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Old 10-09-2019, 11:20 AM   #61
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Rule of thumb

I always heard that it referred to the Roman Emperor’s use of his thumb to determine whether a defeated gladiator was to live or die.
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Old 10-09-2019, 01:25 PM   #62
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I propose a test.
If lower air pressure (65 vs. 80) leads to more "interply sheer" shouldn't it result in more heat?
So those of us with dual axles, and a TPMS, I propose we inflate the front axle to 65 psi, and the rear axle to 80 psi.
Then before driving, note the ambient temperature at rest.
record the tire temperature under way, along with the pressure.

My tires, at 65 psi seem to increase about 7 psi under way.
If you don't see a significant change between 65 and 80, I'd run with the lower pressure for a smoother ride.
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Old 10-10-2019, 06:44 AM   #63
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Quote:
Originally Posted by Mollysdad View Post
I propose a test.
If lower air pressure (65 vs. 80) leads to more "interply sheer" shouldn't it result in more heat?
So those of us with dual axles, and a TPMS, I propose we inflate the front axle to 65 psi, and the rear axle to 80 psi.
Then before driving, note the ambient temperature at rest.
record the tire temperature under way, along with the pressure.

My tires, at 65 psi seem to increase about 7 psi under way.
If you don't see a significant change between 65 and 80, I'd run with the lower pressure for a smoother ride.
2 thoughts:

First, Inner Ply Shear is about the stresses in cornering. Yes, there would be more heat buildup, but in order to test for IPS and only IPS, you'd have to be cornering the vehicle more or less constantly - like driving in a circle. I'm sure you didn't have that in mind.

Second, if you are getting a pressure buildup from 65 psi to 72 psi (10.7%) starting at 75°F, the Ideal Gas Law says you would get a temperature buildup to 122°F (+47°F). In order to get the same energy being generated, an 80 psi tire would have to buildup the pressure to 87.2 psi (+9.0%), with a temperature buildup of 41°F (to 116°F). You might be able to detect the temperature difference, but not the pressure difference.

Please note: A 10.7% pressure build up of an 80 psi tire would be 88.6°F. If we use the Ideal Gas Law to figure out the same temperature buildup (+47°F), then the pressure buildup is from 80 psi to 88.3 psi (+10.4%). however, energy generation is the key value here and that drops the temperature build up by about 15% - and the values posted in the above paragraph..
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Old 10-10-2019, 08:46 AM   #64
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As the OP for this thread, I thought I would weigh in on my observations on my current trip regarding the tire pressure with my Goodyear Endurance tires since my original post.

I've settled on a tire pressure of 65 psi. Sometimes it is 67 when I start to travel, depending on the ambient temperature outside. At 65 psi, a 10% increase would get the psi at roughly 72 as a safe psi. If I understand everything, According to Capri Racer I can have a 3% increase in psi for every 10 degree increase in the ambient temperature. At an initial psi of 65, this is roughly a psi increase of 2 psi due to heat. If the ambient temperature at the start of my trip is 75 degrees and the ambient temperature increases to 85, my psi can go up to 74 (72 psi initial safe psi + 2 psi increase due to heat) and my tire psi is still safe.

I have consistently observed this during my travels so the last 5 weeks in the heat of Texas and cooler weather in the mountains of New Mexico. I appreciate the input from Capri Racer and others in understanding this.

I have adjusted the tire pressure in my TV as well to 65 in the rear and 60 in the front. The ride is much smoother and I don't have thrown pillows like before. The only exception has been the Highways of Louisiana and I don't think any RV can survive the roads in that State.

All psi's are within the MFG's rating for the load I am carrying. While Goodyear says I could get by with 45 psi I don't feel comfortable going that low. I'll leave that for others to try and stick with 65 psi.
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Old 10-10-2019, 09:18 AM   #65
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This is enlightening for me. I’m going to lower down to 65 on the trailer. The trailer is definitely having a rough ride—this summer I got used to having to reorganize the contents of the two upper storage lockers above the lounge whenever we landed somewhere.

But I was approaching this like I approach bicycle wheels—trying to avoid a pinch flat on very rough terrain ( highways in and around Alaska ). It occurred to me on the trip home from Seattle that many highways in the lower 48 afford much more leeway. If I can get a more gentle ride at 65, I’m all for trying it.

