RV Tire Safety
with RV tire expert Roger Marble
“WARNING – Super Technical Post” —Roger
I have had some folks who seem to want to replace science with what they term “common sense.”
In my opinion, this is why some people still think the sun and the rest of the universe rotate around the Earth, or believe the Earth is flat, or the moon landing is a hoax simply because they don’t understand the physics and science behind the facts.
I mention this because I continue to get people claiming that because they checked their tire pressure last week or yesterday and had a tire failure today, some sort of “magic” must have happened to cause their 65-psi or 80-psi tire to suddenly go sky high in pressure and to cause the tire to explode due to high pressure.
You don’t have to do the technical research yourself, just as you don’t go to medical school to learn about some ailment you have. You do have a choice. You can trust your doctor or go to medical school or, in the case of tire failure, you can put your trust in those who have spent years working on and constantly improving tire design. Or you can simply believe that tires fail because of some unidentifiable “defect” that was built into the tire.
I have previously posted on how sidewall flex failures can easily mislead the inexperienced into believing they had a “blowout due to high pressure.” I also have some who do not want to accept the science behind the need for tires in trailer application to run lower speeds and higher inflation in an effort to lower (but not eliminate) the probability of belt separation.
So I decided it is time to get out the “Big Guns” and cite some actual tire science.
Here is a question from someone who took issue with my recommendation to increase the inflation in trailer application: “Did you ever notice that the two rear tires on the tow vehicle are putting hundreds of horsepower to the road? Did you ever notice that the two front tires are steering the whole assembly?”
Here is my reply:
Yes, I have noticed that. I also know that the internal structural forces are different for torque than for high “slip angle,” which is the situation in trailer application. Front tires on cars, motorhomes or tow vehicles do go through slip angle but usually in the one-degree range, while trailer tires are subject to angles in the 10-degree and greater range. The forces are NOT linear. Ten degrees can generate significantly more shear than one degree.
If you want you can purchase the software package here for the vehicle response and handling. The results of this vehicle simulation show the vertical and side loads being applied to tires as you drive around a corner. These forces can then be used as the input into Finite Element software programs to determine the structural loads on tire components.
Here is is a technical paper on “Interply Shear Stresses and Coupled Deformations of a Folded Belt Structure Under Extension.”
Personally, I prefer Dr. Jaehoon Song’s paper on “Fatigue of Cord-Rubber Composites For Tires.” Here is the abstract:
Fatigue behaviors of cord-rubber composite materials forming the belt region of radial pneumatic tires have been characterized to assess their dependence on stress, strain and temperature history as well as materials composition and construction.
Using actual tires, it was found that interply shear strain is one of the crucial parameters for damage assessment from the result that higher levels of interply shear strain of actual tires reduce the fatigue lifetime. Estimated at various levels of load amplitude were the fatigue life, the extent and rate of resultant strain increase (“dynamic creep”), cyclic strains at failure, and specimen temperature. The interply shear strain of 2-ply ‘tire belt’ composite laminate under circumferential tension was affected by twisting of specimen due to tension-bending coupling.
However, a critical level of interply shear strain, which governs the gross failure of composite laminate due to the delamination, appeared to be independent of different lay-up of 2-ply vs. symmetric 4-ply configuration. Reflecting their matrix-dominated failure modes such as cord-matrix debonding and delamination, composite laminates with different cord reinforcements showed the same S-N relationship as long as they were constructed with the same rubber matrix, the same cord angle, similar cord volume, and the same ply lay-up.
Because of much lower values of single cycle strength (in terms of gross fracture load per unit width), the composite laminates with larger cord angle and the 2-ply laminates exhibited exponentially shorter fatigue lifetime, at a given stress amplitude, than the composite laminates with smaller cord angle and 4-ply symmetric laminates, respectively. The increase of interply rubber thickness lengthens their fatigue lifetime at an intermediate level of stress amplitude. However, the increase in the fatigue lifetime of the composite laminate becomes less noticeable at very low stress amplitude.
Even with small compressive cyclic stresses, the fatigue life of belt composites is predominantly influenced by the magnitude of maximum stress. Maximum cyclic strain of composite laminates at failure, which measures the total strain accumulation for gross failure, was independent of stress amplitude and close to the level of static failure strain.
For all composite laminates under study, a linear correlation could be established between the temperature rise rate and dynamic creep rate which was, in turn, inversely proportional to the fatigue lifetime. Using the acoustic emission (AE) initiation stress value, better prediction of fatigue life was available for the fiber-reinforced composites having fatigue limit. The accumulation rate of AE activities during cyclic loading was linearly proportional to the maximum applied load and to the inverse of the fatigue life of cord-rubber composite laminates.
Finally, a modified fatigue modulus model based on combination of power-law and logarithmic relation was proposed to predict the fatigue lifetime profile of cord-rubber composite laminates.
Let’s see if I can help. Here is a key phrase: “… the fatigue life of belt composites is predominantly influenced by the magnitude of maximum stress.” Now, think of the tire side bending when you back a trailer into a campground site.
The video in this post shows the side loading during relatively low angle turns
I apologize for going so deep into tire engineering but sometimes the facts are needed to demonstrate that “common sense” doesn’t always lead to the actual facts. Before writing my posts on interply shear on trailer tires I had both vehicle simulation and tire structural shear forces run. We learned that for tires on multi-axle trailers, like RV trailers, the belt shear forces can be 24% higher than the belt shear forces of identical size, load and inflation tires on a motorized vehicle. This is why I suggest a different approach to tire inflation for trailer application than motorhome applications.
I will try and “lighten up” a bit in the next few posts.
Read more from Roger Marble on his blog at RVtiresafety.net.