Tire pressure and old wives tales.

[Tireman9]

Originally Posted by Flagelpater

FWIW A few years ago I was someplace, can't remember where, talking to a tire engineer. He told me that the most accurate way to measure the proper tire pressure is with a Profile Gage. A device that when held up against the outside of the tire, at the point where the tire is on the ground, matches the profile of the outside of the sidewall of the tire. This is what the tire manufacturer is concerned with. This is what determines the amount of flexing that is going to take place. The PSI Gage is simply a more convenient way to arrive at the same setting. One would have to assume they know what happens after the tire warms up to operating temperature. You could load your axel and apply enough PSI to achieve the proper profile, not to exceed the Max PSI Rating. This should not happen unless you have overloaded the tire.

Well I understand the concept but have to wonder if that "tire engineer" actually was ever responsible for the design and testing of a tire. I have never heard of any test where the sidewall bulge / profile/ shape was used to specify the load.

If that "tire Engineer" worked for a major company he/she should have been very familier with The Tire and Rim Association and the Load and Inflation tables used by all tire companies in the US and many in other countries for establishing the load/inflation relationship. There are no Load and profile tables or formulas anywhere in the standards.

[Flagelpater]

FWIW And according to your reply, NVM "Not Very Much"! This guy, supposed engineer, was not indicating that it was in lieu of the Load and Inflation tables that we all, I assume, use, but one of the methods used to generate the Tables. Would you be so kind as to explain how the flexing is determined with just the Load and Inflation Tables? I do not want to be argumentative, just looking to be a little more educated.

[Tireman9]

Originally Posted by InPursuit

If I'm reading and interpreting the transcribed statements correctly...

minimum psi = the weight based calculation of the rig relative to the tire characteristics (clearly stated on the sidewall) and *in conjunction with* what restrictions the wheel will have (stamped into the metal)

maximum psi = the psi as it is stated on the sidewall...
regardless of whatever voodoo someone may be using elsewise

obsessiveness = worrying over minor variations in gauge accuracy or daily temperature or altitude changes

Well I don't quite agree with all you said

Yes
Minimum tire inflation is based on real load based calculations

Nope
Wheel restrictions are Maximums and as long as you are not loading the wheel more that the stated Maximum there should be no problems. I have never heard of a minimum inflation or minimum load as far as the wheel is concerend.

Almost
Maximum inflation is marked on the tire and the wheel. This is the inflation when measured cold and should not be exceeded when cold.

Right on
obsessiveness

[Tireman9]

Originally Posted by Wayne M

Bryan,

Well, my tires are rated for 5205 lbs single, and 4805 dual at 110 psi

(Actual Weights)
My front axle is 9280 lb (Divided by 2 is 4640 each tire)

My rear axle is 26310 minus 9280 is 17030. (Divided by 4 is 4257.5) and that includes the TOAD hooked up

My gross is 29760 lbs at that weighing.

I was 208 pounds light of fuel so my actual gross was 29967.

And if it were applied to the rear and front axles, it would make them weigh 26518

I'll let you do the math, and tell me, what air pressure would you run?

Personally I'll stay with 110 psi for the safety margin.

I don't portrait to be an expert in this field, by any means, but I do the best I can with what I have. Other RV'ers can inflate their tires the way they want to. I hope they tend to be on the "safe" side.

Thanks.

Bryan
You didn't provide the tire size or Load Range but I believe you have 255/70R22.5 LR-G
Now not knowing where the fuel tank I am going to assume the 208# is evenly split F/R.
So based on your figures when fully loaded (all fuel, food, cloths, stuff, pets & co-pilot) your front axle is loaded to 9280+104 or 9385# If you have 50/50 side to side (which is improbable) each front needs to carry at least 4693. Your tires are only rated at 4675 @95 so you need to consider 100 psi as your minimum cold inflation.

For your rear you confuse me a little. Did the scale read 26310 with just the rear axle on the scale? oe is 26310 the total for the unit?

You say your grose is 29760. Do you mean your "combined gross" which is your RV + toad?

Lets say the scale for the rear was 17030 + 104 fuel = 17134
Divide by 4 = 4284
Your tires are rated for 4275 at 90 so you need to inflate to a minimum of 95 to carry the load.

Now to add a little cushion I would run 105 Front and 100 rear.
I would also get the actual four corner loads with full tank of fuel etc. I have seen somd data indicating some rigs have a 53/47 side to side load.

You always do the calculations on the heavy side.
You always run the same inflation on all tires on any axle.
The inflations are cold which means ambient and not sitting in the sun for a few hours.

