Maximum tire pressure

[Tireman9]

Originally Posted by S.LANGE

Tireman9, in your RV TIRE SAFETY article dated Aug 1, 2017 "How I Program My TPMS" , a good refresher for all, I was wondering if you could offer safety or crucial advice (words of wisdom) in a situation where the CIP-Cold Inflation Pressure is the same as the max pressure listed on the tire's sidewall. I love the idea of programming in a "safety cushion" so the TPMS starts warning you there's a potential pressure problem before tire pressure is no longer at it's weight carrying capacity and it, and it's dually mate in my case, are being stressed and possibly damaged if either fall below.



Tires in question are Firestone Trans Force LT225/75R16 w/80 psi max limit. With MH empty and fuel tank @ 3/4 full, the left rear is weighing in at 4480 lbs or 2240 lbs per tire in the dual pair with Firestone's table max'ing out at 2470 lbs @ 80psi per tire. All related to left side: If we utilize pantry, add waste to black water tank, include it's % of any fresh water in tank, it's share of last 25% to fill fuel tank, etc- tires are near if not overloaded.

Glad you found the post informative. That is my objective.
If your CIP target is the same as the top pressure for the tire. 80 psi in your case, I would set the low pressure warning to 75. Ideally you never want to be below the pressure needed to support but I would not set the warning to 80 as a simple drop in Ambient temperature would set off the warning.


I also suggest you at least get a scale weight with the RV full as you normally travel. You may be overloading a tire even with 80 psi and some action should be taken if that is the case.

[jadatis]

Originally Posted by budh9534

My question regards the desired tire pressure for the four rear tires of a Class C motor home, because as you can see below, the Ford and Fleetwood "stickers" differ (80 psi vs. 60 psi at GAWR)

I recently purchased in a private sale a 2002 Fleetwood Jamboree 29V with a Ford chassis (E450). All six tires plus spare almost a year old with very little use (Mastercraft LT 225/75R16 Stratus HT). Front tires presently at 75 psi and all four rears at 78 psi.

Base weight is 10,140 lbs.

=======================
The Ford sticker on the inside of the driver's door says

GVWR: 14050 lb
Tire: LT225/75R16E
Front: GAWR 4600 lb, 65 psi cold
Rear: GAWR 9450 lb, 80 psi cold dual

A nearby Fleetwood sticker says

GVWR: 14050 lb
Tire: LT225/75R16E
Front: GAWR 4600 lb, 65 psi cold single
Rear: GAWR 9450 lbs, 60 psi
=======================

Why the 20 psi difference shown for the rear tires?

(This is going to carry five adults from Michigan to Florida in a few weeks, and I expect the actual weight carried with a full 55 gallons of fuel and 1/4 tank of fresh water will be near 12,600 lbs, or about 10% less than the GVWR. 10% less of the rear would be about 8500 lbs, divided by four tires is about 2125 lb per tire. Using a generic tire chart for LT225/75R16, since I could not find a specific Mastercraft chart, it showed 65 psi for 2150 lbs. This makes me think 60 psi on the Fleetwood sticker for the rear tire at GAWR is far too low.)

The estimating of totalweight and division over the axles, is dangerous, weighing per axle, better axle-end will mostly prove you wrong.

Further you gave all the needed info , so I can calculate.
Used the GAWR's , and even with no reserve it came to Fr 68 psi and Rr 77 psi .
With the reserves I determined to be needed F72, R 88psi, wich is not allowed rear.

So the rear 60 psi advice must be an error, or yust a misreading, so look again.

Used the michelin list on their site
https://www.michelinrvtires.com/refe...tion-tables/#/
Saw the XPS rib giving 80 psi max pressure and Agillis Crosclimate max 90 psi, while lists where the same. So also in America the referencepressure ( behind AT )is not always the maximum allowed cold pressure .

[Tireman9]

Originally Posted by budh9534

My question regards the desired tire pressure for the four rear tires of a Class C motor home, because as you can see below, the Ford and Fleetwood "stickers" differ (80 psi vs. 60 psi at GAWR)

I recently purchased in a private sale a 2002 Fleetwood Jamboree 29V with a Ford chassis (E450). All six tires plus spare almost a year old with very little use (Mastercraft LT 225/75R16 Stratus HT). Front tires presently at 75 psi and all four rears at 78 psi.

