Why are ST tires better than LT type?

Following some posts on an Airstream forum of tire inflation and type tires. Some are complaining about rivets "popping" when they increase tire pressure. Many questions in the thread and some confusing replies.  I thought some readers might find this information interesting.

Some general observations and comments.
On this RV forum, there are two Tire engineers with significant Forensic Tire Inspection knowledge, myself "Tireman9" and "Capri-Racer" Our focus is on getting a better tire life and seeing fewer structural tire failures.
Out there, I would guess there is a pop rivet expert who would ask why the size, number or type of rivet being used is failing at a high rate on Airstream trailers. Airplanes are riveted together but I don't hear about "poped" rivets in that application, so maybe there is room for some improvement in the rivets called for.

Very few RV owners know the actual load on their tires. Simply dividing the scale weight by two or 4 (the number of tires on the RV) doesn't provide the correct answer, as it is easy to have one tire position to be hundreds of pounds heavier than another and they only respond to the actual load on that tire and not some mathematical average.

Also, I don't know the loaded weight of each year/size/model TT so can't provide an informed estimate on the inflation that TT should run without knowing the actual measured load on each of your tires.
I do know that having a MINIMUM of 10% Reserve Load (tire capacity - actual load) is a new requirement (2017) from RVIA (Recreational Vehicle Industry Association). DOT has no margin requirement ( Zero%). Capri and I are on record of suggesting at least 15% margin. The Interply Shear numbers would suggest that TT need to have a reserve closer to 25% if you want tire life more like a Motorhome gets (5 - 7 years).

Since it is the air pressure that supports the load and not the tire, simply going up in Load Range but not changing the inflation will in all probability not gain you anything in actual load capacity.

The pressure on the tire sidewall isn't really the "Maximum" you can safely use but is the MINIMUM inflation needed to support the MAXIMUM load that is marked on your tire sidewall. Personally, I do wish the wording on tires was more technically accurate but I can only guess that the lawyers and bureaucrats decided to use "max" in relation to the pressure for some reason.

Here is a question I wish someone could answer. Why are ST tires rated to support 10% to 20% more load than the same size LT tire? In the past, the reason was that ST tires were only rated to 65 mph to compensate for the increased load. Now ST and LT tires have almost the same speed rating but ST still carry the extra load capacity? If tire companies could all simply improve their ST tire Speed Symbol, almost overnight in 2017, why don't they use the same "magic" tire construction to increase the load rating of their LT type tires?

Anyone?


##RVT911

What should Airstream and similar type trailer owners do?

One of the Forums I follow is on Airstream trailers. There was a post from an owner who was having problems getting the actual loads on the trailer, hitch, and TV. There were a number of posts on what he needed to do.

However, I saw no mention of tire loading or tire inflation so I added the following post to the thread.

"When you are done making adjustments and moving stuff around and get new scale readings. You need to look at tire loading and inflation.

Ideally, you would get the individual axle load as they are probably not split 50/50

You are also probably not 50/50 side to side loaded either.  The good news is that you don't have slides or residential refrigerator so are probably 51/49 side to side or 48/52%

The best approach is to take the heavier loaded axle and assume a 52/48% split and compare the 52% number with the tire load capacity. You should have at least a 15% load capacity margin.  (52% of the heavier axle is no greater than 85% of tire max capacity.

Finally, with a multi-axles trailer, you should use the tire sidewall pressure number as your "Cold Tire Inflation" to try and lower the Interply Shear forces that are trying to tear the tire belts off the carcass.

You can learn more on my RV tire blog."

Note: While I wrote this in response to a specific post/question on an Airstream forum the information on tire load & inflation would apply to similar trailers that do not have slide-outs or heavy residential refrigerators or other obvious unbalanced equipment.