What is your correct inflation?

[Tireman9]

We talk about knowing your real weights by corner but not everyone gets to large RV Rallies where RVSEF offers their services. Here is a link to CAT scales. Any certified scale will work for getting your RV weighed. You might find a local company if you check your phone book or try this link.

You should NOT adjust your inflation for each tire based on it's real load. Tire inflation affects both load carrying capability as well as vehicle handling (turning right forces should = turning left forces). You should have all tires on an axle at the same inflation +/- 1 psi which is easy to achieve with a digital inflation gauge.

Now knowing that all tires on an axle should have the same inflation we need to be sure no tire is overloaded. The procedure recommended by experienced tire design engineers is to:
1. Get the weights with the vehicle fully loaded (water, fuel, food, clothes, co-pilot, pets etc.)
2. Use a published Load & Inflation table. You should be able to find that info in this thread.
3. Look up the lowest inflation that has the load higher than yours.
4. Be sure to note that if you have Duals there are different load numbers.
5. That lowest inflation is your MINIMUM "Cold" inflation.
6. "Cold" inflation is measured at ambient, not in the sun and at least 3 or 4 hours after it was driven.
7. All tires leak air at about 1% - 3% per month. This is at the molecular level. Also cold tire pressure changes with ambient temperature at about 2% per 10 Degrees F. There is also an altitude effect, so to avoid having to inflate your tires every few days as you travel around, many recommend that you inflate your tires to 5 psi above your minimum. That way it will be longer before you have to add air.

Finally, If you discover you are a few (1 to 5) psi lower than you want to be, but have to drive a bit to get to high pressure air, simply make a note of the number of pounds you need to add for each tire. Drive the few miles at speeds lower than 50 and when you get to the location with air, again measure the now "Hot" pressure. Check your notes and add the number of pounds you want for each tire and add that plus 1 psi to the hot pressure to get your new inflation.
If you follow this procedure I think you will find that you are back to your desired +/- 1 psi cold the next moning.

Hope this helps.

[az bound]

Tireman9; This is the best inflation article I have ever seen. I have to wonder why manufacturers fail to show this iformation along with their charts. The key words that seems to be overlooked by many "minimum cold inflation".
Would you not agree that running a tire at minimim would be detrimental to tire life ?
Thanks very much for the information.

[Tireman9]

Originally Posted by az bound

Tireman9; This is the best inflation article I have ever seen. I have to wonder why manufacturers fail to show this iformation along with their charts. The key words that seems to be overlooked by many "minimum cold inflation".
Would you not agree that running a tire at minimim would be detrimental to tire life ?
Thanks very much for the information.

The minimum is just that. If you set your tire to the minimum there are some instances where you will be overloaded 24 hours later (Temperature changes). Or on some vehicles simply putting five 200# people in the vehicle instead of two at 154# will result in overload as the manufacturer provided essentiLLY NO "RESERVE LOAD".

Sometimes "minimum" is established based on assumptions on how much stuff you put in your vehicle or how much people weigh. Placards and charts are just estimates. When there is data showing that over 50% of RVs have one or more component overloaded I can not understand why people complain when they have a tire failure due to overload and poor or no maintenance. They made a concious decision to not educate themselves about their RV but somehow want to blaim the tire mfg when the tire fails due to overload.
If there were really as many "defective" tires out there as some want to claim there would be many more recalls.
I saw an item this morning where NHTSA is starting an investigation because of 54 complaints out of 80,000 items having possible failure.

After 4 to 8 years usage I wonder if there are only 54 RVs that have had say a water pump or generator or refrig or someother part fail but I don't hear owners demanding better quality from RV manufacturers.

[FastEagle]

Originally Posted by Tireman9

We talk about knowing your real weights by corner but not everyone gets to large RV Rallies where RVSEF offers their services. Here is a link to CAT scales. Any certified scale will work for getting your RV weighed. You might find a local company if you check your phone book or try this link.

