Tire pressure question

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Wagonmaster2

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According to the weight of my coach and the Michelin Tire Chart (with a little self conversion) I should be able to use 75# of air all around, however the Chart only goes down to 85#, which is more weight carrying capacity they I have.  When I questioned Michelin all they would say was that they had never tested this tire below 85#.  I know I probably have more tire than necessary but when I got rid of the original 80 series tire and went to a 70 series to get a little more hefty tire this was what I ended up with.

Now the question:  I run 85# in normal summer driving but going into a mountain campground in Colorado for a few days with temperatures hitting 28-34 degrees in early mornings my Pressure Pro will indicate tire pressures 8-10 lbs lower than normal.  By noon time even in the cooler temperatures the tire pressures will be back up to normal with the sun shinning.    Should I be getting my air hose out and running the engine in the CG to just air the tires back up to 85 then stop in an hour or so after getting down to warmer temperatures and bleed back down to where they should be?  Seems like a lot of work when I have excess tire carrying capacity anyway.

Thoughts from the experts?
 

John From Detroit

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They talk about "Cold" pressure.. But what is "Cold"?  I mean in Arizonia, in August, any number with only 2 digits is "Cold".  But in the Upper part of Michigan ON Feb-1. Well, any positive number is HOT.

IF the pressure is good at say 60 degrees, I'd be happy.  Though, question, when the temps are down, are you parked or driving... If parked, don't worry about it.

If you are driving the tires will very quickly warm up.
 

jc55

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I had a similar question a while back and someone suggested having the tires inflated with nitrogen. Nitrogen apparently doesn't let the pressures fluxuate so much..just a thought.
 

taoshum

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jc55 said:
I had a similar question a while back and someone suggested having the tires inflated with nitrogen. Nitrogen apparently doesn't let the pressures fluxuate so much..just a thought.


air is about 80% Nitrogen, BTW...
 

FastEagle

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Wagonmaster2 said:
According to the weight of my coach and the Michelin Tire Chart (with a little self conversion) I should be able to use 75# of air all around, however the Chart only goes down to 85#, which is more weight carrying capacity they I have.  When I questioned Michelin all they would say was that they had never tested this tire below 85#.  I know I probably have more tire than necessary but when I got rid of the original 80 series tire and went to a 70 series to get a little more hefty tire this was what I ended up with.

Now the question:  I run 85# in normal summer driving but going into a mountain campground in Colorado for a few days with temperatures hitting 28-34 degrees in early mornings my Pressure Pro will indicate tire pressures 8-10 lbs lower than normal.  By noon time even in the cooler temperatures the tire pressures will be back up to normal with the sun shinning.    Should I be getting my air hose out and running the engine in the CG to just air the tires back up to 85 then stop in an hour or so after getting down to warmer temperatures and bleed back down to where they should be?  Seems like a lot of work when I have excess tire carrying capacity anyway.

Thoughts from the experts?
In the tire world for self propelled RVs the proper tire pressure becomes more complicated then necessary. Tire manufacturers become very wishy-washy with their RV tire manuals and not very many of them follow the complete T&RA standards.

According to DOT regulations your vehicle came with a certification label attached to it somewhere. On that certification label is the minimum tire requirements for that particular vehicle accompanied by the - correct - recommended tire pressures for it?s normal load.

When ?plus sizing? up from your original tires, the load capacity for the OE tires is carried over to the replacement tires. In other words, you need to set the tire pressures on your new tires to provide you with the load capacity the OE tires provided. You should never use less tire pressure than that, regardless of actual vehicle weight. It?s a built-in safety margin that should never be compromised. If you are heavy on some axle end (s) you can go up in tire pressure - insuring all tires on that axle have the same psi - not to exceed the rim/wheel maximum or tire maximum. With the ?plus sized? tire you can insure you have the percentage of load capacity reserves the OE tires gave you.

FastEagle 
 

Gary RV_Wizard

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Short answer: do NOT run any tire at any pressure less than the minimum shown in the inflation table. Ever.

