Tire Crack or ?

Saw a post from an RV owner who discovered something on the upper sidewall of a tire when doing a visual inspection.

 He was calling this a "crack" and was looking for input from readers of that RV forum.

A number of readers chimed in with opinions ranging from "Time for NEW SHOES"  and " That one is ready to throw some chunks."  to a discussion on what was an acceptable spray tire protectant.

The good news was that many recognised this damage as "curbing" that can occur when trying to go over a curb at a low angle.

I have outlined the area I looked at which extends well past to obvious deeper damage. Especially off to the right side where the tire just shows signs of surface scrubbing.

If I could physically inspect this tire I would take a close look at the right end (arrow) and confirm the suspect surface abrasion which would confirm no problem internal to the rubber.











Not all tire damage is from a tire "defect".

The owner did the right thing and had the tire inspected by a Michelin truck tire dealer who confirmed no defect but just curbing.

How to avoid tire failure and RV damage. And I had a tire failure!

A general comment on tire inspection and how it may be able to prevent RV damage due to tire failure if done properly.

There are three basic types of tire failures.

- The least likely is a sudden large impact with some object in the road. This is the least likely and is actually difficult to do. By this I mean to have a tire that is OK with no damage having been done due to excess heat, high load, low inflation or improper repair. It could even be a new tire. You drive over something like a Railroad spike or into a foot deep pot hole or over a 10" chunk of scrap steel that fell off a truck. It has to be large and you have to hit is just right for the tire to suffer immediate failure. I know from personal experience (doing a special test project of tire "Rapid Air Loss") that even driving over a piece of 2" pipe sharpened at one end and standing 2" up in the road with sharp end up doesn't always cut through the steel belts of a tire.  Yes you might run over something that cuts the tire but you would see it in the road and hear it hit but most of the time the air loss is not immediate. The good news about this type of failure is that it has a very low probability of happening. I would guess that fewer the 5% and maybe less than 2% of RV tire failures are of this type.

- Next is the belt separation. This is when the tire tread and belts come off the body of the tire. This is usually the result of tire aging and long term cumulative heat related damage that reduces the flexibility of the rubber to the point that rather than bending the rubber develops microscopic crack which do not heal themselves but will grow. Excess heat and tire aging can come from many sources. Even parking in direct sunlight with "tire protectant" spray does not lower the temperature of the tire. Excess heat can accelerate the aging or the tire rubber properties and drastically reduce the tire life. I would expect that if properly diagnosed this type of failure occurs 25 to 40% of the time. The good news is that with proper and frequent tire inspection this can be discovered and the tire replaced before it comes apart enough to cause damage to the RV. I did a blog post just on this topic "How do I inspect my tires" back in Aug 12 2014. You can Google the phrase and find a number of web pages on the topic but many simply are telling you to look at tread depth but this is not sufficient if you want to do a complete and competent inspection. My link included a YouTube video showing the inspection of a tire with belt detachment that has not come apart and the result of the "tire autopsy" I was able to perform. You can even see the separation between the belts in the above post.

- Finally there is what is commonly, and incorrectly, called a "Blowout". This is really a failure of the tire sidewall due to excessive flexing from running with significantly under-inflated ( probably below 50% of the inflation needed to carry the load. For Polyester tires (mainly ST and LT type) This heat due to flexing can be enough to reduce the strength by half and in extreme cases even melt the cord. For Steel body tires the bending of the steel can result in a fatigue failure similar to bending a steel paper clip till it breaks. This type of failure may be 60 to 80% of the failures on RVs. The good news is that if you run a TPMS you will get a warning of the air leak and hopefully you will not ignore the warning as too many do with other warning indicators on their dash, and take appropriate action which is to stop and pull over as soon as safely possible. Amazingly some people, even when verbally warned that they have a tire that is significantly under-inflated simply choose to continue to drive off. This has happened to me a number of times. As the saying goes "You can lead a horse to water but you can't make them drink".

