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Monday, January 13, 2014

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.


  1. So where does one REALLY go to get the load on each individual tire weighed?

  2. You can use instructions at 5th Wheel
    or worksheet from Bridgestone
    or visit RVSEF and see where they will be "Weighing Program

    Or read my posts click on the word "Load" on the right

    1. Roger, I discovered your blog only a few days ago. A an engineer myself, it is refreshing to find a place where I can get meaningful info.

      I have towed smaller trailers for years and take good care of my tires. I bought a 5th wheel last February (big jump for me). It's 27 feet and max of 9900 lbs loaded. The tires (Towmax Power King) are (visuallly) in good shape and I keep them properly inflated. I have read many posts about problems with these tires. I am sure the posters are sincere, but I treat this as somewhat anecdotal because there are too many unknowns (weight, inflation etc). They have however made me a little gun shy. I am planning a 1200 mile trip this summer and plan to weigh the RV before going. But, I generally error on the side of caution and am considering changing the tires even though I will be installing TPM system before I go. I know nothing is certain, but the last thing I want is deal with blown tires while traveling. Do you have any info/insight regarding the viability of Towmax Power Kings?

  3. A. Get a TPMS
    B. Confirm you have a margin of at least 10% on tire load capacity so as a MINIMUM get the individual axle loads
    C. Always run the inflation on the tire sidewall.
    D. If possible get each tire up in the air and do a visual check for out of round.
    if you see "wobble" as seen in the video you must replace that tire and consider the others too.

    email me (address under my picture) if you have other questions.


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