Tire Pressure

[Midniteoyl]

The 'thump' test wasnt so much for low air, but bad tires.. Doesnt really apply now-a-days...

[Wayne M]

Originally Posted by randco

You might be right, but on page 3 of the Toyo - RV, Motor home Tire Inflation, Safety & Maintenance - Techinal Bulletin (December 1, 2004) it says and I quote, "All tires lose air at the rate of 1 – 1.5 PSI per month due to natural permeation of the air through the tire’s rubber membrane. Always check the cold inflation pressure of any vehicle that has not been driven for several weeks and re-inflate the tires to the placard pressure before driving.

Tire pressure is affected by the ambient temperature to the extent of approximately 1 PSI per 10 degree (F) change in temperature. As an example, a 20 deg. (F) drop in temperature will result in a 2 PSI drop. A 20 deg. (F) increase in ambient temperature will result in a 2 PSI increase."

Now who am I to argue with the tire manufacturer?

That's why I don't buy Toyo's. I have Michelin, and after two months of parking in a covered (sides open) area, the pressure is at what it was when it was put in storage. Must be tighter molecules in the Michelin, hey?

1Ciderdog, funny you should mention that. I also attend Camp Freightliner and was taught the same thing. Matter of fact, I have the manual from them provided by Goodyear regarding the 2% change. Marvelous minds think alike.

[randco]

Originally Posted by Wayne M

That's why I don't buy Toyo's. I have Michelin, and after two months of parking in a covered (sides open) area, the pressure is at what it was when it was put in storage. Must be tighter molecules in the Michelin, hey?

1Ciderdog, funny you should mention that. I also attend Camp Freightliner and was taught the same thing. Matter of fact, I have the manual from them provided by Goodyear regarding the 2% change. Marvelous minds think alike.

Tighter molecules??? I guess that's why Michelin's get those cracks in their sidewalls. Perhaps the molecules are so tight they break and not bend. Now I know why I replaced all six of the Michelin's on my coach with Toyo's. It's all about the molecules.

And I suppose you remember that the ambient temperature was "exactly the same" the day you put it in storage as the day you took it out? Remember, according to you 10 deg. difference at 2% could be significant.

You have Michelin's, why do you use a Goodyear RV Tire Guide?

Doesn't Michelin have anything to say about Ambient Air Temp. & Cold Tire Pressure?

[RickO]

Let's keep things friendly here folks. We don't want to get into a "brand war" which is not allowed on this forum.

Rick

[Wayne M]

Randco,

I'll let you know as soon as i have the time to look around some more.

As previously stated in many posts, the best advice is the advice a person gets from the tire manufacturer.

[Midniteoyl]

Originally Posted by Wayne M

As previously stated in many posts, the best advice is the advice a person gets from the tire manufacturer.

It is with the courts too, should that ever be necessary.

[randco]

Originally Posted by Wayne M

Randco,

I'll let you know as soon as i have the time to look around some more.

As previously stated in many posts, the best advice is the advice a person gets from the tire manufacturer.

As I previously quoted from a tire manufacturer, "Toyo - RV, Motor home Tire Inflation, Safety & Maintenance - Techinal Bulletin (December 1, 2004) page 3, "All tires lose air at the rate of 1 – 1.5 PSI per month due to natural permeation of the air through the tire’s rubber membrane. Always check the cold inflation pressure of any vehicle that has not been driven for several weeks and re-inflate the tires to the placard pressure before driving.

Tire pressure is affected by the ambient temperature to the extent of approximately 1 PSI per 10 degree (F) change in temperature. As an example, a 20 deg. (F) drop in temperature will result in a 2 PSI drop. A 20 deg. (F) increase in ambient temperature will result in a 2 PSI increase."

I guess the tire manufacturers also have a difference of opinion.

I think we both agree that it is prudent to check pressures to make certain the coach can be operated safely. No matter what the conditions.

[randco]

Originally Posted by RickO

Let's keep things friendly here folks. We don't want to get into a "brand war" which is not allowed on this forum.

Rick

Trying to keep it friendly, just bringing up a few points. There has been a lot of discussion on this forum about Michelin's getting getting sidewall cracks. There have also been numerous discussions about Goodyear's "rivering" problems. I can't speak about the Goodyears because I have never owned them. I have had experience with the Michelin's.

[RickO]

No problem. My cautionary post wasn't aimed at anybody in particular but just intended as a reminder.

Thanks,

Rick

[Elkhartjim]

Originally Posted by Midniteoyl

The 'thump' test wasnt so much for low air, but bad tires.. Doesn't really apply now-a-days...

Oh no! Jim, are you telling me my air'er up, give'er a thump and let rip theory is hogwash? I guess I'll have to break down and get the four corners weighed, dig out that tire book Mike Cody passed out at Camp Freightliner and pressure the tires to the spec's it calls for.

