Tire pressure and old wives tales.

[chasfm11]

Originally Posted by two-niner

As the OP, I'm still waiting for somebody to find the word MINIMUM in Michelins booklet.
If 5 lbs is good, wouldn't 10, make that 20 lbs extra be better?

I agree with you that the word minimum doesn't appear in the booklet.

As far as 10lbs or 20lbs more, TQ60 points out that other factors like handling, braking and ride come into play. For me, 20lbs more would have brought me close to the 110lbs that the dealer used and that was definitely a bouncy ride situation where I believed that the handling and braking were compromised.

When I spoke with the RVIAA Safety person via phone. He explained that part of the idea of the inflation was to keep as much of the tire tread surface engaged with the road. As the inflation increases to an overinflated condition, it allows the outer edges of the tire to loose contact. From that, one could expect braking and, for the steer axle, maneuvering to be worse.

IMHO, inflation above the specific value recommended is a matter of compromise. I would never drop below that number because of the potential for heat build up. How much higher you go above it becomes a matter of how many unintended consequences develop. I try never to loose sight of the purpose of the inflation setting exercise - to keep the tire operating in its safety range for the load that it is carrying. I say "range" because given the number of variables and the lack of precision in dealing with them (i.e. a tire gauge that is only accurate within 2%, ambient temperature).

Let me use this as an example. Our health professionals have determined that I need to drink water every day. They have developed a recommendation for it. That recommendation could vary by the person's body weight who is drinking the water, how much fluid loss that person has in a given day, etc. We also know that too much water can cause bad things to happen. So one should try to stay somewhere above the recommended daily water intake. Unlike the water example, however, tires don't well tolerate deviations below the minimum. Heat build up causes damage that may manifest itself later.

One of the reasons that I changed my tires almost immediately after buying our RV was that I didn't understand the history of them and, if they had been abused (run underinflated), I wouldn't know it until the blowout had occurred. Since then, I constantly measure the tire pressure, even between uses and don't allow it to drop unnoticed. In practice, I've found very little change except when the outside air temperature changes dramatically. As evidence for my reasoning, I would offer the reported examples of people who had their tires blowout while parked. Since the blow outs occurred without significant heat, the potential causes are limited to overloading, previous damage or a plain defective tire. I personally had one of the latter on a passenger van that didn't blowout but did bulge to the point that I knew it would.

This thread started with the comment that adding 5lbs to the recommended manufacturer was an old wives tale. I don't believe that it is for large RV tires and hope that I've explained why I believe adding the extra 5lbs makes sense. Like the people who choose to run the tires on the RVs for 8-10 years, we all get to make our choices - and live with the results of them. I've made mine and am so far happy with the results of the choice. I don't represent my choice as scientifically based and am always reading threads like this to gain new information that might cause me to change my mind. My 5lbs extra isn't carved in stone but it represents my current reasoning on the matter. With different information, I'll make a different decision.

P.S. I don't universally apply the + 5lb approach to passenger tires. I do monitor them as carefully as the RV tires and have had nothing but puncture problems and the one defective tire in over 35 years on many cars and vans.

[RVDude]

Originally Posted by two-niner

As the OP, I'm still waiting for somebody to find the word MINIMUM in Michelins booklet.

Here it is. By the word "minimum" are you referring to a minimum inflation pressure? If so, while the WORD minimum is not in the Michelin docs, every Michelin inflation table DOES SHOW the minimum inflation for that tire. So yes, there is a published minimum inflation, they just don't use the word.

[Route 66]

Since tire pressure increases as you drive, adding 5# above the recommended pressure for "safety" is not a good idea.

[two-niner]

Michelin has been making tires longer than any of us have been alive.
I give up....it'll be 5 lbs extra for our children, children's children etc,.
This thread has certainly brought out a boatload of why Michelin's 32 page booklet is wrong, Michelin just won't admit it. Plus we now know that every RV wreck was caused by low tire pressure (apologies to the deceased).

"A keyboard is a dangerous device" author unknown.

Thanks guys,... Kerry

[RVDude]

Originally Posted by two-niner

This thread has certainly brought out a boatload of why Michelin's 32 page booklet is wrong, Michelin just won't admit it...

