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Friday, May 18, 2018

ST tire Belt Separation "Autopsy" "How To"


This info is from an inspection I did a while ago. Some may find it informative.  A friend wrote a post on an RV Forum of his experiences and the results of my "Cut Tire Inspection". He posted:

[image]"This summer I had a tire failure, 3 actually in all by the time I was done. If you are into the details of tire failures, this is one type of tire failure. This is a little long of a post, but there are lots of pics to go with the words and some background.

I'm not a tire expert by any means, just a machinery guy trying to figure out what went wrong with my rig so I did not repeat the same problem and could correct what went wrong. So I took this one as far as I practically could.

I caught the first failure here in my yard doing axle maintenance. When I jacked up the camper to put it on jack stands, my left rear tire would not clear the ground on the normal stand height. H’mm OK what’s up?
[image]
I looked at the left rear tire and it looked more round across the face than I remembered. Still did not know what the issue was at this point.
 [image]



I jacked it up some more and started to take the tires off. I took all 4 tires off and I could see one tire, the left rear looked different. It was more curved across the face of the tire than the rest. Laying it on the ground it showed up more not be square to the sides.

Here is a normal one

 [image]
After comparing the other 3 to this one rear left I measured the OD of the tire. Yup, it is 1” larger in OD circumference. OK something let loose inside this tire.
 I put this bad tire aside, finished up the axle work and put the spare on. I then tried to figure out what was wrong with this tire. Looking on the outside I really did not see anything much that was wrong with it other than about 180 degrees around the outside, the tire progressively was getting larger in OD up to a high spot then starting coming back down. Whatever was wrong with it created an out of round tire.

 [image]

I demounted the tire and looked inside. Nothing really looked wrong to me inside. I happened to have a fellow RV buddy who is a retired tire engineer who has done tire failure analysis most all of his career. We hooked up and he told me how to section the tire and send it to him. So here is how we did this. He told me how to cut out the side walls. I was shocked you can cut up a side wall this easy. There is inherent danger in doing this. Heavy gloves and a sharp knife is a must. Once you start cutting it out it sort of unzippers. You start above the tire bead and cut towards the OD, then start and cut around the circumference.
[image]

Now I had a donut. The hard part is cutting through the steel wire in the tire. Ideally you do this on a vertical band saw with a progressive tooth blade. The rubber wants to bite and grab the blade. I did not have a large enough vertical band saw so I used a Sawzz All. Here one really has to clamp this thing down or the rubber will grab and start shaking the saw violently. You do not want any blade pinch as the rubber bites into the blade. Need to back flex it to keep it pulling apart as you cut. I clamped it to some old saw horses and cut it apart.

[image]

[image]

Now I looked at the cross section. Well nothing real exciting at the 90 degree point from the high point.



[image]

 So I curled up the sample to fit in a box I could UPS to him. Here it is
[image]

I  sent him the tire and then started to investigate if I did something wrong to cause this failure. I always check tire pressure and run max cold side wall pressure at the start of every trip. I even have my own small compressor in the truck if I need it. So under inflation was not the problem.

I also do not tow faster than 60 mph. I do not need to, big truck or not this is a safety limit for me. So I was not overrunning the 65 mph max speed rating creating excess heat.

I use white tires covers when the camper is at home. These tires are 3.5 years old at this point. I can say the first year they were only 50% covered until I have my present tire covers in place. There really was not much tire cracking. No side wall cracks, some very fine in tread cracks and there was a number of stone cuts in the valleys of the treads.

Next was weight. I had not been to the scales in about 1.5 years and I added some upgrades. So I loaded the camper with stuff for a campout and full fresh water as we do haul water to some camps. Went to the scales and weighed each axle with WD engaged. I could not get each wheel position at the scales so when I came home I used my force jack to get each wheel position. Here is the weight chart.
[image]

The failed tire location has 12.5% extra tire capacity or 318# from max load. There is some error in this data as my force jacked weighed a little heavy. It did however show me that the 4 tire locations are different. The front axle was pretty equal, the rear axle very different. My fresh tank sits right over the front axle which may have had something to do with the front being more even. At this point, weight did not seem to point to a glaring problem.

Since I could not find anything wrong, yet anyway, I bought a new Maxxis tire for the spare and we headed off on vacation. My tire buddy also was on vacation so he did not yet have my tire done.

On vacation I was 800 miles from home on the NYS Thruway and while gassing up, OMG… This tire right side rear tire does not look good. Dang, it looks like the failed one. So we pulled over in the truck lot and took it off and put the spare on. Yup, it let go too like the 1st one.


[image]
I was lucky a 2nd time I caught the failure before it let go. If you are going to have to change a tire on the road, the NYS Thruway makes a good changing spot… OK so now I have no spare…. I’m thinking of where I can get one when I get to my Mom’s house. Well…. No luck finding one close by. We did make it home OK. Another 800 miles.

When we made it home I was already working on changing tires to LT tires. When I jacked up the camper to take off the ST’s, OH boy, another one…. The tread was bulged. This tire would not even roll correct it wobbled so bad it would fall over.
[image]

So I dodged a 3rd bullet. I could not see this when it was on the camper, only when I took the weight off. The damage did not yet progress far enough yet or it was not out in the open where I could see it.

Soon my tire engineer buddy was done investigating the 1st tire I sent him. The tire failed for what is called “detachment” or what is nick named sometimes “slipped belts”. It is where the tread separates where the steel belts are in the tire tread from the main tire. Basically the tire unbonded itself inside the tread. Here is his analysis pictures.

He sectioned the tire sample I sent him and knowing what to look for started to see clues of the problem. You can see small separations at the yellow arrows.

His description was “Cut 1 was my initial cut away from the identified bulge area. There are small detachments identified. If this was all that was found in the tire it would not be a serious issue.”
[image] 

Then he sectioned again and he found the entire area let go.
His description was
“Cut 2 location was identified by careful measurement of tread depth and I found a location with more wear than in other areas. The large detachments between the belts on both shoulders can be seen”


[image] 

 [image] 
[image] 
 [image]
His description was “Detach 1a & 1b show the length of the detachment to be over 11" long” 
 And this one really shows the detachment separation.