I'm speaking sarcastically of course. I can't find my whacking stick is really the reason.

[Wayne M]

Judy has the whackin' stick! You should know that!

Now teach her that it is the tires she is supposed to be whackin'

Better yet, maybe she does have it right. (hee hee)

[mikron]

Our 22.5 tires on our Windsport F53 22,000 chassis coach after weighing front and back axles at a Pilot CAT scale 20,800 lbs and based on the Michelin chart 80 lbs all around.

[Midniteoyl]

What was the weights per axle.. that what you need..

[Lincolnboy2]

Originally Posted by Wayne M

Aw heck!
Okay, I'm just going to assume some information.

Just for the sake of explanation, let us assume that the tires are Goodyear 225/70R19.5, okay?

Lets start out with a simple chart.

For single tires per axle

PSI---------------70-------75-------80------85------90-------95---------100 (There's more but let's stop there)
Weight(S)------2895-----3040----3195---3315---3450---3640(F)-----3715 (F = Load range F)

Now, let us assume that the weight on your tire is 3040 per tire on a single axle (The psi setting has to be the same for all tires on the same axle)

So, let us assume that you inflate your tires to 75 psi. I mean, after all you just came back from the CAT scale and you know what your weight is. You drive home pick up the wife, 50 pound dog. The dog sits at DW's feet. Total weight is an additional 150 pounds on the passenger tire. (You didn't think I was going to make your DW heavy, did you?) You are 150 pounds overweight on the passenger tire. Even if you had distributed the weight evenly across the axle, you would still be 75 pounds per tire overweight. If you experienced a drop in temperature or altitude, you would be additionally overweight. (Remember, this is just an example - I actually like working with the 22.5 inch tires)

So do the fudge factor:

Inflate the tires to 80 psi, and the tires are now rated for 3195 pounds. Now you have a fudge factor of 155 additional pounds per tire, or a total of 310 pounds you can put in the RV distributed across the axle. See where this is going. No? Okay, now stop at Wall-world for the night, go in, buy some groceries, supplies, whatever, and make sure you go to the bathroom scale and weigh all that stuff, because you are not overweight on the tires. Don't forget that DW and Fido are using up 150 pounds of that, so you only have a margin of 160 pounds. Don't go to Mcdonald's every night and eat hamburgers. Don't fret - more fudge coming.

Inflate the tires to 85 psi, and the tires are now rated for 3315 pounds per tire. That is 275 pounds per tire, or 550 pounds across the axle. DW and Fido can be comfortably safe and do not have to have salad every night.

Now let us say that you are running at 85 psi, and when you checked the tires at that pressure the ambient temperature was 90 degrees (I'm in Texas). You are at sea level. You head North. The temperature drops to 60 degrees. That represents a 6% decrease in pressure. 85 minus 6% equals a 5.1 psi change, and the gauge will only show a 5 psi change. So you are at 80 psi the next morning when you get up. Oops, wait a minute, you gained 2000 feet in altitude. That is a 0.96 psi increase in pressure, so your actual tire pressure reading should be 81 psi. Look at the chart. Are you still within range of the tire manufacturers recommendations? Do you need to change the tire pressure by adding more air? Nope. Smile, be on your way. If you go to the other extremes, like from 90 degrees to 120 degrees you are going to gain 5 psi which will put you at 90 psi plus one for altitude of 0.96 psi so you are at 91 psi. That particular tire in this example goes to 110 psi at a max load of 3970(G Rated tire). Are you still within the parameters. Most definitely.

You can do the math for the duals, and remember their load rating for this specific tire is 3115 per tire. That is a total axle load rating of 12, 460 pounds. Your front steer tires, single tire on each end of a single axle at 85 psi is 6630 pounds.

I cannot express this enough. Never let air out of a hot tire that has not set for several hours and come to ambient temperature. The best time to check a tire is early in the morning. If the sun is hitting the outer dual, it will normally be at a higher psi than the inside dual. If both are on the shady side, the inside dual may be a tad higher that the outside dual, because the inside dual is getting heat from the RV. After about a mile down the highway they will both catch up with each other.

When to check tire pressure:

• Before each trip
• Every morning during long trips
• Before you leave and when you return home on short trips
• Before and after storing your vehicle
• AT least once per month while the vehicle is in storage

The only way to know what the proper inflation for your tires are to weigh the vehicle, preferably at each corner, and use the manufacturer's tire inflation chart.

Happy trails. If I made a mistake in the computations, well, I'm not a tire engineer nor a math major.

Edited: Here is a link to the Goodyear Tire Inflation Loading web site.
Once there click on the blue "Download RV Tire & Care Guide."

Lots of great information. Thanks for the info!