ST Tire "Plies" Question and a PSI Question

Tireman9 wrote:
On the question of what inflation to run. I would suggest that for multi-axle trailers the tires be run at the tire sidewall pressure. 65 psi for the LR-D in this case, even if the OE tires were LR-C and 50 is sufficient to carry the measured tire load.
There are a few advantages.
The higher inflation provides greater Reserve Load (safety factor for some)
The higher inflation lowers the Inter-ply shear which is the force trying to tear the belts apart every time you go around a curve or corner.
This shear loading is unique to multi-axle trailers and has been calculated using Finite Element Analysis to be significantly higher than the forces experienced on identically loaded tires on Motorhomes.

This is one reason trailers seem to only get 3 to 5 years tire life while Motorhomes get 6 to 8 or 10 years.

The only part of this that I have a problem with is ST tire life. If ST's are designed for the unique sheer loading of trailer use, then they should be capable of surviving for more than 3-5 yrs. And if sheer loading is the primary issue with trailers, it would also seem that LT tires used on trailers should have inferior life expectancy compared to purpose designed ST tires. And if LT users are more likely to inflate their trailer tires based on weight while ST users are more likely to inflate to max PSI, then that would further imply worse reliability when using LT tires on a trailer. Yet there seems to be very few issues with using LTs on trailers.

A few yrs ago at a rally I checked the tire stickers on every TT and fiver at the rally. 1/3 specified ST tires, 1/3 specified LT tires and 1/3 did not specify either. Furthermore, most of the ones specifying LTs were heavier, expensive units like DRV. Now if ST tires are really engineered for higher load and specifically higher sheer load, then how could these rv manufacturers specify LTs and not STs??