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Friday, March 27, 2020

"Reserve Load" or Load Capacity Margin

Ran across a post on Reserve Load or Reserve capacity that suggested the RV owner had been given incorrect information. Here is the post and my reply.

Personally, I'd run LTs, simply because of their higher "reserve" capacity; upwards of 30% over the stated load. Given that STs have, at best, 10% (used to be basically 0%), you're still in ST load territory, with a much better tire. Hell, we used to run our old 1/2t trucks with massive loads and just air up to 60-65 psi and go. Yes, it wasn't very far, or very fast, but those tires still lasted 50-60k miles, usually with steel cord showing around the edges. :-) We'd then take them off and put them on a disk or trailer and use them until they sun-rotted.

I think someone miss-informed you about "Reserve Load".
All tires have a stated load capacity for example. "2,340# Max Load" molded on the tire sidewall at a stated inflation level such as  "50" psi.

"Reserve Load" is the difference between the actual applied load and the stated load capacity and is many times stated as a percentage

Example: A vehicle is on weight scales and we learn that a tire has 2200# load on the tire. The tire has a load capacity of 2,750#.   2,750 minus 2,200 = 550#   which is 20% of 2,750. It doesn't make any difference what type tire we are talking about as the math is still the same.

Now, it is true that for a given set of dimensions, e.g., 235/75R15,  the stated load capacity is different depending on type tire and inflation level.   P-type and LT-type and ST-type each have different stated load capacities at their stated inflation pressure. For this discussion, let's keep inflation differences out of the picture.

Let's look at a P235/75R15 at 35 psi is rated to support 2,028# ( In a trailer application P-Type must be De-rated by Load/1.1 giving 1,842# capacity.) An LT235/75R15 is rated for 1,530# @ 35 psi and an ST235/75R15 is rated to support 1,870#

BUT the "Reserve Load" calculation is still  (Tire Load Capacity)/Measured scale Load).

The 10% margin for trailers is the difference between the GAWR and the total capacity of the tires on that axle at their max load.  I have posted in my blog some actual margins showing that many cars have load margins of 25% to 35% while some RVs made before Nov 2017, when RVIA changed the "Margin" to 10%, had margins of tire capacity vs GAWR as low as 1%.

Hope this helps.


Friday, March 20, 2020

Response to some of my information and warnings on tire inflation

After posting on one RV Forum some steps that I felt if taken could result in longer tire life by lowering the Interply Shear Forces, I got this reply:
"Sounds like tires should never be used or stored in any configuration other than the 'as cured' state or they simply self destruct.
Gonna have to figure out how to make these R/Vs hovercrafts."

I offered the following:
A bit of an over-reaction. My advice is intended to offer a series of steps that can be taken to extend the life of their tires rather than actions or inaction that may shorten tire life.

One of the biggest problems is the inability to understanding the real "Root Cause" of tire failure. IMO too many simply assume that somehow, the zip code of the factory where the tires are made is a "cause" for a failure.
Have you read and do you understand the difference between the two major and different reasons (root cause) for failure as covered in THESE posts?

TPMS, when properly PROGRAMED and used  can essentially eliminate one of the two primary reasons for tire failure.

Having a good level (15% to 25%) Reserve Load is the second major thing people can do to get a more reasonable tire life. I would be very surprised to learn that tires with at least 15% actual Reserve Load didn't perform much better than the more normal 0% to 5% level.

Friday, March 13, 2020

Is the Goodyear Endurance ST type tire "Better Quality" than a Goodyear LT tire?

Had a couple of comments on my post on LT vs ST type tires for RV trailers

Alan said

"Now, I am VERY confused. For years, everyone in the RV business, its commentators, its experts and other blogger forums so-called experts “having used this or that for soooo many years without any problems”, etc, had me convinced in believing all the hullabaloo about the ST tire’s sidewalls being much better, much stiffer and much more resistant to the particular type of flexing that occurs when driving, turning and parking travel trailers or fifth wheels than LTs. After the appearance of the “china-bombs syndrome” a few years ago, people started to realize that practically all ST tires were manufactured in China anyway, whatever the
“US brand name” stamped on them, leaving the consumer with very few, if any, alternatives.
Now, co├»ncidentelly, we hear that LT tires are “just the thing” for trailering.
It also appears that a handful of RV manufacturers, like Jayco, have seemingly struck a deal with Goodyear, who now has resumed manufacturing ST tires in North America, to equip their new trailers with “US-made ST tires”. I guess their ST tire prices have dropped again enough to catch the big manufacturer’s attention and make it worth their while.
So I guess the real question should now be, are US-made Goodyear ST tires better, worst or equal quality than their US-made Goodyear LT tires, or why even bother to engineer ST tires at all? We shall see, in time, who wins between; Beta or Vhs, Plasma or Lcd, Android or Apple, etc, etc, etc."

