Goldilocks tire pressue

[TRAFFICAL]

GOLDILOCKS TIRE PRESSUE

I have seen many discussions about the proper tire pressure for Airstreams. I would like to share my thoughts as well and the tire pressure that I have determined to be the best tire pressure for me. I, for one, want the greatest amount of rubber on the road for sudden stopping and drive-ability during wet weather.

What got me looking at this issue was when I noticed that maximum tire pressure on the tire is 51 PSI while the suggested tire pressure on the inside door jam of my tow vehicle is 31 PSI front and 33 PSI rear. This is significantly lower than the maximum tire pressure stamped on my tires. So, what tire pressure is right?

To get started there are the two schools of thought. School one recommends inflating the Goodyear Endurance to the maximum at 80 pounds per square inch (PSI) which is what Airstream recommends. The second school suggest that we use the tire pressure published by Goodyear for the ST Endurance tire. Both have merit and I hope that I can bring my simple rational to the discussion. We can all agree that over or under inflated tires have issues. If under inflated, the side walls of the tire are exposed to excessive flexing that breaks down the tires structural integrity and can lead to tire failure. If over inflated the tire rides on the center part of the tread and prematurely wears out the tire. The trick is hitting the Goldilocks tire pressure that is just right with less wear and stress on the tire and more rubber on the road. The three tire inflation conditions can be seen in the figure.

So let’s take a look at our 2021 - 20 FB Caravel. The Caravel has a Gross Vehicle Weight (GVWR) of 5000 pounds, has one axle and two tires. The Caravel also has a tung jack or is attached to the tow vehicle and the tung weight is 500 pounds. Now, let’s subtract the tung weight and divid the remainder by two for ( (5000 - 500) / 2) = 2250 pounds per tire. Now, let’s assume that I did not balance the load in the Caravel and one side is 10% heavier than the other and results in a tire load of (2250 x 1.1) = 2475 pounds. Looking at the Goodyear Endurance chart for the ST225/75R15, that is on our Caravel, we see that the tire load limit at 65 PSI is 2540 pounds and is a little over our projected load per tire. However, I did what we all should do, I weighed our rig at the beginning of a major trip at a truck scale to see what our weight is per axle. The trailer axle weight connected with weight distribution installed was 4160 pounds. This is less than the projected axle weight of 4500 pounds (5000 GVWR - 500 Tung) and I have over estimated a bit.

We have had our Caravel for two years and have traveled 15,000 miles with the tires inflated to 65 PSI. To see how well I’m doing I checked the wear on the tires by measuring the current depth of tread. The results are shown in the table.

DEPTH OF TREAD
Drivers side Passenger Side
Outside Center Inside inside Center Outside
0.220 0.180 0.210 0.185 0.155 0.195

Based on the data it appears that my tires are still over inflated as they are worn more in the center of the tread. Although it appears that my tires are a bit over inflated I plan to keep my tire pressure at 65 PSI. I think that it is better to be a little over inflated than under inflated. It is also noted that the passenger side tire shows more wear and will require some more research on my part.

For me the Goldilocks tire pressure remains at 65 PSI, not to low nor not to high but just right. I hope that this, although lengthy dialog, will help in understanding my thought process in selecting a tire pressure for my Caravel. I trust that what ever tire pressure you decide on will be right for you.

A final note on tire inflation is to get and use a Tire Pressure Monitoring System (TPMS). The one I have is off by about three PSI but is still worthwhile as it will alert me to significant changes in tire pressure. I always make it a rule to check the TPMS every time I stop for gas, lunch or head out in the morning and I suggest that you do so as well.

[Dennis C]

I think your approach is logical. I like to use actual weights, and I add a 10% safety margin. This is especially important if you have an axle weight vs. individual tire/wheel weights. I then consult the tire manufacturer’s load chart and fill my tires to the appropriate pressure.

[Acheron2010]

I am in that range also. Very comfortable towing, no rivet popping or other anomoly.

