tire pressure?

[jadatis]

You cant go from general advice, many different kind of tires are used in America on T owing vehicles
The advice on car is moastly for P- tires, and afterward often even E-load LT-tires are placed, with 80 psi to carry the larger maximum load.
Then you think you can use the same advice, but an LT- tires are stiffer( to hold more pressure in) , and by that have lesser surface on ground at the same deflection.
So if advice for P is given calculated , you need a but higher for LT.
But often 35 psi , wich is referencepressure of SL P - tire, is given, and then calculated pressure for GAWR's is much lower, and then for LT mayby it will come to that 35 psi, if its calculated, even if you add first some reserve to the load.

Tireman9 has on his site articles about how to handle for determining pressure, and most imporant is weighing in the loading you drive , best per axle-end. Then look up the needed pressure in the list for the used tires.

My input is NOT to use the pressure/loadcapacity- lists. They are made with a formula that leads to lower pressure then the European calculation gives, and for P-tires America stepped over to the European calc as late as 2006, not for no reason.

In my calculations in spreadsheets, I now use my own decided formula , to idea of an article of American IR J.C. Daws, that even comes to higher pressure.
To my conclusions, this leads to the same deflection ( so heatproduction) as when maxload and pressure behind AT on tire.

So resume, what one uses and is safe, can be unsafe for an other.

[jadatis]

Originally Posted by jbrnigan

Following is an example of the weight ratings at inflation pressures for LT275/65R18E tires: (Michelin data, but the same for similarly sized tire)
Single Tire
35 psi 1940 lbs
40 psi 2130 lbs
45 psi 2310 lbs
50 psi 2535 lbs
55 psi 2660 lbs
60 psi 2825 lbs
65 psi 3000 lbs
70 psi 3150 lbs
75 psi 3305 lbs
80 psi 3415 lbs
On my Super duty the recommended pressures are 70psi front and 75psi rear, which I run while towing. Unloaded, I run 50psi all around. This make a significant difference in the ride characteristics of my unloaded truck. On my trailer, 60psi all around is sufficient for the tire size and for the weight of the trailer. (trailer tire max rating is 75psi). There is no problem running max pressures, but not always necessary. Tire inflation data per tire size is readily available online.

I used a spreadsheet , I once made, to determine, what formula was used for making this list.
Came out that they used the in America used for LT- tires and for D-load AT 65PSI MAXLOAD 3000LBS.
Then they overwrote the 50psi with the maxload a C- load tire would have in this sise . And 80psi overwritten with maxload of E-load.

Strange for an European tyremaker Michelin, in Europe they would give for every loadrange a seperate list, and use the european formula ,wich comes to lower loadcapacity's for the pressure.

I think Michenin does this to please the TRA ,and not want to go out of line.

Try to include a picture on mobile( see if it works)

[jbrnigan]

Originally Posted by jadatis

I used a spreadsheet , I once made, to determine, what formula was used for making this list.

Not quite sure what point you are making, so no disrespect. The tire pressure chart listed and referenced, is published data from Michelin (and available from other tire mfr's) for a specific sized tire and load range. Different sized tires and load ranges and speed ratings have different weight and PSI ratings. Each vehicle manufacturer lists the recommended pressures for the specific sized tire, load range and speed rating they deliver on the specific vehicle (as posted on the door sticker). Not to over think the discussion, run door sticker pressures and you're "golden". If you know the unloaded GVW of your truck, you can use the chart to "tailor" your tire pressures and "possibly" improve the handling and/or ride characteristics of your vehicle. Adverse tire wear can happen at either extreme, or poor tire balance, or alignment issues or aggressive driving etc.

[jadatis]

The point , I am trying to make,is that to laws of nature, only one calculation is the right one.
And if you put it in graphics, there should be curve with no peacks or valleys in it.
And the American lists do have those peacks.
The list here given peaks at 50 and valley at 80 psi.
In other lists at those pressures the loadcapacity is fat printed with the , C, D , E etc of the loadrange with it. This list dont give that, but numbers are the same.
And that is wrong,for every loadrange of a sertain sise, a seperate list should be made.
In Europe this is done this right way.
And also a formula used that to conclusion of the article of mr Daws, is closer to the ever to be constructed ideal formula.

So you have the whole story about what data is needed for determining needed pressure , you collected everything right, but then the calculation of the given lists gives to low pressure and so still overheating tires possible.