So how doe this information help you "avoid" failure and RV damage. While nothing is 100% I bet you would like to be able to prevent 90% to maybe 98% of tire failures you might experience.

To do this I suggest that you run a TPMS and to get plenty of advance warning I suggest you set the warning pressure to be the minimum needed to support the load. Then your cold inflation pressure would be about 15% higher.

Next do a "free spin" inspection with lots of light. This is relatively easy with a trailer but harder to do as your axle load increases. You may even need the HD jack from a truck tire center and the spin balance machine to get that big 295/75R22.5 tire spinning at at least 20 to 30 RPM to allow you to see if there is "wobble" similar to the video in my post on How to I Inspect my tires" above. While the tire is in the air also do a slow rotation to inspect 360° of the tire tread as well as 360° of each sidewall looking for cuts and bulges. If any are found you should get the tire to a service center for a more thorough inspection that may include de-mounting the tire.

I believe that if you don't see any bulges or wobble and don't have any localized irregular wear spots in the tread you can be reasonable confident there is no large separation.

Remember nothing is 100% but if you make an effort you can significantly improve your odds of avoiding a tire failure.

Information to show you how this can work. Three weeks ago I was doing my annual 360° inspection. My tires are 7 years old so even though I know they have always been properly inflated (4 corner weights + TPMS from 2nd week of operation), I knew I needed to be sure all was OK. I discovered one of my duals had developed stress cracking from long term parking. This was a real surprise for me but it shows that even with the best of care rubber can get old and tires do need to be replaced.

  This tire was scrapped (I cut two slices from bead to tread to prevent its re-use by a "dumpster diver") as it still had lots of tread but I felt it should not be on the road.

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When is Minimum inflation the Maximum inflation?

Sometimes the information on inflating tires can lead to some confusion.  OK a lot of the time it can be confusing to many.

As many of you know I try and follow a number of RV forums and offer comments. I try and focus the comments to correcting serious or significant errors or misunderstandings, especially when Safety Related.

Recently there was some confusion about the Maximum allowable inflation for a tire. Some wanted to co-mingle some information from PSR or passenger car tires with information about large TBR, truck bus radials. The discussion then went way off track. Rather than limit my audience to those following that thread, I decided a blog post would be more appropriate. Also I wanted to be sure to have all the information up to date and accurate I contacted an "old friend" from the tire industry and he sent me this nice summary. With his permission I re-print it here.

First some definition of terms may be appropriate as "tire engineer speak" may confuse some.

"Seating pressure" this is the inflation needed for the tire beads to "pop" home against the wheel/

"FMVSS" these are various Federal Motor Vehicle Safety Standards. If you need some reading that will put you to sleep HERE is a link offered by NHTSA.

 "NHTSA" NATIONAL HIGHWAY TRAFFIC SAFETY ADMINISTRATION

"PSR"  Passenger steel belted radials or the normal tire you find on todays cars

"TBR" Truck Bus Radial  as found on most Class-A RVs

"LTR"  Light Truck Radials 

"CIP" Cold Inflation Pressure

OK here is the nitty-gritty

"First, the max pressure to seat beads as a matter regarding the technician’s safety is 40 psi, whether passenger, light truck, or truck-bus.  This bead seating pressure is totally independent of the tire maximum operating pressure.

It is important to clarify the differences in sidewall markings of the tires you bring up:

PSR & LTR Tires (load range E or less):  Since FMVSS 139, these tires have sidewall markings indicating maximum load AND maximum pressure.  Maximum load means max [static] load, and maximum pressure means max [operating] pressure (cold).  With respect to the minimum pressure that carries the maximum load there is a difference:

·         Passenger Tires:  These tires are usually marked with a maximum pressure that exceeds the pressure necessary to carry the maximum load marked on the sidewall.  For example, the tire may be marked with 44 psi max pressure, but only requires 35 psi to carry the max load.

·         Light Truck Tires (load range E or less):  These tires are usually marked with a maximum pressure that is also the pressure required to carry the maximum load.  For example, a load range E tire marked with max 80 psi would need that same pressure to carry the max load.

