Setting inflation pressure Step-By-Step

 While I have posted a number of times on how I would set tire pressure this topic seems to be a recurring question, so here is a slightly different approach to the answer where I use information on a specific example

First off.... You will need to adjust for your RV, your weights and your size tire. This should work for owners of Motorhomes but not for towables or 5th wheel RVs. I will try and cover them in a couple of weeks once I get some real weight information from a trailer/5th wheel owner.

So here is the question I got.

Sorry to bother you again but I just realize I should have asked you about CIP (cold inflation pressure)  and ambient temperatures. I was hoping to set my new pressures late this evening or early tomorrow morning when it coos down, but.

As I stated in my first email, we live in the California High Desert and the lowest temperatures we are going to see today, tonight or in the wee hours of the morning is a cool 75. If I remember correctly in one of your posts, you stated CIP of 65 - 70 degrees. I am sure folks in Arizona and Nevada have the same issue where low temperatures may not get below the 80's or 90's in any 24-hour period for weeks if not months. 

Do we just apply the 2% per 10-degree formula when initially setting our tire pressures? Meaning a 90 psi would then be 91.8 @ 80 and 93.6 @ 90 degrees?

For us tonight and tomorrow morning:

90 @ 7 PM

83 @ 10 PM

75 @ 6 AM

79 @ 7 aM

 

Here are my numbers:

2018 Winnebago Vista 29VE

Class A

Two axles, dual rear wheels

Scale Weights:

Total: 16,540 lbs.

Front:   5,620 lbs.

Rear:  10,940 lbs.

Goodyear G670 RV, 245/70R19.5G

Tire sidewall information:

Max. Load Single 4540 lbs@110 psi cold

Max. Load Dual 4410 lbs@110 psi cold

RV placard information:

GVWR:

18,000 LB

GAWR:

FRT  7,000 LB 82 psi  Single

RR  12,000 LB 82 psi Dual

Goodyear chart:

Goodyear Load/Inflation information for unisteel G670 RV:

Max Speed 75 mph
	80	85	90	95	100	105	110
S	3640	3740	3890	4080 (F)	4190	4335	4540 (G)
D	3415	3515	3655	3970 (F)	4115	4265	4410 (G)

Again, thank you for the wealth of information and your incredible blog.

 

 My reply

 

Yes you can use axle weight numbers until you can get "4 corner weights"  (Weight of each tire position)

I suggest you assume one end of the front axle is supporting 52% of that axle.

Similar for the rear. So based on your numbers that would be . 

2,922# on the front tire

5,689# on one end of the rear axle or 2,845# on a tire.  Yes I always round up when calculating loads.

Using your chart the min inflation for the Front would be 80  and the min on the rear would also be 80.

 

BUT don't forget I also strongly recommend we use at least +10% on inflation to avoid the day to day temperature changes that will change the tire inflation. A +10% on inflation means you could experience a change in temperature of 50°F and not be forced to get out and adjust the tire inflation. Don't forget that some folks are where it might be raining in the AM and don't want to be on their hands and knees messing with tire inflation if they can avoid it.

Add 10% to 80 and you get 88 psi. and I see nothing wrong with rounding up to 90 psi

At 90 a front tire would be rated for 3,890#    3890/2922 = 133 so you would have a good 33% reserve load

at 90 a rear tire would be rated for 3,655#    3655/2845 = 128 so you would have a good 28% reserve load

You should be good to go.

 

I hope this real world example shows you that learning the proper CIP is not complex.

 

 ##RVT1063

 

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 While I have posted a number of times on how I would set tire pressure this topic seems to be a recurring question, so here is a slightly different approach to the answer where I use information on a specific example

First off.... You will need to adjust for your RV, your weights and your size tire. This should work for owners of Motorhomes but not for towables or 5th wheel RVs. I will try and cover them in a couple of weeks once I get some real weight information from a trailer/5th wheel owner.

So here is the question I got.

Sorry to bother you again but I just realize I should have asked you about CIP (cold inflation pressure)  and ambient temperatures. I was hoping to set my new pressures late this evening or early tomorrow morning when it coos down, but.