Seems to me the Michelin booklet and inflation tables are exactly CORRECT - it's just that many think they know better and not follow what Michelin writes.

[speed racer]

I would be willing to bet that there have been a lot more blow outs caused from under inflation than over infation.

[chasfm11]

Originally Posted by Route 66

Since tire pressure increases as you drive, adding 5# above the recommended pressure for "safety" is not a good idea.

OK, Dirk. So the Michelin pressure is exact.

So can you help me understand how to deal with

• the + or -minus 2 percent variance on my tire gauge. I haven't found a gauge anywhere that guarantees to measures the pressure exactly
• the temperature variations. I've done a lousy job of the calculations for it on my own. When I read the tire pressures after large temperature changes, I'm not sure what to do with the pressure because they didn't drop or rise as I thought they would.
• what I should do while I'm traveling. Typically, I don't stop and empty my tanks every night so the load of "water" and fuel changes daily.
• What I should do when I bring passengers. I had 5 adults, 2 dogs and an 18 month on board with associated luggage. We drove 3K miles in 8 days in the heat of the summer.

TIA

Charlie

[TQ60]

Given some variables above, the simple statement is fill the tires as required for the maximum weight of the vehicle and any adjustments that may be required for your specifics, (some tires state to add additional pressure if speed is above a certian point) and be done with it.

There are built in safety margins in all parts of the tire system, and unless the tires are under great stresses they air temperature should not rise to an unsafe level after a day's drive.

To be on the safe side one could contact the manufacturer directly with the installed tire model, wheel weight, maximum driving speed and any other considerations.

You could request the suggested operating pressure by axle and for the expected variance in pressure after driving for 8 hours in hot/cold conditions, they have tested their equipment under these conditions (should have anyway) and should have computer models or charts available for them to suggest the correct inflation along with a Fudge factor for personal prefference to ride and a Safety factor for safety concerns with performance issues.

[Russ & Ronda]

Well ......... after reading all of this stuff, I thought I should add my $.02.
Many years ago, I do not recall when it was, but it has been during the last twenty years, as that is how long we have been motorhoming.
I seem to recall, in a number of responses in the tech section of the Motorhome magazine, which seemed to have just as many ?'s about tire maintenance as there is here on this forum. Do not remember who the guy was that provided answers, but he did recomend the inflating of MH tires to 5 - 10lbs over the load pressure; IE: if the proper inflation for the given load was 95lbs, add 10lbs for 105lbs. He indicated that this would stiffen the sidewalls. He noted that auto tires posted the MAX inflation on the sidewalls and RV/truck tires posted the pressure for MAX load. Anyway, that is what I remeber. So, this may be, in part where this type of info was intiated or propagated. Over the years I have replaced three sets of tires, but when
I asked the tire people, they had never heard of it.
Enough said.... at least by me... on this subject.

[azloafer]

The link that follows explains yet another way to check for proper PSI in your tires. I tried it and found that I needed to up the pressure on all 6 tires by 10 pounds. I drove about a half mile for this test. Added air and waited for cool tires and then tested again and they checked out fine. Joe

Optimizing Tire Pressure

[Wayne M]

Adding 5 psi is okay as long as you do not exceed the tire manufacturers maximum pressure. If you weigh your coach and find that you can run at 95 psi and your tires are rated for 110 psi max., then running them at 100 psi is alright. but if you need to run them a 109 psi for your weight, do not exceed 110 psi if that is the maximum psi stamped on the side of the tire.

That's my story and I'm sticking to it.

[chasfm11]

Originally Posted by Wayne M

Adding 5 psi is okay as long as you do not exceed the tire manufacturers maximum pressure. If you weigh your coach and find that you can run at 95 psi and your tires are rated for 110 psi max., then running them at 100 psi is alright. but if you need to run them a 109 psi for your weight, do not exceed 110 psi if that is the maximum psi stamped on the side of the tire.

That's my story and I'm sticking to it.

Right. I've placed a call to Michelin. They are set up to contact me in the morning. I'll reserve my remaining comments until then.

[chasfm11]

I just got off the phone with Michelin. I'm using the information that the representative that I spoke priovide in discussing the points below.

