Tire pressures...again!

[Tireman9]

On Friday Sept 18 2020 I wrote:

Don't get your shorts in a bunch about tire inflation. BUT you still should consult the inflation tables.


I talked about this a number of years ago but it seems it's time to cover this again for those new to RV living.
I have covered what I felt is the "Best" inflation for tire life in my posts where we discuss "4 corner weights" which means learning the actual load on each tire position by getting the RV on a set of individual tire scales. While large RV Conventions such as FMCA or Escapees events sometimes have those scales, many times they are not convenient. But you can find Truck Scales at many Interstate exits, where you can learn the actual load on each axle. Since we know that almost no RV has perfect 50/50 side to side load balance, learning the actual load on each end of each axle is a good idea. Some RVs have been found to be 1,000# out of balance.
If you can only find truck axle scales then I suggest the following rules of thumb until you learn your "4 corner weights".
- Class-A Motorhomes and large (28'plus) 5th wheel Trailers with slides and especially if they have a residential refrigerator, should assume they have 53% of the axle load on one end so should use that heavier number when consulting the tire Load & Inflation charts.
Class-C Motorhomes and trailers shorter than 28' with slides should assume a 52/48% side to side split, While Class-C without slide, Class-B and small single axle trailers can assume a 51/49% side to side load split.
Using the heavier end figure consult the published tire Load & Inflation tables to learn the minimum inflation pressure for the tires on that axle of your RV. This "minimum" inflation is the number you would consider for the morning of every travel day.To avoid chasing inflation changes due to changes in the weather, I suggest you add 10% to the number from the tables so you can simply monitor the inflation using your TPMS and as long as you never drop below the Minimum inflation needed to support your load.

I am also in favor of this plus 10% inflation margin above the "minimum". So you don't find yourself chasing your tail every day by adding 1 or 2 psi when it gets cooler when you find yourself 1 or 2 psi low, or bleeding off 1 or 2 psi when the weather turns warmer. You can simply monitor the morning inflation number and as long as it stays near the +10% and does not drop to +0% or go above +20% you are good to go for that travel day.

With +10% margin it would be easier to discover you are low a few psi and simply wait till the next fuel stop, where there should be high pressure air available if you need to add air.

For those that don't know how to inflate a warm tire here are the steps:

1. Measure the pressure when the tire is at ambient temperature (not warm from driving or being in sunlight). Many consider this their "Morning Tire Pressure".
2. Note the number of psi you want to add to each tire to get to your goal inflation.
3. When you get to a fuel stop measure the warm pressure.
4. Add the number of psi from #2 to the warm pressure in #3 and add air till you get to at least this new warm pressure goal.

This "rule of thumb" will work for pressure changes of 5 psi or less. If you find you need to add 5 psi or more there may be something wrong, i.e., a leak unless you have seen a long term decrease in pressure as the weather cools down.
Don't get hyper about being 1 or 2 psi off. Remember, if you have a 10% cushion, you are good to go as long as you are within a few psi of your goal.
##RVT966

[jadatis]

Originally Posted by drwaynesmith

Yes, that is correct. Add air as you go north, let it out when you return south. Keep the pressure the same as the temperature changes to keep the shape of your tires optimal. Cold air takes up less space, so you have to add more of it to keep your tires correctly inflated.

Read back my ( only ) post #52 , why I dont agree with your answer.
Highening up when its cold outside, you can do for roadhandling , but bleading down to determined advice when it is hot, can be dangerous, and give overheated tires. So example, if 100 psi determined advice, when 100degr F outside , a pressure of 105 psi cold measured is yust needed to prefent overheated rubber when driving, and dont bring it back to 100 psi.
Mt offer still stand to make a Pressure temperature list for your determined best pressure, as I wrote in my post #52.

[RM Art]

Originally Posted by Tireman9

On Friday Sept 18 2020 I wrote:

Don't get your shorts in a bunch about tire inflation. BUT you still should consult the inflation tables.


I talked about this a number of years ago but it seems it's time to cover this again for those new to RV living.
I have covered what I felt is the "Best" inflation for tire life in my posts where we discuss "4 corner weights" which means learning the actual load on each tire position by getting the RV on a set of individual tire scales. While large RV Conventions such as FMCA or Escapees events sometimes have those scales, many times they are not convenient. But you can find Truck Scales at many Interstate exits, where you can learn the actual load on each axle. Since we know that almost no RV has perfect 50/50 side to side load balance, learning the actual load on each end of each axle is a good idea. Some RVs have been found to be 1,000# out of balance.
If you can only find truck axle scales then I suggest the following rules of thumb until you learn your "4 corner weights".
- Class-A Motorhomes and large (28'plus) 5th wheel Trailers with slides and especially if they have a residential refrigerator, should assume they have 53% of the axle load on one end so should use that heavier number when consulting the tire Load & Inflation charts.
Class-C Motorhomes and trailers shorter than 28' with slides should assume a 52/48% side to side split, While Class-C without slide, Class-B and small single axle trailers can assume a 51/49% side to side load split.
Using the heavier end figure consult the published tire Load & Inflation tables to learn the minimum inflation pressure for the tires on that axle of your RV. This "minimum" inflation is the number you would consider for the morning of every travel day.To avoid chasing inflation changes due to changes in the weather, I suggest you add 10% to the number from the tables so you can simply monitor the inflation using your TPMS and as long as you never drop below the Minimum inflation needed to support your load.

