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Old 08-14-2013, 03:30 PM   #1
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Calculating the correct tire pressure to use

This may be covered elsewhere, but I'm not finding it, so I'll ask: How do I calculate the correct tire pressure to use on my "new to me" 1971 Land Yacht Safari 23' Airstream trailer with dual axles? Airstream of Chicago put new Marathon Trailer Service tires (size ST225/75R15, load range D) and new aluminum rims on it. When I asked our dealer, he recommended inflating the tires to 65 PSI when cold. That's the maximum cold pressure.

However, the tires were actually inflated to 42 PSI on delivery, and have been fine for the couple of hours I've had them on the highway thus far, so I'm wondering if there is a way to calculate the correct tire pressure to use more precisely? Since I've also seen bike tires explode on hot days when inflated to their maximum rated pressure, I'm not eager to push any limits unless there is a good reason for me to do so.

The trailer weighs 3530# empty, and has 4 tires to bear that load. I'll try never to let the total weight exceed 4,500#, as that is the maximum rating for my tow vehicle.

Goodyear's RV tire care guide (http://www.goodyearrvtires.com/pdfs/tire-care-guide.pdf) doesn't suggest a specific pressure, but does show what weight each tire can hold at various pressures, and suggests dividing the total load evenly among the 4 tires to achieve that. To me that suggests the idea for filling to 65 PSI is overkill, as each tire can hold 2540# at that pressure, and I'll never need it to hold more than 1125#, which Goodyear's chart suggests can be borne even at 20 PSI.

I'm inclined to stick with the actual delivered 42 PSI, rather than the suggested 65 PSI, but would love to hear from anyone who actually knows the ideal pressure to use.

Thanks much,
-MrJim
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Old 08-14-2013, 04:41 PM   #2
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There are many camps of thought here on tire pressure. Some folks stipulate maximum sidewall tire pressure of 65 psi, some suggest referring to the weight vs pressure tables offered by the tire manufacturer. Others say refer to the Airstream door information label pressure and tire ratings.

Of paramount importance with the GYM ST tires is the maximum speed rating of 65 mph on the tire sidewall.

Check the sidewall on these "new" tires for the date code which is a four digit number which reads wwyy where ww equals week tire was made and yy equals the year the tire was made. You might also find they are made in China.....
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Old 08-14-2013, 04:45 PM   #3
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I run the tires on both trailers at 35 lbs. The 1979 Safari and the 1954 Liner.

Before you rely on the 4500 lbs number, take the trailer to a scale. With water, propane and air conditioner, you will be heavier, the Airstream number is DRY.

Bill
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Old 08-14-2013, 07:22 PM   #4
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After lots of experimentation I fill my Carlisles to 39#. Gives me the smoothest ride. Max tire pressure is for MAX Load which I don't come near to.
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Old 08-14-2013, 07:37 PM   #5
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Every tire made has an inflation chart. All tire of the same type and size are required to meet the same specs so this chart will give you the answer.

http://www.goodyearrvtires.com/pdfs/rv_inflation.pdf

Almost no tire dealer is even aware that these charts exist and will inflate tires to the MAX printed on the sidewall.

Weigh your trailer and than consult the chart for your size.
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Old 08-14-2013, 11:35 PM   #6
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The tag on the side of my 2000 25' Safari gives the tire size as 225/75R15 inflated to 50 psi cold.

The ST225/75R15, load range D Goodyear Marathons
says 65 psi max on the tire....I run 50 psi cold.
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Old 08-15-2013, 07:21 AM   #7
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OK, I am of the opinion that the vehicle tire placard is the proper pressure to use. It has been looked at by the vehicle manufacturer to include all the safety reserves needed for safe operation.

HOWEVER, trailer manufacturers are notorious for using tires that are too small. That is why the tire dealer is telling you to use the maximum listed on the sidewall of the tire.

Charts? Those are MINIMUMS (they even say so). You don't want the minimum. You want a bit of over capacity. That's what the major motor vehicle manufacturers do, and I think they have a lot of experience in this area.
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Old 08-15-2013, 09:11 AM   #8
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We run at 96% of sidewall " cold" PSI at least. My biggest fear is a blowout because of under inflation and over heating of the tire. Some tires are more resistant to these stresses than others based on construction and rating: e.g. "D", "E", etc. A few miles down the road my TPM shows psi's in the "normal" or above normal range. ( expected occurrence)

In storage, I always inflate cold psi to max indicated on the sidewall and move the unit at least monthly.

Bottom line: I run the tires at stated max pressures or as close to those stated on the sidewall. ( on longer trips with varying morning temps, the psi will fluctuate some) I have spoken to several truckers and they all advise the same. IF you run 10% or more under the stated sidewall psi, you may be asking for trouble. ( at least monitor the temps of the tires with a TPM system)

These comments will not doubt "pique" a few.

We also maintain a focus on the age of the tires. Many RV'ers will recommend changing to new tires when the existing ones are 5 years old. Some object to that philosophy.

For every reader of this forum, there is an opinion and certainly not all agree!
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Old 08-15-2013, 09:31 AM   #9
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In my opinion, tires are a maintenance expense item associated with any vehicle. Just like engine oil, they need to be replaced upon both time since installed and percentage of remaining tread, which ever occurs first. A tire study on this forum by "Phoenix" showed the issues really developed by year three/four.

I change oil every 5,000 miles because oil is still cheaper than main bearings. I plan to change the four tires on the road by the third season as the desert heat will do more damage even in storage that the miles actually on the tires. The cost of four tires is still less than the deductible on the Airstream insurance and the hit in the value of the trailer if being damaged by a tire issue.

