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Friday, February 16, 2018

Changing tires on a trailer - NEW load capacity requirement

On many of the RV forums I monitor that focus on trailer application, there is a recurring question about changing tires. Some wonder about going up in Load Range (Ply Rating) some wonder about changing the "Type" 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 responsiblity 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 i.e. 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 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 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. 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% but some have been found as much as 10% off balance i.e. 40/60% As a rule of thumb I suggest you use at least an assumed 47/53% splits you would use the 53% figure.
If making an investment in new size tires & wheels you really 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, that 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 and 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.



Friday, February 9, 2018

Quick post on Belt Separation not a "Zipper" failure

Starting to catch up on posts on various RV forums after 10 days on a beach in Jamaica not thinking about tires BUT here we go.

Post about this tire that lost it's tread.
 

There was a reply about "zippering"

"Your tires look like they threw the whole tread off. I always assumed that the treads I see on the road are bad retreads from truck trailers, but it appears that this is not the only source of thrown treads.

Note that Michelin has documented a "zipper" condition on their motorhome tires. It consists of a circumferential crack on the sidewall just where it meets the tread. It could be the start of a delamination of the whole tread, which is what you seem to have.

You might look this up on the Michelin website. It might result in a warranty claim.
The only good way to protect against such a problem is by inspecting the tire regularly, which is not easy to do with an inside dual, especially since the zipper starts on part of the tire only and doesn't start on the full circumference. I wouldn't have any confidence at all that a TPMS would catch such a problem."



While the reply was well intended it didn't in my opinion offer the correct  answer. A closer examination finds areas that were rubbed smooth.

This is usually an indication of long term separation probably after operating for hundreds or possibly thousands of miles with a small separation at the edge of the two steel belts.

 I commented that "IMO the picture indicates not just a "tread separation" but the top belt separated from the bottom belt. This is a long (thousands of miles) term progressive failure. Many times a close thorough inspection can discover the signs beforthe tire comes apart."
I also advised "RE "Zipper" comment.  A classic "Zipper" failure occurs not at the tread sidewall jusnction and certainly not in a textile body tire as the one seen in the picture but is a steel body tire that has been run severly underinflated. This results in steel fatigue in mid sidewall as seen in the picture in THIS post.

##RVT832