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Old 04-10-2003, 07:06 PM   #1
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Tire pressures

Ok, I was out at a party and got into a conversation about tire pressure. One person said that you fill the tire to the suggested pressure when the tire is cold.

Another said that if you fill it to the suggested pressure, as the tire heats (due to the rolling resistance), it will exceed the max pressure and is more likely to blow out.

I am currently in the camp where I do not fill the tires all the way. For example, the car states 44psi. I tend to fill to 38-40psi so as the tires heat up, the do get to 44+psi. In winter, as it is cold out here, I fill the tires to 44 and as it gets colder out, I need to add about 8-10lbs. When spring arrives, I have to take some out since as it gets warmer, the tire pressure increases and I need to compensate.

On the trailer, I believe it says 2540lbs at 65psi cold. These are the standard "D" rated Goodyear Marathons.

So now I am torn. Do I fill the tires to the 65psi on the camper and 44psi on the car (cold) when on the road with the rig and allow them to heat and possibly exceed the 65 and 44psi or will it not exceed the pressure as it gets hot. I can't see how it can't with physics being what it is......



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Old 04-10-2003, 07:17 PM   #2
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I'm not a tire expert but, I think you are right. The figure is the Max inflation rate, not THE inflation rate.

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Old 04-10-2003, 08:23 PM   #3
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Tire pressures

Greetings Eric!

The inflation pressure listed on the sidewall of the tire is the designed maximum pressure that will permit the weight carrying ability molded in the sidewall. The weight carrying ability of the tire decreases as the inflation pressure goes down. I would suspect that with your new Bambi you would have recommended inflation pressures in your owner's manual. No such information is provided in my '64 Overlander owner's manual, but then it was Airstream's first attempt at producing an owner's manual. Since the manual doesn't contain the information and my Good Year dealer was vague about what reduced inflation pressures would do to the weight carrying ability; I have always run my Marathons inflated to the maximum listed on the sidewall. I carry a small, portable air compressor that can be run from my inverter so that I can fill the tires before the trailer is even on the road at the beginning of the day - - I also purchased a high quality tire guage at a local truck supply so that I am reasonably confident in its readings. It has been my experience that when inflated to the maximum indicated pressure that the tire runs cooler and lasts longer.

(I have run the Michelin tires on my Suburban at the maximum rated pressure since they were installed at 45,000 miles - - the truck now has nearly 104,000 miles and the tires still have about 15% treadwear remaining - - the dealer estimates another 10,000 miles before replacement.)

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Old 04-10-2003, 08:44 PM   #4
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It says 65 PSI COLD, then fill it to 65 PSI and don't worry about it. Like Kevin says, the tire will run cooler and last longer with more pressure in the tire.
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Old 04-11-2003, 07:55 AM   #5
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I agree - tire pressures should be checked and set when COLD - see Tire Rack
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Old 04-11-2003, 11:11 AM   #6
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The Real Answer to Tire Pressure

I worked for Michelin Tire Corporation for 7 years and Yokohama Tire Corporation for 11 years. I have given numerous tire seminars on tire maintenance and especially how to determine the correct tire pressures. So here goes.
The pressure on the sidewall of the tire is the maximum pressure at the published load at approximately 55 mph. (The speed can vary somewhat but it is not important for our discussion).
The air pressure is required to support the load that the tire must carry in such a manner that the tire flexes at the designed place on the sidewall of the tire.
If the load on the tire changes then the air pressure should change accordingly to keep the tire flexing at the proper place.
The reason for correct air pressure is to prevent the tire from overheating. It was put together with heat and it will come apart the same way. An under inflated tire will eventually self destruct due to excessive heat build up. An over inflated tire will ride harshly and be more likely to burst upon impact. Sorry for the long explanation but here is the bottom line.
To determine the correct air pressure, check the pressure when the tire is cold. Run the tire for several miles at highway speed. Stop and immediately check the air pressue in the tire. It should be higher than we cold but no more than 10% higher.
Now here is the hard to believe part. If the pressure is more than 10% higher you must ADD AIR and test again. For example if you start with 50 psi cold. If the pressure is 60 when hot, you have exceeded the (10%) in this case, 55 psi maximum safe heat build up pressure. You must ADD AIR. In this case I would add 5 psi which would take the tire to 65 psi when hot. After you run the tire again you will find the pressure to actually drop because the tire will run cooler. The heat build up causes the tire pressure to increase when under inflated.
On the other hand, if the 50 psi cold pressure does not change when hot. You have more air than needed. You can remove 5 psi or so and test again when they return to cold. Like the next trip you take.
So a fully loaded rig will require more air in the tires than one with empty tanks and a light load on board. Always error on the side of over inflation. Thus the maximum sidewall pressure indicated on the tire is usually used. It usually is more than needed. Each axle has its own requirement based upon the load on that axle.
So how do I get a 2 Rivet rating? Does this long explanation help or hurt?
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Old 04-11-2003, 11:25 AM   #7
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Always err on the side of over inflation.
Great post, thank you.

