80 psi vs 65 psi - are both safe in this situation?

[trispeed]

All tire MFR s provide pressure recommendations online forthe weight on the tire; they know more about that than anyone else. It's usually lower than you think. No mystery or guessing needed

[Mollysdad]

Originally Posted by smithcreek

TPMS sensors inside wheel or stem mounted? Mine are stem mounted. When I stop moving the heat transfers from the rim to the stem to the sensor and there is much less moving air to cool it down.

Stem mounted on metal stems.
TST TPMS

Originally Posted by trispeed

All tire MFR s provide pressure recommendations online forthe weight on the tire; they know more about that than anyone else. It's usually lower than you think.

So you're saying the "tire experts here" are wrong?

Originally Posted by drbrick

According to the Goodyear Endurance chart we run 'Blue Streak' at 65lb -

How did you arrive at that number?
Let's say your trailer weighs in at a svelte 7000 lb.
There's a thousand pounds on the tongue, not the tires so I subtract that.
Leaving 6000 lb. on 4 tires means each tire carries 1500 lbs. Yes?
The Good Year chart tells me I should inflate to 28 psi.
What did I do wrong?

Conversely, at 65 psi, the load per tire is 2540, meaning your trailer SHOULD weigh in at 10,160 + the tongue weight or about 11,500 lbs.

[GOUSC]

Originally Posted by Mollysdad

That's odd. I set my tires at 65 and while driving I see pressures about +9 or 74. I've seen the temperatures to reach 110f. but the pressure doesn't go up much more. The temp actually rises when I stop for gas, which seems counter-intuitive, but I guess the tires stay cooler while rolling.

I saw the same thing. When pulling into rest areas to stop the temp spiked and went down when back rolling with no change in tire pressure. I will say that yesterday we pulled a 12 hour day driving when usually 6-8 hours are our longer days.

We recently were at Jackson Center ( a 8 hr drive). Tire pressure set at 70 psi cold. I never got the 20% alarm but outside temps were only in the high 80's T the most.

I've put about 17,000 miles on my Globetrotter since April of 2018 and am trying to find the right balance of tire pressure to allow a smooth ride / staying within my load limits and preventing blow outs as 80 psi is just too rough for the RV. 65 psi was the first time the tire pressure went above 20% of the cold setting. I am basically asking if this is a safety issue if tires can handle 15-20% above an 80 psi cold pressure setting. If it is a safety issue then I will adjust them up to 70 psi cold next time.

[cazual6]

I have 2018 FC25RT. I use to run 80.

After reading topics about this, I ran it to 70-75. The only thing I feel is "loose".
I've have had a near sway catastrophe incident, here is the thread. So I know what swaying feels like, it feels "loose" so I am amp up on caution when I feel loose.

But I do understand that 80 is max pressure and lowering would make it less stiff thus a smoother ride. I think 65 would be the lowest I would go.

Any thoughts on that?

[perryg114]

As long as the tire is not overheating you should be fine. Measure temperatures of the center and edges with an IR thermometer. If you are running too low of a pressure the sides will be hotter than the center. 80psi is when you are close to max load. It will shake your trailer apart. Small stuff is all tire. Big hits are taken by the axles. My 7000lb trailer is running XL car tires at 50psi and it has shown no signs of problems. Tires are rated at 50psi MAX. I think my trailer came with D rated tires at 45psi.



Perry

[smithcreek]

Originally Posted by Mollysdad

The Good Year chart tells me I should inflate to 28 psi.
What did I do wrong?

Conversely, at 65 psi, the load per tire is 2540, meaning your trailer SHOULD weigh in at 10,160 + the tongue weight or about 11,500 lbs.

The chart does not tell you what you "should" run for psi. It only tells you the ABSOLUTE MAXIMUM weight you can carry at a given psi, or conversely, what the MINIMUM PSI is for a given weight.

So what the chart really says is "Run whatever you want up to 80 psi, but DO NOT carry a heavier load than X lbs. at Y psi".

[jcanavera]

Keep in mind that the tire temperature is controlled by a few factors. One is the pavement temperature and the surface the tire is running on. Second is the load factor that the tire is supporting. Third is the tire inflation level. Finally is the speed at which you travel.