TBR Tires (and light truck load range F and higher):  On these tires the sidewall markings indicate maximum load AT a certain pressure (the word “maximum” is not used in regards to pressure).  Maximum load means max [static] load, but the pressure is not the maximum operating pressure (cold).  This marking just follows FMVSS 119.  Essentially, the pressure marking is informative, simply telling the reader the pressure that is required to carry the maximum rated load.

However, for most practical purposes, on TBR tires the pressure marking is typically considered the maximum pressure recommended in the tire while in ordinary service.  Certain situations may permit cold inflation pressure higher than the marking, usually in consultation with a tire manufacturer for a specific product, application, and service.

Regarding load-inflation tables:  As long as you are looking at the right table, this is where you find the pressures needed to carry certain loads for a given tire type, size, load rating, etc.  Note that for truck-bus, you might need to make sure the tire is a “T&RA tire” or an “ETRTO tire” since the tables can differ, even though the size codes are the same (such as 295/75R22.5).  Also, a tire manufacturer may have unique load-inflation table(s) associated with certain tire models, sizes, etc.

If an operator is running at max load, and the pressure to do that happens to be max pressure, then yes, they need to be diligent.  But it is manageable, and they owe it to themselves and to others on the road to do so.  Pressure loss through permeation requires minimal adjustment approximately once a month.  For predictable swings in temp, set the pressure when it is likely the coldest, and try to consistently check it during those times, such as early in the morning before setting out.  No one is saying that everyone in all circumstances needs to set pressure to +/- 0.1 psi every morning, noon, and night with three hours of ambient cold-soaking before taking measurements.  I agree with you about reducing load; not just due to the tire influence, but also drivetrain/axle and chassis wear and tear, braking performance, fuel economy, etc."

So we see there are similar but different words on the sidewall of tires. Some have a stated Max cold inflation others do not. This is one reason why it best to have tire service done at a store that has the appropriate equipment and training to handle safe and proper mounting and inflation of the type tires you are working with. This does not mean you can't add 5 or even 10 psi to your tires but IMO if you need more than 20% of the CIP there is something wrong and you really need to consider having a professional inspect and re-inflate your tires. Inflating an improperly mounted, improperly repaired or damaged tire can injure or even result in death if not handled properly.
OK now back to our regular programming.
One other comment I have is that many times some think tire failure "Blowout" is caused by too high a pressure but this is essentially incorrect. Unless you have damaged the body ply cords, be they Nylon, Polyester, Steel or Rayon by over flexing and running significantly under-inflated, tires are designed to tolerate the normal pressure increase seen when running highway speeds at the approved load.

But if you have damaged or run the tires in overload or under-inflated for the actual load or perhaps at a speed higher than the tire rating you may have damaged the cord sufficiently that it has lost a portion of its strength so in that case even normal cold inflation may be too high. This is one reason any tire that has been damaged must be rendered un-usable or if it appears to be OK then inflated in a Safety Cage.


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Tire Bulge. Defect? or Damage

Sidewall bulges can be difficult to diagnose. Sometimes it is even possible to misinterpret a depression as a bulge so lets start off today's lesson with clarifying the words.

According to Dictionary.com  bulge means a "rounded projection, bend, or protruding part; protuberance; hump".  I think we can all agree that this is a sidewall bulge.





A depression would be the opposite, or "sunken place or part; an area lower than the surrounding surface". Sometimes I may use the terms "Bulge in" and "bulge out"just to be sure people have a clear understanding.
Look closely at this shot and I think you can see that this is a depression in the sidewall.

 Here are a couple of other shots showing sidewall depressions.






OK So now you are probably asking why are these two conditions in tires and are they defects or what? Lets step back for a moment and consider how tires are made.







The basics apply to all tires, be they small 10" or 12" as seen on micro cars or 22.5 or even large mining tires like this one.






