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Friday, September 28, 2018

Are ST type tires "Required" on trailers?

Had a question:
"My 2012 Airstream Classic came with Goodyear Marathon ST225/75R15 LR-D tires. I would like to tell you how many original installed tire I have had blowout using the original and same size tires as replacements. I even replaced the LR-D's with LR-E's and still only get about 3,000 miles before blowouts. It is the damage caused by the blowouts that concerns me most. I am tired of fixing my trailer after blowouts.

As I alluded to in my earlier message, I owned an Alpha 5th wheel that had 16" wheels as the original size. After 5 blowouts in one 6,000 mile trip I replaced the tires with Michelin LT235/75R16 XPS RIB (All Steel) tires. These tires are made with steel sidewalls. Many thousands of miles later I traded the trailer for my present Airstream without any more blowouts on my Alpha.

I have another question that does not appear to have a good answer....If trailer tires are so good and must be installed on a trailer when truck tires wear so much better, what is the rational for requiring the ST's on a trailer? My truck, 2002 GMC Duallie uses LT tires and I can not wear them out not blow them out before I feel it is safe to replace them (7 years). I am running LR-E' LT types on my truck.

I am sorry to say that I do not agree with the idea that ST tires for trailers is the safest and best option.

I have never seen any "requirement" that trailers run ST type tires. However, ST type tires can only be installed on trailers, so that might be the "requirement" you are thinking of.

The selection of tires to use on any vehicle is up to the RV company. As far as I can see they select tires based only on load capacity, physical size and cost. I don't know what the RV company actually pays for ST tires but it is probably less than an LT of equivalent load capacity.

If your only concern was for lower cost when comparing two tires of equal load capacity and the ST tire was easier to fit (it was smaller and did not require you make larger wheel wells), Why would you select the higher cost tire?  If you don't offer a meaningful warranty, you do not face the expense of warranty costs if there is a tire failure so what is the incentive for the RV company to spend more?

Friday, September 21, 2018

How to take good pictures of a tire tread (Not too technical I hope)

I occasionally  see posts from folks complaining about some tread or sidewall condition but either they can't properly describe the condition or if they have pictures the image is too small, poorly lit or from too far away to allow proper inspection of the condition for me or others to help and offer an opinion.
I recently wrote this post:
Some general guidelines and suggestions for taking good pictures of tire conditions.

Full sunlight and NO flash.
Set the camera to maximum pixels or max quality
Take close enough to only include 8" to 10" of the tire surface. Closer if you have a camera that allows "Macro" or close-up.
If all you have is a phone you need a minimum of 10 Megapixles.
Only use "Optical Zoom" not digital zoom if possible.

Here is an example of a good quality tire picture that would allow someone to make a judgement on a tread surface complaint.

The following, while showing an obvious problem really would not be good enough to show a sidewall or tread cracking complaint.

Friday, September 14, 2018

Do you have to replace valve stems when installing TPM sensors?

Got this question on an RV forum:
"I understand from the thread that you should use metal tire stems with TPMS caps. The ends of the tire stems on our trailer are metal with the only visible rubber part right where they go through the wheel. I am guessing these are still considered rubber stems and should be replaced prior to installing TPMS?"

"Metal" stems can be a bit misleading as some newer stems may have some brass showing. I posted pictures on THIS post showing standard passenger rubber valve, a "bolt-in" metal valve, and a new style rubber/brass valve.
When it is recommended that "metal" stems be used, what is actually meant is "bolt-in". You will see the nut that retains the bolt-in stem on the outside of the wheel for most of these stems.

The concern is that over time (months or maybe years) the extra weight of TPM external sensors may cause some vibration and movement of the rubber stem body which might result is a failure of the rubber portion of the stem.

I am not aware of any testing done on the new style rubber/brass stems with a TPM sensor screwed on, so I and others are erring on the side of caution when we recommend "bolt-in" metal stems.

Cracking and failure of the rubber part of valve stems is another time & temperature aging thing, just as it is with your tires. You visually inspect your tires monthly, as outlined in your RV  owner's manual, so just include the rubber part of your valves and just as you would replace your tires when signs of significant signs or aging (cracks) are found do the same with your valves. 

You could consider running your current valves until you get new tires and at that point have the bolt in valves installed. In the meantime, I would take a close look at your stems when doing your monthly visual inspection. You can push sideways a bit on the rubber/brass stem and look for any cracks. If you find any, that would mean you need to replace them sooner rather than later.

Friday, September 7, 2018

WARNING - Super Technical Post Tire failure and Interply Shear

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 rest of the Universe rotate around the Earth or that believe the Earth is flat or the Moon landing is a hoax simply because they don't understand the Physics and Science behind the stated 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 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, motorhome or tow vehicles do go through slip angle but usually in the 1° range while trailer tires are subject to angles in the 10° and greater range. The forces are NOT linear. 10° can generate significantly more shear than 1°.

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. 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.