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Saturday, October 29, 2022

Tire Autopsies - Run Low Flex Failure and Impact break

 I thought it might help to show a few examples of what was observed while doing a "Tire Autopsy".

A tire would be received with a complaint letter. The letter might say something like " I was driving down Route 66 at 55 mph when suddenly I had a blowout. I wasn't speeding and had checked the inflation just that morning. What is wrong with your tires? There must have been a defect.

The tire looked like this. 







Another view 

On close inspection I found what looked like a hole in the upper sidewall where the

rubber had been worn away.






 Using a bent paper clip,



I was able to confirm the hole


and looking inside we can see where the puncturing item


Note a bent paper clip will fold at the bend if you try

 and push it through a tire sidewall, but if there is a

 hole the paper clip can follow the hole.

For this case I provided the above information and pictures.


Another complaint involve a bulge in the tire sidewall.

 The location was marked by the tire dealer.

On the inside we see a lump

We can also see a dark curved line where the tire

had extra bending.

By cutting away the rubber from the inside of the tire,

down to the body cord we can now see the broken

cords from the impact.

I trust that after seeing these examples you have learned that

sometimes additional examination is required to find and 

confirm the actual failure.

After doing hundreds of similar examinations it allows the

experienced examiner to quickly identify that the reason for

 a tire failure was not some design or manufacturing "Defect",

but the result of external causes.

More like thin in future posts.

Friday, October 21, 2022

"Optimal Inflation" or not?

 Many think the RV sticker (aka tire placard)








or in tire Load & Inflation tables







PSI number is an "Optimum" pressure when it is in fact, it is the MINIMUM pressure needed to support the GAWR which assumes a perfect 50/50 side to side split which is unrealistic.

We tire engineers want to ALWAYS protect the tire from overload. We know that pressure will change with changes in the Ambient (air temperature in the shade). We also know that tires will always get warmer when they are driven.

The pressures found in the Load & Inflation tables are almost universal across all tire companies (a handful of Michelin sizes differ by 5 psi or so, so not significantly different) and they are not playing games with tire engineering science.

Unless there is a specific reason, which should be mentioned in a post, we are ALWAYS talking about "cold" inflation pressure as we know that tire pressure changes by about 2% for each change in temperature of the tire as I covered in these posts.

The Short answer to all tire inflation questions.
1. Get the weight on each RV axle from a truck scale (or similar)

2. If you do not get individual tire position weights (aka 4 Corner weights) assume the heavy end of each axle is supporting 51 to 53% of the axle total.

3. Consult tire Load & inflation tables to learn the MINIMUM inflation to run based on the heavy end of each axle.

4. I suggest that your cold inflation on all tires on that axle, be _at least_ 110% of the MINIMUM psi found in the table in #3 above, but do not exceed the max inflation rating for your wheel. (number may be on the wheel or you may need to consult RV company).

5. *Always* run a TPMS with the low pressure warning level set to the pressure in #3 above ( this might take some calculation effort as different TPMS have different ways of setting the warning level.



Thursday, October 20, 2022

The "safety factor" for tires


Many people ask about the "safety factor" for tires. The dictionary offers this definition for "safety factor": "The ratio of the maximum stress that a structural part or a piece of material can withstand relative to the maximum stress estimated for it in the use for which it is designed." While that sounds reasonable, it really only works when talking about items that fail from simply increasing the load placed on the component.

Items like tires do not really have a "safety factor," as tires generally do not fail by simply increasing the load too much. In a non-rolling situation, I would not be surprised if we could load tires to 200% or maybe even more than 300% of the load marked on the tire sidewall if they never had to roll. However, as soon as you introduce rolling or time or operating temperature, the maximum load before failure is much closer to the max load number molded on the tire sidewall. The exception to Max speed is affected by temperature, time and load. With zero load, many tires can probably handle 200+ mph, but again, for how long and at what temperature?

So you see that tire durability is affected by a combination of load, temperature, speed, and time.

Since tires are basically a structure made of some steel but mostly "organic" components, time and temperature can have a significant impact on the maximum load capabilities of the tire.

If we think of non-organic items like a steel girder or maybe even a stone block as used in a pyramid, we can see that time and normal atmospheric temperatures have essentially no impact on the long-term maximum strength. The exception would be if we were to allow steel to rust or stone to be exposed to water and freeze/thaw cycles.

Tire engineers prefer to use the term "Reserve Load" when talking about the load capacity of a tire. Here we would find a tire engineering definition as the difference between the tire's maximum capacity when inflated to the stated level for the specific application (the inflation on the tire placard) and the actual load to be placed on the tire.

Here are a few comparisons: First some normal car and truck applications.

As you can see, tires in normal vehicle service have their Reserve Load well in excess of the RVIA suggested 10%. I would suggest that in addition to the special, internal tire structure forces such as Interply Shear, this low level of Reserve Load is a significant contributor to the shorter tire life seen in RV service.

While you, the owner, have little input or control over the materials used in tire construction, you do have significant control over the Reserve Load on your tires as well as operating speed and tire inflation. By decreasing the actual load you place on your tires while ensuring the tire has the highest Load Capacity (the "Max Load" number molded on the tire sidewall), you will increase the "Reserve Load" for your tires.

You can also ensure the Reserve Load stays high by not exceeding the speed for the service. For LT and truck tires (19.5" and 22.5"), the max speed for RV service is 75 mph, except for a few tires that have a lower limit of 65 or 62 mph. Check the "Data Book" publication from your tire manufacturer. Goodyear, Michelin, and Bridgestone state their max speed in RV service in their data book (75 mph max). I see no reason to go faster than that unless you find a data book from your tire manufacturer that states faster. If you have a tire salesman that claims faster is OK, ask to see the speed IN WRITING from that tire manufacturer or ask for a signed letter from the RV dealer on their letterhead paper stating the max allowable speed of operation for the tires they sold you.

Thursday, October 13, 2022

Why you should have road hazard tire insurance

 Are you prepared for the cost of tire failure? When buying new tires for your RV, one thing to consider is road hazard tire insurance. That is especially true for ST-type tires you put on a trailer or fifth wheel.

  ( A bad "Fork" in the road)

If you have a failure, it could be blamed on some impact or cut which would not be covered by any manufacturing warranty. But with road hazard insurance any failure would be covered. Even something unusual, as seen here.

Road hazard tire insurance can be worthwhile in all RV applications

Not all brands and not all dealers offer this insurance for RV applications. But when I recently got new tires for my Class C, the Firestone store offered road hazard insurance. I said yes, as I wanted 100% coverage and some assurance that I would not have any expenses related to tire failure. No, my tires are not ST-type. But my advice to consider road hazard insurance applies to all types of tires when used in RV applications.

All I have to do is keep track of the sales receipt and I'm good to go if there ever is a tire problem. The receipt has the full DOT serial along with the notation that I purchased the insurance. So that makes keeping the invoice information doubly important as I do not have to go crawling around under my RV trying to read the DOT identification number aka "tire serial".

Speaking of keeping important papers, I keep copies of papers like car insurance, registration, truck scale weights, roadside assistance insurance, and tire invoices, etc., on my phone in a folder that is backed up "in the cloud" so the information can't get lost.