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Old 09-07-2019, 09:59 AM   #43
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Originally Posted by GOUSC View Post
Looks like I screwed up again and missed the AMBIENT in large caps. My bad. Ambient temp that day was 97 degrees with the highest psi of 84. Using SteveSueMacís #ís to keep it simple;



The increase in Ambient temp was 23 degrees so a 4.4 psi (2.2 at a 10 degree increase = 4.4 at a 20 degree increase in ambient temp) needs to be backed out due to heat.



84 high psi

73 original psi

óóó

11 degree increase

-4.4 increase due to heat

óóó-

6.6 psi increase



6.6 / 73 = 9% increase in tire pressure so tires appear to be in a safe zone.


Just want to be clear - youíre saying that 84 PSI was at 97* ambient temperature on the day you started at 73 PSI at 74* ambient temperature, correct?

If so, then as I understand Capri, yes, 4 of those 11 increased PSI are due to the rise of 20+* in ambient temps. You would back those 4 out of the 11 and say 7 PSI increase is less than 10% of your starting PSI of 73.

My understanding of Capri could be flawed but thatís how I measure mine based on his input over the years in the forums.
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Old 09-07-2019, 12:33 PM   #44
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Originally Posted by SteveSueMac View Post
Just want to be clear - youíre saying that 84 PSI was at 97* ambient temperature on the day you started at 73 PSI at 74* ambient temperature, correct?

If so, then as I understand Capri, yes, 4 of those 11 increased PSI are due to the rise of 20+* in ambient temps. You would back those 4 out of the 11 and say 7 PSI increase is less than 10% of your starting PSI of 73.

My understanding of Capri could be flawed but thatís how I measure mine based on his input over the years in the forums.
Yep- those facts are correct and I appreciate your input in helping me get a handle on this. We are on an extended trip thru Texas when Temps can be hot and I wanted to get a handle on how the heat works in conjunction with tire pressure. I saw 3 blowouts on cars not towing in the heat 2 days ago so I would like to do all I can to minimize that situation.

I also came to realize that trying to read and respond to all of this via Iphone on the road is not the most efficient way for me to do this. But, I have to ask enough questions (stupid or not) until I have satisfied myself that I understand it.

Thanks for being patient with me and, barring some unexpected response from CapriRacer, I would think this closes this issue for me.

Safe Travels,

Jim
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Old 09-11-2019, 11:30 AM   #45
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The original post states 95 degrees ambient temperature. Was the ambient temperature the same 95 degrees when the cold pressure was set and when he towed? Maybe the 95 degrees was the temperature when he was towing, but set the cold inflation psi at a different temp.

For instance, the last time I added air to my tires was done at 68 degrees ambient temperature, 70 psi. By the middle of the day, while towing, it was 93 degrees ambient temperature and 81 psi. More than the 10%, but I assume the 23 degree difference in ambient temp was causing it?

A 23F increase in Ambient would result is about a 4% increase in tire pressure before you started to move.


Running tire temperature can see a rise of 30F to 60F depending on speed & load. This would yield an additional 6% to 12% increase in pressure.


None of the above includes long term exposure to direct sunlight. I know I have seen +40F just due to sun exposure but that was static. Running the Temp increase I have observed in a cross country slog gave me + 15F or so on the sunny side vs shade side.


All of this shows that chasing the hot pressure is way more complex than simply looking at Ambient. The other input is that TPMS temperature readings are really a measure of the metal wheel temperature and not the hottest part of the tire.


IMO If you have 15% Reserve load. Use the individual tire load to check the tables to learn your minimum "cold" inflation, you should not worry about your hot pressure. Unless of course, you are running in NASCAR or at Indy.


Tires do not "blowout" from the increase in hot running pressure. Period.
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Old 09-11-2019, 11:31 AM   #46
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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.

I published the internal vs external TPMS temperature difference in my blog. It is about 20F to 30F different.
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Old 09-11-2019, 11:35 AM   #47
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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.

Yes temp spike is from brake heat into the wheel and measured by the TPMS.
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Old 09-11-2019, 12:30 PM   #48
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The ONLY way of truly knowing EXACTLY what PSI tires should be inflated to is to get your rig a Smart Weigh. CAT Scales are, at best, an APPROXIMATE weight; it will NOT tell you how much weigh each INDIVIDUAL tire is carrying. Smart Weigh is a modestly priced service which is often offered at major RV Rallies. Bothe Escapees and RVSEF can do a professional Smart Weigh. Once done, if you don't make major changes / weight shifts, you're good to go. After being Smart Weighed, a trained professional will go over your numbers and be able to determine EXACTLY what PSIs are optimal. Just do it; you can thank me later.
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Old 09-11-2019, 12:32 PM   #49
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Originally Posted by GOUSC View Post
2018 Globtrotter
Goodyear Endurance 80 psi max cold pressure
Outside Temp 95+ degrees

With 80 psi cold inflation and the above temps my TPMS system has shown psi to go to 94-96 and tire temps to 115-119. At this psi I get thrown pillows and some popped rivers in my overhead cabinets.

With 65 psi cold inflation I have 14% margin for my load as weighed on the CAT scales. PSI goes to 79 and temps to 104-107. No thrown pillows or popped rivers and a smoother ride.

I figure if the 65 psi cold pressure doesnít exceed my 80 psi readings Iím OK. Is this good logic or flawed?
I myself am quite fond of the KISS method. Keep it simple stupid. (This is not a disparaging remark toward anyone).

