RV Tire Safety: More important info on trailer vs. truck tires

11

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.

 

Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.

 ##RVT954

 

Subscribe
Notify of

This site uses Akismet to reduce spam. Learn how your comment data is processed.

11 Comments
Newest
Oldest Most Voted
Inline Feedbacks
View all comments

Sam
4 days ago

I’m looking at a DRV Mobile Suites 38RSSA at 39′. The Dry Weight is 16,300lbs & GVWR is 21,000lbs. 2 Heavy Duty Axels. Are the 17 1/2” Goodyear H-Rated Re-Groovable truck tires good? What is a Re-Groovable tire? I will tow with a 2020 F-350 SRW, crew cab, long bed w/ 5th wheel prep package.

Roger Marble
4 days ago
Reply to  Sam

“regroovable” truck tires are tires with extra rubber under the tread design. This allows a basic tread pattern to be cut into the tire once the original pattern has been worn off. Very unlikely that you would ever be able to have your tires “re-grooved”. One thing to be sure is that you follow the max speed rating for those tires (found in GY Data Book) or by calling GY Customer Service. It is 75 mph or LOWER in RV application

Mike Gregg
4 days ago

I switched my 2017 Keystone 5th wheel 15 inch ST tires from the factory load range D to Load range E. No problems with the new tires. The factory tires were only rated to carry the empty weight of my trailer. How do I determine the load capacity of my 15 inch wheels?

John T
17 days ago

There is no such thing as a 6 or 10-ply radial tire. Radials have 2 or 3 plies at most.

Roger Marble
12 days ago
Reply to  John T

Correct. The term “Ply” was done away with many years ago with the introduction of Radial tire construction. We have “Load Range” which does tend to line up with the old “Ply Rating” numbers. Some folks are just more comfortable with the number rating so they continue to use the term “8 ply tire” when it would be correct to say “Load range D”.

Crowman
17 days ago

I switched my 15 inch rims with China bombs on our Jayco trailer with 16 inch rims with Michelin LT 225/R7516 tires. After 2 China bomb blowouts I made the switch and have not had any problems since.

Roger Marble
12 days ago
Reply to  Crowman

What was the load rating and inflation molded on the sidewall of your old ST tires? and the numbers on your new LT tires?

peterb
17 days ago

Years ago switched my cargo trailers, travel trailer, 5th wheel trailer and car hauler trailer from ST tires to LT tires (taking in account load ratings) and have never had another problem with flats, coming apart, or scrubbing the tread off. I will not go back to ST tires

Don
17 days ago

All good information, Roger. But you didn’t answer the question! Care to elaborate on the differences between trailer rated and LT tires? And whether (assuming the load ratings are as you recommend) there are any reasons NOT to use LT tires on a trailer?

Roger Marble
17 days ago
Reply to  Don

If you address the load rating difference between ST and LT tires and your wheels are rated appropriately i see no reason to not run LT type tires. You will need to increase physical size and or Load Range to do so, as there are no LT tires rated to support ST loads without making these changes.. With LT you get a lot more choices from the major tire companies and if it makes a difference you should be able to find USA made tires.

Gary Broughton
17 days ago

We had US made 16 inch tires on our Terry 5th wheel at 3200# at 80 psi. They were good with no problems.