[image]
His description was “Detach Width shows the width of the two detachments relative to the tread width”
asked him how did this happen? His response:

Why detachment?
There are a number of things that can cause this. They would primarily be a breakdown of the rubber that coats the steel. This could be due to a manufacturing error or simply the selection of a lower strength rubber which cannot tolerate the forces applied to the tire. Only lab testing can determine that. Sometimes detachments can be initiated by tread cuts or punctures but that does not seem to be the case of the tire I inspected. Detachments are one of the more difficult conditions to analyze as there is a need for a lot of additional background information and data.


From this investigation I have a perfect case to file a report with the NHTSA as I used the tires within the ratings. I have since filed 3 complaints, 1 for each tire. I’ll create a thread on how to do this filing. It is the only way we as RV’ers can help this cause of trailer tire failures become more known to the right people who can help. See here How To File a Tire Failure Complaint

There is now a growing understanding that for tandem trailer applications that a 20% more tire capacity reserve is needed to help hold up to the service of a multi axle trailer. In my case I am towing heavy, I am not overloaded, in relation to my tire capacity. When I upgraded I targeted to get as close as I could to the 20% reserve at the heaviest loaded tire. I had a choice of load Range E in the ST or go to 16" LT and deal with tire wheel well issues. I weighed the options and went LT. See here for more on the LT change ST225/75R15 to LT225/75R16 Conversion

While LT tires may not be for everyone, knowing your weights of each tire location and where you are in relation to the tire capacity reserve is something you can do. You can also not tow over 65 mph on ST tires, keep them at max side wall cold pressure at the start of each trip and use white tires covers over them when the camper is in storage.

Hope this helps someone in the future.

 

 

Friday, May 11, 2018

How do I set inflation on my TV and RV trailer?

Got this PM from someone who reads RV forums.

"Good evening, I'm looking for some professional advice. I have a 2500HD pulling a fifth wheel, my steer axle is 4,000 lbs, drive axle is 5,380 lbs, and our trailer is 7,700 lbs. Tires on the TV are rated for 3,100 lbs a piece at 80 psi. The RV tires are rated for 2,800 lbs at 80 psi. Do you think I am ok to run 65psi in the steer axle, 70psi in the drive axle? What I've been doing is 75psi steer, 75-80psi drive, and 80psi trailer. Our TV tires are Michelin MS2 LT265/70-17 the load range chart says I can run much less psi. Also, our trailer tire's are Maxxis 8008 225/75-15 LR E, also the load range chart shows I can run less psi. Any thoughts on what you would do would greatly be appreciated, thank you. New Crusader owner"


To start with it is important to remember that we should never operate tires with inflation lower than what is needed to support the actual load on the tires.
As I have pointed out in my blog we first need to confirm the actual load on our tires. For TV we will find the side to side loading is usually 49/51 to 50/50 split so we can take the measured axle loads divide by 2 and then use the resulting figure when consulting the Load Inflation tables for our size tire.
I have links to many load/inflation tables and related info HERE. Once you find the MINIMUM inflation, I suggest you add at least 10% to that number to allow for day to day variations. Having said this, it is important to remember that the car company spent a lot of time and money evaluating many different tires at different pressures to arrive at the recommendation that is on the Tire Placard on the driver door jam. You should note that the Placard inflation may be higher than my above-suggested calculation as my calculation is only addressing load and not handling, braking or the fuel economy.
The scale weight for the TV for the above needs to be the reading with the RV & TV fully loaded as heavy as you ever expect to tow.

The RV tire inflation is different. Due to SIDE LOADING in trailer application, I strongly recommend running the tire sidewall inflation as your CIP. I think that if you review the RV Placard info from the RV company you will also find that they say to use the inflation number on the tire sidewall.

I know you were hoping I would just tell you to run xx psi here and yy there and zz on the trailer but without knowing tire sizes and Load Range for all your tires, I don't have enough info to provide an informed answer BUT I feel you now have enough info that after reading the sidewalls of your tires - Be sure to check each as they might be different on TV and RV and possibly even different F & R on the TV - You now know how to establish the correct inflation for your current vehicles and for any vehicles you own in the future.

I hope this information and instructions help. If not you can always contact me directly using the email under my video picture to the right.


Friday, May 4, 2018

How much inflation in my tires? Question from new RV owner

Recently read yet another forum post on the topic of "How Much inflation should I run. This post followed the standard format of:  A. I just bought an XXX RV. How much air do I need in my tires?  B. Numerous replies ranging from "I use xx psi" to "You must always run the pressure on the tire sidewall"  to the more correct reply of "You need to know your tire loads first". Some forum threads run to dozens of back and forth exchanges Some have correct info IMO while others are still using what I consider "Old Wives Tales".  I recently replied as follows.



1. Almost all tire Inflation/Load charts have identical numbers (maybe as high as 98% )so if you can't find your brand you can reasonably use another brand till you do.

2. Yes, the charts give the MINIMUM inflation needed but to avoid the need to adjust your inflation every morning (inflation changes by about 2% for every change of 10F) I and others suggest you add a bit so you are running the minimum + 0% to + 15%.

3. You should NEVER run lower than the minimum inflation shown on the chart.

4. Since all tires on any one axle should have the same CIP (Cold Inflation Pressure) you base the minimum on the heavier end. This is why we recommend "4 corner weights" to learn what the heavier end load is. Until you learn the actual load on the heavier end you can assume one end is supporting 53 to 55% of the total on the axle (the axle weight when on the truck scale).

5. How much to add over the minimum? I think you will find various suggestions in the range of +1 psi to +10%. Since I am offering advice to a wide range of users (ST trailer tires, LT and Class-A) with minimum inflations from 50 to 120 I prefer the percentage and if people want to have their CIP ending in 0 or 5 they can add the % then round up. ( Note the +1 Psi is from Tire Rack where they are advising owners of passenger tires that normally are running 30 to 36 psi)

6 Premature wear. Most LT and 22.5 size tires can be good for 60 to 100,000 miles wear so most RV owners will have their tires "age out" before they wear out so the minor wear difference, if any due to running + 5% over the minimum if overshadowed by variations in suspension wear, alignment variations, and driving habits. The center shoulder wear info is based on car wear rates primarily with bias tires or inflation differences greater than +/- 15% from the suggested CIP.

7. "Correct" inflation is not what the charts say. They are providing the "Minimum" cold inflations is every case.