As I stated in my first email, we live in the California High Desert and the lowest temperatures we are going to see today, tonight or in the wee hours of the morning is a cool 75. If I remember correctly in one of your posts, you stated CIP of 65 - 70 degrees. I am sure folks in Arizona and Nevada have the same issue where low temperatures may not get below the 80's or 90's in any 24-hour period for weeks if not months. 

Do we just apply the 2% per 10-degree formula when initially setting our tire pressures? Meaning a 90 psi would then be 91.8 @ 80 and 93.6 @ 90 degrees?

For us tonight and tomorrow morning:

90 @ 7 PM

83 @ 10 PM

75 @ 6 AM

79 @ 7 aM

 

Here are my numbers:

2018 Winnebago Vista 29VE

Class A

Two axles, dual rear wheels

Scale Weights:

Total: 16,540 lbs.

Front:   5,620 lbs.

Rear:  10,940 lbs.

Goodyear G670 RV, 245/70R19.5G

Tire sidewall information:

Max. Load Single 4540 lbs@110 psi cold

Max. Load Dual 4410 lbs@110 psi cold

RV placard information:

GVWR:

18,000 LB

GAWR:

FRT  7,000 LB 82 psi  Single

RR  12,000 LB 82 psi Dual

Goodyear chart:

Goodyear Load/Inflation information for unisteel G670 RV:

Max Speed 75 mph
	80	85	90	95	100	105	110
S	3640	3740	3890	4080 (F)	4190	4335	4540 (G)
D	3415	3515	3655	3970 (F)	4115	4265	4410 (G)

Again, thank you for the wealth of information and your incredible blog.

 

 My reply

 

Yes you can use axle weight numbers until you can get "4 corner weights"  (Weight of each tire position)

I suggest you assume one end of the front axle is supporting 52% of that axle.

Similar for the rear. So based on your numbers that would be . 

2,922# on the front tire

5,689# on one end of the rear axle or 2,845# on a tire.  Yes I always round up when calculating loads.

Using your chart the min inflation for the Front would be 80  and the min on the rear would also be 80.

 

BUT don't forget I also strongly recommend we use at least +10% on inflation to avoid the day to day temperature changes that will change the tire inflation. A +10% on inflation means you could experience a change in temperature of 50°F and not be forced to get out and adjust the tire inflation. Don't forget that some folks are where it might be raining in the AM and don't want to be on their hands and knees messing with tire inflation if they can avoid it.

Add 10% to 80 and you get 88 psi. and I see nothing wrong with rounding up to 90 psi

At 90 a front tire would be rated for 3,890#    3890/2922 = 133 so you would have a good 33% reserve load

at 90 a rear tire would be rated for 3,655#    3655/2845 = 128 so you would have a good 28% reserve load

You should be good to go.

 

I hope this real world example shows you that learning the proper CIP is not complex.

Do you "Read the Label" on your tires as you do when shopping in the Grocery store?

 A question was posed on what would be a good replacement for a GYE tire in trailer application.

Here is the question and my answer.

John said "It is time to replace 4 tires on my Airstream. The current tires are GYE ST225/75/R15 LR-E. My dealer says GY Endurance in my size is unavailable from his distributors and is highly recommending Hercules as replacement, noting that they are cheaper, 10 ply and customers have been happy with results.
Any experience or feedback on Hercules ST tires?
I can get the GYE from an online dealer, so they are available.
I prefer to stay with ST tires.Thanks."

IMO one feature that has allowed GYE ST type tires to have a better reputation than other or older ST type tires is the presence of a Nylon "cap ply". While there are or may be other features used in that line of tires, it is not possible to know what they are without being a GY tire engineer, which I am not.

But when you read the material list that is molded on the tire sidewall you will see "Nylon" layer on top of the steel belts. When I was able to inspect a GYE that was sectioned for analysis and doing an "autopsy", I confirmed the presence of such a construction feature and it would be illegal to say you had a Nylon ply, if there was no such layer, I have every reason to believe that Goodyear is using that construction feature in the entire GYE ST tire line.