Base weight is 10,140 lbs.

=======================
The Ford sticker on the inside of the driver's door says

GVWR: 14050 lb
Tire: LT225/75R16E
Front: GAWR 4600 lb, 65 psi cold
Rear: GAWR 9450 lb, 80 psi cold dual

A nearby Fleetwood sticker says

GVWR: 14050 lb
Tire: LT225/75R16E
Front: GAWR 4600 lb, 65 psi cold single
Rear: GAWR 9450 lbs, 60 psi
=======================

Why the 20 psi difference shown for the rear tires?

(This is going to carry five adults from Michigan to Florida in a few weeks, and I expect the actual weight carried with a full 55 gallons of fuel and 1/4 tank of fresh water will be near 12,600 lbs, or about 10% less than the GVWR. 10% less of the rear would be about 8500 lbs, divided by four tires is about 2125 lb per tire. Using a generic tire chart for LT225/75R16, since I could not find a specific Mastercraft chart, it showed 65 psi for 2150 lbs. This makes me think 60 psi on the Fleetwood sticker for the rear tire at GAWR is far too low.)

LT225/75R16 LR-E are rated in dual application to support 2140 ea @ 65psi psi x4 = 8560# When Ford sold the chassis to Fleetwood the frame did not need that much support.
But after Fleetwood added their stuff and had to meet DOT requirements of being able to support the GAWR.

Also the vehicle MFG, in this case Fleetwood is the one responsible for selecting the tires and specifying the inflation.


At 80 psi the tires are rated for 2,470# ea or 9,880 for 4



Now in 2002 the RVIA requirement was only that the tires be able to support the GVWR so the 10% margin was not provided for. The {10% load capacity was added in 2017 to improve vehicle safety



If I were you I would get on a scale with the RV fully loaded (people, food, clothes, toys and tools) and confirm your actual axle weights and be sure the inflation is sufficient to give an extra margin of 10% tire capacity over your measured tire loading. I do suggest that if possible you get to a +15% tire capacity over the actual loading.



Your tires are rated

Front 2,335 @65 and 2,680 @80
Rear 2,159 ea @ 65 and 2,470 ea @ 80 psi





Also in RV application LT tires have a MAX speed rating of 75 mph no matter what the tire is marked

[jadatis]

TM9 wrote:
Also in RV application LT tires have a MAX speed rating of 75 mph no matter what the tire is marked.

That is new to me.
Does this mean that even if Q speedrated given on sidewall, that theoretically the maximum load is calculated for L = 75mph?

For the semi truck-tires ( from G load and up , and mayby F) I already knew mostly maxspeed 75mph.
But RV tires sometimes 81 or 87( M or N) with lesser maxload then same sise and loadrange truck-tire.

[Tireman9]

Originally Posted by jadatis

TM9 wrote:
Also in RV application LT tires have a MAX speed rating of 75 mph no matter what the tire is marked.

That is new to me.
Does this mean that even if Q speedrated given on sidewall, that theoretically the maximum load is calculated for L = 75mph?

For the semi truck-tires ( from G load and up , and mayby F) I already knew mostly maxspeed 75mph.
But RV tires sometimes 81 or 87( M or N) with lesser maxload then same sise and loadrange truck-tire.

In Europe the "Speed Rating" has some real meaning. As I understand it car owners are suppose to always use same or faster speed rated tires. In the US car tire "Speed Symbol" is really a relative measure of handling capability Higher rating >= better handling. Here the Speed Symbol is really only a relative measure of heat resistance. Most states have a max speed of 70. A few have 75 and only a couple have 80 so who would need 87 or 100 mph+ speed rated tres where no state allows driving at those speeds.


RV application both Goodyear and Michelin in their RV databook clearly state 75 MPH for RV application.

[Tireman9]

To help clarify:


Many time the "application" or use of an item results it additional or different limitations or ratings.


Example
If I have a tire that is a 225/75R15 and inflate it to 35 psi, How much load can it carry?