You should NOT adjust your inflation for each tire based on it's real load. Tire inflation affects both load carrying capability as well as vehicle handling (turning right forces should = turning left forces). You should have all tires on an axle at the same inflation +/- 1 psi which is easy to achieve with a digital inflation gauge.

Now knowing that all tires on an axle should have the same inflation we need to be sure no tire is overloaded. The procedure recommended by experienced tire design engineers is to:
1. Get the weights with the vehicle fully loaded (water, fuel, food, clothes, co-pilot, pets etc.)
2. Use a published Load & Inflation table. You should be able to find that info in this thread.
3. Look up the lowest inflation that has the load higher than yours.
4. Be sure to note that if you have Duals there are different load numbers.
5. That lowest inflation is your MINIMUM "Cold" inflation.
6. "Cold" inflation is measured at ambient, not in the sun and at least 3 or 4 hours after it was driven.
7. All tires leak air at about 1% - 3% per month. This is at the molecular level. Also cold tire pressure changes with ambient temperature at about 2% per 10 Degrees F. There is also an altitude effect, so to avoid having to inflate your tires every few days as you travel around, many recommend that you inflate your tires to 5 psi above your minimum. That way it will be longer before you have to add air.

Finally, If you discover you are a few (1 to 5) psi lower than you want to be, but have to drive a bit to get to high pressure air, simply make a note of the number of pounds you need to add for each tire. Drive the few miles at speeds lower than 50 and when you get to the location with air, again measure the now "Hot" pressure. Check your notes and add the number of pounds you want for each tire and add that plus 1 psi to the hot pressure to get your new inflation.
If you follow this procedure I think you will find that you are back to your desired +/- 1 psi cold the next moning.

Hope this helps.

Hi Tireman, FastEagle Here.


What is your basic explanation as to just how a tire manufacturers tire inflation table is supposed to be applied when used in conjunction with self propelled RV vehicles or RV trailer tires?

We see a lot of experts talking about the reserve load carrying ability tires are supposed to have. But we cannot find it in print. By using a load inflation table as you and many other highly knowledgeable people in the tire industry promote, we can end up with zero published reserves.

Here is a case I have specs for. Sorry it’s for an RV trailer but if helps demonstrate how most of the RV manufacturers view their responsibility when it comes to selecting tires.

An RV trailer with two 7000# axles is delivered to it’s new owner with four ST235/80R16E tires on those axles. The vehicle tire placard tells the owner to air those tires to 80 psi (matching the maximum amount shown on each tires sidewall) to obtain 3520# of load carrying ability from each of the tires. The vehicle manufacturer has satisfied his requirement to equal or exceed the DOT requirement for tires on those axles. The new owner has a total reserve load carrying capacity totaling 80#. I don’t care who made those tires. They are going to fail and early. My guess would be less than a year if the owner is loaded close to GVWR.

The bottom line here is when you tell us to use the load inflation tables as describe in your thread above you are placing us in the same condition the vehicle manufacturer has put the above owner in.

Knowledgeable tire people will say the Light Truck tire has a greater reserve than the Special Trailer tire but NOBODY will publish it as fact with figures. These same people say ST tires degrade faster than LT tires yet they fail just as frequently when used under equal conditions.

When reading through the major 2007 DOT rules changing document many of those giving inputs into the decision making process recommended that RV tires be required to have a minimum of 12-15% reserve load carrying capacity above the GAWR. The rules makers disagreed and left the industry as is until such time as new rules are discussed again.

I’m a firm believer in a large reserve, especially for the RV. I have an RV trailer and I keep tires on it that will always give me a 20% reserve.

FastEagle

p.s. Here is a little wrench for your gears.
When an RV trailer manufacturer puts tires rated at 3420# @ 80 psi on both ends of a 6000# axle and tells the new owner via the tire placard that that is his minimum recommended requirement, what is the owner to do when you tell him it’s OK to use the tire manufacturers load inflation table? According to the DOT the vehicle manufacturer sets the inflation pressures (using the information from the tire manufacturer) for the tires he has selected for that axle.