As I already wrote, in determining pressure, there are a lot of bears along the road.

[Tireman9]

Proper tire inflation for motorized vehicles is different from the inflation for Towed trailers.


Cars, and trucks have a team of engineers who work with tire engineers to arrive at the best compromise for ride, handling and fuel economy but always provide more load capacity that the individual axle load rating (GAWR)


New TT are required to specify tire size and inflation such that the load capacity of the tires at the stated inflation is at least 110% of the stated GAWR as seen on your certification lable.
Now if you want the best durability and to lower the chance of having a belt separation on your TT then you need to lower the Interply Shear.
Here
There are 9 more posts that also talk about Interply Shear HERE

Basically TT should ALWAYS run the pressure on the tire sidewall associated with the tire max load. PLUS ensure your actual load in no greater than 85% of the actual max MEASURED load when the TT is at its heaviest ever.

Now you can guess or depend on info from your brother-in-law or listen to an actual tire engineer. Your choice.

[jbrnigan]

Originally Posted by jadatis

The point , I am trying to make,is there are a lot of bears along the road.

Oh........ OK.

[twdenman]

Ask your tire manufacturer what THEY recommend. THEY want their tires to last. Your tire supplier, wants to sell tires. I weighed my rig, loaded, then checked the tire manufacturer for best pressure. There is a pressure chart, based on rig weight, for you to follow. If you are a class A owner, you will immediately feel better ride, and handling. I did.

[roblaura2]

I recommend weighing your coach at a truck stop, and setting tire pressure for the tires on each axle based on the inflation chart posted online for your brand, size, model and axle weight (individual wheel weights are the best, but harder to get). As a second choice, use the inflation amount posted in your coach. It is the recommended tire inflation for your coach at its maximum safe weight - fully loaded including passengers. Hopefully, the coach is not overloaded. If it is, weighing your coach is the only 'safe' way I know to determine the proper tire inflation for the weight. Of course being overloaded reduces coach safety regardless or tire pressure. The pressure on the sidewall is the maximum safe amount for that tire's structure.

[Rick 17RB]

As you've seen from multiple posts you still don't have a definitive answer. You're going to want somewhat higher pressure than what's on the door. You're not going to run at maximum. Warm air expands and your tire pressure will change as the tire warms. My TV happens to have the tire pressure displayed in the vehicle through sensors on the valve stems on the tires. That pressure changes 3 or 4 PSI or so when they warm up while driving. It's changes about 2 PSI without pulling the camper. So do not fill to maximum because the tires will be overpressure when they warm up.

With higher pressure you feel every little ripple in the road and it will ride rougher. With lower pressure there will be much more bounce caused by the campers weight. Start a few pounds more than the door and find your preferred median between rough ride and bounce.

My vehicle door says 35 PSI and my tires say 50 PSI. My happy medium is 42 to start which increases to 45 when the tires warm up. You'll have to figure that out for your vehicle and camper. Buy one of those cigarette lighter powered pumps. They are slow but do work.

I also leave the camper tires 5 o 7 PSI below max figuring the at pressure will increase some when the tires warm. My camper has a sticker that 40 PSI and the tires say 50 PSI.

[Tireman9]

I have written a number of times on the topic of Interply Shear. This is the force that is tring to tear the belts of radial tires apart.
There are some highly technical papers on the topic and you can review tham after a simple Google search on the term. You can also look here on my blog for the posts where the term is tied to a post by simply checking the list of topics on the left side of my blog. Basically this force comes about when the belts in a radial tire are forced to change shape. This means either when a tire rolls and the footprint changes from curved to flat as the footprint rolls into contact with the ground. This force increases when external forces from cornering are also applied to tires.
So why do trailers seem to have such high Interply Shear forces? Well it's not all trailers as the cornering forces of single axle trailers are much lower than the forces of tandem or triple axle trailers.

What tipped me off to this was an observation at a campground that happened to have a freshly smother gravel driveway and a multi-axle trailer happened to make a 180° turn as I was walking by. I noticed that the gravel marks were not a smoth curve but there was a series of turns interupted by discontinuities.

A short time later I saw a video from Keystone RV Company of wheel lug nut torque (you should watch the full video some time)
While watching their video I recall some special high side load tests we ran in our tire test lab.

The Keystone video that shows what happens to multi-axle trailer tires.