Alain, The biggest issue is, I know of no way to do a direct comparison of ST vs LT tires other than an expensive tire test. We can't depend on the DOT test results. In all probability the tires all pass, simply because the RV industry and ST tire mfg companies were successful in getting ST tires excluded from the significant improvement in testing with updated standards in 2002. There is no question, in my mind, that the tests (FMVSS 571.119) for ST type tires are easier to pass than the tests (FMVSS 571.139) for new Passenger and LT type tires.
I have looked at the construction of a GY Endurance and it certainly looks better than the older Marathon design. Now remember I do not have access to the actual specification or material properties used by GY but in the Endurance tire line, they appear to have added a Nylon cap over the steel belts. You can confirm this by reading the material list molded on a tire sidewall.

"Quality" is a tough call when you have two different specifications to start with. Isn't Quality just a measure of a tire's ability to meet the specification for that product?

You can think of it this way.  If I had two pieces of chain. One rated at 500# and another rated at 550#. They both meet a strength test of supporting the rated load and if you tested 100 pieces of each and found that all 100 passes the rates strength test how would you rate the 'Quality"? You can't say the 550# chain is better quality than the 500# chain as they have two different goals, just as you can't say a 1-ton truck is better quality than a 1/2 ton truck because the 1-ton can handle more load.

Your example of Beta vs VHS is a good example of the confusion possible. Beta was judged technically superior to VHS but Sony made a marketing decision to prevent other companies from using the Beta specification in their video players. The result was that VHS units were less expensive so many people bought on price. Eventually, Sony lost out. Not because the "Quality" of that video format wasn't as good as VHS but because the market was "price-sensitive" and too many people selected lower cost over better quality product.

Gene offered
"OK, color me confused! I have always gotten the impression that ST tires and not LT tires are what I should be running on my RV. After reading this article now I’m not sure. Are you suggesting that all things being equal, LT tires of the same size, load rating, etc are better to run on my RV? I don’t mind the cost, just want to put best possible tire on my RV. Thanks for keeping us informed."

to which I replied

Gene, The problem is that "all things are not equal" as there are no LT tires of the same size, load range, and load rating, etc as an ST tire. There is always something that is different.
Sometimes the only option would be to go with larger size LT tires but in some cases, there is no physical room to run the available larger size tires.

IMO, what you "should" be running are tires built to the latest industry test standards (LT) that can offer at least 15 to 25% "Reserve Load " capacity

Friday, March 6, 2020

Is it against federal regulations to change tires on an RV?

I have been following a series of posts on RV Forums where people ask about changing tires [Size, Type, Load Range or cold inflation setting]. Occasionally I run across some people who have adopted a hobby of commenting on tires in RV application and with a little working knowledge, make pronouncements on the "legality" of making any change in tires, that I do not agree with.

Other times I see a question like this one;
 "My research (curiosity vs need) is that LT and ST tires are not sold in the same sizes, so changing RIMS would be required?"

Here is my reply

Some LT tires show the same "dimensions" for example (235/75R15) as some ST tires but the "Dimensions" are NOT the actual complete size "description" which includes the letters and numbers before and after the dimensions. i.e. ST235/75R15 110/105L LR-C  vs  LT235/75R15 110/107T LR-D.  In this example, the ST tire is rated for 2,340#@ 50 psi while the LT tire needs 65 psi tp support 2,335# (single load capacity shown) Also the ST tire is rated for a max of 75 mph while the LT is rated for 118mph operational speed

Yes there are some ST tires where the dimensions do not match any tire identified as an LT type currently on sale. This is where you have to do some research and learn some facts on what can be considered a reasonable and safe change in tire type or size.