Originally Posted by TQ60

Given some variables above, the simple statement is fill the tires as required for the maximum weight of the vehicle and any adjustments that may be required for your specifics, (some tires state to add additional pressure if speed is above a certian point) and be done with it.
There are no specific speed restrictions for XRVs. The highest possible maximum weight of my vehicle should be the GVW which is about 2,500lbs more than the sum of the actual wheel weights. Assuming the same incremental increase in weight at each corner position, the chart shows a 10lb increase per tire over the specific inflation pressures that Michelin just gave me. I specifically asked the representative about that increase and her response was that it would not be harmful to the tire (because I'm still well below the maximum pressure) and that it will not cause increased over-inflation wear on the tire.


There are built in safety margins in all parts of the tire system, and unless the tires are under great stresses they air temperature should not rise to an unsafe level after a day's drive.
The Michelin representative stressed this point also. She said that even if I needed to inflate to the maximum 110lb pressures for my tires, the tires had sufficient safety margins built in to handle the results of heat build up even in the summer on Texas roads

To be on the safe side one could contact the manufacturer directly with the installed tire model, wheel weight, maximum driving speed and any other considerations.
I just did exactly that.

You could request the suggested operating pressure by axle and for the expected variance in pressure after driving for 8 hours in hot/cold conditions, they have tested their equipment under these conditions (should have anyway) and should have computer models or charts available for them to suggest the correct inflation along with a Fudge factor for personal prefference to ride and a Safety factor for safety concerns with performance issues.The Michelin representative was unable to provide me with any test data or computer modeling. I asked her about braking and handling on various pressures within the inflation range of my tires and she said that she didn't have information on that. I assume it is back to my individual driving needs and experience.

I spoke the representative specifically about my Dallas to Casper, WY experience. Her statement to me is that the tires need to be inflated to the pressure specified regardless of the ambient temperature. That seems to contradict the other information that I've seen but that was more racing related. It said that there was a .8 pound pressure difference for each 10 degrees of change. I specifically ask her for the assumed ambient temperature on which the Michelin charts were developed and she did not have that information.

The representative stressed to me that the tires should never be allowed to fall below the chart recommended pressure for their respective weights. I specifically asked her about the overnight temperature change and the possibility that exactly that would occur. Her statement was that as long as the tires were brought back up to proper pressure the next morning, there was no issue. We did branch the discussion into longer term idle storage and the possibility of the tires, over days or even weeks loosing pressure to under the lower limit. We talked about this being one of the causes of blowouts during storage.

I drew the conclusion from my conversation with the Michelin rep that her company does not recommend a 5lb adder or any other amount. She told me that she understood my concerns about the variables in weight, gauge accuracy, etc. and that my adding 10lbs of pressure to each of the tires under the conditions that I outlined would have no impact on my tires. Impacts on handling and ride would be left for me to determine.

Your mileage may vary.

Charlie

[JManatee]

well there you go, Never below recommended pressure but ok above.
So the adding of a few PSI above the recommended helps with temperature change and slight loss of air over night.

Originally Posted by chasfm11

I just got off the phone with Michelin. I'm using the information that the representative that I spoke priovide in discussing the points below.





I spoke the representative specifically about my Dallas to Casper, WY experience. Her statement to me is that the tires need to be inflated to the pressure specified regardless of the ambient temperature. That seems to contradict the other information that I've seen but that was more racing related. It said that there was a .8 pound pressure difference for each 10 degrees of change. I specifically ask her for the assumed ambient temperature on which the Michelin charts were developed and she did not have that information.

The representative stressed to me that the tires should never be allowed to fall below the chart recommended pressure for their respective weights. I specifically asked her about the overnight temperature change and the possibility that exactly that would occur. Her statement was that as long as the tires were brought back up to proper pressure the next morning, there was no issue. We did branch the discussion into longer term idle storage and the possibility of the tires, over days or even weeks loosing pressure to under the lower limit. We talked about this being one of the causes of blowouts during storage.

I drew the conclusion from my conversation with the Michelin rep that her company does not recommend a 5lb adder or any other amount. She told me that she understood my concerns about the variables in weight, gauge accuracy, etc. and that my adding 10lbs of pressure to each of the tires under the conditions that I outlined would have no impact on my tires. Impacts on handling and ride would be left for me to determine.

Your mileage may vary.

Charlie