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Friday, June 21, 2019

Tire sidewall "strength". What does that mean?

I note that on many RV forum posts on the topic of tire "Blowouts," there many times are posts on the "Strength" of the tire sidewall. Before I start, it's important that we have a shared definition of a few words.
"Blowout" is simply a statement that a tire failed catastrophically. It does not mean it simply exploded as if it were a bomb.

Belt or Tread Separation is the detachment of the belts and or tread in a radial tire. This can lead to a rapid loss of air which can make a loud noise, surprising the driver and leading to the idea that there was an explosion or "Blowout". But not all tires that have belt separations end up with a rapid loss of air. Here is an example of a tire with tread & top belt detached from the rest of the tire. It is shiny as I sprayed water on the tire while inflated on a wheel to inspect for punctures or leaks as I had suspected the tire had been run with low inflation. I "redacted" identifying marks as that information is not important for this discussion.

Sidewall Failure for this discussion means a failure of the tire sidewall not related to a sidewall cut or impact. many times when a tire loses air but is still being driven at highway speeds, the body cord or "body ply" material can fail due to excessive heat.  A more technically accurate term used by tire engineers would be "RLOF" which stands for Run Low Sidewall Flex Failure. Most Passenger, Light Truck and ST type tires made today use Polyester cords the sidewall ply.  With excessive flexing and bending from low inflation, the cord can overheat.








If the temperature gets high enough (300°F to 350°F) the cord can lose half its strength and high temperatures can result in the cord melting just as you have seen when you melt the end of a piece of Nylon or Polyester rope with a match.
 Here is what melted tire cord looks like.

I have shown this condition in a few posts, but probably the one with the best example is my post "Blowout- Real Life Example"

So why does this engineering stuff make a difference? You still had a "Blowout," and are not happy. Probably want to blame someone and the tire company is an easy target. BUT as I have said before if you do not know the real reason for a tire failure you might not prevent another failure from happening.

Imagine you had an RLOF but did not bother to try and learn why the tire was low on air. Puncture, cut, leaking valve, leaking valve core, cracked wheel are all "suspects" and just replacing the tires on your RV with a different brand will not prevent another "Blowout".

So this leaves the question of Sidewall Strength. DOT has a specific test to confirm a minimum "strength" for tires. The test procedure. involves forcing a 34 inch diameter steel rod with a hemispherical end perpendicularly into the tread as near to the centerline as possible,  at the rate of  2 inches per minute. This is repeated 5 locations around the tire and there are published minimum energy requirements (inch-pounds force) that tires must exhibit. These minimums are based on the tire size, type, and Load Range.

Now you may ask: How does the sidewall material strength come into play? Well, the sidewall material runs under the belt material so is part of the total strength requirement. Tire companies also have their own "Burst" test requirements which involve mounting a tire on a special test wheel and increasing the pressure till the tread or sidewall or the bead fails. They use special wheels as most tires are stronger than regular production wheels. The minimum pressure is not published but in most cases, it is in excess of three times the inflation number molded on a tire sidewall. and in some cases, I have seen tires exceed six times the inflation number on the sidewall.

Tire Design engineers have a wide choice of sidewall ply material to choose from. Different types of cord, different sizes and even different amounts or cords per inch circumference can be selected as the engineer works toward the final design specification. Simply claiming that "Our tires have larger and stronger cord" while true doesn't address the question of how much of that cord is used in the tire.

Bottom Line. I hope you now understand how simply claiming the tire sidewall wasn't strong enough will not help you solve your tire failure issue.

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