 

If there are other tires you are considering for purchase, along with confirming the candidates are ST type and of the same dimensions and same Load Range I would recommend that you read the material list molded on the tire sidewall.

Many people have learned to "Read the label" when shopping in the grocery, so it is a similar action to "Read the label" of the reinforcing materials in a tire.

A side note. I doubt that the Hercules are really "10 ply". More likely they are Load Range E the same as the GYE. When you read the label on your current tires and any tire you are considering the label will tell you the actual number of layers of the different materials. If in fact the dealer told you "10 ply" I have to wonder about the competence and training of that salesperson as when you read the tire material "label" on the tire sidewall you will probably see something like "Sidewall 1 layer Polyester, Tread 1 layer Polyester + 2 layer Steel + 1 Layer Nylon"

##RVT1062

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You just had a tire failure. Did you take apropriate action to prevent a re-occurance?

If someone has a problem with a tire it would help us all if some effort was made to learn the real root cause. If the root cause is a manufacturing problem then it should be reported to NHTSA and if enough complaints are received there is the possibility of a recall. The threat and costs associated with recalls can be enough incentive for a MFG to change and improve the durability of their tires. It is also possible that external conditions such as  road impacts caused the failure. Another possibility is a combination of overload and under-inflation. Tires like any other product do have a limit in their ability to tolerate loads greater than their design limit. One thing about tires, unlike items made of metal such as a length of chain, is that tires can tolerate overload for a short time but eventually they will fail and the structural damage is cumulative. Metal items like a piece of chain also have a rated load and as long as you never exceed that load the chain retains it's strength.

 

If you have a failure it is sometimes possible to do an inspection and to learn the reason for the failure. However a detailed inspection requires a close inspection by a trained and knowledgeable specialist. If you had a tire fail and it looked like this tire,

 

 

do you think you could identify the evidence of the reason for the failure? The multiple pictures of this tire (well lit and in focus) provided the physical evidence to allow the root cause of the failure to be identified with a high level of probability as the act of checking tire inflation about 50 miles earlier in the day. This is an example of why it is important to get a number of pictures of the failed tire and to have the tire inspected by a knowledgeable Tire Adjustment Engineer. Each of the major tire companies have a team of experienced tire engineers available to inspect tires that have been presented to a tire dealership run by the tire manufacturer, for an "adjustment" and a request for compensation for the failure.

If you have ever had a tire failure did you just dispose of it as soon as possible and just claim it failed because of the zip code where the tire was made, or did you make a real effort to learn the real cause of the failure so you could take appropriate action to improve your chances of avoiding a repeat?

Just so you know I have identified three different and distinct locations of different evidence of the cause and reasons for this tire failure. All of these have been identified in one or more of my blog posts. Here is one, if you are interested.

##RVT1061

Another question and post on adjusting pressure due to temperature changes

 Reviewed another comment on tire pressure changes due to temperature changes:

The original post

Very informative thread. If I may throw in my recent experience...

One thing I do not see mentioned is how outside temperatures can affect tire pressures. Last Nov-Dec, I had a R/T excursion from upstate SC to the Keys.

Outside temperatures ranged from the mid-30's to the 90's and I found myself either adding or reducing air not only prior to every trip, but on a few occasions during mid-travel. The tires on the side of the trailer facing the sun would increase by 2-3 lbs. alone. This was worrisome as I was constantly attempting to monitor and maintain a constant psi in the tires.

The point I'm trying to make and what I've discovered is that having a margin of safety, especially in temperature extremes cannot be understated.

My reply:

I have covered the effects of a change in Ambient temperature in great detail in this blog with some 40 posts on Cold Inflation pressure or similar related topics. I have also covered my reason for recommending that people run a +10% margin on inflation over the minimum needed to support the load so they do not have to worry about adjusting pressure every time the Ambient changes a few degrees.