8. Adding Load % and adding inflation % is done only to compensate for not knowing the actual heavy end of an axle. If you learn the actual heavy end when the RV is fully loaded to the heaviest you ever travel at, I do not suggest you need to add any more weight when doing your calculations. How much can you be out of balance side to side? I have seen a very small % of RVs with the load at 50/50 side to side. Most appear to be in the 53/47 to 55/45 range but a few have been found with over 1,000# heavier on one end.

9. It is important to realize that when we look at actual weights of RVs in use, over half exceed one or more of the weight specifications for tires or axle loading. IMO this is a clear indication of why so many RVs have tire problems.

Friday, April 27, 2018

Sidewall "Bubble" or ? A Tire Autopsy

Took the wife's Miata down for an oil change and tire rotation. Contacted by the store, that they had discovered a "Bubble" in the sidewall and wanted to know what I wanted to do.
After a quick look at the location of interest I concluded that we were looking at a Sidewall Impact Break. I had the store replace the tire with a new one and asked that the allow me to do a "Failed Tire Inspection".  Here is what I found.

Picture 1 is of the inflated LF tire on the car.
You can easily see the "Bubble" on the sidewall under the molded word"OUTSIDE". What tipped me off to the damage being an impact and not a sidewall bubble or blister was the shape (more radial in nature than round) and the gradual nature of the rise of the shape. Sidewall blisters are normally more abrupt.



The interior picture is what we see inside the air chamber at the location of the sidewall "bump".
I have an arrow pointing at the stress marks of the body cord on the inner liner rubber.






Picture 2 is of my initial examination cuts.   Note I did not cut directly through the area in question as I did not want to destroy or contaminate the "evidence" that I suspected would be found in the area of interest. This is an important point that many inexperienced investigators sometimes miss.


In picture 3 we see the location of interest. I added an arrow that points to area os special interest. If you look closely you will see a short dark line. This is a depression where the interior rubber has been "sucked" into the damaged area of the sidewall,




After removing the section of the tire, close examination revealed some of the initial Innerliner cracks / tears.  Here is a close-up shot.



These cracks would grow and could eventually result is air loss through small cracks in the sidewall. It is important to consider that the cracks will not grow in just a couple of miles or maybe not even in 100 miles but if driven long enough would most certainly result in an air loss. The driver would have no recollection of ever hitting a bad pothole or road debris so in all likelihood simply claim that the "Blowout" was due to some tire defect.

 Finally, we can see the separation of the body cord from the surrounding rubber. It is possible for this type of separation to allow the cords to move independently from the rubber next to the cord and the rubbing can generate heat which under the right circumstances could lead to a cord failure.
Again this could take weeks or months to progress to a loss of air but is another example of why few drivers connect the effect of a tire losing air to the real cause of suffering a sidewall impact break.

This break was actually quite minor. A more severe impact was covered in THIS post.

Friday, April 20, 2018

Belt Separation or not on a towed vehicle?

Saw a post that started out with a claim of a belt separation on a front tire of a 2007Jeep Sport while it was being towed behind a Class-A rig.

Luckily the owner posted a picture that I felt was suspicious. While the owner was going about getting the Jeep repaired and tires replaced I PM the owner and asked if they could provide better pictures. I advised full sunlight and straight on shots showing just the tread.

Here are the two front tires after being removed from the Jeep.

The LF shows the belt separation which led to the air loss when the edge of the steel belts cut & punctured the tire carcass.



If we take a closer look at the LF and note the Tearing and Wearing of the tread elements we can confirm the tire was turned hard right while the Jeep was pulled forward by the tow bar.  I have added a red arrow showing the direction the road surface moved across the face of the tire tread.
 I have circled a couple of tread sipe edges that show the lifting of the leading edge of the slots.
"Sipes" are the name for the small, thin parts of a tire tread pattern that leaves small narrow slots in the tread surface. This "lifting" occurs as the edges of the sipe slot are bent down into the empty space in the tread patern.
The yellow box identifies the relatively smaller level of wear on the trailing edge of the tread.


Here is the RF where I showed the leading edges
of the larger tread blocks that have been worn away from having the tire being pulled sideways across the road surface.




The owners of the Jeep believe the steering wheel lock was not engaged but so far inspections by auto mechanics have not been able to learn why the tires were being dragged straight ahead while they were turned hard right.

As can easily be seen the result of being dragged put more load on the outer edge of the LF  tire which suffered the greatest level of damage. I would estimate that the tires were pulled for a good number of miles based on the severe damage to the thread. Before the vehicles were stopped, the tires did appear to straighten the steering and pull straight ahead for a number of miles as the tread surface does not display surface scrub marks I have observed on other tires that have been slid across the road surface.

BOTTOM LINE
Yes, there was a belt separation but it was caused not by any tire defect but by some mechanical failure of the vehicle steering system.

IMO the above is a reasonable example of a "Failed Tire" Examination and analysis of the evidence thaat leads to a probable cause of the tire problem. Experience in tread wear paterna and anomalies is a definate pluse in completing this examination.


Friday, April 13, 2018

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.

Friday, April 6, 2018

What inflation to run when changing Ply Rating or size?

Following a few different RV Forums, I see a number of people, that clearly do not follow this blog,  have questions on what inflation to run. While many of these folks are discovering that the new Goodyear Endurance line seems to come with higher Load Range or only in larger sizes they end up not knowing what to do about inflation.
In one case the person didn't even know they were buying a larger size so I do wonder sometimes how people managed to ever buy tires for their car if all they seem to know is that tires are round and black and seem to cost too much.

When you replace tires on any vehicle, you should always be sure the load capacity of the new tires is equal to or greater than the capacity of the original tires on the vehicle.

Regular readers of this blog know that load capacity depends on BOTH tire size and INFLATION level. You also know that there is a "Tire Placard" or "vehicle certification label" on or in all your vehicles that identifies the original size tire and the inflation recommended by the manufacturer. In the case of RVs the label also should tell you the GAWR which is the maximum load you should ever have on each axle confirmed by scale readings.

So back to the original question of what inflation to run ?
 .

Example 1:  There was a change in tire size but the Load Range stayed the same.
Original tires were ST205/75R15 LR-C and you mistakenly bought ST225/75R15 LR-C
 A quick check of a Load & Inflation table for ST type tires we see that the ST205  is rated to support 1,820# @ 50 psi and the ST225 can support 2,150#@50 psi. This is a nice 18% increase in load capacity.  I would suggest that in this case the owner continue to run 50 psi and enjoy better tire life.