You said the tires are back up to the normal 85 psi by noon. Is that just sitting in the campground, or does it include some driving. If just sitting, leave the pressure alone. But I would not start driving on them if they were still well under the 85 psi minimum. A couple of pounds, may ok, but not 8-10 under.

The reason is that the tires probably does not have the proper shape (sidewall profile) if below the minimum. That often leads to damage of the cords or a break at the bead where it fits to the rim.
 
B

bucks2

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Many folks believe nitrogen is a good practice to adhere to. Others point out that air is 80% nitrogen, so that 20% oxygen in the air can't cause any harm. If you put 4 blanks and 1 live round in the gun to play russian roulette does that mean there is no danger from the live round?

Oxygen causes oxidation. Rust is oxidation. Fire is rapid oxidation. Your tire is made of rubber which is air permeable and can oxidize. Rubber (compounds used in tires) will slowly allow air to pass thru over time. You know this because you have to fill your tires occasionally. Your tires have steel belts inside. High pressure inside your tire forces whatever it it pressurized with, nitrogen or nitrogen/oxygen mix thru the rubber and steel.

So ask yourself why tires come apart? Was it bad steel? Was it bad rubber? Could it be that both the steel and rubber was weakend by oxidation/rust? A search on tire oxidation brings up many studies like this one http://tirenitrogen.typepad.com/techinfo/Ford%20Baldwin%20TireAging%20%232.pdf and this one http://www.hislop-associates.com/papers_and_talks/conference-01/Tyre%20Aging%20-%20David%20Osborne/Aging%20of%20Tires.ppt .

I have no illusion of changing someones mind that already thinks nitrogen filling is snake oil. If you aren't sure though, do some reading on the subject and see if it makes sense to you. Why would many large tire stores go to the effort to put nitrogen generators on their air compressors and then give it away? Costco is one example of a store that uses only nitrogen to fill tires and does it for free.

Ken
 

s2kskibum

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I think Gary has it right - 8 to 10, don't bother if at altitude (think it is boyle's law). Kept my RV at 8600, driving to Denver would lose about 8#. If I stayed in Denver I'd repressure. Headed back up, no.
 

Gary RV_Wizard

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Why would many large tire stores go to the effort to put nitrogen generators on their air compressors and then give it away?

Good marketing does not have to have good technical substance. It just has to sell more tires.  ;)


One can equally argue that only dry air should be used in tires. Lack of moisture the primary benefit of nitrogen in car and RV tires anyway - all the water vapor gets removed in the process of extracting nitrogen from air.
 
B

bucks2

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I don't think I know anyone that doesn't have an air dryer on their compressor. Do they even make compressors without dryers? But that's a good point, if you see the air tools icing up, don't let them fill your tires with that air.

Ken
 

elkhornsun

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The tire gauge and specs are for internal pressure of the air inside the air as compared to the outside ambient air pressure. The maximum is what I would run with any load involved and with an RV there is always a load involved. I run my tires at the maximum PSI as indicated on the sidewall of the tire. In theory you could run with less than that in the tires but that could compound the problems caused with cold weather and in my own experience I found uneven tire wear (and shorter tire tread life) if I used the RV weight and the manufacturer's chart. The tires would be under-inflated for all practical purposes.

Excessive flexing from under-inflated tires results in "scrubbing" which wears down the tread and more flexing of the sidewalls is what causes  tires to run hotter than they were designed to handle and blowouts result - as with the Bridgestone tires on the Ford Explorers.
 

Gary RV_Wizard

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I don't think I know anyone that doesn't have an air dryer on their compressor. Do they even make compressors without dryers?

Ken, let me introduce myself. You now know someone without an air dryer on his compressor. Few consumer grade compressors come with them, and many owners do not add them. I also wouldn't want to bet on how many gas station air pumps or small shops have them, or at least not ones that have been serviced lately so that they actually dry the air.

But bigger shops probably do use them.
 

Ned

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My compressor at home doesn't have a dryer either, but I can use my motorhome air system, which does have an air dryer, to inflate my tires.  You can also buy an air dryer that goes inline with the air hose and I will probably get one of those this winter.
 