If the intended use is on a passenger car 1,874#
If as a single on a LT it is rated for 1,445#
If as a dual on a LT 1,315#


If a Single on a trailer where having passengers in the trailer is actually prohibited 1,760#
and if in dual application on a trailer 1,570#


Part of the decision process for a tire's capacity in each of these different applications includes such things as:


Will the vehicle carry passengers? Is the "Reserve Load" of the tire in the application normally close to zero or maybe 10% or normally closer to 25%


An obvious example where the application affects the load capacity is well established in the use of "P" type tires on a truck, trailer or multi-use vehicle such as an SUV. In these applications the load capacity is reduced by Industry standards by dividing by 1.10 so the 1,874# capacity becomes 1,703#


For heavy truck there are published guidelines that allow an increase in load capacity if the max speed is significantly reduced. As the speed is lowered the load capacity can be increased up to 16% with no increase in inflation.


Similar to above if the tire is made for a specific market a tire company my have more demanding requirements on the tire during the development process. An example might be it the tire was being made for a market where the speeds and Ambient temperature were both very high the tire might require a more robust construction. Another example might be providing a special tread rubber if the tire was to be used at extreme low temperatures where the tread rubber might even crack is a "High Speed" rated tire tread were used.


Tires are much more complex than many people suspect. Ideally an owner would be more informed and knowledgeable about tires and their limits and capabilities when making a purchase. They are much more than just "Round Black" things that almost no one "wants" to buy.

[budh9534]

Originally Posted by Tireman9

At 80 psi the tires are rated for 2,470# ea or 9,880 for
If I were you I would get on a scale with the RV fully loaded (people, food, clothes, toys and tools) and confirm your actual axle weights and be sure the inflation is sufficient to give an extra margin of 10% tire capacity over your measured tire loading. I do suggest that if possible you get to a +15% tire capacity over the actual loading.

Your tires are rated

Front 2,335 @65 and 2,680 @80
Rear 2,159 ea @ 65 and 2,470 ea @ 80 psi

Also in RV application LT tires have a MAX speed rating of 75 mph no matter what the tire is marked

I’ve already asked the wife when taking it to Florida in a few weeks with five adults to get each axle weighed after filling the gas tank with all passengers on board.

[Crasher]

Roger. When you refer to a 10% safety factor, is the tire any safer at 10% over it's rated load psi? If it is, why don't the manufacturers recommend a higher psi for the load? Or, is the 10% factor to cover the days when the ambient temp is lower which would lower the CIP eliminating the need to adjust the pressure? Whenever, I have run tires above the load charts, the center of the tread will wear more than the outer sides. That tells me that the tire was not making optimum contact with the road for best wear and traction. Admittedly, it's a minor issue, but an inquisitive mind has to ask.

[jadatis]

Its difficult for the tiremaker, when making lists.
When they give fi 1800 lbs maxload and maxpres op 65 psi , then people go asking when at 65 psi in the list , lesser loadcapacity is given then maxload.

But some traveltrailer tyrepressure, calculators on the internet give advice going from using 90% of loadcapacity belonging to pressure.

[Tireman9]

Originally Posted by Crasher

Roger. When you refer to a 10% safety factor, is the tire any safer at 10% over it's rated load psi? If it is, why don't the manufacturers recommend a higher psi for the load? Or, is the 10% factor to cover the days when the ambient temp is lower which would lower the CIP eliminating the need to adjust the pressure? Whenever, I have run tires above the load charts, the center of the tread will wear more than the outer sides. That tells me that the tire was not making optimum contact with the road for best wear and traction. Admittedly, it's a minor issue, but an inquisitive mind has to ask.

My +10% is on the set inflation and is NOT a "Safety Factor" in the normal sense. We know that tire inflation changes by about 2% for each 10°F change in ambient. The intent of this "Flex Range" of inflation is to avoid the need to mess with inflation on a daily basis.
Assume you needed 70 psi to support your heaviest ever expected load (this is why we say get on the scales when fully loaded to your heaviest). So what happens when you set your inflation to 70 psi and the Ambient temperature is 80F if the ambient drops to 70F? Your tire pressure will have dropped by 2% to about 68psi which is below what is needed to support the measured heavy load. So you get out and increase your tire pressure back up to 70psi. A few days later it's 90F so tire pressure is now (90F - 70F = 20F so 2% per 10F = 4% increase of the 70 psi so now your tires are at about 73 psi cold so you drop your tire pressure.
See the problem? You are messing around with your tire pressure. almost every day.


