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Old 01-17-2015, 10:11 AM   #29
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Originally Posted by MN_Traveler View Post
Folks ... I think we are all making WAY too much of this. The way I think of it, the biggest danger to tires is running them at too low a pressure for their loading. The point of checking the pressure "cold" is that from there the pressure will only go up (as temperature rises, either due to ambient temperature, sunlight , or running on the thin ..
MN TRAVELER. ...you hit the nail on the head. Thank You, that makes perfect sense.

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Old 01-17-2015, 10:42 AM   #30
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I set my PSI in the AM before the sunlight can heat the side towards the sun. I adjust up/down based upon the outside temperature. I monitor I set with a tire gauge that is calibrated yearly. I then validate that the tire monitor PSI readings are off the same amount (about 3 psi lower than actual, on all but the LF tire, which is almost spot on.)

While traveling, I do a visual inspection of the tires at the start of each day, and when stopping for fuel of lunch. I also check the PSI on the tire monitor, before heading out. Again, adjusting for outside temperatures, and factoring if one side has been baking in the sun. (Not exact, but at least I make things pass my reasonableness test.)

I find that I can go 6 months and have very little loss of PSI.

I do not adjust my tires based upon location local conditions. If I'm say down 5 PSI due to lower temperatures, I just carefully warm the engine and tires up as I slowly get started on that days drive. Same thing on extreme heat conditions, as long as it passes my reasonableness test, and stays below my max PSI/Temp settings, I leave things alone.

If I were to relocate from say sea level, to 8,000 feet for a long period of time, I would reset my tire PSI for the condition.

This is how I handle PSI!
Best to all,
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Old 01-17-2015, 11:23 AM   #31
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Hi Ho: Tireman9 obviously has it right with this contingency: Our Acura (and many other OEM monitoring systems) yells loud when tire pressure isn't what it think it should be. This results in a big "check tire pressure" being displayed over the top of stuff that I want to know. So every fall we have to add 6 or 8 psi and take 6 or 8 psi out in the spring. This has nothing to do with the tires and everything to do with the car itself.

I have talked with a bunch of "over the road" truck drivers, and most I have talked with just put in 115 psi (if that's the rated tire pressure) and that's it. they look at me kind of funny when I ask about loaded (vs empty) weights or difference between loads on each wheel. I suspect we RV types pay more attention to tire pressure than it needs. More pressure than needed probably doesn't hurt anything, and I for one, can't tell any difference in the way the rig handles or comfort for 10 or 15 lbs. too much.
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Old 01-17-2015, 06:18 PM   #32
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Just thought I'd add an interesting observation (at least to me) I had recently.

I left home here in VA headed to FL a couple of weeks ago. I had my TPMS alarms set to alarm if pressures exceeded 20% of where they were set. My fronts were set at 100 psi and duals at 90.

First night was spent in Georgia. Sometime after crossing into FL on day 2, I started getting high pressure alarms on the TPMS. It wasn't long before all ten were alarming.

It was very interesting to see and experience the impact of warmer temps on tire pressures going down the road.
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Old 01-18-2015, 08:19 AM   #33
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Originally Posted by scep View Post
If you check and adjust the psi before you drive "every time" it wont matter what the temp is.
Quote:
Originally Posted by Gary RVRoamer View Post
Isn't that "compensation"? For temperature and any/every thing else?
Yes but it's automatic, there's no need to agonize over psi "compensation" for temp...

From Michelin...
http://www.michelinrvtires.com/asset...ireArticle.pdf

Always try to check tires when they are “cold” and have not been driven for more than one mile. The stated load capacity for a given cold inflation pressure is based on ambient outside temperature.

So if you simply insure that the tire is at the psi you need for the weight you run then all is good.
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Old 01-18-2015, 08:28 AM   #34
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Don't worry about a few pounds. As soon as you start driving they will warm up and pressures will come back up. Just watch for the one tire that doesn't.
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Old 01-18-2015, 01:21 PM   #35
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For every temperature the tire can produce a sertain energy by deflecting about 10 to 20 times a second of every segment of it.
If ambint temp is higher , inside cold temp is same as outside ( asuming no other influences), and so deflection is less so heat production is less.
When warming up inside tire air by driving ( that deflecting and flexing back of every segment 10 to 20 times a second) the deflection gets less and so heat production gets less. At a sertain moment there is a balance.

But when cold deflection is to much ( aby lower cold pressure) , warm deflection is also to much so the tire inside gets hotter and pressure gets higher untill a new balance is reached.
By this higher inside warm temp the critical area's get to hot so they harden and get damaged in next deflections.
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Old 01-18-2015, 04:07 PM   #36
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The ambient temperature absolutely has an effect on your "cold" tire pressures.

A simple rule of thumb is 1lb of tire pressure for every 10 degrees. For instance, if your took your "cold" tire pressures (not driven on all day for instance) in the afternoon and it is 70 degrees, and then you check them in the morning when it is 40 degrees, you'll see about a 3lb drop (just as the op and Randy noted).

The sun heating up the tire carcass also has the some effect. So, if you did those afternoon tire pressures when it is 70 degrees, but 2 tires are in the direct sunlight, they may really be 80 degrees, so in the morning when it is 40 and the sun is not up yet, the shady tires will be 3 pounds low, and the sunny side tires may be 4 pounds low.

So you should take into account the temps you expect to be driving in. If you start a trip in NY in January and it is 30 degrees when you set your pressures, you will want to adjust them down as you head south as you get to warmer temps. Same situation if you take the temps of your car in a heated garage when it it 40 degrees colder outside - you'll want to be about 4pounds high in the garage, but if you know on average it's generally going to be 20 degrees colder outside, you'd set them 2 pounds high in the heated garage.