[Tireman9]

Originally Posted by FastEagle

Hi Tireman, FastEagle Here.


What is your basic explanation as to just how a tire manufacturers tire inflation table is supposed to be applied when used in conjunction with self propelled RV vehicles or RV trailer tires?

We see a lot of experts talking about the reserve load carrying ability tires are supposed to have. But we cannot find it in print. By using a load inflation table as you and many other highly knowledgeable people in the tire industry promote, we can end up with zero published reserves.

Here is a case I have specs for. Sorry it’s for an RV trailer but if helps demonstrate how most of the RV manufacturers view their responsibility when it comes to selecting tires.

An RV trailer with two 7000# axles is delivered to it’s new owner with four ST235/80R16E tires on those axles. The vehicle tire placard tells the owner to air those tires to 80 psi (matching the maximum amount shown on each tires sidewall) to obtain 3520# of load carrying ability from each of the tires. The vehicle manufacturer has satisfied his requirement to equal or exceed the DOT requirement for tires on those axles. The new owner has a total reserve load carrying capacity totaling 80#. I don’t care who made those tires. They are going to fail and early. My guess would be less than a year if the owner is loaded close to GVWR.

The bottom line here is when you tell us to use the load inflation tables as describe in your thread above you are placing us in the same condition the vehicle manufacturer has put the above owner in.

Knowledgeable tire people will say the Light Truck tire has a greater reserve than the Special Trailer tire but NOBODY will publish it as fact with figures. These same people say ST tires degrade faster than LT tires yet they fail just as frequently when used under equal conditions.

When reading through the major 2007 DOT rules changing document many of those giving inputs into the decision making process recommended that RV tires be required to have a minimum of 12-15% reserve load carrying capacity above the GAWR. The rules makers disagreed and left the industry as is until such time as new rules are discussed again.

I’m a firm believer in a large reserve, especially for the RV. I have an RV trailer and I keep tires on it that will always give me a 20% reserve.

FastEagle

p.s. Here is a little wrench for your gears.
When an RV trailer manufacturer puts tires rated at 3420# @ 80 psi on both ends of a 6000# axle and tells the new owner via the tire placard that that is his minimum recommended requirement, what is the owner to do when you tell him it’s OK to use the tire manufacturers load inflation table? According to the DOT the vehicle manufacturer sets the inflation pressures (using the information from the tire manufacturer) for the tires he has selected for that axle.

Hi FE
Lets see if I can shed some light to help those that are interested understand the various issues.

Lets start with by defining our terms.
"Reserve load" is generally understood as the mathimatical difference between the published load capacity at a given inflation and the actual static load on a tire. The "actual load" is generally considered "curb weight" for passenger cars.

Reserve Load is something most used when evaluating Passenger car applications and for a good number of cars the Reserve Load is in the 10% to 20% range. There have been a few notible exceptions where the Reserve Load ended up being less than the weight of four passengers with no luggage.

Now when we move to Truck appllications the history was that we were dealing with professionals who were required by law to not exceed certain axle load limits. In the Trucking industry, vehicles are load limited so the application was based on the max published load at the max published inflation with 0% Reserve Load.

Light Trucks are really an off-shoot of Heavy truck when it comes to the max capability. The fly in the ointment with LT is that there are a lot of people driving empty pickups around and they don't like the bad ride. If people used their truck as a truck there would be less of a problem. Some manufacturers publish dual inflation to address this problem. Too bad there are so many users who lower the pressure for a good ride but forget to inflate their tires when they load their truck.

Motorized RVs are basically like Light Trucks ( Class B & C) or more like heavy trucks (Class A) when it comes to tire sizing, loads & inflations.
I am not aware of any Official or un-Official "Reserve Load" built into the recommended inflation for these applications.