In this post there are some links with videos showing the basic process of wrapping layers of fabric (or sometimes steel cord) that is in a sheet of rubber, around a drum. The place where the builder starts and stops has a "splice". Now the goal is to have a strong enough joint to keep the uncured rubber together till the tire is cured. In some constructions this means a small overlap of one to maybe 4 cords. If the overlap is larger than desired there is a doubling-up of the cord and this is what creates the depression. I know this is counter-intuitive but you need to remember that when a tire is inflated the rubber stretches and the textile cords stretch a slight bit. However if the splice is "heavy" or larger than desired the forces from inflation are resisted by twice the normal amount of cord and rubber so the stretch is less than in the rest of the tire. There is nothing wrong here other than a visual depression.

Now a bulge is just the opposite. If the splice is "open" or there are cords missing then that area will stretch out more as there is only sidewall rubber resisting the air pressure so the sidewall stretches out just like a balloon. A bulge from an open splice is noticeable as soon as the tire is inflated. If you see this on a new tire point it out to the tire dealer right away and confirm the bulge is below the level of concern for that make of tire. This will probably be less than a tenth inch above the surface of the rest of the tire and less than 1/2" wide. If larger I would request a different tire unless the dealer is willing to put in writing that the tire is safe.  Get a nice close picture of the tire for your records and be sure the bulge does not get any larger.

The other thing that can cause a bulge is broken body cord from some sort of impact such as a curb, pothole or from hitting something on the road. Here are a couple shots of a 22.5 that suffered an impact.

.One thing to point out how I know this was not a factory defect. I have yellow arrows pointing to the small amount to irregular tread wear. You will note that this level of wear is fairly uniform around the tire. If the defect had been in the tire from when new I would expect the sidewall bulge to affect the tread wear. Since it didn't, that indicates to me the break of the body cord is recent.

In case you are wondering what broken cord might look like here are a couple shots of broken Polyester cord from a smaller tire.

.I hope everyone now understands the difference between a cosmetic depression and a bulge due to a tire impact.


If you have a Bulge that looks anything like the examples above I would not drive on the tire. If we are talking about a high pressure tire (75 psi or higher) I would not even stand near the tire while waiting for service. A tire explosion can be damaging or even injure people.



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Why are "Blowouts" of special concern to RV owners.

Here is a video that points out a problem unique to RVs when we are discussing what happens if a tire fails catistrophically.


and this picture shows what might happen to your electric wiring.

I have made a number of posts on Tire Failure and How to lower the chance of failure, but none of the actions you take can completely prevent a tire failure with 100% certainty.

Since RV manufacturers don't have to design or build their product to pass crash or rollover tests as cars do, I have not seen what I would consider a real effort on the part of RV MFG to place electric, fuel or propane lines behind shielding that would protect these potentially dangerous features in the event of an all too often tire failure.

So it is up to you, the RV owner, to take all the steps you reasonably can to lower and hopefully avoid the chance of tire failure so you don't have to worry about damaging these other critical components.

1. Know the actual load on each tire. Axles are seldom balanced 50/50 and it is not unusual for the heavy loaded tire to have hundreds of pounds or even a thousand pounds more load than the more lightly loaded tire.

2. Ensure you have more air pressure than needed to carry the load (I suggest a 15% margin minimum)

3. Always use an accurate digital pressure gauge when setting tire pressure

4. Use a TPMS to provide as much warning as possible of an impending tire "Blowout" due to air loss so you can slow down and stop before damage is done to the RV.

5. When inspecting your tires do not simply look at tread wear or sidewall cracking. Look for bulges and out of round conditions or locations of excessive wear that may be indications of potential tire damage that could lead to a tire failure.

6. At least once a year do or have done a "Free-Spin" inspection to help find structural problems with the tire. If you don't have a jack and the proper equipment, have this done at your tire dealer. Tell them you are looking for a separation that would show up as an "out of round or lateral run-out condition." This is much easier on trailers than Motorhomes but it seems that trailers have a higher probability of having this type of failure.

