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Tuesday, August 2, 2016

Can you "contain" a tire failure ?

I read a story about someone that had a couple of tire failures on their trailer. He was trying to build a solid steel shield over the tires in the wheel well in an attempt to contain the "Exploding " tire.. I offered a reply...

Sorry, but IMO trying to "build a shield" just is not a reasonable approach. I doubt that you can spare the 500 or more pounds of steel it would take for the shield and structure to support the shield. Since a failure of a tire can be explosive event a solid wall will not let the force dissipate. You need an open grate. Check out this compilation of exploding tires to see the forces involved. Note that most of these did not involve a tire spinning at 50 to 65 mph which would add significant force to any explosion.

You need to remember that tires simply do not fail catastrophically without some reason. There is no magic involved.

Starting with a new tire...
1. It can fail in the sidewall if it is run at highway speeds (30+) while significantly under-inflated ( 40 to 80% low). Polyester melts. Steel tires do not need the speed but steel fatigues so after maybe a couple thousand cycles you get a "zipper" failure. Properly functioning TPMS can warn of the air loss in the first couple seconds of a loss ( of just a few psi for some brands). In most cases this early warning will come way before the tire has lost enough air to result in steel fatigue or body cord melting. If you have a TPMS have you tested it? Can you hear the buzzer over the loud radio?

2. Radials can have a belt/tread separation. This takes many hundreds or even thousands of miles to grow large enough for the tire to come apart. This is where the close inspection in my blog post comes in. As I showed a tire with even significant separation does not have to come apart at once but it does leave visible clues.

The reason for belt separation is a combination of initial tire design and material selection and the long term use. Initial design can not prevent all damage done through excess heat and age but current technology in first class radials should deliver 5-6 years or 30 to 60,000 miles at specified inflation and a max of 80% load, except for multi axle trailers.

Due to trailer suspension design there are unique forces "Interply Shear" placed on TT tires that result is about 24% higher shear forces than seen in motorized vehicles. This means you would need to run very much decreased load ( maybe -25% to -50%) to get the same life on a TT application than the same tire on a TV application.

I do not know of any direct comparison real life testing so can only guess at the above figures other than the 24% that comes from Finite Element computer simulation that is a well developed tool in automotive circles  other than the RV industry.

Rubber strength degrades with time and heat with HEAT being an over-riding contributor. Do you cover your tires with white covers? This can result is a very significant lowering of tire temperature. Every hour of full sun exposure can be equivalent to two to 3 hours of use running down the highway at top speed.

Quick example: 8 hours a day 7 days a week for two months each summer can be the equivalent of 10,000 miles use as far as rubber degradation is concerned. So if we assume a tire is good for 40,000 miles  and you park it as in the above example after 3 years you may have "consumed the equivalent of 30.000 mile tire life, just while parked.

IMO making some effort to prevent a failure in the first place (TPMS & frequent inspection) would be a better use of time and money than trying to prevent damage to the RV with some sort of shield. 

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2 comments:

  1. Had a rear tire blow on our 40ft motor home. Inside tire so it didn't affect the outside body of the coach. It did however force both basement sidewalls in, bent the metal side wall framing and worst of all completely took out the "roof" of the wheel well up to the sub flooring of the coach. Gone was the outer black fabric skin, the thin aluminum underneath and at least one of the "2x4" cross studs in the floor. After straightening the metal framing I was able to reinstall the basement sidewalls into their correct positions and waterproof them with caulking and spray seal. Still, replacing the tire did not solve the problem as encountering ANY rain or moisture will result in the subfloor (which is particle board) getting wet and coming to pieces. I found a shop that was willing to try my idea of taking a piece of aluminum diamond plate 1/8" thick and cutting it to fit up into the top of the wheel well which they did using copious amounts of caulking on the back side of the diamond plate to seal it to what was left of the top of the wheel well. They did a tremendous job and the plate fit perfectly. I wish manufacturers would build a wheel well strong enough to contain a blown out tire.

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  2. Well I understand your desire to minimize damage to the RV but the forces involved when a spinning tomes apart are much higher than many people realize. While it may be possible to install some metal plate you then need to consider the structure needed to support the plate. Tire test lab has equipment designed to "contain" the pieces of a tire when it fails but I have never seen a test machine with solid walls as part of the problem is the expanding air needs to dissipate its energy and solid walls (plate) prevent force dissipation. Here is a video of a 33# passenger tire with maybe 40 psi failing. You can imagine how much more damage would be caused with a 100psi 100# RV tire.
    https://youtu.be/lvVf8UZJCrU
    Our test machines are lined on all sides by open steel grating much like the grating seen on the deck of large bridges. This grate is backed up with 1/4" wall 2"x2" square tube. This structure can approach 1,000# in weight and you would need similar on both sides of your RV.
    I think the best policy for RV owners is to try and prevent failures in the first place. Running TPMS can warn the driver of air loss which is the cause of most tire failures. Also close inspection once a year for any signs of localized irregular war or damage to the tire can indicate potential of internal structural weakness.

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