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Friday, January 17, 2020

LT vs ST for RV trailers?

It's no longer 1970 with a National 55 mph speed limit with ST tires being introduced i.e. "pushed" by a large tire co, as an 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+ all day long. Trailers now come with two bathrooms, residential refrigerators, and 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 when the 65 mph speed limit for ST type tires was 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 LT 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 P and LT type tires if all these "improvements" are available to tire engineers?
What "feature" is in ST type tires that gives then 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 Co 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 on the 1970's.

I am not saying that some of the newer ST tires with  newer construction are not significantly better than the same size tire from the 90's but IMO there is still a limit.

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