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Wednesday, November 20, 2013

"Interply Shear" and other Techno Babble

I have been putting this topic off for quite some time as I was worried about overloading you with too much "Techno-babble" but I find myself having to constantly repeat this information in individual posts on various RV forums I monitor so figured it would be easier to do a complete post that I could direct people that want to understand why the loading of some trailer tires is much more complex than the average person thinks.

To make this less painful I will give the Bottom Line info first, so those not interested, can stop reading before I put them to sleep.

When a radial tire is loaded, the belts and body have to bend from a round shape to a flat shape in the area that contacts the road. In addition when you turn a corner the forces generated to move the RV sideways have to be transferred through the tire structure.

This causes additional bending of the belt and body structure. The more the bending the higher the stretching of the rubber. With enough stretch, microscopic cracks form and existing cracks get bigger. Eventually with enough cycles and enough force the cracks may grow and join up with the possibility of tire components separating which could lead to a tire failure. You can lower the stretching if you lower the bending and you can lower the bending if you increase the inflation.

So now on to the Engineer Speak and Techno Babble

If you own a multi-axle trailer these forces can be much higher than those seen on a tow vehicle, motorhome or car, where the tires are not close together but at the corners of the vehicle.
I found an excellent video that shows the results of these forces at Keystone RV. Watch the section from time 0:46 to 1:07 and note that the tires on one axle bend inboard while the others are forced outward.

Special consideration for multi-axle trailers. Warning, this gets technical.
When not driving in a straight line there are special side loads on multi-axle trailers because the tires are fighting each other because they are not "pointed" to the center of the radius of the turn. These loads cause interior structural tearing. Sometimes 24% higher loads than those seen in tires on non-trailer application. Initially tearing is at the microscopic level but with time and repeated cycles these forces grow which can lead to small cracks at the belt edges as seen here at the arrows.

 If not spotted these cracks continue to grow to almost the full width of the tread as seen below.

 If you are lucky you will see the bulge in the tread as seen here and now you know this tire has failed and MUST be removed AT ONCE as the separation can grow and  can cause a belt to come off the body of a tire.
You can lower these forces by either decreasing the load 24% on the tire (probably not something you want to do or may not be able to do) or you can increase the inflation to stiffen the structure and decrease the slip-angle. In this case you could increase the tire inflation from the minimum inflation needed for the static load to the inflation associated with the max tire load as molded on the tire sidewall. BUT you need to be sure you are not exceeding the max rating of the wheel.

So the best recommendation I can give to trailer owners is to run the inflation molded on the tire sidewall. For owners of a TV or motorhomes, I recommend you run the inflation needed to carry the actual measured tire load plus at least a 10% margin.



  1. No wonder so many travel trailers and 5th wheels have blowouts! First most (many?) are traveling with the tires at max inflation and max load, or even overloaded. Add the stress noted above and the tires are ripe for failure

  2. Would rotating trailer tires help, along with higher pressure?

    1. Sorry Smitty I don't see how rotating tires will improve tire durability. It may even out the irregular wear if parts are out of align but not much else.

  3. Thanks for the clear description. I have had a few friends complaining of tread separation and the resulting damage. Now I can direct them to a cause. Ted

  4. Roger, Can you help me with my 15K 5th wheel? It shipped from Sunnybrook with LT235x85R16 LRE and I purchased it used at 6 years of age. I had a blowout halfway into my first long trip and by the time I returned home the other tires were bulging. Well, no wonder there; they were old and needed to be replaced.
    So I put on ST 235x85R16 LRE. After a year of local travel with these, I made a long cross country trip. By the time I returned, I had three blowouts. I weighed the trailer after each blowout and had about 11.5K on the two axles which was within the tires rating at 80 PSI. Not wanting to go through this again I purchased a TPMS and changed tire brand. Returned back across country to stay and traveled locally for a year with no problems until this last local trip when I blew one tire and developed a quick leak 100 miles later in another. Should I go up to a load range G or just go back to the LT tires?

    1. What is the original tire size? There should be a sticker or placard. An LT235/85R16 LR-E is rated at 3042 @80
      while the ST235/85R16 LR-E is rated at 3640 @80.
      From your comments it sounds like the TPMS warned of the "quick leak" (puncture ?)
      Are you using bolt in metal valves? Have you confirmed your axle loads are balanced and that the side to side loads are also balanced?
      Were the "Blowouts" sidewall flex failures or did you have tread separations with the belts and tread separating. You can see the difference in a number of other posts.
      The reason I am asking the questions is I need more information (or real good close-up photos) to know why the tires failed. If I don't know the "Why" I am only guessing on what might fix the problem.

  5. Both were tread separation. The first was one long full width of tread including the belts separating from the carcass. In the second quick leak, I can see where the belts and tread are separating from the carcass, also.
    I have weighed the rig and am balanced and within weight allowance for the axles and the tires.
    I would upload pictures but can't figure out how.

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