Correct tire pressure for aftermarket tires?

[motozen]

Well, the OEM Goodyears finally bit the dust, fortunately before they came apart and damaged the trailer. I had a slow leak in one tire and our rv repair shop noticed that the tire carcass was separating. Quite timely as when they removed all four tires, two of them were several inches taller/shorter than the others due to the tire construction failing.

At any rate, I bought a set of National tires (which I've never heard of, but our rv guy stands behind what he sells). He said to run them at 80psi, but the Airstream manual says to run the tires at 65psi for our 27FB. Do I use the tire manufacturers recommendation or Airstreams??? The National tire site does not have a load/inflation chart online. Any knowledgeable help appreciated.

[HowieE]

Tires should be inflated to a pressure set by the load they are carrying. Weigh the trailer as loaded for a trip and use this chart.

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

Yes it is a Goodyear chart but the Federal regs require all tires of a given size be made to the same standards. Not sure how it happened that Goodyear tires have so many problems if our Government is really looking out for us.

65 is probably closer than 80 to the correct pressure.

[Sphere Guy]

Motown,

Look at the side wall of your replacement tires. It should have the Max pressure on it. 50 for a "C", 65 for a "D", 80 for a "E" rated tore.

For your 27, it had "E" rated tires, so the 80 psi. Your tire shop should have replaced them with a comparable tire rating capable of your trailer weight rating, GVWR. If not, the they could be at fault and you could have poor handling, or much worse!......

Sphere

[AWCHIEF]

Do keep in mind that the listed max air pressure may not be the correct tire pressure. Over inflation in my opinion is a major cause of many trailer tire issues. Right along with gross under inflation and excessive speed. Maybe Tireman will jump in on this.

[CapriRacer]

Motozen,

Tires are standardized, so the inflation requirements doesn't vary by brand. So if you didn't change size, there is no reason to change inflation pressure.

- BUT -

Collective experience says that trauiler manufacturers have not done a good job of sizing tires and many trailer would benefit from a larger load carrying capacity (and one way to do that is increase the inflation pressure.)

Do we know your trailer is one of those that could benefit from more load carrying capacity? No, we don't. But you can, if you weigh your trailer. Just keep in mind you need to account for side to side and front to rear variation, and speed. (ST tires are restricted to 65 mph!)

[motozen]

The tires are 'E' rated. I appreciate all your input, but there is no clear consensus. The most logical, but perhaps not optimal, seems to be to use the max rating on the sidewall.
Tireman?

[HowieE]

If you want a reasonable and detailed explanation of tire pressure you may want to read this site.

https://toyotires2-1524598101.netdna...0623_Final.pdf

It all come down to the pressure is a function of the load on the tire.

[nrgtrakr]

Another that talks about trailer tires specifically.
http://m.discounttire.com/dtcs/infoTrailerTireFacts.do
Our new to us 75 Overlander with twin axles had 7.00x15 STs, 27.8" diameter (one of which could have been original).
The replacements are slightly smaller in diameter, 27.1", (to prevent contact with the front edge of the wheel wells) ST205/75 load rating D, 2,150# @ 65 psi (as indicated on the side wall of the tire). The GVWR of the AS is 6,500#.
I have no idea what the owners manual says about tire inflation for the 7.00x15s, their load rating, or psi from the sidewalls.

[Jacob D]

This seems to be a never ending debate. I have been running GYM's for nearly 15 years on a '67 Overlander with never a problem. The set on now was made in China in January 2015. The set before was made in Canada. Both appear to have been well made and I take care of them by shading them when in storage, spraying them monthly with ArmorAll Tire Foam when using the trailer and ensuring correct inflation pressures even when stored . The tire pressure chart shown by HowieE above is universal and is the same chart shown to me by my tire dealer. My trailer weighs approx. 5,600 lbs with the weight pretty evenly distributed on its 4 wheels. At 40 psi, the chart says I could weigh 1880 lbs per tire, so I have a safety factor of approx. 480 lbs per tire. I see absolutely no reason to beat my trailer to death running max pressure (65psi) on normal roads at speed limit or less. Approximately 60K trouble free miles on one set of tires confirms it.

I too am puzzled by the stories of GYM's and others not giving good service unless there is something in the way the tires are cared for before or during service on an A/S. My guess is that such tires may have been stored poorly and/or for a long period of time before use. BTW I am a retired Mechanical Engineer but certainly not a tire expert.

[senioryakker]

To add to the confusions; our new to us '07 Classic 29' load range d tires were replaced with load range e. Ok to run at 65# per AS recommendations, even though sidewall max is 80#? Don't want to over heat, nor do I want to beat the trailer apart.

Thanks.

[Howard L.]

Originally Posted by motozen

The tires are 'E' rated. I appreciate all your input, but there is no clear consensus. The most logical, but perhaps not optimal, seems to be to use the max rating on the sidewall.
Tireman?

Trailer Tire Load/Inflation Chart | Maxxis Tires USA

Here is Maxxis chart for 15 inch ST tires rated at E (80 lbs). As you see you gain several 100s pounds of carrying capability with the E tires at 80. If you want, as some suggest, to lower the pressure to 65 or even lower, for a smoother ride and because you are still legal to carry your trailer weight, that's fine. However, understand that you have given up the tire's ability to carry that extra weight and in essence, at 65 psi you are running a D tire. If you lower psi to 50 you are running a C tire that will carry 2150 pounds. If you buy a 10 ply tire that has way more muscle to carry your load than needed and makes you feel comfortable when and if you do have a flat and need to run on the one tire for a while, air your E tires up to E tire pressures--or at least close. We run Maxxis 225/75 15 "Es" on our 2012 30 FC. I run them at 75 cold and nothing moves inside the trailer during tow, so can not comment on why others get a rough ride with 10 ply tires.

