Tires and Burst Pressure, What causes Blowouts?

Recently saw a post on another blog Tires and Burst Pressure, What causes a "blowout". 

Now there was nothing really wrong with what was posted, it was just not as accurate as it could have been.

I have many posts on tire blowouts and their most likely cause, but as long as I continue to hear people claim or read blog or forum posts on blowouts followed by partially correct information I will continue to try and provide the facts based on the thousands of tire autopsies, I have done in my career.

Tire Maximum Pressure

Many times people incorrectly think the pressure molded on the tire sidewall, similar to what we see in this picture,

to be the maximum tire pressure a tire can tolerate when in reality you should consider it the minimum pressure needed to carry the load indicated 

Tire companies design and test their tires to tolerate pressures much higher than the number molded on the sidewall. For regular passenger tires, it is probably close to or above 150psi. Light truck tires will probably be a bit higher and truck tires higher yet. In almost all cases I am aware of, there is a good possibility that the wheel might fail from high pressure before the tire simply explodes from high pressure.

SAFETY WARNING  DO NOT TEST THIS YOURSELF. The explosive force can take out concrete block walls and could kill you.

The correct terminology for the pressure on the tire sidewall is: The tire pressure molded on the tire sidewall associated with the maximum load capability for the tire. Now this is admittedly a real mouthful so I can understand it being shortened to "Tire pressure associated with max load" or even Tire pressure molded on the tire sidewall. but it is incorrect to say or think that the pressure molded on the tire is the maximum the tire can stand.

Now it is important that you not confuse "The tire pressure molded on the tire sidewall associated with the maximum load capability for the tire." with the number that might be part of the tire "Safety Warning" 

many passenger and light truck tires have a separate warning about tire pressure not associated with the tire load capability. This has to do with the pressure used to "seat" the beads. This is the "pop" you hear in the tire store when the tech is inflating your new passenger tires.

Important Tire Safety information. If you do not understand the safety warning you should never be inflating tires from flat. Leave that job to the professionals.

Bottom Line

Tires can handle pressures higher than the number molded on the tire sidewall that is associated with the maximum tire load, so do not bleed down your tires when the hot tire pressure increases above the number molded on the sidewall. Pressure increases of 10 to 20% are not uncommon. Always inflate your tires to specified pressure when they are "cold". That means at air temperature, not having been driven on or in sunshine or even partial sun for at least two to three hours.

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.

Tire Inflation Check TEST

One of the Myths that many people have about tires is that they believe that they can tell if their tires are properly inflated simply by looking at them. After all they look at their tires so they believe they know what they look at when properly inflated so being able to see say a 10% loss in air is easy.
 Check out the presentation below and see for yourself how well you do.

You can view different slides by moving the bar on the side of the image below.

How Do You Check Your Inflation

Tire Inflation accuracy

I recently read a number of posts on some RV forums on tire gauge, TPMS pressure accuracy and proper inflation levels. I will try to answer a number of questions in this post.

At a number of RV events I have tested different gauges against a digital 0.5 psi reading gauge that has been checked against QS9000 ISO/IEC 17025 approved master laboratory pressure gauge and found accurate to +/- 0.5psi. We have found that about 15% of the tested gauges were wrong by more than 5psi with a few off by more than 12psi. In general the most accurate were digital. The least accurate were the "stick" gauges with the sliding scale.

I have suggested that if you are accurate to within 5% of your goal cold tire inflation you are probably OK but obviously more accurate is better. The 5% tolerance is based on an assumption that you have added 10% to the minimum required per measurement of actual tire load and consultation with your tire companies Load/Inflation tables. It has also been suggested that using any gauge is better than not checking your tires at all.

Now how to know the accuracy of your gauge in day to day use. I suggest the following based on personal observation (no sponsorship or compensation received).

 Get two digital gauges. One can be like Accutire MS-4021B as seen on the right

or similar for your "master". The choice of the 2nd gauge depends on how you can access the valves on your rear tires.

I have hose extenders as seen below

that means I do not need the more expensive dual foot gauge like Accutire MS-5515B seen on right so I use two of the 4021B. If you don't have valve extenders your day to day gauge would be the dual foot gauge.
My master is packed away in a box so it does not get damaged or knocked around. Once a month I measure one front tire with both gauges and record the readings. The difference between the two readings should not change by more then 0.5psi from new.  If I notice a change in tire inflation not explained by significant temperature change or puncture I will compare the daily gauge against the master gauge. I figure the chance of both gauges going bad the same amount and in the same direction at the same time is vanishingly small.  The 4012 is available at $10 and occasionally for less than $7.

TPMS readings have not found to be as accurate as hand gauges. Their purpose is to be a warning of a loss in pressure. You should set tire pressure with the hand gauge. You should see a fairly constant difference between the TPMS and the hand gauge.

It is important to measure your tires when they are at ambient temperature. This means the air temperature. NOT exposed to the sun or even under a tire cover or having been driven on for at least two hours.

Tire pressure changes with temperature and you can use the rule of thumb of 2% for every 20F.