Example 2:  There was a change in tire size AND in Load Range as can happen with the new Goodyear Endurance.
 Original tires were ST205/75R15 LR-C and you mistakenly bought ST225/75R15 LR-E
Back to the Load & Inflation table for ST type tires we see that the ST205  is rated to support 1,820# @ 50 psi and the ST225 can now support 2,830# @ 80 psi. This is a 55% increase in load capacity.  If the owner were to run the 80 psi as indicated by the tire sidewall the "ride" would probably be hard on the TT. In this case I would not follow the inflation on the tire sidewall. I would suggest that in this case the owner continue to run 50 to 55 psi and enjoy better tire life. In this example the owner expressed concern about running lower inflation than marked on the tire. Some people even incorrectly said that running less than 80 psi would somehow overheat the tire. I pointed out that I saw no problem with running less than the sidewall inflation as long as the actual load had been confirmed with actual scale readings and that there was a good margin of capacity over the actual loading.

Example 3. A Class-A owner wanted to change from a Michelin 275/80r/22.5 to a Firestone FS 591 295/75r/22.5. His reasoning was two-fold. First he was concerned about the historical reports of  delays in getting the sometimes hard to find the Michelin size and second was the significant for him, cost difference.
Both tires were LR-H . The owner did a good job of consulting the tire manufacturer's data sheets to learn the actual tire dimensions. One poster had offered a generic tire size comparison web page but the numbers the page generated were clearly different and could lead to problems.
The load capacity for the Michelin and Firestone tires is 7,160 @ 120 The owner had also confirmed the dimensions were acceptable for the application.








Friday, March 30, 2018

More on TPMS Temperature reporting

Continuing my series on TPMS.  Last time I covered temperature reports when it was moderate to just cool outside. This report is different. On my way South to GA from OH it got downright cold.
 

I stopped for the night in KY and in the morning I obtained these readings.

Internal System.
RF 30   LF  32    RRO 32    RRI  32    LRI   30  LRO  32

External System
RF  32   LF  32   RRO  32   RRI  32   LRI  32   LRO   32

As expected the temperature numbers were essentially the same. To me this confirms that the sensors can report ambient temperature if given sufficient time to cool to ambient temp.

After about an hour running down I-75 I observed these temperatures when the outside temperature was in the 40's.

Internal System.
RF 86   LF  83    RRO  85    RRI  86    LRI   80   LRO  80

External System
RF  48   LF  48   RRO  46   RRI   44   LRI  57   LRO   55


These numbers clearly show that the external sensors are reporting significantly cooler temperatures than the internal sensors. With cooler ambient the difference internal to external is greater than when the temperatures were warmer. I previously suggested that people running external semsor TPMS might consider adjusting theit high temperature warning level down by 10°F to 15°F.  While the above might suggest lowering from 158°F to maybe 135°F I am not sure if this would be too cool for people that are traveling when the abmient is above 85°F.

For now you might just want to be aware of this difference. I am hoping that later this summer when I am traveling I will be able to get measurements when the ambient is above 85°F.

Friday, March 23, 2018

How accurate is your TPMS - Part two

Last week I posted on the question of "How Accurate was your TPMS".  This week I am continuing on my close examination of TPM systems.

We saw that the variation in pressure readings were Measurable but that IMO the differences were not meaningful.

I recently traveled from Akron, Ohio, to a large (2700+) RV Convention held by Family Motor Coach Association (FMCA) in Perry, GA. There was a large variation in ambient temperature during the trip with a low of 26°F to a midday high of 78°F observed. I felt that this trip was a good opportunity to take a first look at the temperature numbers reported by an internal TPMS vs. an external TPMS.

To start out let's look at the morning temperatures in GA before I started driving home.
 All temperatures are in °F
Internal System.
RF 61   LF  61    RRO  61    RRI  61    LRI   63   LRO  61

External System
RF  60   LF  62   RRO  60   RRI   60   LRI  62   LRO   62

I feel it is reasonable to say that for all intents and purposes the temperature readings are the same.

After heading out on my trip North  I stopped a couple times and recorded the readings.
The ambient temperature was 74.0°F
Internal System.
RF 108   LF  106    RRO  111    RRI  115    LRI   111   LRO  108

External System
RF  80   LF  84   RRO  84   RRI   84   LRI  88   LRO   85

I made a second stop and this time was able to learn the ambient temperature was 60.9°F
Internal System.
RF 95   LF  90    RRO  93    RRI  99    LRI   100   LRO  91

External System
RF  62   LF  68   RRO  66   RRI   66   LRI  73  LRO   71


I consider the above temperature differences to be both measurable and meaningful.

All the above data provides some interesting information.  All along I have thought and said that I felt that the temperature reading from external TPM would be affected and cooled by the outside temperature and rapid air movement around the sensor. Thinking about the heat flow the heat is generated in the outer edges of the belts under the tread in the radial tires. While some of this heat flows out through the tire sidewall the insulating properties slows this heat flow. Most of the heat energy flows into the air chamber where it is transferred to the metal wheel and then to outside air.

An internal sensor mounted on the wheel will be measuring the temperature of the air on the inside of the tire but since the sensor is in contact with the wheel it will be cooler than the hottest part of the tire.
An external sensor is depending on the heat to transfer from the wheel to the brass valve stem and then to the metal base of the sensor and finally to the thermocouple inside the sensor to report the temperature. But along the way heat is being lost to the outside air from the wheel outer surface and through the brass valve stem and from the brass sensor base to the cooler outside air.

BOTTOM LINE
I believe that the default high temperature warning for all or most TPMS is 158°F ( 70°C). With this initial data I plan on suggesting that the high temperature warning for external sensor TPMS be lowered by 10°F to 15°F.   I will be repeating this temperature comparison later this summer when I travel from Akron to Yellowstone. I will be looking for the numbers when ambient is in the 85°F to 95°F range to see if external sensors continue to be cooled as much at these higher ambient temperatures.

Friday, March 16, 2018

How accurate is your TPMS?

I have seen a number of people express some concern about the accuracy of the pressure readings from their TPMS. You can read my previous responses HERE  and HERE

I have also pointed out that the primary purpose or "job" of a TPMS is to warn the driver of a pressure LOSS.

