Changing trailer tires – new load capacity requirement

Changing trailer tires – new load capacity requirement

RV Tire Safety
with 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.

##RVT833

 

 

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7 thoughts on “Changing trailer tires – new load capacity requirement

  1. Roger

    If you have a specific tire question please provide the complete size & Load Range of the tires you are talking about.

  2. Bill Watson

    I have heard of a company offering a 225/70R19.5 replacement for the 235/85R16 wheel &tire found on most 5th wheels. Would this be smart, would it effect handling or ride? It should decrease side to side tire movement but is it safe?

    1. Roger Marble

      You didn’t say the Load Range but 225/70R19.5 LR -F @ 95 psi are rated for 3640 ST 235/85R16 LR -E 3640 @ 80 psi so without more info I don’t understand the offer as it doesn’t provide more load.

  3. Terry Brown

    Roger, Will you be addressing issues about the rim or the need to upgrade your rims if you change to larger tires?

    Example: My cougar weighs 12200 lbs (dry weight w/o anything loaded) and I now have GY Endurance 235/80R-16. An “F”rated tire. Weight rated at 3420 lbs. each and a max psi of 80.

    I have considered Sailun S637s and will have to get 635/85R-16s. The load capacity is 4400 lbs. at 110 psi.

    The concern that my present time will not tolerate the higher psi. When the tires were changed to the Endurance tires there were no markings or numbers to pressure rating of the rims that are the originals.

    Your opinion would be appreciated and that you for being with RV Travel.

    1. Roger

      Some have suggested that it is the load that “kills” most wheels. However, without some statement from the wheel company, I would hesitate at increasing the inflation 30 psi.

  4. STEPHEN P MALOCHLEB

    Roger you would know better then me but have’nt the rv builders been putting on the cheapest tires then can find. I have been in the auto industry for over 40 years and still to this day I see tire companys selling tires by price not capability of vehicle load.I’ve always believed that if my RV came with an 8 ply rating that replacement would be 10 ply or more. Better to have more then not enough. Thanks for your continued articles.

    1. Roger

      Stephen, Before jumping to the conclusion that increased Load Range “D” to ‘E” is the only option. You need to remember that unless you increase the CIP you will not increase the load capacity. I tried to lay out what I consider is the proper process in step by step format.
      Another observation on the new tire load capacity rule. Just because tires are supposed to have a 10% margin that doesn’t mean tires on new RVs will actually have more capacity. The 10% margin can also be achieved by simply lowering the GAWR (reducing RV load capacity). It is quite possible that some buyers will assume they can put the same load in their new model as they had in the old one when in reality it is possible that some RV assemblers simply are changing the brochure and tire sticker but delivering the same old tires.

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