Another issue, if you run over 65 mph with your ST tires, you need to add 10 or even 20 psi to chart listed pressures for the weight you are carrying with those tires. In doing that, you need a tire that you can add that air pressure to and still be under the sidewall PSI. So whether needed or not, a E/10ply tire gives you extra security in a number of ways--but only if the pressure is there.

[motozen]

A couple of more data points, just in case someone will actually venture to recommend a definitive # for the tire pressure. Fully loaded the 27FB wights in at a hair under 6,500 lbs, and I believe we have 16" aluminum wheels. We run exclusively out west and often at 70 - 80 mph...except when in California with the (unbelievable) 55 mph limit.

[nrgtrakr]

I think you'll need to provide the exact and complete tire data if you want a specific psi recommendation.
Me, again I'd go with what the psi max from the sidewall info, use trailer tires and never exceed 65 mph. My father was a chemical engineer who worked product improvement for Gates rubber and an avid RVer. On his towables he used STs and stayed under 65 mph - never had a blow out or other tire problems, good enough for me.

[CapriRacer]

Originally Posted by senioryakker

To add to the confusions; our new to us '07 Classic 29' load range d tires were replaced with load range e. Ok to run at 65# per AS recommendations, even though sidewall max is 80#? Don't want to over heat, nor do I want to beat the trailer apart.

Thanks.

Yes. In effect you are running the Load Range E tires at Load Range D conditions - and the tire doesn't know the difference.

[m.hony]

I say inflate to the pressure on the placard on the side of the trailer if there is a placard on the side of the trailer. On my trailer, the placard says inflate to 65 psi.
I recently removed my tread-separated Marathons, installed 16" wheels/LT tires/TPMS/Centramatic balancers, and sold my 15" wheels here on Airstream Classifieds to a man in Indiana restoring a 1972 27' trailer. I am still running 65 psi per the placard on the side of the trailer, although the sidewall says 80 psi max.

[motozen]

We still seem to be going between two opinions: 65psi as recommended by Airstream, and 80psi, the max pressure on the sidewall.

Is there no one who can provide a definitive answer? Someone mentioned a forum member 'Tireman'...perhaps his moniker is based on his being an expert? Any way to engage him in this discussion?

[nrgtrakr]

Not an "expert" ... but there is this ...
http://m.discounttire.com/dtcs/infoTrailerTireFacts.do

[Tireman9]

Motozen
You didn't identify the actual tire size but from the comments it appears your original tires were Load Range-D (65 psi max) and the replacement tires are Load Range-E

Here are some facts.
1. It is the RV company responsibility to select the tire size and specify the minimum inflation that can support the axle load or GAWR.
2. Many times it appears that the RV companies select the minimum tire possible to meet the requirements so they specify a tire size, Load Range and inflation that may be only 1# above the GAWR.
3. RV companies can only guess at how much "stuff" you will carry in your RV. You may have more load in your RV than expected. The only way to know for sure is to get the RV on a scale when fully loaded and get the facts.
4. The tire company has no idea where the tire will be used so only makes tires that meet published tables for load & inflation.
5. The Max load capacity is marked on the tire sidewall along with the inflation needed to support that load.
6. As a MINIMUM you must inflate your tires to a level shown in the tables
that can support the actual measured load.
7. If your tires are made by a company that will not put their name on the sidewall that may tell you a bit about their quality. Do you have a tire warranty of a number of years? or in a number of weeks?
You may want to invest in a "Road Hazard " warranty so there is no argument about why your tire failed if it does.
8 If you provide the full DOT serial from your tires we can tell you who actually made the tires
9. Your tires should have the same capacity as Goodyear or Maxxis publish


Now my recommendation as a tire design engineer and RV owner.
1. For multi-axle TT you want to lower the "Interply Shear" as much as possible.
You can Google "Interply Shear RV Tires" to learn more if you want.
It is Interply Shear that is trying to tear the belts off the tire body.
2. If your tire size is an ST type (tire size has the letters ST in front of the numbers then it has a MAX speed rating of 65 mph unless the tire company has marked ON THE TIRE a Max Speed in MPH or a Speed Symbol letter such as K or L or M etc.
3. I suggest you ensure your measures load on the tires is no more than 85% of the load capacity for your size tire as seen in the Load Inflation tables at the inflation you will be running.
4. In general tires do not fail due to over-inflation as long as your cold inflation does not exceed the pressure on the tire sidewall.
5. If you want to run faster than 60 then IMO your only option is to switch to a speed rated tire which means probably an LT type. This will require load measurement and size & inflation adjustment and calculation UNLESS you can find one of the newer ST type tires that are rated "L" (75 mph max)

[CapriRacer]

Originally Posted by motozen

...... Is there no one who can provide a definitive answer? ........

If we were talking about a car or a pickup, the definitive answer would be the vehicle tire placard. Even then, you will find folks who disagree. This topic seems to be one of those that everyone has an opinion on, whether they are an expert or not.

But we are talking about a trailer - and trailer manufacturers haven't always done a good job of sizing their tires. To get to a definitive answer, we need to know the loads on the tires - you need to weigh the trailer tire by tire. Then do some math, apply some engineering judgement, look up tire load tables, etc.

So I suggest you weigh the trailer. If you are willing to accept a less than definitive answer, then use what Airstream published on their vehicle tire placard - but recognize there are some risks associated with that.

[motozen]

Tireman9: Thanks for weighing in on this and adding some clarity. I will heed your warning about ST tires only being rated for 65mph. I also heard that tires generally do not fail at max cold inflation pressure, thus I will go with the 80psi printed on the sidewall as I do not see any downside to doing so. Perhaps it will be a marginally 'rougher' ride, but in all probability we will not be able to notice the difference. Thanks again!