[4RXLA]

I’m between 55-60 psi on 80 psi max tires based on the weight rating chart of the tire manufacturer. Much smoother ride and even tire wear.

[John Ware]

I've experimented with tire pressure on my 23'FC and have settled on 65#'s as well. I ran 80#s like the book says on my 2018 single axle Sport and that trailer bounced down the road like an overfilled basketball on a cement driveway.

[ROBERT CROSS]

15% less than 80....COLD.
Never 'adjust' while on the road.
Once before starting in the shade.
No Charts needed.

TETO

Bob
🇺🇸

[gypsydad]

Another tire/PSI thread...how many does this make this month?

[Tireman9]

Originally Posted by TRAFFICAL

GOLDILOCKS TIRE PRESSUE

I have seen many discussions about the proper tire pressure for Airstreams. I would like to share my thoughts as well and the tire pressure that I have determined to be the best tire pressure for me. I, for one, want the greatest amount of rubber on the road for sudden stopping and drive-ability during wet weather.

What got me looking at this issue was when I noticed that maximum tire pressure on the tire is 51 PSI while the suggested tire pressure on the inside door jam of my tow vehicle is 31 PSI front and 33 PSI rear. This is significantly lower than the maximum tire pressure stamped on my tires. So, what tire pressure is right?

snip.

You appear to have two different topics. First is the TV pressure.
The Door jam pressure is the correct COLD pressure for the TV. It appears you have XL P type tires on the TV. This has nothing to do with tire pressure on the RV.


Your RV has a Certification label on the outside , driver side toward the front of the RV. Publishing a picture of that label would give us all a lot of useful information. This label gives the RV cold inflation recommended by the RV MFG.


Related question have you ever had the TV and RV weights measured on a truck scale? The individual axle weights would be ideal but that is hard to get but individual axle weights can be learned at any CAT scale or truck stop. The TV and RV should be loaded to the heaviest you ever expect to be (Full fuel, water, Propane, food clothes tools etc.) You may need two weighings as you need to get the reading with only one axle on each scale pad, so you may need to re-position between a couple of readings.


Here is a video on a TV + RV https://youtu.be/dVc72YKk1_I
There is one thing missing. They did not get the individual axle weights on the tandem axle RV. They are assuming a perfect 50/50 axle load split which almost never happens.
I have attached a weighing worksheet that will help you get the correct INDIVIDUAL axle loads.

[CapriRacer]

Originally Posted by TRAFFICAL

GOLDILOCKS TIRE PRESSUE........ [verbage omitted]

2 thoughts from another tire engineer.

1) The assumption you are making is that the tire was designed to give even wear at the "goldilocks" pressure. This isn't necessarily true.

2) There's another indicator you ought to be using: Pressure buildup.

Measure the pressure immediately before starting out, and then after an hour at highway speeds. You don't want more than a 10% build up. If you have more, than you need to take action - like a larger capacity tire or more inflation pressure.

If it's more 15%, then you need to take immediate action - slow down until you can get larger capacity tires.

[LNBright]

Rather similar setup, we have a 20FB FC w/ the same Endurance tire. Day before a trip, when I pull the camper out of its parking spot, I check the psi, I shoot for 68, but if I'm between 65 and 70, I'm comfortable w/ that.


I've noticed the upward shift in psi once on the road (using the TPMS), but I've not done my math on it to verify that it's under 10%... Have to admit, I don't know what other tire I'd go to from the Endurance, I think it's the right tire for the job.

[jadatis]

I calculated that 62psi would be right for the 4160lbs axleload. Assuming your Endurance 2830 lbs AT 80 psi.
So 65 psi should not give centrewear.
But did you measure the sides close to the edge mayby?
Could be that if you measure half an inch more to the centre, difference between centre could be marginal.
Strange is the difference between R and L.

[thiel]

For my Goodyears, 65psi was max and worked great.

