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Friday, November 26, 2021

Adjusting tire pressure because it's cold outside

 I ran across a series of posts on an RV forum on the need to adjust tire pressure because of changes in ambient temperature. 

First lets be sure we all are using the same definition for "Ambient temperature" For our purposes we should consider it "the outdoor air temperature in the shade".

An RV owner noticed that his tire pressure had dropped based on his TPMS readings, almost 7 psi when the temperature dropped to the 40's. He did not provide his "normal" pressure or temperature. But he did claim " But within 10 driving miles they are back to normal pressure." He asked if he needed to add air, when it is cold, to bring pressure back to normal. There followed over 100 posts which is not uncommon when tire pressure is the topic. As expected, some offered correct information on the need to adjust pressure to accommodate the normal drop when it gets cold or increase in pressure when the ambient temperature rises. This immediately lead to various suggestions on the amount of adjustment needed.

I was heartened to read the comments from many, that knew it was important to adjust tire pressure to compensate for changes in ambient temperature. There were a couple of people that thought there was some special temperature for adjusting pressure. Replies to these people were quick and correctly pointed out that there was no special temperature such as 68F or 72F when tire inflation could be set but that a tire pressure can be set when the tire was not "warmed" by being in sunlight or having been driven on more than a mile.

Adjusting pressure when traveling

A few offered information on how to calculate the amount of air pressure that needed to be added but this concept quickly into discussions of Pressure vs Temperature formulas.

Bottom Line:

Tire inflation pressure will change about 2% for each change in Ambient Temperature of 10°F (6°C).

I advise that you use your TPMS to check inflation at the start of each travel day. This might be in the Morning but whenever you check you need to ensure that no tire has been warmed from being in direct sunlight for the previous couple of hours. Then set your tire inflation when the tires are at Ambient Temperature. It's just that easy.

There are charts on the Internet that cover how to adjust tire inflation under extreme conditions such as the RV being a garage heated to 65°F but the outdoor Ambient is way below freezing. I posted a chart last March but I bet that there are not many RV owners that need this information.


Friday, November 19, 2021

Another question on "4 corner weights"

 On Tue, Nov 9, 2021 Andy  wrote:

Good afternoon Roger,

 I wanted to double check myself regarding tire position weights.  In reading your blogs you said for double-axle trailers the tire pressure should be the max cold pressure amount as stamped on the tire because of trailer sway and turning forces on those tires is different than on a motorhome.  Is it important to get individual tire position weights for trailers?

 As I just got a new 5th wheel trailer and I am getting ready to install a Tire Tracker tire pressure monitoring system I have a few questions:

1.      Do I need to get 4 corner weights (or in this case 8 tire position weights)? 

2.      Is load on each tire position important for trailers vs. motorhomes, or is axle weight sufficient?  (I can get overall and axle weights easy enough.  And, I just got an appointment for tire position weights with a SmartWeigh club in FL, if you think it is important.)    

3.      When I go to get the weights, should ALL holding tanks be full or only Fresh Water? (The SmartWeigh group literature states that they only want the fresh water tank full.)

 The 4 tires on my 5th wheel trailer are: (And the spare, too.)

Carslie    CSL16   ST235/85R16

Load index 132/127

Speed Rating: M (81 MPH)

Max Load Single 4400 lbs. at 110 psi. cold

Max Load Dual 3860 lbs. at 110 psi. cold



The 6 tires on my Ford F450 DRW are:



Max. Load Single 3970 lbs. 110 psi. cold

Max Load dual 3750 lbs. 110 psi. cold

Then they have LRG in an oval followed by 128/126N in another oval and then DOT B6 YB NFL X 0621 in various ovals


I don’t have weights on the trailer yet but the GAWR is 7,000 lbs.   GVWR is 17,000 lbs.

 Thank you very much,  Andy

I wrote:

The "4 corner weight" is usually talking about Class-A motorhomes and the data shows that some number of those units can be significantly (1,000# or more) unbalanced side to side on an axle,

Smaller RVs can probably get away with just learning the weight on each axle with the RV and TV loaded to its heaviest.
Then calculating 51% or 52% for each axle and then using that weight number to confirm you are not exceeding the load capacity of the tires.
For towables, including 5vers, your calculated heavy weight should be no more than 90% of the tire capacity when inflated per the Load & Inflation tables. I covered the reason for this 90% limit in  my blog post on Interply Shear and the RVIA (see that Gold sticker near the door on the trailer) has a requirement that tire capacity should be = 110% of GAWR.
The max load capacity for a tire is load in pounds shown on the tire.

Your Dry weight is not important when we are talking about tire loading. The GAWR is just a number for all the tires on any one axle. The tire on the Passenger side has no idea what the load on the driver side is so you can't average the axle weight to learn the max weight on the heavier loaded tire.

"Dual" loading is when 2 tires are mounted side by side on one end of an axle as we see on the rear of most Class-C motorhomes and you have on "Dually" Pick-up trucks like your F450. Dual does not apply to RV Trailers. LT tires do have different load capacities if mounted on the front (single) or mounted on the rear (dual).
Yes, you do not need both freshwater and holding tanks full. Think of your loading when you start a trip. Fresh water is normally full, so is propane, gas or diesel and food pantry but holding tanks are empty, don't forget your tool box. That would be what I would call "Heaviest expected weight".

This is when you get on a truck scale and get readings with just one axle on a scale pad. You may need a couple of readings depending on pad spacing and your axle locations. Get both trailer and truck weights as you should also confirm you are not overloading your TV tires either.

