Changing tire types

[Bill]

The forum has had a lot of discussion about changing out TM's OEM tires for something else, usually for reasons of reliability. A recent issue of RVTravel.com included an article by Roger Marble, an acknowledged and experienced tire expert for the RV Industry. This article originally appeared in his blog called http://www.rvtiresafety.net/. With the permission of both RVTravel.com and Roger Marble, I have copied the entire article here in the Trail Manor Technical Library.

In his discussion, Roger uses the abbreviations ST, P, and LT. ST stands for Special Trailer, meaning tires that are specifically designed for use on trailers, P means tires designed for passenger cars, and LT means light truck tires. Tires of these three types are not necessarily interchangeable, even in the same size made by the same manufacturer. And remember that when Roger uses the term "MEASURED LOAD", he specifically does NOT mean the "dry weight" listed on the manufacturer's spec sheet. Read Roger's article below to find out more.

Bill

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Changing trailer tires – new load capacity requirement by Roger Marble
February 14, 2018 RV Staff

RV Tire Safety by RV tire expert Roger Marble

On many of the RV forums I monitor that focus on trailer application, there are recurring questions about changing tires. Some wonder about going up in Load Range (Ply Rating). Some wonder about changing the “Type” of tire: P > ST, or P> LT, or ST > LT. Others want to change the tire dimensions. While there are many reply posts, I do note that not everyone offering answers has worked as a tire design engineer. It takes years of working with the engineering and scientific knowledge before you can be given the responsibility to develop a new tire capable of passing various company and DOT regulations and be produced for sale in the tens or hundreds of thousands.

While I have tried to provide answers, I seem to end up saying the same things over and over, so this post is intended to be a go-to post for those asking tire change questions.

First, it is important that the owner know the ORIGINAL tire size including the Type and Load Range and the recommended inflation from the RV company along with the GAWR for their specific trailer. Finally, if considering a change, we need to ensure that the new tires can properly support the ACTUAL load on the new tires.

I will start off assuming the owner is keeping the tire dimensions the same, e.g., 225/75R15 > 225/75R15. Note I said “dimensions” not the “size.” As a tire engineer, “size” to me includes the Type + Dimensions + Load Range.

For P > ST or P > LT you need to remember that application of a P-type tire on a trailer required that the RV company “De-rate” the load marked on the tire sidewall. Sidewall/1.10 = load capacity of a P-type tire on a trailer.

For ST > LT you will probably need to increase the dimensions and or Load Range to achieve sufficient load capacity.

The general rule of thumb: “Any replacement tire MUST be capable of supporting an equal or greater load than the original tire.”

Another rule: You need to ensure that any tire you use is capable of supporting your actual MEASURED load, not the load your neighbor said he has and not an estimate or the measured load someone posted on a forum. The load measurement ideally should be obtained with your trailer at its heaviest, i.e., with fuel, water, propane, clothes, etc. If you can’t get individual one-side weights, DO NOT assume a 50/50% side-to-side load split. While some trailers may be balanced at 49/51%, some have been found as much as 10% off balance, e.g., 40/60%. As a rule of thumb, I suggest you use at least an assumed 47/53% split, and you would use the 53% figure.

If making an investment in new size tires and wheels you need to learn the real loads before making the change or you may discover you bought tires you should not be using.

You will need to consult the published Load & Inflation tables for your old and new tire to confirm load capacity numbers. I have this post with links to many different tire companies. Be sure you understand how to read the tables as while most provide load capacity per tire, some load figures are per axle. DO NOT use the “dual” load numbers as these only apply when there are two tires mounted as a pair on each end of an axle.

Comment on valves: I always recommend that whenever changing tires, even if you are just replacing with same size and type, rubber valves be replaced with bolt on metal valves, and if you already have metal bolt in valves that you get the various rubber gaskets and “O” rings replaced as these rubber parts age out just as tires age out. It’s awful to read about a $2 valve failing which can result in hundreds or thousands of dollars in damage and costs.

Finally, some new info that all RV owners should consider. Late last year RVIA updated the tire type and load spec such that “based on the rating of the axle the tires have to be 10% greater than the axle rating.” You will note that RVIA decided to ignore the reality of load unbalance.

Clearly, if you are getting new tires it makes sense to incorporate this new safety margin in your calculations.

I want to thank my fellow RV owner and tire design engineer CapriRacer for doing a bit of technical editing on this post. He also has a blog on tires.

Next week I will do a post on trailer tire inflation.

If you find this post helpful and happen to see someone posting questions about changing tires, please consider posting a link to this post as I don’t see every tire question posted by all RV owners.

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