I am inclined to think that some are still concerned with their exact pressure reading. I also have to wonder how some people are comparing various pressure readings reported by their TPMS. If they have external sensors, the simple act of removing and replacing a sensor can allow some air to escape. While I doubt that the small loss of some air will result in a meaningful pressure differential it does add some uncertainty.

So the engineering DNA in me kicked in and I a devised a plan to test 12 sensors. These come from two different companies. One set of 6 external sensors is from Tire Traker and one set of 6 internal sensors is from Truck Systems Technology.

The question is: How do I make the test both fair and useful. I decided to eliminate as many variables as possible and get all 12 readings from the same air chamber at the same time and compare them all against my personal digital hand gauges that I have checked against an ISO certified laboratory gauge.
Note my hand gauge reads to 0.5 psi which is way more precise than anyone needs for checking tires in normal highway use.

Here is the test fixture I made.



It has 6 bolt in valves for mounting the external sensors plus a valve for me to use with my hand gauge. Also since safety is always of concern when dealing with a pressure vessel I added a pop-off valve.  One end of the fixture has a cap that can be removed so I can place the internal TPM sensors inside the 4" tube. It also has a regulated air supply to compensate for the very slow leak around the cap and an analog dial gauge that allows me to constantly, visually monitor the air pressure to ensure the use of my hand gauge does not result in a pressure change that isn't immediately compensated for by the regulated air supply.

Here are the results of my comparison test. The target pressure is 80.0 psi as reported by my handheld digital gauge.

Set A  1 reading of 78 psi,  5 readings of 79 psi
Set B  2 readings of 78, 2 readings of 79 and 2 readings of 80 psi

I also recorded the temperature.
Set A  4 readings of 66 F, one each of 64 and 68F
Set B  4 readings of 69 F  and 2 readings of 68

I do not consider any of the differences in the readings of pressure or temperature to be significant or meaningful for a TPMS.  I would consider the pressure readings from all 12 sensors to essentially be equivalent.
You can learn a bit more about what a "meaningful" measurement is HERE.

Tuesday, March 6, 2018

How would I set inflation on a smaller single axle trailer?

Got this question from a reader. 

Our Jayco Hummingbird came from the factory with P235/75R15SL tires. The TT GVWR is 3,750 lbs and the GAWR for the axle is 3,500 lbs. This is a single axle TT. The P-rated tires were like a pogo stick at max inflation.

We changed from the factory tires to Maxxis 8008's in ST225/75R15 size. The factory aluminum wheels are good for a max of 80 psig according to the stamp inside.

Also converted to metal valves stems for running our TPMS....because I'm an engineer who tends to overdo everything I touch .

The heaviest CAT scale weight has been 3,320 lbs on the axle and 3,780 lb GVWR. We've since removed a few items to stay within the 3,750 lb. GVWR.

I've always kept the tire pressure at the minimum sidewall stamp of 65 psig (Max load of 2,540 lbs at 65 psi cold). After reading some of your blogs and looking at the Maxxis load chart, if I assume an equal split weight on each wheel we would have a worse case of 1,660 lb load. Of course a perfectly balanced load isn't likely to ever happen. But even with adding 10% it would put us at 1,826 lbs per wheel. Maxxis says that for our particular tire 40 psig would give us 1,880 lbs capacity.

I can't say that I'm comfortable going all the way down to 40 psig, but I feel ok with 50 psig, even though this is grossly over-pressurized for the given load. I know that at 65 psig the TT rides like a log wagon and we recently bent a spindle on the axle without even knowing it, I wonder it the limited travel of the torsion axle combined with the tire pressure came into play because we were under the GAWR of the axle and never even felt anything out of the ordinary during the trip, of course we are pulling a 3,750 lb trailer with a '17 GMC 2500HD w/ Duramax so we don't feel much anyways.

So if it were yours what pressure would you choose? I've been running at 65 psig and I think that's too much, 50 psig sounds good to me, but it's still too much pressure according to the weight charts.....

http://www.maxxis.com/trailer/trailer-tire-loadinflation-chart

Thanks!"

Here is the answer I gave him.

My approach
OE tires P235/75R15 are  rated for 2,280#@35 psi but on a trailer we need to De-Rate the load capacity so 2028/1.1 = 1844# load capacity.

Your measured axle load was 3,320#
If we assums a nominal 53/47 side to side split we get 1,760# for heavy end  and a 60/40 split gives 1,982# for the possible heavy end of the axle.

An ST225/75R15 LR-C is rated 2,150# at 50 psi. Since we are looking at a single axle trailer we can check the tables and find 40 psi is rated to support 1,880# and 45  psi can support 2,020#.

Since we always select the pressure needed that can support the heaviest end of an axle and we always inflate all tires on any one axle to the same inflation we could select  40  to 45 psi for our CIP.
I would set my TPMS Low Pressure warning level to 40 psi and my CIP to 45psi.


If this was a multi-axle trailer we would want to lower the special belt shear forces and run a higher inflation. Maybe 50 psi minimum


Thursday, March 1, 2018

Can I run my LR-E at 65 psi? or is this overloading the tire?

As trailer owners start applying the new Goodyear Endurance ST tire, many are discovering that for some sizes the Endurance tire is only available in a Load Range that is higher than their OE tires. Some are concerned about what inflation to run. I have even seen some claim that running a LR-E at LR-D inflation i.e not 80 but at 65 that the "tire will be overloaded, heat up and fail".

While I understand some of the confusion I do not agree with some of the concern or replies.

Tire load capacity is a function of the tire size and inflation level as long as you stay in the same "type" tire.  By "type" I mean P type or LT type of ST type or for large RVs "truck" type.

If you stay with the same type and use the same numeric "size" then the only thing left to change is the Load Range or "Ply Rating". While I do not like using Ply Rating as it is an old and discontinued nomenclature it may help for better understanding in this post for you to think of the old term.

Important Point. "It is the air pressure that supports the load NOT the Ply Rating." This statement is supported for every tire made by every tire company in the world through the use of Load & Inflation tables. These tables show a size and then for different levels of inflation the load capacity of that tire when inflated to that level. You will never see a tire shown where a LR-D at say 65psi can support 1,500# and for the same size the same tire when having a LR-E rating shown a higher load capacity at 65 psi. Not even just 1 pound more.