You don't need to get carried away with it, just be cognizant of it and manage your pressures accordingly.

Regards!
Sorry but 1psi for 10 degrees is the wrong thumb.
Even TireRack changed their prior 1psi for 10 degrees F in April 2014 after I gave them the correct math. That may work for passenger cars that are talking about 35 psi where the 2% is 0.7psi rounded to 1psi.

If you bothered to read my post that has the math proof you will see the real number is 1.89% which gives the "rule of thumb "2% for 10F change".

Your garage suggestion is reasonable but few RVs are kept inside a garage. If however you check the pressure on each travel day you will be close enough assuming you have included a margin above the minimum needed to carry the load in the motorhome.
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Old 01-18-2015, 04:13 PM   #37
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So let me see if i have this right tp 80 at 75 would equal 65 at 55, so do i fill back to 80 when temp is 55, as outside temp will climb during the day or what
Sorry I think your math is off a bit.

75F to 55F is 20 degrees. At 2% for each 10 degrees I would expect approximate 4% change in pressure.

4% of 80 = 3.2 psi so I would expect the tire to read about 77psi +/- a bit at 55 NOT 65psi. The "bit" is because we are not talking about a "perfect" dry gas. Even Nitrogen follows the gas law.

The variable is how much moisture is inside the tire. Its the moisture that messes up the nice math.
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Old 01-18-2015, 04:23 PM   #38
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Talking about 2 or 3 psi in 100psi is somewhat meaningless given the specified accuracy of any normal means of measuring the pressure. TPMS accuracies are worth having a look at because they are often nothing to write home about either.


"Fill the tires with dry nitrogen"

Can't be done.
Agree that 2 or 3 psi is not worth worrying about. That is why my recommendation for MOTORHOMES is
1. Establish the real minimum inflation to carry the measured load of the heavier end of each axle.
2. Add 10% to that pressure to establish your "Cold pressure target".
3. Set your TPMS to warn when the pressure ever drops below the minimum inflation so you can safely stop.

With the 10% margin built into your calculations you don't have to worry about day to day variation of a few psi. If you see the morning pressure getting lower each day you have advance warning that you will need to air up at a truck stop or if one tire is loosing but the rest are not you now have advance warning of a very slow leak.

Trailers are different. Multi axle trailers should try to never get below the inflation pressure on the tire sidewall for ST type tires. Details in THIS post.
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Old 01-18-2015, 04:30 PM   #39
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Some good info here. Tires don't fail because they are 4 lbs low, they fail because they are grossly low, as a general rule. The air pressure gauge you use is going to have be a very good one to be better than 3% accurate, or 3 psi at 100psi. That means you don't know whether you have 97psi or 103psi, or somewhere in-between, it's close enough.
Accurate tire gauges are not that hard to find. I have three digital gauges that cost me $9.95 to $12.50 and all measure +/- 0.5psi against ISO certified laboratory gauges.
See my latest blog post post
Basic tire inflation procedure - Motorhomes
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Old 01-18-2015, 04:34 PM   #40
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Hi Ho: Tireman9 obviously has it right with this contingency: Our Acura (and many other OEM monitoring systems) yells loud when tire pressure isn't what it think it should be. This results in a big "check tire pressure" being displayed over the top of stuff that I want to know. So every fall we have to add 6 or 8 psi and take 6 or 8 psi out in the spring. This has nothing to do with the tires and everything to do with the car itself.

I have talked with a bunch of "over the road" truck drivers, and most I have talked with just put in 115 psi (if that's the rated tire pressure) and that's it. they look at me kind of funny when I ask about loaded (vs empty) weights or difference between loads on each wheel. I suspect we RV types pay more attention to tire pressure than it needs. More pressure than needed probably doesn't hurt anything, and I for one, can't tell any difference in the way the rig handles or comfort for 10 or 15 lbs. too much.
RE Truck drivers. Don't forget most try to run loaded as much of the time as possible ans their Load to empty varies a LOT.

RVs on the other hand are running most of the tire at or above the tire max load capability so these are really two different markets.
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Old 01-18-2015, 04:38 PM   #41
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You're going to drive yourself crazy chasing your tire pressure. I usually add air to my tires once a year in the spring and don't use a tpms. Never had a problem, always check them before a long teip and usually never have to add air.

Cheers!
Well, that's just silly. I'd use another S word but don't want to offend you. I seriously hope nobody takes your advice. Just because you've been S lucky doesn't mean you aren't one mile away from a blowout.

The most important thing about tire pressure is NOT to run them UNDER inflated. Over inflation will cause some issues (uneven wear, hard ride, etc) but the tire is unlike to fail. Under inflation will cause the tire to fail - often catastrophically. So, you MUST adjust your cold tire inflation pressure to match the outdoor temps. No, you don't have to get crazy chasing tire pressure. As long as you stay in within a few pounds you'll be fine.

Tire pressure is important. For safety and for performance. If you don't have a TPMS you have no clue what's going on with your tires. You are simply guessing and good luck with that!
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Old 01-18-2015, 06:31 PM   #42
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[QUOTE=Tireman9;2388127]Sorry I think your math is off a bit.

75F to 55F is 20 degrees. At 2% for each 10 degrees I would expect approximate 4% change in pressure.

4% of 80 = 3.2 psi so I would expect the tire to read about 77psi +/- a bit at 55 NOT 65psi. The "bit" is because we are not talking about a "perfect" dry gas. Even Nitrogen follows the gas law.

The variable is how much moisture is inside the tire. Its the moisture that messes up the nice math.[/QUO
I agree ,go read my correctionion post #27
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