Tires naturally flex as they rotate. The byproduct of that flexing is the generation of heat. The inflation factor of the tire can minimize the amount of flexing that occurs as the tire contacts the ground. Tires can overheat due to excessive flexing that occurs due to under inflation. Tied to that closely is speed. A tire has to disperse that heat. If it can't, the tire will overheat and you will get a failure. Underinflation and/or excessive speed are two factors that will overwhelm a tire's ability to dissipate that heat. Road temperatures and surface add to the tire heat load. Interestingly enough if you look at tire charts related to speed, size of tire, and inflation level, you will see the load capacity of a tire will decrease as the speed increases. If you wonder how Airstream supports towing with a tire removed on a tandem axle, you will also realize that Airstreams notes that the max speed limit in that situation is 45 mph. Part of that reasoning is the axle with the remain tire has to support more weight than it did in a tandem configuration. Slowing down naturally allows that tire to carry that extra weight because the tire is rotating a lower speed thus reducing the amount of flexing going on over the course of time. This slower speed off sets the higher heat load that might have been generated if you maintained a higher speed.

I think in everyday use many of us will see stickers in our automobiles or trucks that show pressure labels in our cars or trucks that may show inflation numbers front and rear axles. In my 3/4 ton van I use LT 225x75R16 tires. The same as my Airstream. However the tire recommendation is not 80 psi on all 4 wheels. That's due to the amount of weight each axle has to carry. So the rear tires are recommended to be set at 80 psi (5360 lbs), the fronts are recommended at 55 psi (4100 lbs). So we have evidence that setting pressures below side wall recommendations are safe if we know that those tires are being used in that format.

However a travel trailer in a tandem axle set up is exerting forces on a trailer, very much unlike a 4 wheeled car or truck. And that's where our tire engineers on the forum have pointed out that those forces are attempting to pull that sidewall away from the wheel during turns. That's where that extra pressure is needed to resist those forces. That's why many have espoused running at higher pressures (both to resist that force), but to also accommodate having proper reserve levels to compensate for load inbalances that occur on a per wheel basis and the lack of knowledge that you know truly what your trailer weighs and the load on each wheel.

So bottom line if you want to carry lower inflation levels in your tires on your trailer, you really must know what that trailer weighs on a per wheel basis when you have your trailer loaded to what you see as your heaviest towing weight. You also must factor in what your speed rating is for your tire. If you exceed that, you may generate more heat than the tire can safely dissipate. It's all a balancing act and if my 31' Classic slide out was a lighter weight trailer, I might have considered lower pressures. But the reality is I don't have a firm grasp on my trailer on that individual wheel weight in a worst case situation, so I play it safe and set mine to 80 psi. I finished 6 seasons with my Michelin LT 16" tires without a failure. They've been replaced with a second set at the beginning of this year. I've seen no structural failures in my trailer other than some sliding drawer issues. Those however were due to a poor design of the system to keep the drawers in place, with no positive locking mechanism available. I don't consider a slightly raised area on the rail a positive method of keeping a drawer in place. Especially with the condition of some of the Interstates that I have traveled on over the years.

Jack

[dixvan]

Get the Goodyear endurance tire inflation guide from their site. Then based on the weight in each tire look up the recommended inflation. For your rig it probably will be approx 55 psi

[GOUSC]

I need to correct some info at my first post for this issue. I stated my margin was 14%. This was the margin on my TV truck tires. Per the Goodyear Endurance load charts, ( if I read them correctly), 65 psi should support 2,540 per tire. 4 tires = 10,160 lbs. My RV fully loaded for a 5 week trip last year weighed 6,380 with weight distribution. So, my margin would be in the area of 40%. I run 64 mph when towing. Based on this I would think 65 psi is more than enough.

Lot of good info here but I'm still looking for the answer of;

If the tire at cold psi of 80 can handle psi of 94 psi in 100 degree weather, I would assume that the tire at 65 psi cold could handle a psi of 79 in 96 degree weather without failure ( even though the 79 psi is an increase of >20% when cold).

Is this a valid assumption- yes or no?

If not, why not?

[majorairhead]

I've read extensively on the tire pressure issue, right here on the airforums.

All I'll say is this. I'm running a 2019 International 30 with the Goodyear Endurance tires. I'm loaded with a full tank of water, 5 boxes of books, full fridge, full propane, clothing, dishes, pots, pans, etc.

Tire pressure = 80 psi

I've run the Endurance tires as low as 72 psi. I've run 'em at 80 psi. I've never had issues with popped rivets, doors opening, or stuff getting beat up.

I'm not certain the tire pressure has as much to do with the ride of the trailer as the hitch setup does. I know there is no shortage of folks on this site that will disagree with that, but that's my story and I'm stickin' to it.