The Key items to confirm:
1. Is the Load Capacity of the new tire equal or greater than to original tire when you consider your new intended cold tire inflation? This does not mean +/- 25#. It means "equal or greater"
2. Is the Speed Rating equal or better than the original tire when you consider your intended new cold inflation level? (yes some tire load capacities are a function of speed)
3. Are your wheels rated for the inflation level you intend to run with your new tires? This limit may not be easy to learn but wheels can fail from too high a pressure just as they can fail from too many pounds load. The pressure we are talking about here is always the COLD inflation pressure.

You may need to be smarter than the tire guy if you want or have to make a change.

Remember there are some who post on various RV forums who would tell you that you would be violating Federal Regulations if you change from a Goodyear Marathon made in 2016 to any Goodyear tire made in 2019.

While it is definitely true that some people make changes in tire "dimensions" or "type" or inflation level which IMO as an actual tire design engineer, I would consider unwise. BUT that does not mean you can not consider making changes as long as you follow the guidelines posted above that follow the published guidelines from major tire companies.

Friday, February 28, 2020

YES You do need "Bolt-in" metal valve stems with your TPMS

Story on a Sprinter RV forum.
Reply to posts on TPMS
"My new tire pressure monitoring system just saved our just-completed trip to Florida. Thanks to you all I thought I had, and paid a lot, to Mercedes to replace my rubber valve stems and add metal valve extenders for my sensors. If you remember, I did see the front wheel valves Mercedes did NOT replace with steel. I did get BORG valve stems from, as suggested, for the front and my local repair shop installed them. I could not see into the rear dual's to check. Well, you were right, after about 2000 miles into our trip, I turned on the TMPS to check pressures. The warning came up right away on one of the rear tires, 7 PSI. Sure enough, after five hours just off of interstate 75 in Florida, Mercedes had roadside assistance change to the spare (still have warranty). I added some air to the flat tire and soaped the rubber valve to verify that's what caused the leak. It didn't seem like a rub through but I did see cracks (must be from vibration). We drove 160 miles to a tire repair shop in Georgia. They put in small rubber valve stems in the spare and all four rear tires because I couldn't trust the rubber valves. All TPM sensors were removed from the rear wheels. Lesson learned, TIRE PRESSURE MONITORING IS A MUST ALONG WITH STEEL VALVE STEMS. Rubber ones will crack and leak. I will order BORG steel valves for the rear wheels."

I did offer a comment that you need the TPMS on for it to provide a warning and I was a bit concerned about the comment on "turning on the TPMS" to check tire pressure.

I am posting this comment to point out what can happen when you use rubber valve stems and an external TPM sensor.


Friday, February 21, 2020

Why inflate your tires to their max, when parking for long time?

I had a question about what inflation to run when parking your RV or vehicles for a long time. I initially said that would lower the Interply Shear.

Then, I was asked
"Could you explain interply shear on a parked R/V?"

Any time a tire is deformed (loaded) the cured rubber in the belt area moves away from the "as cured" shape. Even if not rolling, the area that is now flat on the road has been "bent" from the as-cured original curved state.
This bending causes shear (tearing) forces between the belts, which, at the molecular level, can result in bonds breaking between the carbon, hydrogen and sulfur atoms. If the bending is sufficiently large, the tear gets larger.
If you have higher inflation in the tire, the bending is less than when you have lower inflation which takes us back to decreasing the molecular tearing.
Other things happen too. "Cold Flat spotting" where a portion of the tread ends up flatter than the portion not loaded. This difference can result in vibration and shaking once you start driving. The "flat spot" also has to "work itself out" when you start driving. Again the change in shape when driving is rapid which can result in those broken molecular bonds. Slower changes in shape as when you inflate a tire allow the rubber to move or even "flow" a bit so the atoms have time to re-arrange.

If you ever played with "Silly Putty" as a kid you have experienced this effect. When you slowly pulled the putty it would stretch but if you yanked it quickly it would break. Silly Putty is a form of synthetic butadiene rubber which is very similar to the synthetic rubber used in tires

The whole concept of getting a longer tire life and better belt durability is to decreases these shear (tearing) forces that come about because of the changes in the shape of the belt package.
Now I don't expect people to inflate your tires every night or even every time you go camping for a few days. But if you are parking for a month or more over the Winter, it might be worth the effort. Only you know how much effort it would take to adjust your tire pressure and if you think a few extra weeks or months of tire life is worth the effort.