However if you have a +10% of your needed inflation, or in our example + 7 psi You can ignore the day to day pressure variation unless or until the temperature has dropped 50°F.


Tires can tolerate the increase in pressure with essentially no damage but low pressure can result in increased operating temperature which accelerates the "aging" of the belt rubber which can shorten tire life.


Also if you have to mess with your tires a lot soon you will tire of the chore and stop monitoring and adjusting tire pressure which can lead to low inflation.




RE center wear. That was an issue with bias tires but I do wonder what micrometer you are using to measure tire tread wear to 0.001" especially given that tire tread wear is normally in the .001" per 1,000 mile range and I doubt that your pressure remains constant over each thousand miles operation. Road surface (concrete vs asphalt) has a much bigger impact on tread wear.

[Crasher]

That's pretty much what I thought. The extra 10% is for the convenience of not having to chase the pressure. I ran the original set of Michelins for 5 years and 50,000 miles set at 10% above my heaviest weighed load @ 70 degrees. It worked well and had to adjust pressure only once in 12 months. When I replaced the Michelins. the inner tread was worn to 9/32" while the outer tread was at 12/32". Starting depth was 16/32". Assuming the tread depth was even when new, it is obvious the inner was carrying slightly more weight. I'm not concerned about it, but just relating what I observed. Much of the time, The weight was below the max loaded weight, so was more than 10% above the minimum needed.

[jadatis]

Reaction to post #80 of TM9.

Then if you add 10% to the calculated pressure( or search pressure for 10% more load, wich has about same effect), then you should not do it double. So then not go highering and lowering the pressure for the ambiënt temp. Yust note the temp you filled them at, and use the rule of tumb to check , if the filled pressure at the filling temperature is still the same.
Dont go for the last psi, the reserves cover that.

But my idea was( for what it is worth ) that when axles only weighed, it was to cover the weightdifference R/L . In my spreadsheet for motorhomes, I have a part 3 for when axle end weighed. Then I calculate pressure for average 90% used of the loadcapacity for the pressure, but if one side then would go above 95%, I use that axle-end for pressure calculation, and base it on 95% used of loadcapacity for the pressure.
Then the "lightest" side uses less then 85% of the loadcapacity for pressure, and I once determined that for discomfort-border. So that side can give some bumping, but " heaviëst side still some reserve for things like inacuracy of pressure and weight-reading, pressure dropp in time, etc , etc.

So my conclusions in time are that to prefent overheating of tires, you have to calculate back the cold measured pressure back to 65/68 degr F, and compare that to the advice-pressure.

Cold measurement is simply, when the temp of the air inside the tire is the same as outside the tire. And that is when the tire has not moved long enaugh, and no external factors like sunshine on tire.
Its the most reliable way.

Goal is to give the tire a pressure warm, when driving speed for wich pressure is calculated, that gives a deflection, so heatproduction, that wont give overheating of the rubber.

Never blead air from a warm tyre is given because adding a few psi to the advice, is un-reliable, because you then dont know howmuch it already cooled down, so what the real temp in tire is at that moment.

So that advice is not given, because you can burn your fingers then.

Only when warm pressure is lower then the advice cold pressure, its a usefull measurement, and you should at least fill up to that advice, mayby a bit more, but then already a bit quesswork.

For RV's you can not rely on the temperature reading of the tmps-system, because they mostly are screwed on the outside of valve. Temperature the sensors give is pretty accurate, but they give the temp something in between the inside- and outside-tire air.

[mlpeloquin]

I add 5 psi for the same reason. Go to a cooler place or down in altitude you do not have to adjust the pressure. If I clime and the temperature goes up, who cares if it is eight to ten psi higher. It will go down when the temperature or altitude changes as you travel.

[jadatis]

That is 10% of 50 psi, but only 5% of 100 psi.