Here is an example of a trailer tire that looks OK until it is rotated. 

You can see the wheel is OK but the tire is clearly no longer round.

I had an opportunity to do an autopsy on this out of round tire and confirmed that the belts had separated. You can see the separation in this picture below, that shows the belts are no longer attached to each other.

Clearly this is a complete separation waiting to happen but just looking at the tire as it sits on the ground will not make this problem noticeable.

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Why do tires fail?

Some comments on the general question of tire failures and why they seem to be so prevalent on RVs.

Previously I posted on how tires may be selected by the RV manufacturer. Lowest cost with the least margin of safety allowed by law seems to be the general trend for many manufacturers. We also know that well over half of all RVs that even bother to get weighed have one or more tires overloaded. These facts seem to be something that many want to ignore. Many blame the geographic location i.e. China where the tires were manufactured as the cause of the failures. Some think that the tire mfg intentionally makes poor quality tires. The fact is that tires do not fail for no reason at all. They do not fail because the factory was painted green instead of blue. They fail because their finite life is used up or they suffer some external damage or puncture which lets the air out. Punctures are not always obvious as this one with a 6" piece of aluminum from a broken truck rim poking through the tread.

Some punctures are quite small. So small that you need magnification to see them.

 This puncture resulted a 2 psi per week leak and I only found it after noticing the very slow air loss and the need to add air every week to 10 days.

Impacts do not always result in immediate loss of air. Some tires can run for many miles, days or even weeks and still might not “Blowout”.

 Can you honestly remember every pot hole or curb or piece of road trash you ran over in the last 10 or 20 miles or yesterday or 5 days ago as you pulled into the campground?

 

Valves can fail because the rubber components age-out just like tires can but how often have you replaced the rubber gasket in your metal valves? Do you even have bolt in valves? Standard “snap-in” valves are rated 60 psi Max but since they don’t come out of the wheel immediately after you inflate your tire to 65 psi how many owners assume all is OK for the next 5 years and 40,000 miles?

Valve cores can develop slow leaks if a single grain of sand gets lodged in the valve core seal. How many make sure they have a good o-ring in their metal valve caps, or do they simply use the cheap plastic caps as seen on most cars?

Tires age out. This does not mean that they will fail after 2782 days. It means that after sufficient time has elapsed at elevated operating temperature (which accelerates the aging process in a non-linear and ever increasing rate as the temperature goes up) the strength and elastic properties of the rubber degrade to the point that microscopic cracks that start out tearing molecular bonds apart, grow. The cracks never get smaller. They only grow. The rate at which they grow also depends on the physical properties of the rubber and the properties change with the temperature. Once the cracks grow sufficiently large, the structure starts to lose its ability to retain its integrity and things start to come apart in an ever accelerating manner.

I personally have run tests where I exceeded design range for load and inflation. The tires did not fail for 18 days when the load, inflation, speed and temperature were controlled in a laboratory. This test is repeatable so this is not magic. It is sound science. How many people would relate a sudden tire “Blow-Out” with conditions established 18 days previous?

 I do not intend my comments to mean that there has never been a tire failure that was caused by a manufacturing defect. What I want to show is that simply because you do not find a railroad spike lodged in your tire does not mean there was no reason for the failure due to external causes. It takes years of experience and sometimes hours of detailed examination sometimes including chemical analysis, X-Ray and even Electron Microscope examination to find the real reason for the failure. I know of no tire dealership that has any such equipment or that has provided man-weeks of training in forensic analysis to any of their employees that would allow them to arrive at the real answer of whay a given tire failed.

Should I jack up my RV?

I see this question come up each year when some people are parking their RV for the Winter. There is a quick and simple answer and a more involved answer,

Quick Answer:
I do not jack up my Class-C RV over the winter but I do inflate the tires to 80 psi (the pressure on the tire sidewall). I do cover the tires with white covers over the winter (and whenever I camp at a location for more than 1 day).

