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Old 09-28-2015, 10:11 AM   #15
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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.
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Old 09-29-2015, 10:19 AM   #16
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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?
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Old 09-29-2015, 10:33 AM   #17
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Not an "expert" ... but there is this ...
http://m.discounttire.com/dtcs/infoTrailerTireFacts.do
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Old 09-29-2015, 10:29 PM   #18
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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)
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Old 09-30-2015, 07:36 AM   #19
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Originally Posted by motozen View Post
...... 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.
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Old 09-30-2015, 02:40 PM   #20
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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!
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Old 09-30-2015, 11:17 PM   #21
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Tireman9. Maxxis ST tires, for the most part, are speed rated "Q". At least the 225-75-15s I have on the 30 FC are Q tires. What is their max operating speed?

And what are your feelings of Goodyear Marathon, General, and Maxxis stating that 75 mph and even 85 mph are permissible speeds if a 10 psi or 20 psi additional pressure is added above that required for a given weight per the inflation tables? (That additional pressure of course can not exceed sidewall max)

See:

https://www.onlinetires.com/search/v...ds/page_1.html

Thanks Howard
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Old 10-01-2015, 10:57 AM   #22
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Originally Posted by motozen View Post
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!
Your welcome. Be sure to check out my RV tire blog
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Old 10-01-2015, 11:18 AM   #23
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Originally Posted by Howard L. View Post
Tireman9. Maxxis ST tires, for the most part, are speed rated "Q". At least the 225-75-15s I have on the 30 FC are Q tires. What is their max operating speed?

And what are your feelings of Goodyear Marathon, General, and Maxxis stating that 75 mph and even 85 mph are permissible speeds if a 10 psi or 20 psi additional pressure is added above that required for a given weight per the inflation tables? (That additional pressure of course can not exceed sidewall max)

See:

https://www.onlinetires.com/search/v...ds/page_1.html

Thanks Howard
Thanks for link. Wasn't sure if MAXXIS had finished adding speed symbol to their tires. Not sure if I would go with towing at much above 75 (or for that matter much above 65 simply for vehicle control and safety issues. Stopping distance when towing is poor enough.

Have you ever tested to see what happens even at 30 mph with an emergency stop? It would be 4 times worse as you get to 60 and them much worse with additional speed. Do you stop in straight line with the brakes locked?

BUT each tire company may or may not apply speed restriction on their tires but teh speed rating on tires is like the engine redline. If you think it is good practice to run near, at or above engine redline then you probably believe running tires at or near or above their max speed rating.

On ST type tire without the speed symbol MOLDED ON THE SIDEWALL you should consider 65 the max.
Goodyear has a document allowing up to 75 if you add 10 psi but I have never seen anyone going beyond the +10 psi and + 10 mph so don't know where you got the 85 mph and +20 psi. Can you provide a link to the document you are quoting?

Please remember that ST type tires have no requirement to actually pass a high speed test and even tires that do have a speed rating (P & LT) only have to run 75 for 30 min + 80 for 30 min + 85 for 30 min and this at 88% of the Max tire load.

So I would first ask if you are running at no more than 88% of tire max load (measured not estimated) and if you are willing to accept 1.5 hours as the life of a tire?
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Old 10-01-2015, 02:35 PM   #24
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This is how I decipher how the tire standards and regulations are supposed to work.

Replacement tires, at a minimum, need to provide the load capacity - via inflation pressures - that the OE tires provided.

The FMVSS says - in part - that vehicle manufacturers MUST set your trailer’s GVWR and each GAWR. To that is added the responsibility to select a tire/rim fitment that is appropriate for each GAWR and set the recommended tire inflation pressure (s) for the selected tires. That’s not an arbitrary statement. The standard directs the vehicle manufacturer to do that. It’s a vehicle manufacturers responsibility and once made becomes a minimum standard for that vehicle.

Although the same FMVSS’s are used for automotive and RV trailers they do specify differences and it’s in those differences where the confusion often starts. Automotive fitments are required to have reserve load capacities. RV trailer fitments do not. Therefore an additional measurement is provided for RV trailer fitments and it says that the RV trailer manufacturers’ published hitch/pin weight (and they must publish one) when added to the trailer’s total GAWR must equal or exceed the trailer’s GVWR. That in reality allows the trailer manufacturer to fit two 1500# tires to a 3000# axle. Kick-in the ST tire manufacturers on the one hand saying their tires are good to go all day on maximum allowed tire inflation pressures and you get inferior fitments that are surely going to fail early. And, they only have the option to set the recommended tire inflation pressures to maximum.

Setting RV trailer tire inflation pressures to accommodate the load carried is a poor recommendation by whoever makes it. A 1 psi loss of inflation pressure will cause such tires to be overloaded. You can lose a single psi by hitting a curb or pothole or just dropping off the slab you were parked on all weekend. Sure, it’s uncommon but it can happen.

Look on your tire placard, certification label or in the owner’s manual and find the size of the OE tires. Determine how much load capacity they provided at their recommended inflation pressure. Your new tires need to provide that much load capacity. Determine the inflation pressure you’re going to set to equal or exceed what the OE tires provided and jot it down in your owner’s manual. Auxiliary tire placards are allowed and it’s recommended that you make one and display it adjacent to the original tire placard or certification label.

I’m providing the following quote for those that misunderstand the purpose of the FMVSS standards. They are for vehicle manufacturers but are often quoted as something we, the users owners have control over.

FOREWORD
The National Highway Traffic Safety Administration has a legislative mandate under Title 49 of the United States Code, Chapter 301, Motor Vehicle Safety, to issue Federal Motor Vehicle Safety Standards (FMVSS) and Regulations to which manufacturers of motor vehicle and equipment items must conform and certify compliance.
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Old 10-02-2015, 09:32 AM   #25
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Quote:
Originally Posted by Tireman9 View Post
Thanks for link. Wasn't sure if MAXXIS had finished adding speed symbol to their tires. Not sure if I would go with towing at much above 75 (or for that matter much above 65 simply for vehicle control and safety issues. Stopping distance when towing is poor enough.

Have you ever tested to see what happens even at 30 mph with an emergency stop? It would be 4 times worse as you get to 60 and them much worse with additional speed. Do you stop in straight line with the brakes locked?

BUT each tire company may or may not apply speed restriction on their tires but teh speed rating on tires is like the engine redline. If you think it is good practice to run near, at or above engine redline then you probably believe running tires at or near or above their max speed rating.

On ST type tire without the speed symbol MOLDED ON THE SIDEWALL you should consider 65 the max.
Goodyear has a document allowing up to 75 if you add 10 psi but I have never seen anyone going beyond the +10 psi and + 10 mph so don't know where you got the 85 mph and +20 psi. Can you provide a link to the document you are quoting?

Please remember that ST type tires have no requirement to actually pass a high speed test and even tires that do have a speed rating (P & LT) only have to run 75 for 30 min + 80 for 30 min + 85 for 30 min and this at 88% of the Max tire load.

So I would first ask if you are running at no more than 88% of tire max load (measured not estimated) and if you are willing to accept 1.5 hours as the life of a tire?
Tireman, I think that 20 psi and the 85 mph came from Maxxis. Let me research some history here and get back.
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Old 10-04-2015, 10:35 AM   #26
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Your welcome. Be sure to check out my RV tire blog
10-4...link for your tire blog?
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Old 10-05-2015, 12:39 PM   #27
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10-4...link for your tire blog?
Check contact info in my Profile.

I am not allowed to post the link to my blog directly in a thread.
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