Example:  Suppose you have tires that say 3,900# @ 80 psi. Your GAWR on the certification label is 7,000#   RVIA requires the tires have a capacity of 110% of the  7,000 or 7,700 total or 3,850# each so you might think you are OK but if your RV axle is "out of balance" side to side by 1% or 3,885# on the heavy end you would have a tire in overload. Not a great deal but in overload assuming the scale is accurate +/- 1% and you have a 100% accurate pressure gauge.  What if your axle is unbalanced by 200# or 300#?  TV are normally more balanced so just take the axle load and divide by the number of tires on that axle.

BUT the RVIA 110% requirement went into effect in Nov 2018 so there are many RVs out there that only require that tires be capable of supporting 100% of GAWR so that 10% "cushion" is gone.

I strongly support the RVIA's 110% load capacity. In fact my Interply Shear data suggests that something closer to 125% would be desirable but the RV companies simply have not designed their vehicles with large enough wheel wells to allow that large of a tire even if the cost penalty was less than $200 an RV.

Back to your original question: You do not need to learn the individual loads on each tire position. BUT I do suggest you assume at least a 1% out of balance and apply that to the weight reading you can get at truck scales (just need to pay attention to where each axle is on which scale pad)


Friday, November 12, 2021

Are IR guns good for detecting low inflation in tires?

 Comment of Facebook about RV tires

"I carry a heat gun with me. When I stop I check all tires. If one is a lot hotter than the rest I have a problem."

and another

"A cheap harbor freight gun is a good investment. We carry two when I tow my boat to the keys. Each stop my buddy and I have a gun in each door. We each shoot all four wheels then compare noted about what we got as far as readings."

 My response:

As a tire engineer I can advise that IR guns are OK for checking the temperature of metal items such as wheels, hubs or brake drums that conduct heat but rubber is an insulator and very poor conductor of heat so no IR hand gun I know of can detect a meaningful temperature at a rest stop as it isn't the average temperature of a tire that can result in a failure but a spot, maybe smaller than a quarter that you have to worry about. What testing have you done to learn that exact spot over the entire surface of your tires?

The hottest location (most likely to fail or cause problems is also about 1/4" deep inside the structure). Heat is the result of running overloaded or under-inflated. Scale measurement is the only reliable way to know if you are overloaded. Pressure readings are the only reliable way for a consumer to know if they are running under-inflated. You can pick up a nail or cut or have a leaking valve and be underinflated within a few minutes and have a failure a few miles later. The tire I covered in my detailed post where I did a visual "Autopsy"reportedly had it's air pressure checked just 50 miles before the catastrophic "Blowout" run low flex failure. A leaking valve core from improper seating is one possible reason for the air loss the led to the failure. A TPMS is the only reliable way to know the actual running pressure unless you stop every 5 to 10 minutes and check with a gauge and confirm your valve core has properly sealed after you check the pressure. A TPMS can even sometimes warn of brake or bearing problems because the heat is transferred through the metal parts to the metal base of the TPM sensor. 

I do think these low cost temperature sensors have other uses such as checking the temperature of your furnace output or confirming the electrical connections in your 'fuse box" are not getting  overheated. Even the heating element in you absorption refrigerator can be checked. I did use my IR gun to check the surface temperature of my tires when I did my test on tire covers but running tires have a wide range of temperatures as seen here with the output from a $10,000 IR camera. Note the wide range of operating temperatures (80F to 130F) in close proximity ( 0.10") when the passenger tire was run with 27 psi.

It only takes a few seconds for the surface temperature to drop significantly which would be less time than it takes you to come to a stop in a rest area so your readings would not reflect the important temperatures near the tire hot spot.


Friday, November 5, 2021

Why so much confusion on tire inflation vs Max Inflation vs my recommendation of +10%

A question posted on an RV forum

Tom said "So, I see some who are saying to set pressure to max cold pressure recommended, and others talk about “minimum +10%”…I’m confused."

Starting at the end.

Assuming you know the actual load on each tire from your measurement on a scale (ya I know about assuming but every RV owner has been told at least once to learn their actual loading).
You take the load on the heavy end of an axle as there are almost zero percent RVs with the load exactly at 50/50% side to side.

The load number is then found in the Load/Inflation charts for your size tire and you go up (to the right) till you find a block with at least or more load than what you measured on the scale. NEVER go lower than your scale reading. DO NOT average the reading from each end of the axle weight measurement. DO NOT try and calculate a pressure between the 5 psi increments.    Then look up in the chart to find the PSI.  That is the MINIMUM inflation you should ever run in the tires on that axle.

I suggest you add 10% to that inflation number to offer some "protection" in case the temperature drops. If you have added my recommended 10%, you will probably see that you do not have to add air every day the temperature drops 10 degrees.

RVs have Certification labels AKA Tire Placards that have tire size, type, Load range and inflation numbers. They also have GAWR which is the MAXIMUM load you should ever have on that axle.  The RV company is required by DOT, to post on the sticker, an inflation number that is sufficient to support 100% of the GAWR.  RVIA (a standards organization's sticker on the side of your RV now requires inflation level good enough to support 110% which is better than the DOT requirement)   Because of these load capabilities most RV companies select the smallest (lowest cost for them) tire that can just barely meed these requirements. The result of this purchasing decision is that you will need to inflate your tires to the level needed to support the tire's MAXIMUM load capacity which is the number on the sidewall of the tire.

Side issue. The wording on the tire sidewall is confusing. The reality of what it means is that any given tire has a MAXIMUM load capacity and an inflation (minimum) required to support that load. What is not printed on the tire sidewall is the fact that there is no increase in inflation that will result in that tire ever being capable of supporting more load. Therefore  the "max inflation" wording that was decided upon by some committee 50 years ago.