Friday, February 14, 2020

Heat, High Speed and the "Magic" in ST tires.

Heat generation is primarily the result of a combination of High Speed, High Load, and Low Inflation. Factors that can counter some of the negative effects of heat on tires can include the use of more expensive rubber compounds, reduced thickness tread and other construction features that are out of our control as tire owners. Without access to secret formulas and other proprietary information, I know of no way to learn which of these features is in which tires if any. There is one exception, of the inclusion of full Nylon Cap Ply. This would be identified in the material list molded on the tire sidewall along with the number and material in both the sidewall and under the center of the tire tread.

However, the driver] has complete control over the Speed/Load/Inflation factors.

IMO it would be better if all tires had at least a 15% margin on load capacity, with 20 to 25% being better margin. We need to remember that road slant & crown and curves along with significant sideload from wind can easily result in overloading tires that were measured on a level scale. Multi-axle trailers should shoot for a 25% load margin. (read about Interply Shear to understand why )

Ever wonder why RV trailers seem to have a lot of tire problems but your daily driver (car or SUV) doesn't?
Let's look at Reserve Load Capacity of your car:
Many / most have 30% to 40% Reserve Capacity on the tires.


Now when we look at an RV trailer we see only 6% with many having 0% margin.

 If we assume we want to have 25% Reserve load capacity on the above RV, then 13,290/(Tire max load X 4)= 0.75
and each tire max load shall be 4,430 lb.    So the right choice would be an all-steel
ST235/85R16 132/127 (14 with a max load of 4,410 lb.)

Previously I indicated that Inter Ply Shear can be the equivalent of adding 24% more load on the tire while in operation. Using the above tire the IPS effect is reduced considerably.

NOTE in 2017 RVIA and the National Fire Protection Association, guidelines for tire selection for RVs was updated to require a MINIMUM of a 10% margin in tire load capacity over the GAWR. Some RV companies achieved this margin by upgrading their tire load capacity while others simply lowered the GAWR number on the vehicle certification label. Do you know what your RV company did?


Speed rating. We should think of this like the engine red-line. Everyone seems to understand that running faster than red line will shorten engine life but that doesn't mean that 5,900 on a 6,000 redline is good to run for hours on end and may contribute to shorter engine life just as running 62 mph on tires rated, based on their original load calculations, for a max of 65 mph.
The high-speed test is a 30-minute step speed test of a brand new tire on a smooth test wheel. No potholes or curb damage. All that is needed to "pass" the test is to meet the target speed and not come apart. The tire is considered scrap after the test.  Would you consider your tires to be "scrap" after you run 70 or 80 mph for an hour or so?
Remember damage (internal cracks) done, even at the molecular level, in tires never repairs itself but only increases in size.

While this post is primarily about Trailer application the limit on high speed also applies to 16 through 22.5" tires that have a max rating of 75 in RV application which would be their "Red Line" but I hear many Class-A owners talking about running over 70 for miles on end but being surprised when they suffer a tire failure.

LT tires would be my first choice in any trailer application where heavy loading was required. I do not understand why so many think that there is some magic engineering used on ST tires that allow them to support 10% to 20% more load than an LT tire of identically dimensions. The load formulas used today for ST type are the same ones used in 1969 when they were limited to 65 MAX (red-line).

I would be very interested to hear an answer from the engineers at the companies that make ST type tires why they were able to suddenly, almost overnight, increase the speed rating from 65 to 75, 87 or even 99 on their ST type tires. Do they have some new materials? Why don't they put those same super materials in LT tires and increase the load capacity of LT tires to be the same as or near to the loads seen claimed for ST type tires?

Sorry, I just don't buy the new claimed high-speed capabilities of ST type tires that exceed the load and speed capabilities of equal size LT tires.


Friday, February 7, 2020

Air Compressor - How Big do you need?

While air volume output might be a consideration, IMO if you are properly inflating your tires and properly (TPMS) monitoring inflation I don't understand how anyone can get in a position of needing more than about 5 psi unless you have an active leak.

If you are only adding 5 psi then tou do not need to worry about how long it would take to inflate a 275/80R22.5   All you need is a compressor rated for more psi than what your tire needs. I would think that a 150 psi rating for the compressor fro tires needing 125 psi   or 100 psi rating for 80 psi tires would be enough.

