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Friday, August 6, 2021

Should I run ST tires on my trailer or can I use LT type?

 When it comes time to replace your tires on your RV trailer you may have a few options. Sometimes you may also be confronted with a limited supply problem that limits your selection. What can you do? What are your options?

Now I am assuming you have followed the general instructions to get your RV on a truck scale and as a minimum confirmed the load on each axle when the trailer is loaded to its heaviest. I have numerous posts in this blog on scale weights and how to calculate and estimate the load on the heaviest individual tire

So knowing you need a tire that can support at least 115% (125% is better) of your measured or calculated heaviest load you are ready to start shopping.

I had one reader express some concern about what he called "Sidewall Shear" forces and thought a stiff sidewall would be a better selection. I have no idea how he was going to measure the inflated sidewall stiffness and pointed out that un-inflated sidewall stiffness was not a reliable predictor of the inflated stiffness.

Here is what I told him:

I would not worry about "sidewall shear" as the real culprit for belt forces is Interply Shear at the steel belt edges which can lead to a belt separation.
Sidewall failures, which some confuse with "blowout" is the result of running at highway speeds with significant (40% to 60% or more air pressure loss).

THIS post shows the forces that cause belt Interply Shear. Don't be mislead and think its the sidewalls that are being overloaded. It's not.

You can get warned about the Sidewall Run Low Flex with the use of a TPMS that is properly programmed to alarm as soon as air pressure drops below the level needed to support the measured load on your tires. This comes from scale measurement and consultation of tire Load & Inflation tables.

Belt separations develop over hundreds to thousands of miles. A TPMS will not warn of impending belt separation BUT sometimes it is possible to "see" indications of possible or probable belt separation with a close visual inspection to check for run-out. Either Radial or lateral. I posted a link to a video on a "Free Spin" inspection in THIS blog post.

The GY Endurance tires I have inspected included Nylon or Aramid belt cap in addition to the 2 steel belts. If you find an LT type tire that can support 115% or better of the scale load on your heaviest tire that might be an acceptable alternative if it also has Nylon or Aramit cap belt.  LT tires have to pass more difficult DOT testing than ST type tires so that should give us the confidence to use those tires in RV applications.

Finally, you need to limit your MAX operating speed to 75 mph with LT type tires and to 65 with ST type tires. Even though some ST type tires have a "Speed Symbol" letter that implies speeds higher than 65 we need to remember that the test for those speeds is only 30 minutes and in reality the Speed Symbol is really just an indication of high heat resistence. The load capacity of ST type tires is based on a formula that specifice 65 mph as the max operational speed.


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