I am also in favor of this plus 10% inflation margin above the "minimum". So you don't find yourself chasing your tail every day by adding 1 or 2 psi when it gets cooler when you find yourself 1 or 2 psi low, or bleeding off 1 or 2 psi when the weather turns warmer. You can simply monitor the morning inflation number and as long as it stays near the +10% and does not drop to +0% or go above +20% you are good to go for that travel day.

With +10% margin it would be easier to discover you are low a few psi and simply wait till the next fuel stop, where there should be high pressure air available if you need to add air.

For those that don't know how to inflate a warm tire here are the steps:

1. Measure the pressure when the tire is at ambient temperature (not warm from driving or being in sunlight). Many consider this their "Morning Tire Pressure".
2. Note the number of psi you want to add to each tire to get to your goal inflation.
3. When you get to a fuel stop measure the warm pressure.
4. Add the number of psi from #2 to the warm pressure in #3 and add air till you get to at least this new warm pressure goal.

This "rule of thumb" will work for pressure changes of 5 psi or less. If you find you need to add 5 psi or more there may be something wrong, i.e., a leak unless you have seen a long term decrease in pressure as the weather cools down.
Don't get hyper about being 1 or 2 psi off. Remember, if you have a 10% cushion, you are good to go as long as you are within a few psi of your goal.
##RVT966

Dear Mr. Tireman, "[Retired Design & Quality Tire Eng. 40+ years experience. You can listen to self appointed "Know it all" or someone that is recognized by the court as an actual "Expert"]".

I agree with much of the detailed, somewhat complicated to keep up on, tire pressure recommendations in your quote above.

As a long time masonry and concrete material installation contractor: I've been hauling heavy loads for well over four [4] decades in many size and type trucks and on work trailers. As well I have towed travel trailers, sizable boat trailers and now have a relatively small Class A RV. Also, end dump and belly dump transfer trailer trucks often delivered some 25 tons material at a time to my construction projects. During my 48 years of contracting I got to know drivers of BIG trucks as well as BIG truck owners. I've also had discussions with truck tire shop installers and owners.

Reason I give my background in last paragraph is so it can be known that by 2020 I personally have had much "tire pressure" experience and have discussed tire pressure requirements with many others who have had much tire pressure experience.

So... to you the "Expert", I ask: By experience during my 4 plus decades of daily interacting with the need for "safe" tire pressures - such as this "general" rule of thumb I have learned and follow - isn't it true that keeping tires inflated [during early morning ambient temperature with tire having rested for 6 hours or longer] at the printed-on-tire maximum pressure is a safe way to go regarding tire pressure for vehicles with loads... including weighty RVs?

Looking forward to your expert input/answer. Thanks!

[wolfe10]

Originally Posted by RM Art

... isn't it true that keeping tires inflated [during early morning ambient temperature with tire having rested for 6 hours or longer] at the printed-on-tire maximum pressure is a safe way to go regarding tire pressure for vehicles with loads... including weighty RVs?

Not Tireman9, but have worked with him for years and have conducted the Suspension Seminar at FMCA Conventions.


I will say, in other countries, long-range buses are often equipped with systems that do allow the driver to raise tire pressure from the driver's seat.



Totally agree with you statement IF (yes, big IF) you are near the max carrying capacity of the tires.


But, hopefully, the chassis/RV manufacturer did not make the tires the "weak link" meaning that you needed to carry that PSI.


In fact, I much prefer to see a tire with at least 15% "reserve capacity". In that case one should consult the tire manufacturer's Inflation Table for their recommendation (after all, they are the engineers who designed the tires. If they though one PSI fits all, they would not have wasted their time developing the Inflation Tables). Over-inflating (based on the tire manufacturer's recommendation, not mine, not anyone elses) can lead to harsh ride and "darty" handling.



This IS different than trucks where the tire pressure should reflect what is needed to carry their GVWR. Agree, it would be crazy to lower truck tire pressure when unloaded and than have to add air to all 18 tires when they pick up a load.


BTW, some long-range buses DO change tire pressure for different loads-- right from the driver's seat: https://www.alamy.com/stock-photo-bu...-57599248.html

[richard5933]

One doesn't need to have 20+ years of experience for this. And, having 20+ years of experience doesn't necessarily make one an expert.