For my Carlysle tires, they are load range E with much higher max psi... and I am finding that 50psi gives me the best result. I confirmed on the tire manufacturers load charts that that pressure is appropriate for the load, and the towing has been great.

[TRAFFICAL]

Originally Posted by Tireman9

You appear to have two different topics. First is the TV pressure.
The Door jam pressure is the correct COLD pressure for the TV. It appears you have XL P type tires on the TV. This has nothing to do with tire pressure on the RV.


Your RV has a Certification label on the outside , driver side toward the front of the RV. Publishing a picture of that label would give us all a lot of useful information. This label gives the RV cold inflation recommended by the RV MFG.


Related question have you ever had the TV and RV weights measured on a truck scale? The individual axle weights would be ideal but that is hard to get but individual axle weights can be learned at any CAT scale or truck stop. The TV and RV should be loaded to the heaviest you ever expect to be (Full fuel, water, Propane, food clothes tools etc.) You may need two weighings as you need to get the reading with only one axle on each scale pad, so you may need to re-position between a couple of readings.


Here is a video on a TV + RV https://youtu.be/dVc72YKk1_I
There is one thing missing. They did not get the individual axle weights on the tandem axle RV. They are assuming a perfect 50/50 axle load split which almost never happens.
I have attached a weighing worksheet that will help you get the correct INDIVIDUAL axle loads.

Thanks for the input.
Yes, I measured the axle load on a CAT scale and measured each axle independently.
I am not hear to preach one school of thought over the other. However I did want to explain my logic and to do what no one has done before, provide real tire wear data based upon my theory. A problem with the recommendation to inflate to 80 PSI is that so far I have not seen data to support the 80 PSI MAX theory. Has anyone out there actually performed tests, wear or otherwise?

I am only a simple licensed Civil and Traffic Engineer and thrive on data. I have a theory, applied that theory and tested it to assess the theory. I can't do much more than that.

[TRAFFICAL]

Originally Posted by CapriRacer

2 thoughts from another tire engineer.

1) The assumption you are making is that the tire was designed to give even wear at the "goldilocks" pressure. This isn't necessarily true.

2) There's another indicator you ought to be using: Pressure buildup.

Measure the pressure immediately before starting out, and then after an hour at highway speeds. You don't want more than a 10% build up. If you have more, than you need to take action - like a larger capacity tire or more inflation pressure.

If it's more 15%, then you need to take immediate action - slow down until you can get larger capacity tires.

Thanks.
1) This maybe true but I have provided data on tire wear given axle load, tire pressure and mileage. From the data I provided it appears that tire wear is slightly more in the center than on the edges which means a slight over inflated condition. I general the wear seems to indicate that I have full road contact which is important to me.

2) Yes Pressure Buildup is important. I have, as stated in my original post, use a TPMS. I check the TPMS at departure and on the road. Poorly inflated tires can lead to heat and pressure buildup. I have not seen a significant change in temperature or pressure. Although I don't have a log of my temperature and pressure while on a trip but I will start as the data could be useful.

What I would like to see, if anyone out there as it, data on tire wear with a 20FB Caravel using the maximum 80 PSI. Until then I am staying at 65 PSI as I have data to support my theory.

I am just a simple Civil and Traffic Engineer and am data driven. I will do my best to collect additional data to add to the discussion.

[Boxite]

From the good folks at E-trailer:

“When it comes to trailer tires you always want to have them inflated to their max psi when cold. Not only do you get the full capacity at the max psi, you also will generate less heat in the tire since there won't be as much flex in the sidewall, you'll get better wear and better fuel mileage.”

However, the same E-trailer “expert” also posted:

“Goodyear tires are different then most other manufacturers when inflating tires to its maximum PSI. Since Goodyear developed and tested their tires to work properly based on their recommended tire pressures, I would stick with their recommendations.

The Goodyear Endurance # LH33FR for example has a 2,830 pound max load capacity, going off Goodyear's PSI chart you should have the tire at 60 PSI. “

[jadatis]

I wrote it before.
Airstream TT's have as exeption to the rule,
1.often comfortable reserves in GAWR('s) to the GVWR, and
2. comfortable reserves in the tires to the GAWR.