HOWEVER

There are some issues unique to RVs. Primarily those are related to a significant portion of RVs being operated with tires in an overloaded condition.
Unlike Heavy Trucks, few Class-A are pulled over and weighed and I do not think that Class-A have to go through weigh stations so the owners have no idea what their rig weighs and the do not check their inflation so don't know when they have lost pressure. These problems affect Class-C & B but to a lesser extent.

OK
I think this post is long enough. I will post Part #2 in the next day or so and will cover the effects of Load Calculations on the different applications then go into the unique ST applications.

[Tireman9]

Reviewing some old posts and realized I never provided the part #2 to FastEagle's question.

I previously covered motorized RV load ^ inflation and suggested that after learning the MINIMUM inflation needed to carry your load you add a few psi (+10% is a nice calculation) This will give you Reserve air or Reserve inflation and this means you ahve reserve load. Now some suggest a specific reserve load % but I don't thine the end result will be much different.

Now multi-axle towables are a completely different story.
You should still get on a scale but this is to make sure you are not in the 57% of the trailer community that are overloaded to start with.

The cornering forces on multi-axle units place enormous side loads on the tires when cornering. These forces are trying to tear the belts and tread off the tire. The tire industry has the Finite Element software to predict these forces and they exceed a 20% increase over the side forces seen in other RVs. The only way an owner can help decrease these forces other than reducing the load by a very large amount, is to increase the inflation pressure to the max for the tire.
This is almost always reflected in the tire placard inflation which for many ST tires is 65 psi (read your placard and read your tire to learn what your max tire inflation is).

Hope this helps.

[PHCM USN RET]

Just some more food for thought about tire pressure. I recently purchased a used Chinook 24 Ft motorhome. The tires were in ok shape (14000mi) but were old so I replaced all six. Then the inflation odessy began.

The tire place said to run the Michelins (LT225/75R16) at 65 F 80 R
While getting some work done the RV dealer set the tire to 60 F 65 R
The door sticker says (which is what the RV dealer used) 60 F 65 R
After an oil change the Chev dealers said to use 75 F 75 R
Using the Michelin chart it says to use (minumum) 45 F 50 R

The corner weights on the Chinook are (weighed fully loaded to go)
Driver side front 1725
Passanger side front 1600
Driver side back 3350 (Dual)
Passanger side back 3525 (Dual)

How much air would you put in the tires?

[DAN L]

Originally Posted by PHCM USN RET

Just some more food for thought about tire pressure. I recently purchased a used Chinook 24 Ft motorhome. The tires were in ok shape (14000mi) but were old so I replaced all six. Then the inflation odessy began.

The tire place said to run the Michelins (LT225/75R16) at 65 F 80 R
While getting some work done the RV dealer set the tire to 60 F 65 R
The door sticker says (which is what the RV dealer used) 60 F 65 R
After an oil change the Chev dealers said to use 75 F 75 R
Using the Michelin chart it says to use (minumum) 45 F 50 R

The corner weights on the Chinook are (weighed fully loaded to go)
Driver side front 1725
Passanger side front 1600
Driver side back 3350 (Dual)
Passanger side back 3525 (Dual)

How much air would you put in the tires?

check this resource:

Michelin North America RV Load & Inflation Tables

ie, minimum of 45 psi front cold, 50 psi rear cold. i add 10% for 'cushion' in my tires. that would be 50 front, 55 rear.

[Tireman9]

I agree that you should consider your "minimum" to be 50/55 but assuming you have Load Range E (information you did not provide) I would suggest you run 60/65 as you will see slight improved fuel economy.
My Class-C RV needs 50/55 based on load but I run 65 all around and find the Chevy 3500 chassis provides good ride at that level. I am getting 11+ mpg.

Also get a TPMS to help avoid tire problems from punctures and slow leaks etc.