Thanks, GOUSC.
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Old 10-10-2019, 09:25 AM   #66
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Quote:
Originally Posted by Mollysdad View Post
I propose a test.
If lower air pressure (65 vs. 80) leads to more "interply sheer" shouldn't it result in more heat?
So those of us with dual axles, and a TPMS, I propose we inflate the front axle to 65 psi, and the rear axle to 80 psi.
Then before driving, note the ambient temperature at rest.
record the tire temperature under way, along with the pressure.

My tires, at 65 psi seem to increase about 7 psi under way.
If you don't see a significant change between 65 and 80, I'd run with the lower pressure for a smoother ride.

The test idea is reasonable as long as we consider the items CapriRacer mentioned. Plus, your test would assume a perfect 50/50 split between axles on load which is not likely so any load difference could confuse the data. You might get around this by repeating the test a number of times ( 4 or 5) when the Ambient is also similar (+/- 5°) and the loads have not changed but swap the inflation number between axles.
Controlled testing is not always easy to "control".
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Old 10-10-2019, 10:30 AM   #67
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Originally Posted by Tireman9 View Post
Controlled testing is not always easy to "control".
My proposal was for many owners to try this on their own and come to their own conclusions.
I'm amazed at the difference in pressure having the Sun shine of one side makes!
But still, I've never seen temperatures over 110, even when the ambient temp was 95. (and that was stopping for gas, where the tire seems to absorb more heat. (brakes?) or dissipate less from moving.

I once had a motorhome weighed by the Smart Weigh folks, and they sent me a detailed report. I recall I paid $25, and it was worth every penny! A rally like Aluminina would be an ideal location.
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Old 10-10-2019, 12:07 PM   #68
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55 - 60 psi cold
70-75 psi hot
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Old Yesterday, 09:58 PM   #69
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Originally Posted by Bob662 View Post
We have participating on Air forum at least three true experts -- two (Capri and Tireman) are retired tire engineers. The third is Andy Thomson, at CanAm RV. Andy has tons of actual experience picking up and delivering all sorts of RV's all over North America with a wide variety of tow vehicles, and he has installed thousands of tires on RV's and has experimented with differing tire pressures and their effects on tire longevity and trailer ride. Having read most of the tire posts, at least over the past four years, here's my synopsis:

The tire engineers suggest tire pressures for maximum load -- usually 80 psi. They're right. That pressure minimizes the risk of interply sheer and tire failure. BUT -- they have never commented (that I've seen) on the effect of that pressure on travel trailer ride, failed rivets, thrown pillows, etc. That doesn't seem to be their focus. Their focus is (I believe) tire strength, not ride quality, or even a combination of the two.

Andy Thomson, with enormous actual experience setting up trailers and experimenting with different tires and different pressures, seems to me to focus on ride quality AND enough tire pressure to assure no tire failures. So strong enough; not necessarily maximum strength.

Andy suggests 45psi for GY Endurance tires on 25' trailers and 55psi for those tires on longer trailers. With Michelin Defenders (his favored tire over the GYE), he suggests 44psi. Andy has had great experiences with the Michelin tires; and, so far, good experience with the Endurance, but they haven't been on the market long enough to conclusively recommend them.

So that's my summary of hundreds of posts. If I got it wrong I'm counting on others to correct me. Andy doesn't post a lot, but when he does it's worth paying careful attention to what he says.

As to owners on this Forum -- they seem to use tire pressures all over the place. You can take advantage of the preceding PLUS the tire manufacturers' tire pressure vs. load tables to form your own opinion. For me, I'm in the Andy Thomson camp. But there may be no wrong answer, so long as you are above (plus a reserve) the manufacturers' tire pressure charts, as are the pressures that Andy suggests.
I have set my Safari 25SS tires (Goodyear Marathons) at 45-46 psi (cold) for the last 5 years, ever since they were new . My goal was to make them last out to their economic life of 5-years or so. The tires still have quite a bit of tread left so I am pleased with the result. (We do have some rivet loss in the interior but for me this is not an issue.
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