with RV tire expert Roger Marble
It’s no longer 1970 with a national 55 mph speed limit and with ST tires being introduced, i.e., “pushed,” by a large tire company as an alternative to the real LT truck tires of the day. I have been told that one of the “selling points” for this then-new type of tire was that “no one would ever pull their 15′ trailer faster than 55 with their bumper hitch.”
Well, times have changed. We now have “1-ton” diesel pickups that can pull a 35′ trailer up the side of a mountain and run 75+ mph all day long. Trailers now come with two bathrooms, residential refrigerators, two AC units, multiple TVs and other heavy equipment that we never considered possible in 1975.
What hasn’t changed is the fundamental science that a tire’s load capacity is still basically “Load = Air Volume x Air Pressure.” In fact, the actual load formula still used for current ST-type tires is identical to the one used in 1970 with the 65 mph speed limit still in the industry standards.
I can find no mention of alternate materials delivering increased load capacity in those standards. Yes, materials have improved and radials are better than bias tires, but basically the only benefits all these “improvements” delivered is longer life and better fuel economy and tread wear. Some construction features such as the addition of nylon cap strips or full cap ply have allowed an increased resistance to the heat from higher speeds, but I haven’t seen any increase in load capacity in either passenger or L- type tires over the past 40+ years.
As I have posted in this blog, “There is no free lunch.” If there is, why haven’t tire companies increased the load capacity of passenger and LT-type tires if all these “improvements” are available to tire engineers?
What “feature” is in ST-type tires that gives them the +10% to +20% more load capacity over an LT tire of the same physical size?
IMO there is no reason why RVs could not be supplied with LT tires other than it would increase the cost of the tires to the RV company.
Remember, it is the RV company that is responsible for selecting the tires they provide. We all see numerous posts from some who are running truck 17.5 size or LT 16″ tires with improved durability, so I see no reason to believe that the entire market could not benefit from a switch from ST- to LT-type tires.
One thing is that the LT tires that have to meet the new DOT standards (FMVSS 139) that were introduced in 2002 are probably more durable than the ST tires that are still only required to meet the standards of the 1970s.
I am not saying that some of the newer ST tires with newer construction are not significantly better than the same size tires from the ’90s, but IMO there is still a limit.
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