So even a C-load ST tire can do with, even my extra tight calculation and reserves, do with lower pressure then 50 psi.
They recomend in that case the 50 psi, wich then often give popped rivets.

So then you do the lower pressure, for the TT and not the tires.

Then going to D- or E-load, my calculations give higher pressure, but still much lower then the referencepressure belonging to the loadrange.

For singleaxle TT of topicstarter with his weighed loads, I calculated 62 psi on a AT 80 psi E-load.
Then 80 psi would give many popped rivets.

So the TT staying at one piece is also important, and with my calculations no exessive heatproduction, and even wear.

Using the Goodyear list or whatever ST list ( they are all the same) , is not wise to my opinion.

1. They are calculated for one loadrange, and the other loadranges maxload writen over the calculated loadcapacity for the pressures belongiing to those loadranges.
I still wonder how the engeneer who calculated the first list, handled with that. He must have seen the ilogicality of it. In Europe for every plyrating/loadrange of same size, a seperate pressure/loadcapacity list is made, in wich the incements between the loadcapacity's are logical.

2. Calculated with a formula introduced in 1928 for diagonal tires, wich lead to higher loadcapacity in the lower pressure, then the nowadays used formula for P- tires in US since 2006, and also higher then the now still used formula for LT and many trucktires.

3. Calculated for 65mph, so leaves no reserve for a bit higher speed, fi 70mph.

The smaller sises are given maxload same or yust a bit lower as LT for 99mph, but the larger sises have 6 loadindex steps higher then comparable LT tire, because maxload is also speed related.

Compare the 235/85R16 in ST and LT in attached pdf
The ST is maxload 4400lbs/ LI 132 AT 110 psi, and the LT is 3750 lbs/ LI 126 AT 110 psi. And this is only because of the difference in reference-speed, they calculated the maxload for.

I think general rule of using referencepressure belonging to loadrange of OEM tires for TT, is courced by the experiënces with other TT brands wich mostly have no or poor reserve in tires, and the 3 things I mentioned here.

[Tireman9]

Originally Posted by TRAFFICAL

Thanks for the input.
Yes, I measured the axle load on a CAT scale and measured each axle independently.
I am not hear to preach one school of thought over the other. However I did want to explain my logic and to do what no one has done before, provide real tire wear data based upon my theory. A problem with the recommendation to inflate to 80 PSI is that so far I have not seen data to support the 80 PSI MAX theory. Has anyone out there actually performed tests, wear or otherwise?

I am only a simple licensed Civil and Traffic Engineer and thrive on data. I have a theory, applied that theory and tested it to assess the theory. I can't do much more than that.

Engineer to Engineer I suggest you not be so concerned with tread wear as you will soon learn that most tires in RV service "Age-out or fail before they wear-out" I have a number of posts, with the science behind the topic, on Interply Shear. I suggest you check them out for a better understanding of why running with a + 15% or better Reserve Load can significantly decrease the probability of you suffering a Belt Separation.


This forum does not allow me to post a direct link to my tire posts so you will need to Google " Interply Shear Tires' to see the science.

[gypsydad]

Originally Posted by jadatis

I wrote it before.
Airstream TT's have as exeption to the rule,
1.often comfortable reserves in GAWR('s) to the GVWR, and
2. comfortable reserves in the tires to the GAWR.

So even a C-load ST tire can do with, even my extra tight calculation and reserves, do with lower pressure then 50 psi.
They recomend in that case the 50 psi, wich then often give popped rivets.

So then you do the lower pressure, for the TT and not the tires.

Then going to D- or E-load, my calculations give higher pressure, but still much lower then the referencepressure belonging to the loadrange.

For singleaxle TT of topicstarter with his weighed loads, I calculated 62 psi on a AT 80 psi E-load.
Then 80 psi would give many popped rivets.

