The main reason for ST-type trailer tires to gain more pressure than the LT tires, is that they are forced to support more load relative to their size and inflation than LT tires are.
If you ever look at the Load & Inflation tables and find an LT-type and an ST-type tire of identical physical dimensions, you will see that the ST tire is “rated” to carry more load than the LT-type tire is. The basic theory behind that increased capacity is that ST tires will be traveling slower because people should not be traveling as fast when towing as when just driving the car or truck. When ST tires were introduced, they were limited to 65 mph MAX in an effort to offset the damaging effects of higher loading than seen in LT tires.
Tire load capacity
Tire load capacity is basically a function of volume and pressure as seen below, where Load = K x P (Air Volume x Air Pressure), with different type tires having different a “K” factor. Tires in LT applications are required to support lower load as a percent of their volume and pressure so they do not have to “work” as hard, so they do not generate as much heat.
More heat means a greater increase in pressure. (Approx 2% pressure increase for each increase in temperature of 10 F.)
Please note that the actual load calculation is much more complicated as the response to air pressure is not linear and different aspect tires, i.e., 75 series vs. 85 series, etc., have some different factors that are applied to the actual calculation.
Please note that the above is just one page of many used to calculate load capacity for tires not already published in the TRA Yearbook, so do not go and try to check every size. I show this to demonstrate that the calculations are complex.
Have a tire question? Ask Roger on his new RV Tires Forum here. It’s hosted by RVtravel.com and moderated by Roger. He’ll be happy to help you.
Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.
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“I show this to demonstrate that the calculations are complex.”. No kidding. Sometimes Roger, your technical descriptions leave me in the dust . . . Just sayin’. One takeaway from all your articles is that I will always buy ST tires for my trailer.
My 2nd 5th wheel ‘99 Carriage Carrilite came from the factory with 285R78x16LT the same size my GMC truck used.
OK FYI Your LT285/75R16 LR-E have to pass more stringent DOT tests than the ST type tires do. What brand is it? Design name ?
That’s an interesting size. 78 series is an older series than most new tires and 285 is very wide BUT as long as the load capacity can support 110% or more than the stated GAWR and 110% or more of the GAWR you are probably better off than with an ST type. Could you go and read the tire material list molded on the sidewall? Do the belts say 1 or 2 ply Polyester + 2 ply steel + 1 or 2 ply Nylon?
Thanks Tony. I learned something!