Tire inflation not the same for all trailers

I have written a number of times on the topic of Interply Shear. This is the force that is tring to tear the belts of radial tires apart.
There are some highly technical papers on the topic and you can review tham after a simple Google search on the term. You can also look here on my blog for the posts where the term is tied to a post by simply checking the list of topics on the left side of my blog. Basically this force comes about when the belts in a radial tire are forced to change shape. This means either when a tire rolls and the footprint changes from curved to flat as the footprint rolls into contact with the ground. This force increases when external forces from cornering are also applied to tires.
So why do trailers seem to have such high Interply Shear forces? Well it's not all trailers as the cornering forces of single axle trailers are much lower than the forces of tandem or triple axle trailers.

What tipped me off to this was an observation at a campground that happened to have a freshly smother gravel driveway and a multi-axle trailer happened to make a 180° turn as I was walking by. I noticed that the gravel marks were not a smoth curve but there was a series of turns interupted by discontinuities.

 

A short time later I saw a video from Keystone RV Company of wheel lug nut torque (you should watch the full video some time)
 While watching their video I recall some special high side load tests we ran in our tire test lab.

Here is a short out-take from the longer Keystone video that shows what happens to multi-axle trailer tires.

In the video you can see how the two tires are fighting each other with one bending in as the other bends out. You can imagine that if the turn is made on gravel at some point the high sideload would result is a sudden breakaway or slip. That's what I observed on the gravel turn.

Back at work I had some high powered Finite Element computer programs run to simulate the side load of a multi axle trailer and the results showed that the side loads on a trailer could be 24% higher than on a standard vehicle even with identical radius and loads on the tires.

This post shows why tires get different side loading.

Further analysis showed that increasing the tire inflation could lower the extra shear, but sadly not eliminate it.

Bottom line
In a number of posts I have recommended that Motorhomes set their inflation based on measured static load plus a margin of at least 10% additional PSI. This would also apply to single axle trailers. BUT for Tandem and triple axle trailers I strongly recommend that the tire cold inflation be set to the inflation molded on the tire sidewall associated with the tire maximum load capacity. I also recommend that the measured static tire loads on these trailers be no greater than 85% of the tire maximum with a 20% margin being better.

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.

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"

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"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

Not a "Blowout" but definitely was a Run Low Sidewall Flex failure. No TPMS was cause

Read the following on an RV trailer forum:
"BOOM it happened again, however, this time it was with the upgraded Maxxis tires, seems to me that a lot of these "trailer tires" are just plain garbage. 
The only positive thing I can say is that the Maxxis tires did not blow up like the TowMax tire did thus I suffered zero damage this time, call that a win-win.
All tires were checked prior to departure and filled to 80PSI, less than 20 miles to our destination (220-mile trip) I saw a bit of smoke coming from right side pulled over and the side wall gave out. 
Changed it with the spare Goodyear Marathon that camping world lovingly overcharged me for last year ($197.67 to be exact) and went on my way very slowly. 
Found a tire shop called Gattos in Palm Bay FL that had my size 235/80R16 and installed four new Goodyear Endurance tires (FYI tire shop was way less than camping world by $34 dollars per tire!)
Why 4 you might ask, well it just made sense to me kids, wife & dog in the truck better safe than sorry. Took the three good Maxxis home with me as they look fine, may use one and make a second spare tire. 
Let's hope the Goodyear Endurance tires are as good as they say at least they are made in the USA VS the Marathon that's made in China & had a really bad track record. Anyways just sharing my experience will be purchasing a TPMS system very soon before our next trip that's for sure. Any recommendations? Seems many offer a six-tire system all I need is a 4 tire system."

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My reply

Sorry to see you had another tire failure but there is no such thing as a "failure-Proof" tire. All tires need to be properly inflated. ALL THE TIME. Not just at the start of a trip.
The pictures provide the evidence with that nice 360 flex that resulted in one side ending up with just the bead area and lower part of the sidewall. This piece of convincing evidence is seen in upper left part of the picture.

I hope you held onto the other 3 tires as there is no reason to expect them to fail as they apparently didn't lose air like the failed tire did.
In case you wonder about my diagnosis you might review THIS blog post where I was provided with a number of sharp pictures of the failed tire.

As covered in THIS post, 99+% of tire failures are due to one of two different causes. Your tire has clear and convincing evidence of failing due to Run Low Flex Failure of the sidewall.

I would strongly suggest a TPMS be installed so you could get advanced warning of air loss.

Note: I have even seen instances where the valve core sticks open and if the valve cap is not metal with an internal "O" ring the tire just takes longer to go flat as plastic caps are really, IMO only reliable at keeping dirt and small birds out of the valve core sealing ring. You might review THIS post on valve cores to understand why metal caps with 'O"rings or TPMS sensors are the only items I consider acceptable to use on the end of a valve stem.