I can probably shed some light on FEs excellent question just from a practical standpoint. The average consumer is not a tire expert. They don't know the difference between an LT, an ST, or a P. So they decide they need to get new tires. It is logical to them they need to get the same size. So they go looking at tire sizes and see all of these numbers there like 225/75R15. The ST and LT just does not register -- those aren't numbers. So they go get something with the same numbers. Since they don't understand tires, they don't realize there is the whole load range and maximum load rating issue. If they replace an ST with an LT of the same size, they are going to a tire with a lower maximum load rating. This is frequently a safety problem. I think the RMA publication is just trying to educate these people and they are trying to say in other terminology "don't replace your trailer tires with ones that have a lower load rating".
I would not be surprised if someone out there has not replaced (or tried to replace) a set of ST225/75R15LRD tires with a set of ST225/75R15LRC tires because in their comparison shopping they found them in some other brand at some other place much cheaper -- not realizing part of the reason for the price difference is the difference in load range.
As NC Hauler points out, when attempting to go to a different type of tire, the consumer would need to do some research. In that research, they will learn all of these issues so they can make the right decision.
Nearly every day we have new members (and even some senior and full members) come here when doing their research. There are about a dozen to two dozen members here, including FastEagle, who bring all of this information to light. Although they don't all agree with each other, the OP who posted the question leaves knowing full well what they are getting into and where the issues are. It is my gut feeling that there are fewer safety issues on our highways due to their input.
All of that said, there are some lingering technical questions that those of us who regularly participate in these threads want to know. We now have an engineer currently working in the field who has stepped up to answer those questions. I'm waiting on my answer about the difference in the size of the cords, but I'm starting to perhaps see some implications of the information he has already provided and would like him to confirm that I got this right:
When comparing an ST and LT of the same size, the ST is allowed to flex more and has a higher load rating but a lower speed rating. We know the more a tire flexes and the faster you drive, the more heat is generated. Too much heat leads to the blowout. OK, then there is the advice from Goodyear that you can increase the inflation pressure of the Marathon by 10 PSI and increase your speed up to 75 mph as a result. So applying some logic here as to how this works. All you are doing is reducing the deflection and thus the ability to produce heat when you increase the pressure. If you drove the same speed, the tire would run cooler. But then you increase your speed to 75, and go back to the same heat generation. Now I extrapolate this. I have an ST225/75R15 load range C tire inflated to 50 psi. I run it at its max load rating at 65 mph and have no problems. But if I live in a western state where 80 mph is the norm (I would never tow that fast), if my wheels will take it, I can put the ST225/75R15 load range D on and inflate to 65 psi. Running at 80 mph, I am probably generating the maximum heat that tire can withstand. To the OP . . . did I get that right? It is all really about the heat limits?
FastEagle wrote: ........This is an excerpt from page 49 of the reference provided below..... My question for the OP is: How valid is this information and why isn’t it widely known?......
Just so you can put this in perspective - I have sat in as a substitute on the committee that writes this stuff. BTW, there is a new booklet that will be published very soon. The whole thing has been reorganized and updated. The wording has been changed slightly to make it a bit clearer.
The problem being pointed out here involves a specific issue - and there is very little overlap between ST and LT tires. The point is almost non-existent.
The real problem lies with the smaller ST tires - and substituting passenger car tires for ST tires. This is a much more common, and potentially more dangerous, problem. That was the intent of the quoted sentences.
Why is this not as well known?
To put this in context, it is diffcult enough to get tire dealers to know how much inflation pressure passenger car tires should have. Tire placards have been on cars since they were required in the 1970's by NHTSA (National Highway and Traffic Safety Administration) - the US regulatory body for car and related equipment. As a result, in 2008, NHTSA required the tire placard to be of a standard form and placed in the driver's door or doorframe - where before it could be found in a variety of places. This was a direct result of the Ford/Firestone thing that exploded (pun intended) in August of 2000.
In spite of all that, it is still common for tire dealers (but mostly the tire buster who sets the pressure at the tire shop) NOT to know this. To address this, the RMA (meaning the tire manufacturers as a group) intends a mass mailing to every RMA member - and they have asked the TIA (Tire Industry Association - the tire dealer organization) for a list of their members as well. This should help, but it means the guy at the shop level has to read them - an unlikely prospect. It is hoped that TIA will start to write articles - which they've said they would - in their magazine.
Let's face it, the tire store manager has much more on his mind than just the technical aspects of tires. He has to manage the flow of money, personnell issues, shop equipment issues, customer issues. His plate is pretty full with the day to day running of his shop.
You have said; "The real problem lies with the smaller ST tires - and substituting passenger car tires for ST tires. This is a much more common, and potentially more dangerous, problem". So putting a P rated car tire in place of a ST tire is dangerous and that makes sense. However what about replacing a ST with a LT?
This much more like the first 24 hours. I can't even write responses fast enough. I've been at this for over 2 hours now and I really must be moving on.
So just one more:
ExRocketScientist wrote: .........To the OP . . . did I get that right? It is all really about the heat limits?
Not exactly - and I am not sure exactly how to address this as this is quite complex. But esssentially you're correct.
But there is another major factor involved - and that's the manufacturing end of things. Please give me a couple of days to write this up as I want to get it as accurately as I can.
But let me leave you with one thought: There is nothing inherent in the design of ST tires (compared to LT tires) that requires a larger cord - and I would amend that to say the sum of the "strength" of the cords, accounting for differences in the cord itself, the density of the fabric as measured in ends per inch, and the number of plies.