Bottom line: Once you have learned the MINIMUM inflation to run on your RV based on actual scale measurement of the heaviest loading expected and consulting the load inflation tables, you simply add 10% to that  pressure number. With tire pressure changing at about 2% for each change in Ambient of 10°F, that 10% margin should cover you for a drop of as much as 40 or 50 degrees.

Using the readings from my TPMS in the morning of each travel day, I can monitor my pressure change and know if I need to plan on adding pressure if there has been a significant temperature drop or just keep an eye on the pressure if it has gone up. Most days I make no adjustment at all.

You do not need to worry about a change in pressure of + a few psi as long as you know you are still above the MINIMUM inflation and have not seen a pressure increase of 20% or more.

I don't see myself ever adjusting pressure in the middle of a travel day. I am going to adjust tire pressure, I will only adjust it when I am done traveling for that day. Usually the next AM when the tires have cooled down to the surrounding Ambient temperature.
Remember that the inflation pressure in the tables or on your RV Certification sticker are when the tire is not warmed up from being driver or in direct sunlight for the previous 2 hours.

In my post on how I program my TPMS I cover the margins by setting the Low pressure warning level to protect the tire from ever being lower then the minimum required. My high pressure warning is set to +20 to + 25% as undamaged tires can easily tolerate that higher pressure assuming they were set correctly in the first place.

##RVT1060

Why is my TPMS giving different readings than my hand gauge?

 That question was asked:

TPMS vs. Tire pressure gauge. The readings between my TPMS and my quality tire gauge, are always different. On cold days there is even more of a difference. Which one do I rely on?

Here is my answer:

Yes, the inflation reported from TPMS are not identical to the numbers found on your hand gauge. There are a variety of reasons for this.

1. Hand gauges can and do vary, with many off by +/- 1 % or more

2. TPMS are primarily designed to monitor pressure change, not absolute pressure. I have seen statements of +/- 2% accuracy

3. Temperature does affect the tire pressure reading at about a 2% pressure change for each change of 10°F so it doesn't take too long for a tire exposed to direct sunlight in the morning to see a measurable change in pressure.

Stack all of these variables together and I think you can understand why your numbers do not always match.

So "Which gauge should I trust when setting tire pressure?"

 

My quick and short answer is: Rely on your digital hand gauge when setting tire pressure.

For a little more background I suggest you review the information and numbers in this blog  on the subject. Here are some direct links. Note some of these posts also have additional links with more data, that relate to this basic topic of pressure and gauge vs TPMS accuracy.

What is a good tire gauge?

Gauge accuracy

TPMS Comparison

How accurate is your TPMS?

As far as I know, I am the only person to do a direct comparison of different TPM systems. As you can see in my posts with data, I found no meaningful difference between the two I tested and I have no reason to suspect any other TPMS to deliver much different results.

 

As a tire design engineer (ret) I am comfortable with readings that are within a couple of percent (i.e. 2 psi on an 80 psi goal). Part of the reason for this is that I also am a strong advocate for including a margin on the inflation required to support the measured load on your tires. I suggest that if possible a +10% margin on inflation as long as you do not exceed the rating for your wheels. Un-damaged tires can easily tolerate a +10% on the inflation found in the Load & Inflation tables and if you are using bolt-in metal valve stems you have the bases covered.

The practice of checking the tire pressure on the morning of each travel day is a good one and is advocated by many people that are knowledgeable on tires in RV usage. My personal practice is to get up, turn on my TPMS and go make the coffee. By the time I can pour my cup, I can go back and watch my TPMS monitor report the inflation in each tire. I have learned that my system takes about 10 to 15 minutes to get all 6 sensors to report the current pressure. Note this timing is normal. I know this from TESTING my system The low-pressure report only takes a few seconds to register with the monitor. This 10 to 15 minute timing for regular checks, is designed into TPMS to preserve battery power. I advocate that all owners of RV TPMS should test their system annually.

While you did not specifically ask about temperature effects, HERE are a couple of posts from my blog on that topic.

Ya, my wife says if you ask me what time it is I will tell you how a watch is built. That's just the engineer in me.  If you have other questions I believe that you can find the answer in one of my blog posts.

Good Travels.

##RVT1059