So a LR-E can support the same load at 50 psi as a LR-C or the same load at 65 psi as a LR-D at 65.

You will not be overloading the LR-E if you load it to the 65 psi rating shown for thet type & size tire and inflate it to 65 psi as you would for a LR-D. Since you are not overloading the LR-E tire it is not going to overheat at 65psi with the 65 psi load so the LR-E tire is not going to "overheat" at 65 psi any more than the LR-D will "overheat" if it is loaded to the 50 psi load rating and inflated to 50 psi.

When going to a higher "Ply Rating" you can then increase the CIP which increases the tire Load Capacity which means it will actually be running cooler because of the greater "Margin". The higher inflation will also lower the Interply Shear which may lead to longer tire life.

When making the change you do need to confirm the upper inflation level for the rim. The wheel manufacturer should provide that information. As an alternative the wheel will have a max load capacity stated. Looking at the OE tire size that comes on that wheel look for the inflation that corresponded to that load and I would consider that to be the wheel inflation rating.

Thursday, February 22, 2018

Tire inflation not the same for all trailers

I have written a number of times on the topic of Interply Shear. This is the force that is tring to tear the belts of radial tires apart.
There are some highly technical papers on the topic and you can review tham after a simple Google search on the term. You can also look here on my blog for the posts where the term is tied to a post by simply checking the list of topics on the left side of my blog. Basically this force comes about when the belts in a radial tire are forced to change shape. This means either when a tire rolls and the footprint changes from curved to flat as the footprint rolls into contact with the ground. This force increases when external forces from cornering are also applied to tires.
So why do trailers seem to have such high Interply Shear forces? Well it's not all trailers as the cornering forces of single axle trailers are much lower than the forces of tandem or triple axle trailers.

What tipped me off to this was an observation at a campground that happened to have a freshly smother gravel driveway and a multi-axle trailer happened to make a 180° turn as I was walking by. I noticed that the gravel marks were not a smoth curve but there was a series of turns interupted by discontinuities.
 

A short time later I saw a video from Keystone RV Company of wheel lug nut torque (you should watch the full video some time)
 While watching their video I recall some special high side load tests we ran in our tire test lab.

Here is a short out-take from the longer Keystone video that shows what happens to multi-axle trailer tires.


In the video you can see how the two tires are fighting each other with one bending in as the other bends out. You can imagine that if the turn is made on gravel at some point the high sideload would result is a sudden breakaway or slip. That's what I observed on the gravel turn.

Back at work I had some high powered Finite Element computer programs run to simulate the side load of a multi axle trailer and the results showed that the side loads on a trailer could be 24% higher than on a standard vehicle even with identical radius and loads on the tires.

This post shows why tires get different side loading.

Further analysis showed that increasing the tire inflation could lower the extra shear, but sadly not eliminate it.

Bottom line
In a number of posts I have recommended that Motorhomes set their inflation based on measured static load plus a margin of at least 10% additional PSI. This would also apply to single axle trailers. BUT for Tandem and triple axle trailers I strongly recommend that the tire cold inflation be set to the inflation molded on the tire sidewall associated with the tire maximum load capacity. I also recommend that the measured static tire loads on these trailers be no greater than 85% of the tire maximum with a 20% margin being better.

Friday, February 16, 2018

Changing tires on a trailer - NEW load capacity requirement

On many of the RV forums I monitor that focus on trailer application, there is a recurring question about changing tires. Some wonder about going up in Load Range (Ply Rating) some wonder about changing the "Type" tire P > ST, or P> LT, or ST > LT. Others want to change the tire dimensions. While there are many reply posts, I do note that not everyone offering answers has worked as a tire design engineer. It takes years of working with the engineering and scientific knowledge before you can be given the responsiblity to develop a new tire capable of passing various company and DOT regulations and be produced for sale in the tens or hundreds of thousands.

While I have tried to provide answers, I seem to end up saying the same things over and over so this post is intended to be a go-to post for those asking tire change questions.

First, it is important that the owner know the ORIGINAL tire size including the Type and Load Range and the recommended inflation from the RV company along with the GAWR for their specific Trailer. Finally if considering a change we need to ensure that the new tires can properly support the ACTUAL load on the new tires.

I will start off assuming the owner is keeping the tire dimensions the same i.e. 225/75R15 > 225/75R15. Note I said "dimensions" not the "size" as a tire engineer "size" to me includes the Type + Dimensions + Load Range

For P > ST or P > LT you need to remember that application of a P-type tire on trailer required that the RV company "De-rate" the load marked on the tire sidewall   Sidewall/1.10 = load capacity of a P-type tire on a trailer.
For ST > LT you will probably need to increase the dimensions and or Load Range to achieve sufficient load capacity.

The general rule of thumb "Any replacement tire MUST be capable of supporting equal or greater load than the original tire".
Another rule: You need to ensure that any tire you use is capable of supporting your actual MEASURED load, Not the load your neighbor said he has and not an estimate or the measured load someone posted on a forum. The load measurement ideally should be obtained with your trailer at its heaviest i.e. fuel, water, propane, clothes etc  If you can't get individual one side weights DO NOT assume a 50/50% side to side load split. While some trailers may be balanced at 49/51% but some have been found as much as 10% off balance i.e. 40/60% As a rule of thumb I suggest you use at least an assumed 47/53% splits you would use the 53% figure.
If making an investment in new size tires & wheels you really need to learn the real loads before making the change or you may discover you bought tires you should not be using.


You will need to consult the published Load & Inflation tables for your old and new tire to confirm load capacity numbers. I have THIS post with links to many different tire companies. Be sure you understand how to read the tables as while most provide load capacity per tire, some load figures are per axle. DO NOT use the "Dual" load numbers as these only apply when there are two tires mounted as a pair on each end of an axle.

Comment on valves. I always recommend that whenever changing tires, even if you are just replacing with same size and type, that rubber valves be replaced with bolt on metal valves and if you already have metal bolt in valves that you get the various rubber gaskets and "O" rings replaced as these rubber parts age out just as tires age out and it's awful to read about a $2 valve failing which can result in hundreds or thousands of dollars in damage and costs.