As I have covered a number of times in this blog, I recommend your cold inflation pressure to be at least 10% above the minimum needed to support your actual measured tire loading (Minimum inflation would be based on the heaviest loaded tire on any axle or lacking individual tire loading numbers,  then using an assumed 53/47% side to side split for motorhomes and trailers with big slides or residential refrigerators and at least 51/49% load split for smaller trailers)

So assuming you have LR-C or LR-D tires you would be inflating to 50 or 65 psi with your TPMS warning set to no lower than 49 or 64 with your minimum inflation in the load tables being 45  and 58. So how would you ever need to add more than 5 or 6 psi assuming you let your tires get that low? Why not do your "top-off" as soon as you need 3 psi? Now you do need to consider the 1 or 2 psi difference between your calibrated hand gauge and the TPMS reading. I set the warning based on the TPMS reading AFTER setting the tire using my certified hand gauge.

Yes pressure changes with temperature (about 2% for change of 10°F Temperature)  A change in morning temperature of 40F from day to day is unusual and that would only result in a pressure drop of 5 psi on your LR-D tires.

Motorhomes should be running a +10% margin on air pressure based on the measured tire loading which means there would need to be a 50°F drop in temperature for them to need to add 10 psi (assuming a 100 psi minimum).

If you need to add more than 20% (20psi) of the needed pressure in your tires with steel body ply, that means you have technically been operating on a "flat" tire according to tire industry standards and you should have a professional inspection and have them re-inflate your tires AFTER the reason for the sir loss was identified and repaired. Large 19.5 and 22.5 tires should only be re-inflated in a cage just in case there was damage to the steel body cords which can lead to an explosion due to zipper rupture.

LR-E (80 psi polyester body tires) as found on most Class-C and some larger trailers need to consider the above information and adjust for their higher cold inflation numbers. I would consider a 20% drop to put you in the safety cage re-inflation level if you drove on the tires when that low. While they are not likely to suffer a true "zipper" failure from fatigued steel body cords, there can still be internal structural damage to your tires.

Bottom Line:  Monitor your tire pressure and don't let the pressure drop more than 10% before you re-inflate your tires. Know why the pressure dropped and if not due to a drastic change in temperature overnight, inspect for leaks. I find that spray cleaner like Windex or other cleaners tend to foam at the location of the leak.


Friday, January 31, 2020

RV Tire warranty. How do engineers "read" tire conditions?

Read the following on an RV forum
"Carlisle Tire's warranty used to state "tires must be inflated to sidewall stated pressure or warranty is void"; I don't know about now though.
How in the world do they determine if the tires are inflated to sidewall pressure? Is there a little gremlin with a report card in each tire. Some manufactures just never get out of the box.
Unless it was a bad tire, had been run flat previously, and most likely a bunch of other possibilities.  At the time of a blow out there is no way they would know other than by assumption.
Yes, most probably operator error, but only assumptions."

Sorry, but there are ways to identify the probable inflation & load history of a tire. Just as a Medical Examiner can do an autopsy and identify the signs of bad diet and poor or no exercise, or years of smoking, it is many times possible to see the physical signs of low inflation and high load.

These signs can show up in the indentation into the tire left by the wheel.

Here are examples of improper inflation or a Run Low Flex Failure of a P type, ST or LT type tire.

Melted body cord is physical proof of extreme run low 

Also, the different flex markings can be seen on the interior of a tire. In extreme cases the Innerliner (special rubber that holds air in much like a tube did in tube-type tires)

Manufacturing "Defects" will usually result in early life failure i.e. <1,000 miles.  Tire failure is in itself not proof of some nebulous "defect" even though lawyers and those not experienced in failed tire inspection want to think so.
Once you examine, in detail, a few thousand tires from both controlled testing and from day to day use & abuse the conditions seen in tires tell a story of the tire's history.
Too often people simply think of the conditions (load, speed, inflation, road) at the moment the tire fails as the "facts" to be considered when trying to decide the "why" a tire failed. In reality, the damage might have been done hours, days or even months earlier.
See THIS post on a study of pot hole damage and how long it took for some tires to fail from the impact.