There are other things you can do such as keeping the tires out of contact with wet sand and dirt. I covered this with pictures in my post on “Winterizing” 

More involved answer:
Jacking up the RV and removing the tire is one of those things that in a perfect world would be easy to do but in reality it isn't as many RVs don't have good jacking points or are so heavy you need very HD jacks so it may just not be safe to do..
I do understand the theoretical damage done from long term parking in one spot but again theory and reality clash. Lots of actions can theoretically prevent microscopic problems but if you extend the life of a tire by 1 month if you were to jack it up every time you were planning to park for more than 3 months, would it be worth the effort? Probably not.

There are actions that are easy to do and relatively inexpensive that can provide real "bang for the buck". I have covered these in various posts on my blog but will touch the big ones here.

1. Your actual static load should not exceed 85% of the max load capacity on any individual tire for the inflation you run..
2. You need to weigh and learn the actual load, when fully loaded and not simply take the total axle load and divide by the number of tires. You will NOT get the correct tire load by doing the simple division. You can download a worksheet on how to do the math HERE Or HERE

3. Get a TPMS so you will get a warning when (not if) you start to loose air due to puncture or leaking valve
4. Cover your tires with WHITE tire covers if they will be in the sun for better part of the day. A couple of hours in full sun does about same damage as a full day in the shade in Phoenix.
5. Replace the snap-in valves or rubber parts of your bolt in tire valves whenever you buy a new tire.
6. Get and use a digital hand held gauge at least once a month, even if you have a TPMS. You will probably be adding 1 to 3 psi each month anyway to maintain the tire inflation. This will also serve as a check on the TPMS.
7 NEVER drive on a tire that has lost 20% or more of its air. Structural damage will be done. Such damage is cumulative and this damage does not repair itself. (See post on Potato Salad)

8 Do not believe everything you read on RV Forums. Ask the poster for their actual training in failed tire analysis. Simply having owned tires from company XYZ does not make that person a knowledgeable source.
9. If you have a multi axle trailer your cold inflation should be the inflation on the tire sidewall

10 You might put on your To-Do list to read the posts in my blog (including the ones where I later point out my errors), then you could subscribe so you get a notice when I do a new post. I do about 3 a month so you will not be overloaded.
11.If you have a multi-axle trailer, learn how to do a rotating tire inspection and do your tires at least once a year. Watch this VIDEO  read the background in this POST to see what a failed tire looks like before it comes apart.

Defective tire or incorrect diagnosos? (Part 2)

Last time, I discussed how a defective tire could be made. Today I want to show how a failure that is the result of external cause can be easily misdiagnosed. Since there are so many different possible external causes I will only be able to cover a few of the multitude of possible external causes. Please note that these pictures are only a few of those collected during my investigation and many times an on-hand examination is necessary to arrive at the real reason.   Lets start with the example from last post.

Here we see a tire with a sidewall split.

Here is an X-Ray showing the broken bead wire

 

 Here we see the ends of the bead wire showing tensile (pulling) not cut ends.

 Here we see a special tool used to measure wheel diameters. If the wheel were of correct diameter the front piece would line up with the slot in the back piece.

This shows the same tool on a reference rim and you can see an in specification wheel.

While this is an extreme example I think you can agree that the engineers at the car company who are responsible for the wheel being made to specifications didn't want to accept the possibility that their manufacturing plant had made a mistake.  I liken this to RV owners who do not want to accept the possibility that by driving at speeds higher than the tire is rated for or at lower pressure or higher load they might have contributed to the tire failure.

Here are so other examples. 

This tire had physical evidence of puncture and run low but the owner wanted to claim the tire was 100% OK up until the instant the tread came off.

This tire had numerous marks of damage on the inside proving run low.

The tire with this tread throw had an old un-repaired puncture with signs of sidewall contact with part of the vehicles.

 In closing I think a review of an earlier post of a "blowout" will show you that it is a rather common mistake to incorrectly identify the reason for a tire failure.

Defective tire or incorrect diagnosis?