Finally, some new info that all RV owners should consider:
Late last year RVIA updated the tire type and load spec such that "based on the rating of the axle the tires have to be 10% greater than the axle rating,"  You will note that RVIA decided to ignore the reality of load unbalance.
Clearly, if you are getting new tires it makes sense to incorporate this new safety margin in your calculations.

I want to thank my fellow RV owner and tire design engineer CapriRacer for doing a bit of technical editing on this post. He also has a blog on tires. 

Next week I will do a post on trailer tire inflation.

If you find this post helpful and happen to see someone posting questions about changing tires please consider posting a link to this post as I don't see every tire question posted by all RV owners.



Friday, February 9, 2018

Quick post on Belt Separation not a "Zipper" failure

Starting to catch up on posts on various RV forums after 10 days on a beach in Jamaica not thinking about tires BUT here we go.

Post about this tire that lost it's tread.
 

There was a reply about "zippering"

"Your tires look like they threw the whole tread off. I always assumed that the treads I see on the road are bad retreads from truck trailers, but it appears that this is not the only source of thrown treads.

Note that Michelin has documented a "zipper" condition on their motorhome tires. It consists of a circumferential crack on the sidewall just where it meets the tread. It could be the start of a delamination of the whole tread, which is what you seem to have.

You might look this up on the Michelin website. It might result in a warranty claim.
The only good way to protect against such a problem is by inspecting the tire regularly, which is not easy to do with an inside dual, especially since the zipper starts on part of the tire only and doesn't start on the full circumference. I wouldn't have any confidence at all that a TPMS would catch such a problem."



While the reply was well intended it didn't in my opinion offer the correct  answer. A closer examination finds areas that were rubbed smooth.

This is usually an indication of long term separation probably after operating for hundreds or possibly thousands of miles with a small separation at the edge of the two steel belts.

 I commented that "IMO the picture indicates not just a "tread separation" but the top belt separated from the bottom belt. This is a long (thousands of miles) term progressive failure. Many times a close thorough inspection can discover the signs beforthe tire comes apart."
I also advised "RE "Zipper" comment.  A classic "Zipper" failure occurs not at the tread sidewall jusnction and certainly not in a textile body tire as the one seen in the picture but is a steel body tire that has been run severly underinflated. This results in steel fatigue in mid sidewall as seen in the picture in THIS post.

##RVT832

Friday, February 2, 2018

Not a "Blowout" but definitely was a Run Low Sidewall Flex failure. No TPMS was cause

Read the following on an RV trailer forum:
"BOOM it happened again, however, this time it was with the upgraded Maxxis tires, seems to me that a lot of these "trailer tires" are just plain garbage. 
The only positive thing I can say is that the Maxxis tires did not blow up like the TowMax tire did thus I suffered zero damage this time, call that a win-win.
All tires were checked prior to departure and filled to 80PSI, less than 20 miles to our destination (220-mile trip) I saw a bit of smoke coming from right side pulled over and the side wall gave out. 
Changed it with the spare Goodyear Marathon that camping world lovingly overcharged me for last year ($197.67 to be exact) and went on my way very slowly. 
Found a tire shop called Gattos in Palm Bay FL that had my size 235/80R16 and installed four new Goodyear Endurance tires (FYI tire shop was way less than camping world by $34 dollars per tire!)
Why 4 you might ask, well it just made sense to me kids, wife & dog in the truck better safe than sorry. Took the three good Maxxis home with me as they look fine, may use one and make a second spare tire. 
Let's hope the Goodyear Endurance tires are as good as they say at least they are made in the USA VS the Marathon that's made in China & had a really bad track record. 
Anyways just sharing my experience will be purchasing a TPMS system very soon before our next trip that's for sure. Any recommendations? Seems many offer a six-tire system all I need is a 4 tire system."



=============
My reply

Sorry to see you had another tire failure but there is no such thing as a "failure-Proof" tire. All tires need to be properly inflated. ALL THE TIME. Not just at the start of a trip.
The pictures provide the evidence with that nice 360 flex that resulted in one side ending up with just the bead area and lower part of the sidewall. This piece of convincing evidence is seen in upper left part of the picture.

I hope you held onto the other 3 tires as there is no reason to expect them to fail as they apparently didn't lose air like the failed tire did.
In case you wonder about my diagnosis you might review THIS blog post where I was provided with a number of sharp pictures of the failed tire.

As covered in THIS post, 99+% of tire failures are due to one of two different causes. Your tire has clear and convincing evidence of failing due to Run Low Flex Failure of the sidewall.

I would strongly suggest a TPMS be installed so you could get advanced warning of air loss.

Note: I have even seen instances where the valve core sticks open and if the valve cap is not metal with an internal "O" ring the tire just takes longer to go flat as plastic caps are really, IMO only reliable at keeping dirt and small birds out of the valve core sealing ring. You might review THIS post on valve cores to understand why metal caps with 'O"rings or TPMS sensors are the only items I consider acceptable to use on the end of a valve stem.




Thursday, January 25, 2018

Another question on why Trailer tires fail more often than tires on motorized vehicles.

On a thread on an RV trailer forum Peter made this statement
"At highway speeds interply shear is not as much of an issue, as I understand things."
I posted this reply:
Speed itself is not the issue that causes Interply Shear in Radial tires. Two things happen to radials as they are driven.
One is the tread (and belts) must flatten out when the tire contacts the road. This results in the steel cords moving relative to each other. This is a shear force.
Second is that when the contact patch or "footprint" is forced to turn a corner there is some slipage between the direction of travel that aligns with the center of rotation and the actual direction of travel.

On motorized vehicles the front tires have a slip angle and the side forces are what actually results in the vehicle turning. But of you were to project the center of rotation toward the center of the driving radius you will find that those centers are close together This is due to the "Ackerman" designed into the front end alignment.
Yokohama has this nice graphic.

This image has been resized. Click this bar to view the full image. The original image is sized %1%2.



Multi axle trailers however have two axles and 4 tires, with no tire rotating around a centerline that points to the center of the turn radius.

Here is what happens to trailer tires.


These tires are forced to higher than normal "Slip Angle" through any turn, Not just the extreme tight turns when backing into a parking space.
Damage to tire structure is cumulative and while a small turn imparts less shear than t tight ture even small forces can do damage on a molecular level.
Here are some links to various articles on Interply Shear.