As I point out in my "RV Tire Knowledge" Seminars at RV Conventions, tires are like potato salad, putting the salad back in the refrigerator after it was left for hours in the hot sun does not "fix it" and make it good to eat the next day any more than taking the burnt hot dog off the grill and letting it cool down before serving it makes for a good meal. Adding the correct air in a tire after running it low for thousands of miles, does not repair the damage. Slowing down to 50mph after hours of speeds of 70 to 80+ over the preceding weeks and months does not "fix"  or heal the thousands of microscopic cracks that were formed in the overheated and overstressed belt rubber. Once a crack is initiated it does nothing but grow. If a person stopped his smoking addiction of 2 packs a day for 40 years a couple months ago, will his lungs be clean and clear today? Not a chance.

When examining a tire, I look at the physical condition of a tire and specifically what evidence there might be. Years of experience has taught me what to look for and allows me, as a court-certified "Expert", to form an opinion that is based on the examination of many thousand tires.


Friday, January 24, 2020

I bleed off the high air pressure so I don't exceed the "Max Pressure"

The above statement or a variation of it seems to be made about once a week on one or another of the various RV forums I frequently review. It appears that many people incorrectly believe that exceeding the 'Max Pressure" number molded on the tire sidewall is going to result in tire failure and explosion.
It may help to let people know about some of the testings that tires undergo during the development phase.

First some background. The "Max Inflation" statement on your tire sidewall really is the inflation associated with the Maximum Load capacity. If you actually read your tire sidewall you will see that is what it says. So the number of PSI is really the minimum needed to support the stated load. It is important to understand that we are always talking about the "cold" inflation and not the inflation of a tire that is running down the highway or has been in direct sunlight or driven on in the prior couple of hours. "Cold" inflation does not mean the tire needs to be refrigerated but it means the tire is at the prevailing Ambient temperature.

With the introduction of aftermarket TPMS that report both tire pressure and the temperature of the sensor, people are now being exposed to numbers, they have no experience with. They see tire pressure rising as they drive down the highway. Some may see a 10% rise in pressure from the pressure they set their tires to just a half-hour earlier. Others may see a 20% or even a 25% increase in pressure and for some, this increase was a cause for concern. Not because they had any working knowledge of tire operating temperature or pressure but simply because the pressure was higher than they expected. A few people have decided that they need to bleed down the high pressure because they thought the pressure number of the tire sidewall was the max it should ever see.  Of course, the action of bleeding down the hot pressure was exactly the wrong thing to do as that meant that the tire would no longer be operating at the pressures expected by the tire engineer when they originally designed and tested the tire specification.

Obviously, this raises the question of how much pressure a tire can tolerate before the owner should be concerned. Well, I am going to give you some numbers but these are only examples. I cannot speak for every tire companies process of specifications but if we start with a few guidelines I think you can get comfortable and hopefully, you will believe that tire design engineers do have a reasonable idea of what they are doing.

First off we are only going to discuss regular production street tires that have passed DOT testing. The numbers I will use would basically be seen on new tires. I cannot advise on a tire's capability after it has been damaged or run for many thousands of miles. Damaged tires can fail when re-inflated as seen in THIS video. Heavy truck multi-piece rims MUST be inflated in a safety cage. If you suffered a puncture and drove any distance with the tire underinflated you may have permanently damaged the body cords which is why you should always tell the tire service person you drove on the under-inflated tire so they know to use a cage or other restraining safety equipment. Age and miles can reduce the strength of any tire but we do consider this degradation when approving a design. Another consideration is the availability of high-pressure air. Most home compressors or those that supply air to hoses made available to consumers have an upper limit of about 150 psi so we don't expect any consumer to have access to or to use industrial air pressure which can be 400 psi or even special inflation units as used on aircraft tires.

Basically many tires are tested to 300 to 400% of their rated inflation or above 200 psi whichever is lower. I had a number of tires that could contain over 300 psi when new but if they had been damaged the same tire might fail at less than 200psi but I also never, in my 40 years saw a tire fail from normal pressure increase and I was involved in the testing and evaluation of thousands of tires.

The BOTTOM LINE is Please do not bleed down your hot inflation pressure. You should ONLY set and adjust inflation when the tire is cool and at Ambient temperature. Doing otherwise MAY result in you having a failure days or weeks later because you were running lower pressure than what was needed for the tire load you were applying.

Friday, January 17, 2020

LT vs ST for RV trailers?