[FastEagle]

Originally Posted by PHCM USN RET

Just some more food for thought about tire pressure. I recently purchased a used Chinook 24 Ft motorhome. The tires were in ok shape (14000mi) but were old so I replaced all six. Then the inflation odessy began.

The tire place said to run the Michelins (LT225/75R16) at 65 F 80 R
While getting some work done the RV dealer set the tire to 60 F 65 R
The door sticker says (which is what the RV dealer used) 60 F 65 R
After an oil change the Chev dealers said to use 75 F 75 R
Using the Michelin chart it says to use (minumum) 45 F 50 R

The corner weights on the Chinook are (weighed fully loaded to go)
Driver side front 1725
Passanger side front 1600
Driver side back 3350 (Dual)
Passanger side back 3525 (Dual)

How much air would you put in the tires?

The correct tire pressure is the recommended cold pressure (s) found on the tire placard. Some times vehicle manufacturers will make exceptions and those exceptions can be found in the owner’s manual.

There are other procedures to follow when deviating from the OE tire sizes found on the placard. However, the load capacity provided by the OE tires would then be the factor to use when setting new recommended tire pressure (s) for replacement tires.

When using tire manufacturer inflation tables don’t forget to insure the load capacity reserves set by the vehicle manufacturer are honored.

FastEagle

[Mr_D]

The vehicle placard is only valid if loaded to the max weight, with the original equipment brand, model and size. Any changes to that requires using actual vehicle weights and the tire pressure charts from the tire manufacturer.
You take the heaviest weight on an axle and use it for all tires across that axle. Cold pressure is at ambient temp and driven less than one mile.

[FastEagle]

Originally Posted by Mr_D

The vehicle placard is only valid if loaded to the max weight, with the original equipment brand, model and size. Any changes to that requires using actual vehicle weights and the tire pressure charts from the tire manufacturer.
You take the heaviest weight on an axle and use it for all tires across that axle. Cold pressure is at ambient temp and driven less than one mile.

When you use that method what happened to the load capacity reserves the vehicle manufacturer provided?

FastEagle

[jstdreamin]

Newbie here. I've read the threads and need some help deciphering the load guides and determining tire pressures for my 40' DP. I've read about how to weigh my coach and have done so on a scale that will weigh each axle and provide the total.

My coach has a
GVWR of 32,000lbs
GAWR Front of 12,000lbs (6,000lbs per tire assuming equal weight?)
GAWR Rear (Dual but no tag) 20,000lbs (5,000lbs per tire assuming equal weight?)

I weighed it with full fuel, Propane, wife, me, 1/3 tank fresh water
Front : 10,260lbs
Rear: 18,720lbs.

I realize I'll need to weigh it again with a full load, but for purposes of this explanation,,, and making the ASSumption that the weight is equally distributed left to right, do I simply divide the

front weight by 2 10,260lbs / 2 = 5,130lbs
Rear Weight by 4 18,720lbs / 4 = 4,680lbs

and refer to the chart using the Single weight/PSI value for the front and use the Dual weight/PSI value for the rear?

Goodyear S670 275/80R22.5 H load rating

S 5,500lbs 90psi (use this on front tires since it is the minimum pressure?)
D 5,080lbs 90PSI (use this on each of the four rear tires since it is minimum pressure?)

Additional tire pressures below:

S 5,745lbs 95psI
D 5,305lbs 95PSI

S 5,985lbs 100PSI
D 5,530lbs 100PSI

S 6,225lbs 105PSI
D 5,750lbs 105PSI


Thanks!

[Beak12]

I know how important it is for proper tire inflation. I have had 10 different types of RV's. But with dual rear tires ---just try and find a gas station or ??? that will have the proper fitting on the air hose. I know you can install adapters that will turn the salve stems so that a normal air hose will work---I had it done on my 37' Itasca. But one side developed a slow leak. So it required checking more often. Any Ideas?