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Old 11-23-2008, 04:22 PM   #1
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Tire Pressure

I always thought that if I filled my tires at the recommended tire pressure while cold, I was good to go, at least for the day. Lately, however, I have read some posts where people fill their tires to the recommended inflation while cold, yet check the pressure again while warm, and make adjustments. Is this really necessary?

I did some testing and filled my tires to 65 PSI this morning at about 40 degrees farenheit. This afternoon, after warming up to 65 degrees, I rechecked and they were 70 PSI. The way I see it, this allowance for pressure difference is built into the recommendation so I don't need to worry about it, and as long as if I check the tire pressure at 40 degrees the next day, and it's still 65 PSI, I'm OK.

Does Mongo's logic need rethinking?
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Old 11-23-2008, 04:36 PM   #2
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I do much the same. I fill them cold and check them cold.

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Old 11-23-2008, 05:15 PM   #3
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It is not a bad idea to recheck your tires as the seasons change but within a given season there is no need to check them with a given day between morning and afternoon.

Have I not you are inflating your tires to 65 lbs. Unless you have a single axle trailer this is most likely over inflated. Tires should be set by load not by the legal number stamped on the sidewall.

The attached chart will give you correct inflation for a given axle load
http://www.goodyear.com/rv/pdf/rv_inflation.pdf

While in general each manufacture has the same inflation for the same size and rated tire it is a good idea to get the chart directly from your manufacture.

Don't go to a tire dealer for this because they will not even know what it is.
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Old 11-23-2008, 05:51 PM   #4
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Check and adjust inflation in the morning, then you can check throughout the day. But, DO NOT reduce the tire pressure over the course of the day, just monitor to make sure no one tire is drastically lower than the others. Tires will increase pressure as they warm up, if you reduce pressure to cold amount when warm, it will probably be badly underinflated when cold.
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Old 11-23-2008, 09:07 PM   #5
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I had the understanding that checking the tire pressure when "cold" really has less to do with the actual air temperature and more to do with checking the pressure "before driving" (as in when the tires are not warmed from driving on them). We check before we travel (and I try to check then while in the shade if possible, especially in SoAZ), but we do not decrease tire pressures while we travel...the increase in pressure due to travel (for us usually 4-6 lbs) is taken into consideration by the manufacturer... We use PressurePro tire pressure monitors so we can push the button and get a tire pressure reading at any point in transit...
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Old 11-24-2008, 12:07 AM   #6
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"Cold" tire pressure is considered to be before driving, no matter how "cold" (or not) it is outside.
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Old 11-25-2008, 06:53 AM   #7
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'round these parts, in may be June or July cold tires could be in the triple digits temp. before driving.

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Old 11-25-2008, 08:14 AM   #8
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Many individuals are of the opinion that a lower tire pressure results in an over heated tire that fails in a blow out and thus tend to grossly over inflate their tires. Yes under inflation will cause a tire to fail do to heat build up. However that is not a reason to Over Inflate a tire.

If you look at the inflation chart I post above you will see that the tires I use on my trailer LT 225 75 16 has an inflation range of from 35 to 80 lbs. Based on the weight of my trailer I inflate them to 40 lbs. If I were to inflate then to the 80 lbs. printed on the side of the tire I would not have a dish or glass when I got to the campground.

As for tire temperature when properly inflated. My tires generally run at 105 degrees F on the shaded side and 110 degrees on the sunny side on a hot summer day while starting out at 40 lbs.

Unless you have a known leaker there should be no reason to have to check your tires more often than once month or when subject to a significant ambient temperature as in a seasonal change.
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Old 11-25-2008, 10:33 AM   #9
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Manufacturers tire pressures are stated cold -- 4-hours after driving minimum, overnight preferable -- and the "coldest" point is around dawn. Adjusting pressure at this point is the recommendation. Adjusting pressure during a break in driving is at least inaccurate, at worst counter-productive.

