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Friday, August 23, 2019

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?




  1. Might it have something to do with the fact that vehicle manufacturers generally have a recommended pressure for tire inflation, regardless of the maximum pressure listed on the tire? Just guessing, but if load capacity is based upon air pressure, and the auto manufacturers specify pressure less than max listed for the capacity of the tire, could they have a concern that the auto should not carry more capacity without additional pressure (which they are setting a limit against with their little sticker in the door)?

  2. Light truck tires are made differently than trailer tires. The side walls are stiffer on a trailer tire because they are designed to handle more weight. A light truck tire has flexible side walls that provide a smoother ride.

    1. While you comment may be true i am wondering what data you have that supports your claim. You need to remember that it is the air pressure that supports the tire load not the tire construction. If you were correct then we would have Tire Construction / load tables not Inflation / Load tables.

  3. I forgot to add, trailer tire Side walls are stiff because they are designed to handle the extreme side load force caused by tight turning with dual axles.


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