with RV tire expert Roger Marble
Here’s an original post and question on a 5th wheel RV forum I follow.
“Searched this forum (no, didn’t search travel trailer forum) for any info on using LT (truck) tires instead of “trailer” tires on a 5th wheel. Nope, didn’t find anything. So, any problems with using 6 or 10 ply LT tires on my ’99’ trailer instead of the replacement tires for trailers?”
The poster gave the dry weight and stated GVWR but no actual scale readings. He also did not provide information on his current size or type or load range of his tires. He did provide info heard around various campfires. In the forum, there were a number of replies: a few OK and, sadly, a number just plain incorrect.
In the ’60s, RV trailers were mostly smaller single axle and towed behind a car or 1/2 ton pickup. RV companies wanted less expensive tires. Speed limits and actual travel speeds were lower so Goodyear came up with “Special Trailer” tires with a 65 mph max speed as part of the load formula specification. This speed reduction and decreased tread depth theoretically offset the increased load capacity when compared to the same dimension LT-type tires.
In 2002 after the Ford Explorer fiasco, new, tougher standards for tires were implemented by DOT. RV companies fought to keep the old test requirements for ST-type the same as they were in 1968, while P-type and LT-type had to meet new, tougher standards for the 21st century.
In 2017, China was accused of “dumping” cheap tires of all types into the U.S. market. Trade restrictions by FTC (not tire safety standards) identified speed-rated tires as not having to pay the import duty. Almost overnight most ST-type tires became speed-rated. The SAE test for speed rating is stated as a “passenger car tire test” but was applied to ST-type. It only requires tires to run for 30 minutes at the stated speed and to not come apart.
Back to the original post. “Dry weight” and GVWR (gross vehicle weight rating) are of no value when selecting tires. GAWR (gross axle weight rating) is almost useless as it is well documented that a majority of RVs have tires and/or axle in overload when actual loading is measured on scales. The only weight number that really means anything is the actual scale reading for each tire, as RV weight is almost never exactly split 50/50 axle to axle or side to side. Some big RVs have been found to be 1,000# or more out of balance.
Reserve load is the extra load capacity above the actual load. Cars and pickups have reserve load in the 20% to 40% range, while RVs have 0% to maybe 10%. That is a MAJOR reason for the difference in durability. There are also the shear forces seen in trailers that can be 24% higher than an identically loaded tire on a motor vehicle. This is a function of suspension dynamics.
Tire interply shear was a complete unknown back in the ’60s and not as well known or understood even in the ’80s. You see the large tread distortion due to lateral loading when backing into a parking spot when the tire bends sideways. You never see this bending on a motor vehicle. The shear is always there in every curve or turns, not just when backing into a parking spot. Even the normal sway observed when a trailer is towed down the highway is generating high interply shear. You can lower but not eliminate the interply shear by increasing your reserve load.
So how do you improve your reserve load? An increase in load capacity would be a good approach. For most trailers, there are a number of options. Increased load range is one as long as you confirm wheel limits on load and inflation. Changing to larger tires. Even changing to larger-diameter wheels might increase load capacity. I have seen some that increase all three specs, with reports of eliminating all tire failures other than road hazard or valve-related issues that can occur on any size, brand or type tire.
If or when you replace tires, the new tires should ALWAYS at a minimum have equal load capacity.
I have covered the above in numerous posts on my blog if you care to learn the facts from an actual tire design engineer. Or you can listen to the guy in the camping space next door or the salesman at “Billy Jo Bob’s Cheap Tire and Bait Emporium” – your choice.