I have been following a thread on a forum for an iconic RV. The company makes trailers that are all silver, but I won't name names. A few of those posts insist the tires are defective because first they failed and second because the owner didn't want to admit the possibility of some external cause as they want to find someone to blame. They will insist that there's "not a scratch on the sidewalls anywhere, and they have never hit a curb or anything else with them."  But somehow two tires failed after 9,000 miles.

The tire pictured here with a very obvious failure was identified as "Defective" by engineers at a large car company. I will cover the real reason in the next post.

I thought this would be a good opportunity to discuss the realities of tire failures and possible defective tires.

While tires are built individually, the various components are made in batches. Depending on the component these batches may be used in 100 to many thousands of tires. So let’s discuss how a "defective" tire might be made.

If it's a design defect then we should see the failure occur in a large percentage of the tires that are manufactured to that specification. Given that in regular production it is not unusual to see 10,000 to 1,000,000 tires made to a given specification it should be easy to see thousands or tens of thousands of identical failures.

A specification identifies which type of rubber is to be used for each of the 15 to 20 different components as well as the type of steel, polyester or nylon to be used. Now it is actually pretty unusual to have a design go into production that results in tires that will fall apart in service. Dozens if not hundreds of tires made to a new specification are tested before a specification is released to production. Additional testing is done by the producing plant to "qualify" the plant to make a specific tire. Even after a spec is released at a plant, modern quality control methods dictate spot checks of production tires. If you research tire recalls over the past few decades you will find that they are not tire design related but either service related or occur in relatively small batches of tires where there was human error in the producing plant on a single batch or two of component rubber not being made to spec.

Another thing that is basically true about tires made with an off-spec material is that they have a very high failure rate, very early in life, sometimes approaching 100 percent. Also the condition of the tires will be essentially identical. You will not see some tires throw a tread off while others have sidewall bulges. Sidewall bulges would all be of approximately the same size and location. If the error was, say, something like the wrong rubber used for the sidewall then I would expect almost all tires made with the wrong compound to suffer the same fate at about the same number of miles and that this failure would be early in life.

Next time I will show some examples of failures that were improperly diagnosed as “defective tires” by tire dealers and engineers working for the auto companies. The tire above is but one example.

Tire Crack or Cut ? CSI - Akron

More than once I was presented with an almost new tire with a claimed "defect crack". I believe that the owner was many times asking for a refund. While the decision to refund the price of the tire or to replace the tire or to deny the claim was not my job. My job was to do a detailed "autopsy" and issue a report. Here is one such case.

A tire was presented with the identified area of concern as seen in this picture. You can see the area of concern does appear to be a jagged crack about 1.2" long.


TIP. To enlarge the pictures to see the "Witness Marks" left by the cutting object right click and select open in new  Tab or New Window. Then you can close the Tab or window and return to this post.


Looking down into the "crack" I noted what appeared to be "witness marks" that would be left by a cutting object. You can think of these much like Ballistic fingerprints left on a bullet by the rifling of the gun. as seen in this picture of a bullet.

For examples and detailed explanation see pg 8 of THIS presentation.

If some object cuts a tire it will leave straight line marks much like those seen on the bullet. If there is simple rubber tear or a molding defect, there will be no such straight lines.



 After deciding where to cut so as to not destroy the evidence, I was able to spread the area open to confirm the presence of the witness marks. There have the appearance of small ridges or lines in the rubber.
Here is the piece I dissected out of the sidewall. You can easily see the lines of the witness marks left by the knife I used to remove this piece.



In this picture you can see the witness marks in the "cut"..







Then I looked at the side of the area and discovered that at its deepest point the body cord was cut.







Cracks in a tire can grow through the rubber but not through body cord.

 The external cut would be in the top of this picture and the "examination" cuts I made are from the bottom.




Closer examination confirm the body cords were cut from some external sharp object and not from any manufacturing defect as seen in these final pictures.

I think you will all agree that there was no defect and that some sharp object cut the tire sidewall.