Ownership of an Airstream does involve the "pay to play" thought and all these ancillary operating costs should be considered along with the purchase price, storage fees and insurance.

At least Airstream ownership and operation should not be as expensive as owning race horses, the sport of kings.

My truck rear tires have two psi settings on the door post, running virtually empty at 45psi and loaded at 70psi (where the two tire load capacity happens to equal the axle rating). Their maximum rating is 80psi.
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Old 08-15-2013, 06:35 PM   #10
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Quote:
Originally Posted by MrJim View Post
This may be covered elsewhere, but I'm not finding it, so I'll ask: How do I calculate the correct tire pressure to use on my "new to me" 1971 Land Yacht Safari 23' Airstream trailer with dual axles? Airstream of Chicago put new Marathon Trailer Service tires (size ST225/75R15, load range D) and new aluminum rims on it. When I asked our dealer, he recommended inflating the tires to 65 PSI when cold. That's the maximum cold pressure.

However, the tires were actually inflated to 42 PSI on delivery, and have been fine for the couple of hours I've had them on the highway thus far, so I'm wondering if there is a way to calculate the correct tire pressure to use more precisely? Since I've also seen bike tires explode on hot days when inflated to their maximum rated pressure, I'm not eager to push any limits unless there is a good reason for me to do so.

The trailer weighs 3530# empty, and has 4 tires to bear that load. I'll try never to let the total weight exceed 4,500#, as that is the maximum rating for my tow vehicle.

Goodyear's RV tire care guide (http://www.goodyearrvtires.com/pdfs/tire-care-guide.pdf) doesn't suggest a specific pressure, but does show what weight each tire can hold at various pressures, and suggests dividing the total load evenly among the 4 tires to achieve that. To me that suggests the idea for filling to 65 PSI is overkill, as each tire can hold 2540# at that pressure, and I'll never need it to hold more than 1125#, which Goodyear's chart suggests can be borne even at 20 PSI.

I'm inclined to stick with the actual delivered 42 PSI, rather than the suggested 65 PSI, but would love to hear from anyone who actually knows the ideal pressure to use.

Thanks much,
-MrJim
You are talking about ST tires here. They are all designed to operate very well at the vehicle manufacturer’s recommended tire pressures or the maximum amount depicted on their sidewall. In my opinion you will soon join the GYM failure group if you continue using 42 psi tire pressures and thus over ruling dealer and manufacturer recommendations.
BA

On edit I went back and looked at the specs for your trailer. It came equipped with 7:00 x 15" rims fitted with 8 ply nylon tubeless tires. I'm hoping Capri Races can provide us with the load capacity for those tires.
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Old 08-15-2013, 08:16 PM   #11
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Running a tire at max air pressure (as stated on the tire) might be the cause of some of these blow outs people are experiencing.
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Old 08-15-2013, 09:33 PM   #12
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One more wrinkle - pressure rating of the valve stems

Thanks everyone for all the good info on this. A comment by "Phoenix" alerted me to a possibly-important additional factor: ordinary automotive wheel valve stems are only rated for 44 PSI, and I'm not sure whether my new wheels came with those or with special high pressure valve stems. I have a question in to my Airstream dealer (who supplied and installed the wheels) as to the type of valve stem he used.

Here's a photo of one of the valve stems, in case anyone can tell its rating by looking at it.
Click image for larger version

Name:	tire valve.jpg
Views:	302
Size:	160.7 KB
ID:	193278

Oh, and one other bit of news FWIW: My trailer's new tires were indeed made in China.
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Old 08-15-2013, 11:54 PM   #13
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Quote:
Originally Posted by MrJim View Post
Thanks everyone for all the good info on this. A comment by "Phoenix" alerted me to a possibly-important additional factor: ordinary automotive wheel valve stems are only rated for 44 PSI, and I'm not sure whether my new wheels came with those or with special high pressure valve stems. I have a question in to my Airstream dealer (who supplied and installed the wheels) as to the type of valve stem he used.

Here's a photo of one of the valve stems, in case anyone can tell its rating by looking at it.
Attachment 193278

Oh, and one other bit of news FWIW: My trailer's new tires were indeed made in China.
It looks like a 60 psi snap-in. Inexpensive and easy to install. See the reference.

TR-413 Low Pressure Valve Stem (0-60 PSI) | RecStuff.com

BA
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Old 08-16-2013, 09:04 AM   #14
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Quote:
Originally Posted by LFC View Post
Running a tire at max air pressure (as stated on the tire) might be the cause of some of these blow outs people are experiencing.
Truckers consistently state that the main culprit in tire blowouts is under inflation and over heating due to the under inflation. They run their tires near or at max side wall recommendations. Daily temps will vary the cold PSI readings, but as long as the PSI is no more than 10% less than max, the tires will heat sufficiently to bring the PSI up to normal within a few miles of travel--usually about 20 miles.

I have seen this occur with my own TMS.

Someone mentioned that part of the problem is the difference between ST and LT tires...that thread is discussed elsewhere. We run Michelins LT's. Rating E

Some have suggested damage to the AS unit because of the "rough ride" on "fully inflated" tires. My only anecdotal comment is that we have encountered no problems with this philosophy.

We have not encountered "very hot" running temps, and have seen the tire temp approach 96 degrees with outside temps at 101 degrees. The danger zone is about 125 degrees-- tire temps. At that point, we would stop and enjoy a beverage in the shade.
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