So the TT staying at one piece is also important, and with my calculations no exessive heatproduction, and even wear.

Using the Goodyear list or whatever ST list ( they are all the same) , is not wise to my opinion.

1. They are calculated for one loadrange, and the other loadranges maxload writen over the calculated loadcapacity for the pressures belongiing to those loadranges.
I still wonder how the engeneer who calculated the first list, handled with that. He must have seen the ilogicality of it. In Europe for every plyrating/loadrange of same size, a seperate pressure/loadcapacity list is made, in wich the incements between the loadcapacity's are logical.

2. Calculated with a formula introduced in 1928 for diagonal tires, wich lead to higher loadcapacity in the lower pressure, then the nowadays used formula for P- tires in US since 2006, and also higher then the now still used formula for LT and many trucktires.

3. Calculated for 65mph, so leaves no reserve for a bit higher speed, fi 70mph.

The smaller sises are given maxload same or yust a bit lower as LT for 99mph, but the larger sises have 6 loadindex steps higher then comparable LT tire, because maxload is also speed related.

Compare the 235/85R16 in ST and LT in attached pdf
The ST is maxload 4400lbs/ LI 132 AT 110 psi, and the LT is 3750 lbs/ LI 126 AT 110 psi. And this is only because of the difference in reference-speed, they calculated the maxload for.

I think general rule of using referencepressure belonging to loadrange of OEM tires for TT, is courced by the experiënces with other TT brands wich mostly have no or poor reserve in tires, and the 3 things I mentioned here.

This again, is a crazy argument, IMHO. LT tires designed/mfg. with different use/application in mind vs ST tires. Agreed? If so, then "why" would you not follow the "tire mfg" load chart for PSI? Does the actual real world experience of many of us here account for nothing? Many of us have been towing AS's or other TT's, with several different types of tires, over the years. My experience is not unique: if you run your tires at "max PSI", you will likely experience popped rivets, broken hinge/latches, hanging cupboards coming loose, etc.. Decrease the PSI and you these issues will all but perhaps a rivet or two, disappear. If that doesn't seem to be an issue for you, run at the PSI you are comfortable with. As for "wear" on the tread when running below MAX PSI, I have not experienced any "wear" issues that were obvious while running my Endurance LT's.

[jadatis]

I mean to remember that you, Gypsydad, also use about 58 psi on a E-load ( AT 80 psi) ST tire, and that is about what I calculated once for your configuration.

In Europe ST is not used on TT, mostly C-tyres is european equvalent of LT.
Organisations who sell tires, long sayd to replace them after 6 years, but nowadays they have come back at it and say to let them be checked every year on aging signs, and certainly replace them after 10 years of use.
Average replacement now is after 8 years.
So LT do better then ST.
Special Trailer tires sugest an upgrade.
But the European version " for trailer use only" N speedrated and maxload for 140kmph/87mph given so 2 LI steps higher., give better the limitation.

[Air2Dream]

Originally Posted by Tireman9

Engineer to Engineer I suggest you not be so concerned with tread wear as you will soon learn that most tires in RV service "Age-out or fail before they wear-out" I have a number of posts, with the science behind the topic, on Interply Shear. I suggest you check them out for a better understanding of why running with a + 15% or better Reserve Load can significantly decrease the probability of you suffering a Belt Separation.


This forum does not allow me to post a direct link to my tire posts so you will need to Google " Interply Shear Tires' to see the science.

Thanks - I Googled the information as suggest and found the Interply Shear Tire information. Very interested.

I am though confused by your pressure recommendation. Based on experience, the smaller trailers seem to “bounce” more when the tires are inflated to the max. We have a 23 FB with dual axles and a published GVW of 6000 lbs or about 1500lbs, give or take, per tire. To be safe from shear, should no allowance be made for trailer weight? According to Goodyear the recommended load at 60 psi is 2380 lbs which is significantly more even allowing for variants in tire loading.