Along with the pressure gauge, one may use an IR thermometer to measure tire sidewall temps during the driving day, looking for significant differences among TT or TV tires. This is a better manner of spending time on "maintenance" issues than messing with accurately-set tire pressures (+/- 1/2-lb of target).
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Old 11-25-2008, 11:04 AM   #10
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Every tire is designed to work with a certain amount of flex. Too hard, and you have a hard ride and poor traction for braking and cornering. Too soft and you have danger of accelerated wear, overheating, and possibly a blowout.

You need to have the appropriate pressure to support the weight on it. More weight, more pressure. Less weight, less pressure.

Too much pressure is safer than too little.

All tire manufacturers recommend setting tire pressure cold, meaning before driving. As you drive the tire will heat up and pressure will rise. If it does DO NOT adjust the pressure (down). Leave them alone. The extra few pounds will do no harm.
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Old 11-25-2008, 11:30 AM   #11
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I fill my tires to about 5 lbs less then tire sidewall says. That is what I feel comfortable with.

Underinflated tires I think are more dangerouse then slighly over filled tires. Because they will build heat.

In drag racing application. I set the slicks at 7psi then after I do my burn out and make a pass my pressure is up to as much as 15 psi. I let them back down to 7 psi and they are fine for the rest of the day, but I still check them.

I bet trailer tire pressure would rise from morning to mid day sun and heat.

I dont think there is a right answer. just my opinion
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Old 11-25-2008, 12:36 PM   #12
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Don't use drag tires as a model for maintence on trailer tires. In fact trailer tires may have enough difference in usage that they operate at temps different than the TV.

Interesting comment from HowieE above. Most of my trailer tires operate at a level of 85 to 90 of capacity all the time. So the inflation I use is the max. Tire pressures need to be matched to load ratings of the tires.

Where as my TV and car almost never have a full load. Only when towing does the TV get any where near the max load of the tires.

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Old 11-25-2008, 01:59 PM   #13
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Interesting comment from HowieE above. Most of my trailer tires operate at a level of 85 to 90 of capacity all the time. So the inflation I use is the max. Tire pressures need to be matched to load ratings of the tires.
I think you missed my point.

Tires pressure should be matched to the "LOAD" not the load rating. The load rating is the designed MAX tire pressure at MAX load for that tire.

Yes if one were to run a C rated tire on a trailer the tire pressure would most likely be close to the Max rating printed on the sidewall. In fact the max for a C rated tire is 55 lbs. and that would be at the limit under a 7.700 lbs. trailer with 4 tires. However if you are running E rated tires under the same trailer you would still inflate to 50 lbs. even though the tire has a max pressure of 80 lbs. My trailer with those same E rated tires and weighing 8,500 lbs and 6 tires calls for only 35 lbs.

Now I do put 40 lbs. in my tires only to give a margin of safety if I had to run in on 5 tires.
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Old 11-25-2008, 02:57 PM   #14
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Originally Posted by HowieE View Post
Have I not you are inflating your tires to 65 lbs. Unless you have a single axle trailer this is most likely over inflated. Tires should be set by load not by the legal number stamped on the sidewall.
I use 65 PSI in my tires because that is the tire pressure that Airstream recommends for my particular tire/trailer combination. It is stamped into a small plaque on the side of my trailer. I have a heavy trailer because of the slideout.
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Old 11-25-2008, 03:11 PM   #15
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Mongo like candy.

Steve, as Howie indicates, tires should be inflated to the manufacturer's recommendation cold (prior to use) for the weight that the tire is expected to carry. As an example, most manufacturers' E-range 275 75R16 tires are rated from 50-95psi depending on the load, and they have a chart showing what the inflation should be for the load.

If your Airstream's plaque says run 'em at 65 psi, that should be good, and it should be cold. It's smart to check them from time to time to make sure that they're all the same temperature when rolling (if one is hotter, that's a good indication that either the pressure is lower than the rest, or that there's a bearing or brake problem), but I wouldn't worry about what the pressure is hot, as long as they were at or near 65psi cold when you started.