In the video you can see how the two tires are fighting each other with one bending in as the other bends out. You can imagine that if the turn is made on gravel at some point the high sideload would result is a sudden breakaway or slip. That's what I observed on the gravel turn.

Back at work I had some high powered Finite Element computer programs run to simulate the side load of a multi axle trailer and the results showed that the side loads on a trailer could be 24% higher than on a standard vehicle even with identical radius and loads on the tires.

There are posts that shows why tires get different side loading.

Further analysis showed that increasing the tire inflation could lower the extra shear, but sadly not eliminate it.

Bottom line
In a number of posts I have recommended that Motorhomes set their inflation based on measured static load plus a margin of at least 10% additional PSI. This would also apply to single axle trailers. BUT for Tandem and triple axle trailers I strongly recommend that the tire cold inflation be set to the inflation molded on the tire sidewall associated with the tire maximum load capacity. I also recommend that the measured static tire loads on these trailers be no greater than 85% of the tire maximum with a 20% margin being better.




Feb 22 2018

[Atlee]

Fortunately, I have a good "margin of error" with my tires. My trailer came with ST205/75R15 LRD GY Endurance. They have a combined 8600# capacity. The GVWR of my trailer is 6500# GVWR. That equates to a margin of over 32% if I'm doing the math correctly.

I've always believed that one of the reasons so many tires (including off brand Chinese) went bad early was because manufacturers tended to put the smallest cheapest tire they could that just met the weight of the trailer in question. Maybe with a margin of only 5% say.

Originally Posted by Tireman9

I have written a number of times on the topic of Interply Shear. This is the force that is tring to tear the belts of radial tires apart.
There are some highly technical papers on the topic and you can review tham after a simple Google search on the term. You can also look here on my blog for the posts where the term is tied to a post by simply checking the list of topics on the left side of my blog. Basically this force comes about when the belts in a radial tire are forced to change shape. This means either when a tire rolls and the footprint changes from curved to flat as the footprint rolls into contact with the ground. This force increases when external forces from cornering are also applied to tires.
So why do trailers seem to have such high Interply Shear forces? Well it's not all trailers as the cornering forces of single axle trailers are much lower than the forces of tandem or triple axle trailers.

What tipped me off to this was an observation at a campground that happened to have a freshly smother gravel driveway and a multi-axle trailer happened to make a 180° turn as I was walking by. I noticed that the gravel marks were not a smoth curve but there was a series of turns interupted by discontinuities.

A short time later I saw a video from Keystone RV Company of wheel lug nut torque (you should watch the full video some time)
While watching their video I recall some special high side load tests we ran in our tire test lab.

The Keystone video that shows what happens to multi-axle trailer tires.


In the video you can see how the two tires are fighting each other with one bending in as the other bends out. You can imagine that if the turn is made on gravel at some point the high sideload would result is a sudden breakaway or slip. That's what I observed on the gravel turn.

Back at work I had some high powered Finite Element computer programs run to simulate the side load of a multi axle trailer and the results showed that the side loads on a trailer could be 24% higher than on a standard vehicle even with identical radius and loads on the tires.

There are posts that shows why tires get different side loading.

Further analysis showed that increasing the tire inflation could lower the extra shear, but sadly not eliminate it.

Bottom line
In a number of posts I have recommended that Motorhomes set their inflation based on measured static load plus a margin of at least 10% additional PSI. This would also apply to single axle trailers. BUT for Tandem and triple axle trailers I strongly recommend that the tire cold inflation be set to the inflation molded on the tire sidewall associated with the tire maximum load capacity. I also recommend that the measured static tire loads on these trailers be no greater than 85% of the tire maximum with a 20% margin being better.




Feb 22 2018

[oldmanAZ]

Originally Posted by Tireman9

I have written a number of times on the topic of Interply Shear. This is the force that is tring to tear the belts of radial tires apart.
...snip...
Bottom line
In a number of posts I have recommended that Motorhomes set their inflation based on measured static load plus a margin of at least 10% additional PSI. This would also apply to single axle trailers. BUT for Tandem and triple axle trailers I strongly recommend that the tire cold inflation be set to the inflation molded on the tire sidewall associated with the tire maximum load capacity. I also recommend that the measured static tire loads on these trailers be no greater than 85% of the tire maximum with a 20% margin being better.

Feb 22 2018
(Bold and underline added by oldmanAZ aka SRA)

Thank for posting this, Tireman9. It's gratifying to see your knowledge and research agreeing with what I have been advocating for some time.