as other posters say is size/weight. If you look close at the different threads folks that have tire problems are usually pulling heavier trailers with ST tires. Check out the TH forums with the many blown ST tire threads last summer. Some of the THs run 15-20000 GVWR . Some folks with lightweight trailers 5-6k and under can use STs but heck even a 44 psi rated P tire works better as their not limited to 65 mph. Thats one reason lots of lower priced lawn mower/utility type trailer with just two 3500 lb axles come with 44 psi P tires. Plenty of capacity and enough tire pressure to keep the tire from poping a bead on sharp turns when max loaded.
One posters comment on sidewall flexing more on a ST tire is right on. BUT that flexing causes heat. One reason the ST tire has more flex is it has a narrow tread design on a 75/ aspect ratio tire which leaves the sidewall basketball shaped. Our better weight carring tires for trailer has a flater sidewall profile, such as a LT235/85-16 or the smaller LT215/85-16, just like the sidewalls on a semi traier. The bigger the bulge the better chance of debree/road hazard/curbs sidewall damage.
'03 2500 Dodge/Cummins HO 3.73 6 speed manual Jacobs
'97 Park Avanue 28' with two slides
You probably find more Fifth Wheelers using LT tires because more of them come with 16" wheels. As someone mentioned, it is very hard to find LT tires in 15" sizes. Because my trailer is heavy, when I started having tire issues, I switched to 16" wheels and LT tires so I could get a higher load range and a greater variety (and availability) of tires. An unexpected added bonus is that my trailer tows even better than before.
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