When a child opens a lemonade stand in 80 degree heat they know full well they will need ice. When a child opens a lemonade stand in 90 degree heat they know full well they will need MORE ice.

I want to believe these high powered corporations with their highly paid engineers exercised as much common sense and knowledge as the child opening their stand in 90 degree heat. Hopefully, knowing many people live and travel in hotter, drier climates 90+ PSI will get the job done.

IMHO
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Old 09-11-2019, 12:33 PM   #50
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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?
Calculated with this 6380lbs on 4 tires, tires 2830 AT 80 psi , with my pigheaded system , a needed pressure of 58 psi.
The old tires 2540 AT 65 psi gave 53 psi.

Calculated if filled at 68 degrC pressure of 65 psi, and rising to 79 psi, temp then in tire is 161 deg F.
80 psi filled at that temp rising to 96 , temp 157 degr F.

So this makes me conclude you have external sensors, because you read much lower temp.
Temp measurement is probably accurate, but sensor measures someting in between in and outside tire.

So leaves the pressures, and I think the conditions where not the same between the 2 values you gave.
For instance higher cold temp for the 65 psi situation then the 80 psi situation.

Also braking a lot can highen up the inside tire temp, so also pressure, this then in the 80 psi sit.

I discovered on the Airstream forum, that tandemaxle trailers, have comfortable reserves. First thought it was an airstream thing, but single axle AS i last calculated higher pressure needed then the AT 80 psi( wich is not allowed anymore).

So my advice is 58 psi at 65 degr F , and at 95 psi you should measure higher.

Now 95 degr ambiŽnt temp is somewhat extream, so mayby then 65 psi at 65 degr , calculated that for you that at 98 degr F then you should measure 70 psi cold pressure.....cold? Yes cold is when in and outside tire temp is the same.
My advice is with max reserve and livetime of tires, with still no rivets loose .
The 58 psi advice at 65 degr F, gives 62.5 psi at 98 degr F.
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Old 09-11-2019, 09:42 PM   #51
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Originally Posted by CapriRacer View Post
There are a few things that bother me.

1) A pressure buildup from 80 psi to 95 psi is almost 20%. The rule of thumb says not to exceed 10% and anything over 15% is dangerous.
Just curious, where does this rule of thumb come from?
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Old 09-12-2019, 04:19 AM   #52
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Originally Posted by CapriRacer View Post



Rule of thumb: Every 10įF 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.
Sounds to me better tires are needed.
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Old 09-12-2019, 06:30 AM   #53
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Just curious, where does this rule of thumb come from?
I don't know. I first became aware of it when I became a tire engineer some 40 years ago. I've since verified that the Ideal Gas Law says that this 10% keeps the operating temperatures within a reasonable range.
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Old 09-12-2019, 06:33 AM   #54
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Sounds to me better tires are needed.
I think this is a Physics kind of thing - that this is just the way things work and better tires would not result in significantly lower operating temperatures, but the tire would be better equipped to handle those temperatures that are being generated.

So I think the 10% rule is still valid regardless of the brand of tire, and is a good tool to determine if the tires are overloaded.
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Old 09-15-2019, 01:16 PM   #55
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I'm running 62 psi on my Goodyear Endurance 205-75 R/15s (max inflation rated at 65 psi). The single axle 68 Caravel weighs 3000+- loaded for camping. The Endurance load capacity/pressure tables say that the correct pressure per tire for each tire to support it's share of 3000 pounds should be 35-40 psi which seems rediculously low. I set them to 62psi at 68F at sea level. America's Tire wanted to set them at 65psi but I felt that it might induce too rough a ride for the little 17 foot trailer.

For it's TPMS, TST says to set the upper pressure limit alarm at 20% above ambient temperature inflation pressure, which in my case the limit was set to 75 psi, and and to set the temp max at 158F (their standard recommendation). I'm currently at 6800 at 60F and the tire pressures are at 62F.

Driving across the Mojave Desert from California the tire temps were in the low to mid teens when ambient was 102-107. Pressures in those conditions reached 76psi.

I'm seeing suggestions here that my limits may be out of whack and that I could be damaging my tires.

Any input would be greatly appreciated. We'll be heading back to California from Santa Fe once again via the hot Mojave Desert.

Thanks
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Old 09-16-2019, 03:25 AM   #56
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Used in my made calculator next.
Tires 2150 lbs AT 65 PSI, 2 tires of TT on the road while driving, 3100lbs total weight.
It gave 55 psi advice , with max reserve and livetime, without bumping .
This is for cold measured at 65 degr F .
At altitude this pressure also needed for 65 degr F, but if you fill it at sealevel, the pressure then will rise, because ambiŽnt pressure drops a little.

If you then measure " cold" at ambiŽnt temp of 102 degrees in the dessert, the 55 psi at 65 , will rise to 60 psi. The 62psi then to 67.5 psi.
Hot , when driving inside tire temp of 154 degr F
55》 67 psi, 62》 75psi.

This 154 I estimated to get inside tire air when heat only created by the bending of rubber of tire, when driving about 55mph, so no sunshine on tire, or severe braking , for instance when descending .

Do wit it what you want, but realise that external sensors read temp something in between in and outside tire, so unreliable. If then reads158 degrF there is certainly something wrong.
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