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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"?