B

bucks2

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Gary RV Roamer said:
Ken, let me introduce myself. You now know someone without an air dryer on his compressor. Few consumer grade compressors come with them, and many owners do not add them. I also wouldn't want to bet on how many gas station air pumps or small shops have them, or at least not ones that have been serviced lately so that they actually dry the air.

But bigger shops probably do use them.

Well that does it then Gary, I'm not buying my tires from you!!!  ;D 

As a former service station owner, and having worked in several others and in repair garages, we always had air dryers. Without them, any heavy use of air tools would cause freezeups of the tools in cooler weather. And if they weren't serviced as needed, we'd notice.

I guess I haven't paid any attention to commercially available air (small gas stations or especially the self service air pumps over in the corner of the lot) since I have my own compressor with dry air on board and my home compressor has an air drier. I learn something new everyday.

Ken
 

Wagonmaster2

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s2kskibum said:
I think Gary has it right - 8 to 10, don't bother if at altitude (think it is boyle's law). Kept my RV at 8600, driving to Denver would lose about 8#. If I stayed in Denver I'd repressure. Headed back up, no.

Just setting in a campground, both when we were in Colorado recently and now in Arkansas,  the tire pressure will be back up to normal by noon or so for the week or two we're there, and is fine if I wait that long to leave for the next destination.  It's when I manage to leave around 8:30 to 9:00 or so then with the cold ambient temperatures the tires will still be 6-8 # low at that time from what I normally would run them.  Earlier in the morning than that the pressure could be down 8-10#.  If I took the time to air them up to normal that early in the morning, which is a little undesirable in freezing temps,  then when ambient temps rose and the mileage driven was 50-90 miles down the road then I would be driving them considerably over pressured for my coach weight.  And this could go on every day while on the road this time of year if it was several days travel to the next destination.

I kind of feel its one of those "darned if you do and damned if you don't".  Guess I'll know I've done it wrong if I start having tire problems, which I hope I'm avoiding when I went from G rated XRV (the reputed zipper tire) to J rated XZA2 Energy tires on my last new tire purchase.  Probably more tire than I really needed but there didn't seem to be any in-between with the Michelin tires  ( had to stay with them since I was getting a good rebate due to premature weathering on the XRVs).

Appreciate all the thoughts, ideas, and suggestions.
 

Mavarick

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Ok, I guess I?m in the camp that says nitrogen is better marketing, period. I have to put it in the category of MOST oil/fuel additives. If they make you sleep better at night then use them. If you are sleeping better then they are doing more good for you than what you are putting them in!
I think TPMS systems allow us to over think things. If you were still using a tire pressure gauge what would you do? You would check and fill the tires to your load specs in the am no matter where you were or what the ambient temp was and then drive.
Almost every industrial and private shop I have been a part of does not use an air dryer. Compressors don?t come with air dryers, they are an add on for the system if you want one. All shop air systems use many water separators in different locations, auto drains and most if not all lines have oilers installed to keep the air tools operating. At home I use the same thing with a couple dry lines (no oiler) for tires, paint etc. Air dryers are a pain to maintain properly especially with a large volume of air like shops use.
Air dryers are great for contained systems like Rv?s, cranes, OTR trucks, etc that use pneumatic valves with small ports (think treadle valves etc) and a limited volume of air. These valves will be the first thing to freeze in bad weather, shop air systems don?t worry about this. Trucks, cranes, etc have to have the tanks drained and dryer serviced or the system starts freezing up. When that happens the operator will dump antifreeze in the air line to get to the shop. This makes the shop happy because the alcohol in the antifreeze will now kill all the diaphragms/seals it touches. It?s much easier to just maintain your system. Ever wonder what those steel cords are for behind your front tires, pull them occasionally to drain your tanks. Even I remember sometimes! I know, the smarter Rv?s never go into freezing weather, it?s all tongue in cheek anyhow. Just something else to think about.  ;D
 
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