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Old 11-25-2008, 03:40 PM   #16
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Too much pressure is safer than too little.
Some years back in another life, I was involved in a test w/ the Goodyear folks regarding tire pressure and stopping distance. According to them, the optimum stopping ability w/ the highest coefficient of friction is right at the "max pressure" imprinted on the tire.

That's not to say it will be the smoothest ride, the longest life, or the best handling but the optimum ability to stop! That was what the testing was about. Hundreds of tires and lots of miles that day.

As an aside, boys will be boys and the conversation turned to over inflation and under inflation. Yes, under inflation is bad and over inflation is not good but is not dangerous. Before the day was out, we were riding around on tires w/ 200 psi (yes, that is 200) and it was interesting ride. They stopped like we were on ice and under steered through turns because of the narrow footprint and they wore incredibly fast. No blowouts other than a couple of times the pull through stems left at a high rate of speed but it had the roughest ride imaginable. According to the engineers, they are capable of 350+.
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Old 11-25-2008, 04:17 PM   #17
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I use 65 PSI in my tires because that is the tire pressure that Airstream recommends for my particular tire/trailer combination. It is stamped into a small plaque on the side of my trailer. I have a heavy trailer because of the slideout.
OK lets bring in another variable. If your trailer has a slide out it most likely has a different weight per axle side to side. Under this condition you shold be inflating the tires on each side by the load on that side.

It is common with motor homes to have a different tire pressure at all 4 corners. If you have ever been to a Family Motor Coach rally one of the biggest points stressed is proper tire inflation based on individual wheel load.

Airstream has an inflation of 50 lbs. for my tri axle, the max pressure on the tire is 80 lbs. and I use 40 lbs based on the load.

I suspect any testing that inflated tires to 200 lbs was done for a early Candid Camera show. If it was done by a tire company it has been replaced by one with more engineering expertise and is reflected in the current inflation charts.
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Old 11-25-2008, 04:32 PM   #18
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I suspect any testing that inflated tires to 200 lbs was done for a early Candid Camera show. If it was done by a tire company it has been replaced by one with more engineering expertise and is reflected in the current inflation charts.
I think you missed the point, Howie. The high inflation deal was the result of questions asked by the drivers about how much the tires would hold. It was not part of the actual tire testing for research purposes. The engineers already knew those results from previous efforts but wanted to demonstrate it to the drivers at the end of a very long day.

There was never any mention that that much tire pressure should be used on any vehicle on any highway nor was I suggesting you do it.
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Old 11-25-2008, 05:16 PM   #19
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HowieE, I agreee with your view point.

To be blunt, I am just too cheap to part with the green backs it would take to obtain tires with a far greater rating than the max load I carry. I could move to a larger rim and get a greater load capacity in the larger tires.

Example, my boat trailer came equipped with 14" rims. The biggest load rating on the biggest 14" dia tires that are available are 1880 #'s a tire. 4 tires and that is 7520 # load rating. The actual load with a full fuel load and with out (a possible 26 gals of) water ran across the scale is 7350. If I added the water this would be another 200+ pounds of weight. I never tow the boat with any water in it and frequently the fuel tank has a less than full condition.

Observations;
I have to run max air pressure to get the load rating I desire out of these tires
To get more load capacity out of the wheel/tire combo, I have to fork over $ (may be a couple hundred +) to go to 15" rims with new tires and a spare. I also have to live with dropping the boat trailer further in the water because likely it would sit higher off the ground.

Mostly it is the money from stopping me from getting tires with a signficant greater load rating than the load. If I had these tires I would run them at a less pressure. However from my point of view is, if I have tires with that amount of reserve, it is a bit of waste in the job called to do. Like sending in an M1A1 task force group to serve a National Guard function. It can be done however it is a resource not needed for the job.

I believe trailer manufacturers think in the same patterns. The falacy becomes when they make an engineering error and cut the margin too thin. Or the consumer doesn't understand the complex nature of the parts and pieces and over loads. And to make another observation most consumers usually elect to pay less if they don't need more. This is the market place for many years.

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