However, there is something inherent in the difference between ST tires and passenger car tires that affects the sidewall (and belt) cords. I'll explain that later.
In the tire threads here on RV.NET, we see a lot of controversy about replacing ST tires (any ST tire) with LT tires. Many first time posters are of the opinion that a LRE ST tire can be safely replaced with a like sized LRE LT tire.
Are you saying, from an engineer’s point of view that is an acceptable replacement tire scenario? Is the real strength of the LT235/85R16E tire equal to the (smaller) ST235/80R16E tire?
The tire industry constantly polices itself. Of all the SOP manuals I’ve read, from all of the major brands, none, approve of violating the vehicle tire placard and have made it a point to put strict compliance “cautions” and “safety warnings” in their manuals. Some will even refer to the NHTSA safety standard (s) where it is a prominent statement to use replacement tires with equal or greater load capacities found on the vehicle tire placard. Vehicle owner’s manuals will also mime the NHSTA standard.
Chris wrote: .........One of our posters took great exception to that Barry Smith quote. Now here we are with a real tire engineer posting something similar.....
Unfortunately, Chris, you are quoting the same guy.
Cool so you are saying my quote of a Barry Smith item in 2009 was Barry quoting your statement about ST tires, load intensity and non passenger aspect of ST tire ratings.
So it does not appear from you recent post here that you have changed your opinion of ST tires. I look forward to reading more of what you have to say.
From my view it appears that ST tires are more about low cost provider than a quality product. I see a lot of trailers come into our Snowbird park with damage from these inferior tires. How it is not only ST that fail.
The Guy across the street lost a G614 coming down this year with a lot of damage. He said he bought this tire a year ago, and when I looked at the date code it was a 2007 tire. 2006-7 appears to be a bad time frame for G614's. The G614's are strange as on paper they havd all the right things going for them, steel carcass and lots of.meat at 61lbs. For those that travel a lot the LRH 215/75R17.5's seem to be the ticket.
The Guy next to me lost a ST with little less damage 3-4k.
It is a bit of a catch 22, with many builders spec'ing their trailers to use up most of the inflated weight ratings. Montana/Keystore derates (on placard) Dexter 7k axles to 6750 so they can use Marathon ST rated to 3420. A full 90 extra capacity per axle.
These trailers would be better served with GY G614 tires. However the G614's cost at least twice as much and has had some issues on heavier trailers.
Maxxis appears to be the only remaining ST manufacturer that tried to build a higher quality ST in those from Thailand. At 43 lbs for their ST235/80R16E they are not one of the bunch of 35 pounders. Chris
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My Rig
2001.5 2500 STD CAB AUTO SLT 4x4, CTD 4:10's, Bomb'd to Tow
2005 Cardinal 29WBLX.
FastEagle wrote: In the tire threads here on RV.NET, we see a lot of controversy about replacing ST tires (any ST tire) with LT tires. Many first time posters are of the opinion that a LRE ST tire can be safely replaced with a like sized LRE LT tire.
Are you saying, from an engineer’s point of view that is an acceptable replacement tire scenario? Is the real strength of the LT235/85R16E tire equal to the (smaller) ST235/80R16E tire?
The tire industry constantly polices itself. Of all the SOP manuals I’ve read, from all of the major brands, none, approve of violating the vehicle tire placard and have made it a point to put strict compliance “cautions” and “safety warnings” in their manuals. Some will even refer to the NHTSA safety standard (s) where it is a prominent statement to use replacement tires with equal or greater load capacities found on the vehicle tire placard. Vehicle owner’s manuals will also mime the NHSTA standard.
FastEagle
.................FE , you need too differentiate between American made tires and Tires imported from China...........American mfgers May very well be self policing , Chinese made tires and their lack of quality are NOT self policing ! WHY , do you continue too lump everyone together ? , jf
Jimmyfred, many of us will argue that ST made in the US, Canada and other non Main Land China sites failed at rates that were not acceptable also. Goodyear only does a Marathon run about once a year and will do it at the plant that has capacity available. Chris
FastEagle wrote: In the tire threads here on RV.NET, we see a lot of controversy about replacing ST tires (any ST tire) with LT tires. Many first time posters are of the opinion that a LRE ST tire can be safely replaced with a like sized LRE LT tire.
Are you saying, from an engineer’s point of view that is an acceptable replacement tire scenario? Is the real strength of the LT235/85R16E tire equal to the (smaller) ST235/80R16E tire?.......
I have to be very careful here to get this right, but what I am saying is that from the engineering prespective, there isn't a difference in the strength needed for ST vs LT tires. I point out, though, that there COULD be differences between manufacturers, between manufacturing plants, etc.
FastEagle wrote: ....The tire industry constantly polices itself.......
There are those who would disagree and point to the Ford/Firestone thing some years back.
FastEagle wrote: .....Of all the SOP manuals I’ve read, from all of the major brands, none, approve of violating the vehicle tire placard and have made it a point to put strict compliance “cautions” and “safety warnings” in their manuals. Some will even refer to the NHTSA safety standard(s) where it is a prominent statement to use replacement tires with equal or greater load capacities found on the vehicle tire placard. Vehicle owner’s manuals will also mime the NHSTA standard.
FastEagle
I think all this business about not substituting ST tires for other types is mainly directed towards Passenger Car tires.
Remember I said I sat in on the RMA committee meetings. One of the things that gets brought up a lot is accuracy vs complexity. If you say something short and simple, people are more likely to remember it. Unfortunately, simple statements are not always accurate. But the simple statement about not substituting other types of tirers for ST tires is very close to accurate - because there is very little overlap between ST and LT tires.
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