It's no longer 1970 with a National 55 mph speed limit with ST tires being introduced i.e. "pushed" by a large tire co, as an an alternative to the real LT truck tires of the day. I have been told that one of the "selling points" for this then-new type of tire was that "no one would ever pull their 15' trailer faster than 55 with their bumper hitch."
Well, times have changed. We now have '1-Ton" Diesel Pickups that can pull a 35' trailer up the side of a mountain and run 75+ all day long. Trailers now come with two bathrooms, residential refrigerators, and two AC units multiple TVs and other heavy equipment that we never considered possible in 1975.
What hasn't changed is the fundamental science that a tire's load capacity is still basically   "Load = Air Volume x Air Pressure".  In fact, the actual load formula still used for current ST type tires is identical to the one used in 1970 when the 65 mph speed limit for ST type tires was in the industry standards. I can find no mention of alternate materials delivering increased load capacity in those standards. Yes materials have improved and radials are better than bias tires but basically the only benefits all these "improvements" delivered is longer life and better fuel economy and tread wear. Some construction features such as the addition of Nylon cap strips or full cap ply have allowed an increased resistance to the heat from higher speeds but I haven't seen any increase in load capacity in either Passenger or LT type tires over the past 40+ years.
As I have posted in this blog, "There is no free lunch". If there is, why haven't tire companies increased the load capacity of P and LT type tires if all these "improvements" are available to tire engineers?
What "feature" is in ST type tires that gives then the +10% to +20% more load capacity over an LT tire of the same physical size?
IMO there is no reason why RVs could not be supplied with LT tires other than it would increase the cost of the tires to the RV company.
Remember it is the RV Co that is responsible for selecting the tires they provide. We all see numerous posts from some who are running truck 17.5 size or LT 16" tires with improved durability so I see no reason to believe that the entire market could not benefit from a switch from ST to LT type tires.
One thing is that the LT tires that have to meet the new DOT standards (FMVSS 139) that were introduced in 2002 are probably more durable than the ST tires that are still only required to meet the standards on the 1970's.

I am not saying that some of the newer ST tires with  newer construction are not significantly better than the same size tire from the 90's but IMO there is still a limit.

Friday, January 10, 2020

"I never hit a pothole"

The title for this post is a direct quote made by many people who have suffered some tire failure. If you think about this claim for a moment and then think about the road conditions we all see in our day to day driving experience one has to wonder just where these people are driving.

In our life as drivers, I am sure that all of us have hit some objects other than simple 1" deep pot hole. Some objects could include bricks, lumber, railroad tracks, curbs, and puncturing or cutting objects such as nails or screws.

Here are some examples of tire damage I discovered in my work. The first three were each submitted with a written claim that the tire was "defective" so I can only assume the driver made little or no effort to inspect the tire or wheel for evidence of what might really have happened.
The last tire, with the wooden stake through the tread was submitted with a claim that the tire was defective because it was "making noise". IMO the noise would have been from the wood hitting the pavement at speed.

I have no expectation of changing the minds of those who want to make a claim of never hitting a curb or pothole or another object. I do want to relate the findings I recently discovered in a technical paper on tire forensics and impact damage.

In the paper, the reference studies involved a process of obtaining a couple dozen both new and used tires P type, LT type and sizes appropriate on Some Class-A RVs. The rim diameters ranged from 14" to 19.5". Tires came from 8 different tire companies. Each tire was inspected and no externally visible signs of damage were found.  Each tire was run on the appropriate DOT laboratory tests to confirm the tire was in good condition. Each tire was then mounted and hit with a heavy pendulum to simulate hitting some object or pothole on the road using a machine like this from STL company

 The tires were again visually inspected and any evidence recorded. Finally, every tire was run on a drum durability test under a load until failure occurred.

Failure Rate  100%
Miles to failure for new tires   1,826 mi  to  41,400 (avg. 11,586)
Miles to failure for used tires  5  mi      to    7,458   (avg. 2,092)

Can anyone here list every object they knowingly or unknowingly hit over the past 2,000 miles?

I know that I have posted a number of times that tires do not always fail at the instant of impact or damage. Clearly, these above studies confirm these observations.