It's just not that complicated and doesn't take an expert to know that putting the sidewall max into tires is going to result in a harsh ride and will negatively affect handling and braking.

My coach is wearing 12R22.5 tires with a max pressure of 120 psi. The load & inflation charts show appropriate pressure for my coach to be around 90 psi. I added a small buffer and am way under the max. I've tried higher pressures, and the result is a very harsh/noisy ride and steering that was prone to wondering. The closer I get to the sidewall max of 120 psi the worse things got.

If the 'expert' way to do this was to simply inflate to the sidewall max, then why in the world do the manufacturer's even both to publish load & inflation tables?

[RVPioneer]

Originally Posted by RM Art

Dear Mr. Tireman, "[Retired Design & Quality Tire Eng. 40+ years experience. You can listen to self appointed "Know it all" or someone that is recognized by the court as an actual "Expert"]".

I agree with much of the detailed, somewhat complicated to keep up on, tire pressure recommendations in your quote above.

As a long time masonry and concrete material installation contractor: I've been hauling heavy loads for well over four [4] decades in many size and type trucks and on work trailers. As well I have towed travel trailers, sizable boat trailers and now have a relatively small Class A RV. Also, end dump and belly dump transfer trailer trucks often delivered some 25 tons material at a time to my construction projects. During my 48 years of contracting I got to know drivers of BIG trucks as well as BIG truck owners. I've also had discussions with truck tire shop installers and owners.

Reason I give my background in last paragraph is so it can be known that by 2020 I personally have had much "tire pressure" experience and have discussed tire pressure requirements with many others who have had much tire pressure experience.

So... to you the "Expert", I ask: By experience during my 4 plus decades of daily interacting with the need for "safe" tire pressures - such as this "general" rule of thumb I have learned and follow - isn't it true that keeping tires inflated [during early morning ambient temperature with tire having rested for 6 hours or longer] at the printed-on-tire maximum pressure is a safe way to go regarding tire pressure for vehicles with loads... including weighty RVs?

Looking forward to your expert input/answer. Thanks!

As a non-self appointed know it all, I do not believe it is necessary to achieve “court sanctioned” expert status, in order to determine correct tire pressure for a vehicle. Honestly, a “C” average 5th grader should be able to figure it out.

There is a possibility of, what at least one expert would consider, over inflation by using your “Maximum side wall pressure” methodology.

Our “weighty RV” has a recommended rear tire pressure of 90psi. Many tag-axle coaches will have recommended pressures well below maximum side wall pressure. Our tires have a maximum side wall pressure of 120psi. Were I to inflate them to 120psi, I’d be running on tires that are 33% above the recommended pressure.

“Expert” advice for inflating RV/Motorhome tires is to keep them between the recommended pressure from the charts, with known weights, and up to 20% above that recommendation. Preferably keeping them closer to 10% above the recommended pressure.

To summarize, inflating tires to maximum side wall pressure would not necessarily be “safe” in all circumstances.

[jadatis]

And this non professional " pigheaded selfdeclared tirepressure specialist" can add that sometimes even max cold pressure given on tire, or belonging to loadrange, can even give tirefailure.

This then for the older TT, wich have ST tires with yust enaugh maxload to cover the GAWR.
But also the newer TT with 10% reserve maximum load, can still give tire-failure.

ST tires are calculated in maxload for 65mph, even the Endurance with speedcode N ( max 140kmph/87mph) .
Then yust a bit weightdifference R/L , and or little overloading, and or higher speed then 65mph, and or inacurate to high read pressure, and or etcetera, and at least one tire overheats, wich gives beginning cracks, that end up in time to tire-failure.

For a LT tire 10% reserve is enaugh, because calculated for 99mph/160mph.
ST tires should have 30% reserve to give them the reserves, I determined, so you can use referencepressure, and maximum reserve without bumping, for GAWR maximum used.

Mind that you, determine pressure with all my determined reserve, but by the inacuracy's you end up with only yust enaugh to prefent overheating.

[Spdracr39]

Thread Warning

There is a lot of back and forth here that is getting out of hand. Post your opinion respectfully and move on. If people don't agree with your viewpoint let it go. Thanks to those that are posting within the rules.

[RM Art]

I've mentioned my feelings/experiences in this thread regarding tire pressures and respectfully appreciate [as well as closely listen to] all other's feelings/experiences regarding how to establish and what may be the correct tire pressures for circumstances at hand.

I plan to no longer post here but will continue reading as others' posts occur.

Cheers! and Best Luck!!

Art

[Gary.Jones]

On forums like this, you can get great advice, and you can really bad advice.

When you get the latter, it behooves those who know what they are talking about to try to correct the bad advice..

I appreciate Tireman for occasionally showing up here.

