but doubtful 65 psi. Definitely not 80 psi.

He has Goodyear Endurance tires, rated for 80 Psi. He'll be just fine at 80. That will give him the best handling. Will rivets start popping out? Probably not.

[CBWELL]

I have pulled Airstreams of 30+ feet length for 30 years and have always run tire pressure between 55-60 psi. I run this pressure to keep from popping rivets, to prevent shaking the trailer to pieces, and to keep things in place inside the trailer. Today's highways are awful and they are causing enough damage to our units without the high pressure adding to the problem. If you plan to keep your trailer for several years, keep the pressure at a functional level, and you will not have to replace screws, or re install cabinets that are falling off the walls. Many of us on this forum have years of experience pulling Airstreams, and quite often "experience" is better information than charts. We have seen the sway, tried different hitches, experimented with different tires and pressures, and done hundreds of other things to keep our tarilers in top shape. JMHO

[sfranklin]

Originally Posted by Action

Any data pressure data after tires are no longer cold is not relevant.
Not relevant because there is no data from the tire manufactures on hot tire inflation pressures.
Only cold tire inflation is given.

The only thing one can use a hot tire pressure data is to determine if the tire is under pressured. That is because the hot tire pressure has increase be something greater than 10%.

A hot tire inflation pressure (which will be greater than cold inflation) does not change the load capacity of the tire. There is no data to support that. Because there is no tire data on hot tire pressures. Only cold inflation pressure.

Action

So you are saying that if I inflate to 70 psi and meet the weight requirements that as the tire heats and increases pressure by 10% on average that extra 7 lbs of pressure does not increase my tire capacity from 2620 at 70osi cold to 2750 at 77psi warm?? Even as the tire heats the tire capacity is still based off 2620 psi cold?

[Action]

Originally Posted by sfranklin

So you are saying that if I inflate to 70 psi and meet the weight requirements that as the tire heats and increases pressure by 10% on average that extra 7 lbs of pressure does not increase my tire capacity from 2620 at 70osi cold to 2750 at 77psi warm?? Even as the tire heats the tire capacity is still based off 2620 psi cold?

No statement can be made as to any tire load capacity after the tires can be heated up. Tire manufactures make no claim/statements about tire loading EXCEPT at cold pressures. NOTHING, ZIP, ZILCH can be assumed inferred about a hot tire as to load capacity! Because NOTHING is published or stated by any tire manufacturer about loading except on a cold tire.

Kind of related, if the tires are inflated to side wall max, there is no additional load capacity of the tire, no matter if the tire is cold or hot. The max side wall loading is based on max tire pressure. No additional load capacity will be achieved with greater pressure.

One last piece, let's assume tire load capacity increases on a hot tire because the pressure increases. (I do not believe it does and that is a guess)
How does one add more weight to the trailer when the tire gets hotter and the pressure increases?

The only thing an owner of a vehicle with tires can know, is based on COLD tire pressures. Beyond that is a pure guess or speculation. That data is not available.

Action

[CapriRacer]

Originally Posted by sfranklin

So you are saying that if I inflate to 70 psi and meet the weight requirements that as the tire heats and increases pressure by 10% on average that extra 7 lbs of pressure does not increase my tire capacity from 2620 at 70osi cold to 2750 at 77psi warm?? Even as the tire heats the tire capacity is still based off 2620 psi cold?

I would have phrased this differently than Action - not that his answer is wrong.

The load carrying capacity of a tire does NOT increase due to the increased pressure when a tire is operating.

The chart is based on a cold starting position. The pressure buildup when the tire is operating is functionally the same as the cold starting. If we wanted to create a load table for "Hot", we need to know more than just the pressure. We would need to know the speed, because the rate energy is being added to the tire is what creates that additional pressure.

That means we would have a series of load tables with different speeds - all of which would result in the "Cold" load table, when the tire was allowed to cool.