[Mollysdad]

Originally Posted by majorairhead

I've read extensively on the tire pressure issue, right here on the airforums.

I know there is no shortage of folks on this site that will disagree with that, but that's my story and I'm stickin' to it.

Amen, brother!

[Action]

Originally Posted by majorairhead

I'm not certain the tire pressure has as much to do with the ride of the trailer as the hitch setup does. I know there is no shortage of folks on this site that will disagree with that, but that's my story and I'm stickin' to it.

My guess is there is some truth to both. Tires and hitches.
And different trailers behave differently.
A 16' trailer and a 30' trailer are just not the same beast.


>>>>>>Action

[GOUSC]

Originally Posted by Action

My guess is there is some truth to both. Tires and hitches.
And different trailers behave differently.
A 16' trailer and a 30' trailer are just not the same beast.


>>>>>>Action

I can’t disagree with the statement re: hitch set up as I have, and continue, to make adjustments in that area. However; I’m still trying to make adjustments in the tire pressure area as well as 80 psi doesn’t work for my set up. If the hitch set up makes my tire pressure go up from 65 to 79 then any suggestions in that area will be appreciated. Otherwise I’d like to keep this post related to the questions I previously posted.

At this time I feel like I’m following the advice early on in the post re keeping pressure above what Andy recommends and above what the tire mfg recommends which makes sense to me. Others have since stated some other opinions and I’m trying to get addt’l info to get a yes or no answer as to why 65 psi may or may not be safe if the pressure goes up > 20% from its initial cold reading.

[majorairhead]

GOUSC,

I'm no tire expert. But I have been given the gift of common sense.

When looking at the Goodyear Endurance tire charts, and given your trailer weight of 6,380 pounds, you can run your tires at 30 psi.

I guarantee you I'm not following the tire chart on that one

https://www.goodyearrvtires.com/tire...n-loading.aspx says this:

IMPORTANT: It's a common practice for RV owners to lower tire pressure in their search for a smoother ride. This is not only dangerous, it's relatively ineffective, as the difference in ride quality is not significant. When minimum inflation pressure requirements are not met, tire durability and optimum operating conditions are compromised. Tire inflation pressure should always meet at least the minimum guidelines for vehicle weight.

Anyway, after all I've read regarding tire pressure.....which is a lot.....I've concluded, for me, it is better to be closer to that 80 psi max than to air on the low inflation side of the equation.

[CapriRacer]

Originally Posted by GOUSC

To hopefully clarify, I set the 65 and 80 psi 1st thing in the morning, tire cold. On this trip the 65 psi was at around 66-70 degree temp...…

If I use 65°F as the starting temperature , then the pressure rising from 80 psi to 95 psi at 95°F makes sense. I get operating temps about 108°F - which is about the 117°F reported.

However, it doesn't explain why the use of 65 psi resulted in a LOWER operating temperature (105°F) - it should be higher than the 80 psi.

So while this additional information helps clarify things, it still doesn't make complete sense.

Originally Posted by GOUSC

….. I will admit I’m not an expert. If I set the tires at max pressure cold, 80 psi, and the tires rise above the 20%, I would think I would be getting close to exceeding the tires capacity. ……

No, the burst limit for the tire is likely near 200 psi, so you are nowhere near the tire's pressure capacity.

But the important parameter is temperature. We use the pressure buildup as an indicator for the temperature build up. We do this because pressure is sooooo much easier to measure - particularly in the past where we only had a pressure gauge to work with. (and, yes, it does matter WHERE you measure the temperature.)

Originally Posted by GOUSC

…….. In cooler weather the tires may rise 6-8 psi max.

That's much more reasonable.

So a quick review for those following this thread:

When reporting pressure build up in tires, it is important to report both the cold setting ambient temperature, and the ambient temperature at the built up pressure.

Rule of thumb: Every 10°F change in ambient temperature results in a 3% change in inflation pressure. That needs to be backed out to determine if the pressure build up is signaling a tire problem.

Another rule of thumb: Pressure build ups shouldn't exceed 10%. Above this point, the higher operating temperature more rapidly degrades the rubber, and results in tire failure.

At 15% buildup you need to take immediate action to reduce the operating temperature. Slowing down is the first step, but you ultimately need to get more load carrying capacity by either using a higher starting pressure or a larger load capacity tire.

---- and please note: You have to back out the pressure caused by the increase in ambient temperature BEFORE you apply the 10%/15% buildup rule.

[Mollysdad]

BTW, The Good Year tire chart for the Endurance is the same as the chart for the Marathons.