BOTTOM LINE
Tire inspection is a real science and knowing what to look for and how not to contaminate or destroy the evidence as an inspection is done is what takes years to learn. I think you can also see that having pictures of the whole tire cannot provide sufficient detail to arrive at the "Root Cause" of the tire condition.

Impact damage or Defective tire? How can fishing line help explain which?

Many times the root cause of a "blowout" is impact damage. While there are many variable conditions such as speed, size of object being hit, tire inflation, spring rate, shock absorber stiffness, angle of attack, tire size and many others, I think we can all understand that it is possible to damage a tire if the right combination of factors occurs.

Many times you hit something or drop into a pot hole and don't know it because the impact is small. Other times you may feel the impact but if nothing happens right at the instant of the hit, the event seems to get erased from your memory. While it is true that most of the time when you hit something no significant damage occurs, sometimes there has been damage and you may not see the result of the damage for many miles, hours or even days so you may not link the impact event with the tire failure.

If there is damage to the tire, many times the damage is limited to the tire sidewall and could be in the side under the RV so you would not observe it unless you removed the tire from the RV. Sometimes the damage can damage the tread and belts. The main reason you don't always have an immediate loss of air due to the impact is because the damage occurs to the structure buried deep within the tire. In the case of sidewall damage it is the body cord that suffers a shock load and "snaps".

Sometimes when it is the steel belts that are damaged you might have managed to bend the steel beyond its yield point so now it is kinked and experiencing a concentrated load at the location of the bent steel.

The best way to think of how you "snap" body cord is to think of fishing line. Fishing line comes in different strengths just as tire body cord comes in different strengths. There are a number of the tradeoffs with going to higher strength cord than what you expect to need. Cost, weight and flexibility are some obvious examples so the tire manufacturer just like the fisherman tries to find a balance between stronger cord and the other measurable or desirable characteristics. Even if you don't fish you may know someone that does so you can check with them on this example.

You can confirm that it is possible to catch a 10# fish using 5# rated line but it is also possible for a 5# fish to break a 10# line if the fisherman doesn't play the fish properly and there is a shock load because the fish jumped or the fisherman jerked the line too fast. The same thing happens with the cord in the tire. A cord that would normally be more than strong enough can be snapped if all the conditions as named above happen to result in a shock load.

Here is an example to the left. This tire was labeled "defective" as the owner saw the bulge in the sidewall but did not remember hitting anything. However I noted the witness marks on the rim (arrow) where the tire was forced down over the rim, leaving black marks on the rim, so I did additional investigation. I discovered broken body cord on the inside of the tire, as seen below right

Here, below left, is a close-up showing damage to the outside of the tire at the exact location of the broken cord on the inside.
 



Now once the cord is broken the tire may continue to hold air with the remaining unbroken rubber as seen in the above picture. Only some time later when the rubber is heated up from driving at highway speed does the force of the inflation air exceed the decreased strength of the rubber. This delay can easily occur if the damage occurred shortly before you parked the RV for an extended stay at a campground. Days later as you pull out you would have completely forgotten about the deep pothole you hit as you turned off the freeway a mile from the campground. The result is a rapid loss of air. This event is often considered a "blowout" by the driver. A TPMS will not provide advance warning of the sudden failure because there was no loss in pressure till the rubber sidewall tore open.

Tire engineers experienced in root cause analysis can sometimes find the evidence that established that there was an impact that ultimately lead to the sudden loss of air. Other times the less experiensed will jump to the erroneous conclusion that the tire was "defective" because they do not know the tell-tail signs to look for or really aren't inquisitive enough to make a sufficiently detailed examination.

I hope this example demonstrates how it is possible for the owner to arrive at the wrong conclusion of what really caused the tire failure.

BOTTOM LINE
Discovering the real "root cause" of a tire failure requires detailed examination of all the evidence. Seldom is the RV owner in the mood to spend an hour or so examining the tire or he may not have the equipment needed to examine it in sufficient detail to arrive at the real answer.