Some reference material for those interested:
Price, V & Follen,G (2019). Road Hazard Impacts: Their Influence on Radial Passenger Tires and the Forensic Signs They Leave Behind
Bolden, G. C., Smith, J. M., & Flood, T. R. (2001). Impact Simulations in the Lab. Tire Technology International.
Bolden, G. C., Smith, J. M., & Flood, T. R. (2005). Impact simulations - what happens when a tire/wheel impacts a road hazard. Tire Technology International, 44.
Bolden, G. C., Smith, J. M., & Flood, T. R. (2006). Structural Impact Damage Under Varying Laboratory Conditions. Tire Technology International, 10.
Gent, A. N., & Walter, J. D. (2005). The Pneumatic Tire. Washington: National Highway Traffic Safety Administration, U.S. Department of Transportation.
Giapponi, T. R. (2008). Tire Forensic Investiagation Analyzing Tire Failure. Warrendale: SAE International.
McClain, C. P., & DiTallo, M. A. (2001). Tire Examination After Motor Vehicle Collisions. In K. Baker, Traffic Accident Collision Investigation (9 ed.). Evanston, Il: Northwestern University Center for Public Safety.
Tire Industry Association. (2005). Passenger & Light Truck Tire Conditions Manual.


Friday, January 3, 2020

Are your tires "Defective"? (updated post from Nov 2018)

People making the claim that their tires were somehow "Defective" is an all too often occurrence on various RV forums. I even hear this complaint at my RV tire seminars. I do note that when there are tire problems, including actual "failure" it's common for some to say "My xxx brand tires failed, I will never buy xxx tires again".

It's important that we have a mutual understanding of what is meant by tire "failure". For some, this means the tire simply came apart. The term "failure" could also be applied to a snow tire that didn't provide enough traction to prevent getting stuck on slick ice. To a driver of a race car, it might mean that the cornering on one specification tire wasn't as good as with a different specification. It could even mean the dry slick tread pattern failed to provide enough traction when it started to rain during the race.

Well sorry to tell you but there is no such thing as "Fail-Proof" tire. This was even said, under oath by DOT spokesperson during the Ford Explorer rollover fiasco of 2000.

Even the new "run-flat" tires available on some expensive cars can "fail" if driven too fast or too far when "flat".

Today's tires are amazingly robust. Even when they are made in some country other than the US. I am sure that many of us remember how bad "Made In Japan" was considered when we were growing up, but just think of the quality perception is today of cars made or designed in Japan.  As I pointed out in this blog previously it is important that the tires you are using are appropriate for your ACTUAL loads and usage. If you have a heavy trailer application and both the tire type and the size was wrong and the tread pattern was wrong. for the application. Why would this be the tire's fault?

If you put a truck tire with a heavy off-road mud traction tread design on the front of your 40' DP and had loud noise and vibration and the harsh ride would that be the tire's responsibility?
Would simply changing tire brands from say Bridgestone to Michelin solve the problems? No of course not.
From my experiences as a tire engineer, I can tell you that I can probably "fail" any tire in under an hour and under 50 miles if you let me set the conditions.

A tire is just a tool you use to get a job done. If you don't select the correct tool that is appropriate for the job you want to be done why is it the fault of the tool manufacturer? Think of the absolute best tool company. Now select one of their flat blade screwdrivers. OK now start using it as a chisel and pound on it as you try and cut through some rusty bolts. After cutting through a few bolts would you blame SK or MAC or  Snap-On or ???? if the point of the screwdriver is dented and chipped?

All too often tire selection seems to consider price as the number one concern. This observation is certainly supported by almost every post that asks for tire suggestions from RV owners. Sometimes it seems as if the price might also be the only consideration for some RV assemblers. Now I would be the last to offer that simply having a higher price doesn't automatically make any given tire "better" and I am not suggesting that price should not be considered.

For some, any tire "failure" is considered proof that it was somehow "defective" even is the tire was

We need to smarter consumers when it comes to tires if we want to avoid ending up with tires we consider "Defective". To me, a "defect" would mean there is an identifiable condition, in the tire when new, that would prevent the tire from performing as expected for the stated life of the tire "when applied and used properly for it's intended purpose".
This means we should not expect a tire to run overloaded or underinflated or at excessive speed for years at a time.

The various RV owners' manuals I have seen, include warnings and advice on load, inflation speed, and tire life. Have you read that information? Do you follow the recommendations? If you don't and the tire "failed" do you accept responsibility or do you simply claim the tire was somehow "defective"?