[GOUSC]

Thanks CapriRacer!

That is very informative and something I can quantify (I think). Let me see if I can.

For the 2nd leg of my trip I upped my cold psi to 73 per my TPMS. Ambient temperature in the morning was 74 degrees. The higher psi reading for any tire was 84 psi at a tire temp reading of 104. This was an increase of 11 psi.

The 30 degree temperature increase should reduce my psi buildup by 9% (3% for each 10 degrees. Based on this, my psi buildup is only 2 psi, 11 - 9. So a psi buildup from 73 to 75. The 10% increase from the original 73 psi would be around 80 so all would appear to be safe for yesterday’s trip if I am understanding it correctly?

[waninae39]

PV=NRT is Boyl'e Gas law equation

P is the pressure of the gas,
V is the volume of the gas,
n is the amount of substance of gas
R is the Boltzmann/constant,
T is the absolute temperature of the gas.

In simple terms
if the temp increases ^ so will the pressure or volume ^ in a proportional way
assume the tire volume to be constant, them Temp ^ means Pressure ^
also as Pressure ^ so does temp ^

If temp V as pressure ^, then you you may have a tire issue as the volume is the only item that can change
Note, temp is in kelvin from absolute ZERO. You need to rebase your temp to that scale to determine the proportional increase for each side of the equation
i.e. if the temp went up 10% (k) then the pressure would go up 10%
you CANT use F or C , only K.
C ->K is easy, i.e. K = C + 273.15
F-> K needs a formula ie K = (y °F + 459.67) x 5/9

[SteveSueMac]

Originally Posted by GOUSC

Thanks CapriRacer!



That is very informative and something I can quantify (I think). Let me see if I can.



For the 2nd leg of my trip I upped my cold psi to 73 per my TPMS. Ambient temperature in the morning was 74 degrees. The higher psi reading for any tire was 84 psi at a tire temp reading of 104. This was an increase of 11 psi.



The 30 degree temperature increase should reduce my psi buildup by 9% (3% for each 10 degrees. Based on this, my psi buildup is only 2 psi, 11 - 9. So a psi buildup from 73 to 75. The 10% increase from the original 73 psi would be around 80 so all would appear to be safe for yesterday’s trip if I am understanding it correctly?

I’m not Capri or a tire engineer - but I’m also not following your math....

Ambient temps when you started were 74*. I’m assuming it was roughly 74* out when you got that temp measure from your TPMS and the unit hadn’t been moving for several hours.

You inflated to 73 PSI. So your “cold” (un-driven) starting point is 73PSI at 74*.

After some period of driving your PSI rises to 84 and tire temps reach 104*. Thats a 15% rise in PSI (11/73) and a 30* rise in temps.

What I believe Capri is saying about the 10* and 3% issue is related to AMBIENT temps - meaning - if the AMBIENT temp went from 74* to 84* then you should expect a 3% rise in PSI (or a 2.2 increase over your starting point of 73). So in theory, if ambient temps rose like that, you’d back out 2.2 PSI from the rise of 11 and would say you had an 8.8 rise in PSI which would be a 12% increase (8.8/73).

You don’t include the ambient temp change in your note so we can’t know the impact of that. You seem to be trying to calculate the increased temps inside the tires to back off the PSI rise but that’s not the source you’re looking for.

[GOUSC]

Originally Posted by SteveSueMac

I’m not Capri or a tire engineer - but I’m also not following your math....

Ambient temps when you started were 74*. I’m assuming it was roughly 74* out when you got that temp measure from your TPMS and the unit hadn’t been moving for several hours.

You inflated to 73 PSI. So your “cold” (un-driven) starting point is 73PSI at 74*.

After some period of driving your PSI rises to 84 and tire temps reach 104*. Thats a 15% rise in PSI (11/73) and a 30* rise in temps.

What I believe Capri is saying about the 10* and 3% issue is related to AMBIENT temps - meaning - if the AMBIENT temp went from 74* to 84* then you should expect a 3% rise in PSI (or a 2.2 increase over your starting point of 73). So in theory, if ambient temps rose like that, you’d back out 2.2 PSI from the rise of 11 and would say you had an 8.8 rise in PSI which would be a 12% increase (8.8/73).

You don’t include the ambient temp change in your note so we can’t know the impact of that. You seem to be trying to calculate the increased temps inside the tires to back off the PSI rise but that’s not the source you’re looking for.

My facts that you restated are correct. It looks like both of us are looking to CapriRacer to clarify our differing interpretations.