Gary

[Tim Fox]

I second Gary’s sentiment and also value Tireman9’s input.

[Tireman9]

Originally Posted by RM Art

Dear Mr. Tireman, "[Retired Design & Quality Tire Eng. 40+ years experience. You can listen to self appointed "Know it all" or someone that is recognized by the court as an actual "Expert"]".

I agree with much of the detailed, somewhat complicated to keep up on, tire pressure recommendations in your quote above.

As a long time masonry and concrete material installation contractor: I've been hauling heavy loads for well over four [4] decades in many size and type trucks and on work trailers. As well I have towed travel trailers, sizable boat trailers and now have a relatively small Class A RV. Also, end dump and belly dump transfer trailer trucks often delivered some 25 tons material at a time to my construction projects. During my 48 years of contracting I got to know drivers of BIG trucks as well as BIG truck owners. I've also had discussions with truck tire shop installers and owners.

Reason I give my background in last paragraph is so it can be known that by 2020 I personally have had much "tire pressure" experience and have discussed tire pressure requirements with many others who have had much tire pressure experience.

So... to you the "Expert", I ask: By experience during my 4 plus decades of daily interacting with the need for "safe" tire pressures - such as this "general" rule of thumb I have learned and follow - isn't it true that keeping tires inflated [during early morning ambient temperature with tire having rested for 6 hours or longer] at the printed-on-tire maximum pressure is a safe way to go regarding tire pressure for vehicles with loads... including weighty RVs?

Looking forward to your expert input/answer. Thanks!

Yes for Heavy trucks that are normally operated at full load use the pressure molded on the tire sidewall associated with the tire max load.



I find that a good number of RV owners complain about ride and want to run lower tire pressure. My step by step instruction is aimed at finding the compromise between durability (higher inflation) and comfort (lower inflation). One big difference between your experience and an RV is that you load and unload your trucks and trailers while RVs tend to get loaded and almost never run empty.

[richard5933]

The flaw in the theory that tires do best at the sidewall max pressure is that it assumes most will operate their vehicle at or near the max load capacity.

The semi I drove recently for a major carrier had tires which collectively had a load capacity far in excess of the legal limit of 80,000 lbs. The tires had a sidewall max much higher than the 105 psi we typically ran them at, and we still had load capacity to spare when the full 80,000 was on the truck and the tires were set to 105 psi. There was no need to put higher pressures in the tires, and nothing would have been gained.

The same is true for my coach (and many other RVs on the road). For example, the total capacity of my 6 tires at full sidewall inflation of 120 psi is nearly 42,000 lbs. My coach only weighs 30,000 lbs fully loaded and ready to hit the road. What would be gained by inflating them to 120 psi rather than the 85/95 psi indicated on the load & inflation tables? (And that includes a buffer to accommodate daily temperature swings and slight load variation.)

In my experiment running higher pressure, what I found was a harsh ride and poorer handling - the steering wandered quite a bit and the coach was more difficult to control. The ride was uncomfortable and having the tires inflated to the higher pressure negated the benefit of having a coach with air ride suspension that normally floated down the road quite comfortably. Then I went back to the pressures in the load & inflation table and was able to enjoy the coach again.

[KanzKran]

Originally Posted by richard5933

The same is true for my coach (and many other RVs on the road). For example, the total capacity of my 6 tires at full sidewall inflation of 120 psi is nearly 42,000 lbs. My coach only weighs 30,000 lbs fully loaded and ready to hit the road. What would be gained by inflating them to 120 psi rather than the 85/95 psi indicated on the load & inflation tables? (And that includes a buffer to accommodate daily temperature swings and slight load variation.)

In my experiment running higher pressure, what I found was a harsh ride and poorer handling - the steering wandered quite a bit and the coach was more difficult to control. The ride was uncomfortable and having the tires inflated to the higher pressure negated the benefit of having a coach with air ride suspension that normally floated down the road quite comfortably. Then I went back to the pressures in the load & inflation table and was able to enjoy the coach again.

That’s what the thread I started about the Vanco tires on my 3500 Sprinter was all about - I had been using door post pressures, which are high but still not as high as the 80 psi they’re rated for, and it was like driving on train car wheels.

At full sidewall pressure, they’re good for a total of 15,240 lb. It has a GVWR of only 11,030 lb, and I’m actually ton lighter than that, per a CAT scale check with full tanks and the usual ‘stuff’ we bring. I’ve dropped the pressure to that which corresponds to 110% of each axle load, per the inflation tables. No more twitchy, busy wandering, no more bowling balls for tires sensation at every little pavement joint, much better ride overall.

The door post sticker does actually use the word “laden”, so I take that as license to adjust for partially laden. I just got back from an 800 mile round trip for work, and got almost exactly 18 mpg, which has been typical, so I’m happy.