[Wayne M]

Tireman9,
I think you were referring to me, Wayne.
See below:

Originally Posted by Tireman9

Bryan
You didn't provide the tire size or Load Range but I believe you have 255/70R22.5 LR-G
They are 255/80R22.5 XRV LR G Michelin's


Now not knowing where the fuel tank I am going to assume the 208# is evenly split F/R.
I did base that on the same assumption that they are evenly split.

So based on your figures when fully loaded (all fuel, food, cloths, stuff, pets & co-pilot) your front axle is loaded to 9280+104 or 9385# If you have 50/50 side to side (which is improbable)(Agree on improbable - I will get each corner weight at my first opportunity - again I assumed equal distribution.) each front needs to carry at least 4693. Your tires are only rated at 4675 @95 so you need to consider 100 psi as your minimum cold inflation.

For your rear you confuse me a little. Did the scale read 26310 with just the rear axle on the scale? oe is 26310 the total for the unit?
All I can give you at this time is what the weight person put on the ticket:
It was with the rear on the scale but the toad was attached but not on the scale. So that would be with tongue weight.

You say your grose is 29760. (Yes Combined Gross) Do you mean your "combined gross" which is your RV + toad?

Lets say the scale for the rear was 17030 + 104 fuel = 17134
Divide by 4 = 4284
Your tires are rated for 4275 (4805 @ 110 psi) at 90 so you need to inflate to a minimum of 95 to carry the load.

Now to add a little cushion I would run 105 Front and 100 rear.
I would also get the actual four corner loads with full tank of fuel etc. I have seen somd data indicating some rigs have a 53/47 side to side load.
(Agree that all corners need to be weighed)

You always do the calculations on the heavy side. (I do)
You always run the same inflation on all tires on any axle. (I do - 110 psi)
The inflations are cold which means ambient and not sitting in the sun for a few hours. (Early morning after sitting all night. I always expect those with a little sun on them will be a few pounds higher. Also, the inner dual can vary from being protected from the elements some. Everything being as equal as possible I run 110 psi.)

[FDchief]

Originally Posted by JimM68

I'm leaving today, ultimate destination in 3 days is "the mountains."
Should I prefill my tires with "mountain air"?
And where along I80 thru Illinois, Iowa, or Nebraska can I get some?

Hmmm, do be careful. If you fill your tires at sea level, then head up to altitude, you will have a significant increase in p.s.i. Do not lower your pressure at altitude to counter this effect, because as you come down back to sea level you will be severely underinflated! You must take altitude and temperature changes seriously.

[Always-RVing]

You should use clean, cold mountain fresh air instead of city polluted air. When used, your tires will love you and add an extra MPG.

[Tireman9]

I believe that your pressure gage will read about 2 to 4 psi difference as you travel from sea level to 5,000 to 10,000 ft altitude. Tire Tech Information - The Influence of Altitude Changes on Tire Pressure

Now it is possible to make such a drive in a single long day. The tire would of course be hot so the next morning when the tire is now at ambient you would of course need to adjust your inflation.

The other thing to remember is that it was probably 80°F at the sea-level and is probably close to 30°F at high altitude so you have also seen a loss due to temperature of about 10% which in all probability is a greater change than that due to altitude.

Bottom line. If you drive normal distances of 200 - 400 miles per day and check your inflation each morning you will probably almost always be within the +5 psi cushion above your minimum inflation I and others have previously suggested.

Now about that smog you inflated your tires with in LA vs the nice clean fresh pine sented air in Colorado.....
Your tire probably can't smell the difference. Remember the compressor adds a bit of oil vapor to the air so when you smell the air that comes out of your tires I think you will seen it isn't important.

[JoeT]

Originally Posted by two-niner

I had just seen (on another forum)(which was taken down quickly) an old wives tale about tire inflation.
The gist of the matter was to inflate tires per manufacturers spec's, then add five (5) lbs. Personally I have never done that.
I am looking at Michelins RV Tires 32 page booklet. Nowhere do they say, state or intimate to add five pounds of air above their recommendation.
It's time to put this chestnut to bed, at least for Michelins.
My old computer will freeze if I try to make a clicky but here it is:
www.michelinrvtires.com
Dag nag it, the clicky worked.

Kerry

A regular tech column in Motorhome Magazine recommended just that. Inflate tires to manufacturer's inflation table and add 5 lbs.
The reasoning was, you don't have to adjust tire pressure as often due to changes in the weather/season.