Mike always makes sure that our tires are inflated to the recommended max. The problem we often encounter when having a tire mounted, is a lot of shops refuse to inflate that tire according to manufac. specs. !!!
We have to insist, or add air ourselves afterwards. A lot of people ride with under-inflated tires because they like a soft ride. As you said, they don't understand that not only their tires will over heat, but they will get lower gas milage.
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Old 04-11-2003, 12:06 PM   #8
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Thanks for the detailed post. All tires will have the correct inflation now.

Thanks a bunch!

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Old 04-12-2003, 09:08 PM   #9
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Interesting posts on a subject I knew little about. Great info as I read thru the list last night. got to thinking that I now need to buy a digital pressure gauge, etc. & start paying more attention to the tire pressures. Today in the mail what appears, but a mini air compresser & a digitial tire gauge, a belated birthday gift from my daughter in Virginia. Now I need to go back over the posts & see what I missed. Appreciate all those who gave freely of their advice, on the importance of the proper tire pressures.
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Old 04-12-2003, 10:57 PM   #10
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Great information AM. Thanks.
I have heard so many things and some of it even printed. One method was to fill tire to max pressure and roll it (with the weight of the vehicle of course) accross a peice of paper or some calk powder and look at the pattern. If the pattern shows lack of full contact on the sides to let some air out. This was printed in a popular 4x4 mag.
I run BFG 32x11.5 MT's and 31x10.5 A/T's on my 4x4's. The tires I was running were rated 50psi max but the added gross total of the weight capacity was well in excess of the vehicles listed GVWR. The second issu was the max PSI rating of the stock Chevy rim they were on was 40PSI.
I ran those tires at 36PSI and I have tested the pressure after running them. I have never found the tire to gain no more then 3PSI due to heating so I guess I was pretty close. 2more PSI and I would proably have been right on. The tires have been wearing even and extreamly well. I put 20k on one set of MT's and they still had another 15-20k left to go and actully sold them to somebody else who is still happy with them.

I guess I'll play with the Load range E Michelins on the new burb. Previous owner has been running them at 55psi and they are firm but not adversly except on choppy pavement. I havent even read the sidewalls yet so not sure the max rating but I think it's near 65psi. The door sticker recomends 44 front and 60 rear with 4 passengers. But that was the original tires and may not be the same requirments as these tires.

Since you have some knowledge here what is the max width rim I could run these 235/85/16's on safely? They are on a set of stock GM steel rims 6.5 inches wide and I would like to dress the truck up some (hate thos huge pizza pan hub caps). I know somebody with a very nice set of Aluminum rims for sale but they are 16x8 and I was concerned that would be a little wide for these tires.
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Old 04-13-2003, 12:10 AM   #11
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Inflation pressure

As mentioned the load rating of the tire changes with inflation pressure - more pressure more load capacity. Manufactures can provide the load rating at different pressures for tires so you can be sure you are not running the tires undreinflated. They are most likely to have the info available to the public for tires that are used over a wide range of loads such as in trucks and motorhomes etc. I was able to get a load rating for the Michelens I use on my truck from the lowest recommended pressure to the highest by making a few phone calls.

The procedure of adjusting the pressure until the pressure rise is in the correct range would seem to be pretty good advise.

Would the amount of moisture in the air (air used to inflate the tire) change the pressure you should run? I know it will change the amount of pressure rise. Thats why racers fill tires with Nitrogen.

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Old 04-13-2003, 09:38 AM   #12
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For a discussion I will call interesting , check this: Nitrogen in the tires
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Old 04-13-2003, 08:51 PM   #13
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In replay to 59toaster question regarding rim width acceptable for your tires: Each tire manufacturer has a published acceptable rim width size for their tires by size. It is sometimes difficult to find a tire dealer that will show you in writing the correct answer. The manufacturers have the information readily available if you contact them directly. I have been out of the tire business too long to make a specific recommendation in this regard. Just contact a knowlegeable tire dealer or the manufacturer. Thanks for the good words. Good luck.
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Old 04-13-2003, 09:00 PM   #14
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The problem that moisture causes in a tire is that it promotes rust on the rim. It should have no effect regarding the correct air pressure. The correct air pressure caues the tire to flex in the correct area of the tire sidewall. The tire is a rolling spring that needs to flex in the proper range for best results.

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