Duals on a large truck slip on the pavement when forced through tight turns and with inflations over 100 psi they do not deforn as much as trailer tires with lower levels of inflation.

Increased inflation will lower the interply shear. It will never lower it to the level seen on a motorized vehicle Only Passive Steer axles on trailers similar to wat is seen on the back end of cement trucks can lower the shear too but I don't see any RV trailer company offering that expensive alternative. Afterall they know you can't make a warranty claim on failed tires on the RV company as belt failures from Interply Shear is a long term proposition.

Friday, January 19, 2018

Snow chains in RV application

Most of us only deal with ice in our drinks or as used to get our beer cold but some folks may find themselves wanting? to drive in areas where roads get snow and ice covered and even in areas where the use of chains is required.
The question is; What does the RV owner do when it comes to using chains in the dual application?

If we choose to travel in such areas, it is important to have the correct type and size of "chains" for our application or we may end up doing damage to both our tires and the side of our coach.
There are a variety of types of chain set-ups and even some alternatives such as "cables". I cannot address the legality of the alternate styles in all areas other than to offer that if there is an advisory that chains must be used and you get stuck and do not have chains on the unit you may face some fines and other penalties so you need to confirm the details of the requirement in your area before spending your money or before traveling out and about on snow and ice covered roads.

If you do not have duals your choice is to be sure you select the equipment correct for your size tire.

Duals have a few options. There is going to be a difference in cost, weight and ease of install so you need to do some research.
There are "Triple Rail" as seen in this video. There are some "cable" type systems as seen here, and some newer designs as seen HERE .

I have not been involved in any direct comparisons so cannot offer any advice on relative performance. For that, you may need to do some additional investigation on the internet or even talk with some over the road drivers. Looking at the different videos it appears to me that some designs may be more durable than others.

I do know that driving on dry roads with chains can cause some serious tire damage so only you can properly evaluate what system would be best for your situation.

Friday, January 12, 2018

Why do trailer tires fail more often than tow vehicle tires?

Here is the question posted on a travel trailer forum.
Why so many trailer tire failures and so few, if any, tow vehicle tire failures?

This post could be considered Part 2 of my post of last week. 

The primary reason IMO is staring everyone in the face and the information is molded onto the tire sidewall so you do not need to be an Einstein, but almost everyone chooses to ignore the science so they can save a few bucks.

Fact: Tire load capacity is a function of air volume (tire size) and inflation pressure. This is well established and is confirmed by every Load & Inflation chart printed. Increase size or increase pressure and you increase load.

If we all drove around on tires with LT level sizing and inflation there would be almost no failures other than those caused by road hazard or punctures.

BUT
People want softer ride, better fuel economy and lower cost so alternatives were introduced into the market.

LT tires can be considered the standard for tires designed to carry load at highway speeds.

P type tires have a higher load capacity than identical size LT tire but if you check the actual load on most car tires, you will see they have 15% to 25% or more "Reserve Load" capacity (more capacity than needed). Inflation is set for cars based not on load but ride, handling and fuel economy. You also know that P tires must have their load capacity de-rated when P tires are used in truck, SUV or trailer application. When P type tires are run with zero reserve load you will probably get more failures. (See Ford Explorer crashes.)

ST tires have a higher claimed load capacity than LT tires and TT companies use the smallest (lowest cost) tire that can, most of the time, meet the specification for load.

How then is it that ST tires are rated to carry more load?
The load formula for ST tires is based on a 65 mph Max operation speed and an expected shorter life. The formula was developed back when bias tires that wore out at less than 15,000 miles were the norm

Since 1990 when ST tires were still "Alpha" sized ( H78-15ST) the load formula has not been changes as far as I can determine.

The science indicates that if you want tire durability on your trailer as you get on your truck maybe you need to consider running the same type tires i.e. LT type and of course that means you also need to use LT Load & Inflation limits.

Here is some info I posted Sept 14 2011

"Here are some facts from the Tire & Rim Association industry standards book 

P235/75R15 105S (Standard Load -35 psi @ max load)
2028# 35 psi 112mph on a Passenger car
1844# 35 psi 112mph on a SUV or P/U or trailer (no Dual)

LT235/75R15 LRC
1512#single 1377# Dual 50 psi 85mph

ST235/75R15 LRC
2340#single 2040# Dual 50 psi 65mph
"

NOTE I did not mention the actual load on a tire but only the specified max. Weight data indicates many TT have one or more tire in overload.


Friday, January 5, 2018

ST type tire max speed

Originally Posted by LI Pets 
The 65mph is not really correct it depends on load and air pressure.

They can go to 80mph.
The above is from an online forum.

As I have pointed out previously:

The load formula that populates the Load/Inflation table, used by the tire industry for ST type tires is based on the assumption that 65 mph would be the MAXIMUM operation speed. If the operation speed was to be between 65 and 75 then users were to use a 10 psi higher value for their measured load.

Example ST235/75R15 with 2,030# measured load needs to run 50 psi minimum cold not 40 psi cold.

If running 75mph to 85 the Load capacity number must be reduced by 10% in addition to the +10 psi adjustment.

Example ST235/75R15 would be rated to carry (2,030 x.9) or 1,827# at 50 psi.

I know this adjustment process seems backward but that is the way tire loading is calculated.

Now I am sure some will say that the new ST tires come with speed ratings faster than 65. IMO many of these ratings were applied just to avoid import taxes. I know of no magic rubber that somehow gives an ST235/75R15 LR-C the ability to support 2,340# at 87 mph with 50 psi in it while an identical sized LT235/75R15 LR-C is only able to support 1,985#.

Before you say "Ya but the tire companies probably made big improvements in the new ST tires", I would ask what makes you think the tire companies would not want to be able to offer better load capacity to their LT tires?

Load capability is basically the tire air volume x air pressure, with adjustments for speed and expected service. So if you have a P/U pulling a TT the "service" would be the same and the speed would be the same so how can the tire with "ST" on the sidewall carry more load at the same speed?

Magic?


Now you are more than welcome to believe in magic or marketing claims but IMO using the load /infl tables without doing the adjustments will probably result in an increased likelihood of belt separation. So when you have a failure please do not post something here along the lines of  "I just had a blowout. I didn't hit anything and always check my air".  Tire failures usually occur because of cumulative internal structural damage from heat and time. The excess heat comes from the combination of speed/inflation/load.