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Tire Air-Presure Preparation: timing

stevenicol
Explorer
Explorer
Silly question:
We are getting ready for a short journey tomorrow afternoon; is it okay to check and fill the air in the trailer tires today; or, should I wait until tomorrow? Does it really matter?
21 REPLIES 21

CapriRacer
Explorer II
Explorer II
JimK-NY wrote:
..... BTW, did you encounter any data about the age of tires? .....


Yes, but nothing useful for the consumer.

But if this project was about trying to determine when tires should be removed from service due to age, there are a number of problems:

Ambient temperature plays a HUGE!! role in this. Tires in Phoenix behave differently than tires in Minneapolis. Those of us in the tire industry have the data that says that most tire failures occurs in (and in pretty much this order) AZ, CA, TX, NV, and FL.

FL seems the odd man out, but if you compare the temperature profile of FL over the span of a year, it is fairly flat but elevated. The net heat input to the tire is pretty high compared to - say - MN.
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CapriRacer

Visit my web site: www.BarrysTireTech.com

JimK-NY
Explorer II
Explorer II
CapriRacer, thanks. That is interesting information. BTW, did you encounter any data about the age of tires? I have seen my RV tires, dry rot pretty quickly. I suppose part of that is intermittent use that is not good for tires. I had a badly cracked and dry rotted spare that had been under my RV for 4 years. I just had to replace 2 more tires that were 5 years old and only had about half of the tread wear. For RVs I suspect a lot of tire issues are due to age instead of load or wear.

CapriRacer
Explorer II
Explorer II
Allow me to tell what I know about the 15% reserve capacity.

Remember the Ford/Firestone controversy some years back? Shortly after the recall was announced, the Feds turned their attention to the rest of the industry and asked some very probing questions. The company I worked for had a large database of tire returns. I was one of the guys who did the data analysis - and among the things we discovered was that the greater the reserve capacity, the fewer the tire failures - and 15% seemed to be the point where no failures were reported.

We adopted the stance that, in the OE market, we would not supply tires to any vehicle that didn't have a 15% reserve capacity. When we announced that to Ford, GM, Toyota, etc., their response was that other tire manufacturers had found similar things, and that they were taking that under consideration - which they (the vehicle manufacturers) by and large did. Today you will find that the GAWRS for their pickups and SUV's are at about 85% of the load carrying capacity of the tires at the pressure specified. This started about 2008.

That's where I think that 15% came from. I also think trailer manufacturers - who weren't subjected to the intense strutiny in that time frame - didn't pick up on the message. Certainly, there is a disconnect between the tire manufacturers and the trailer manufacturers, unlike car and truck manfacturers.
********************************************************************

CapriRacer

Visit my web site: www.BarrysTireTech.com

myredracer
Explorer II
Explorer II
JimK-NY wrote:
myredracer wrote:
......This is important and you want at least 15 percent capacity and more is better. Running on psi lower than the max. sidewall rating will reduce the reserve capacity and can result in premature tire failure.

......

According to my Firestone dealer, LT tires are load rated so that they are considered safe at 100 percent of the load rating for the life of the tire.

Where did you come up with the 15 percent number? I did not see it on the webpage you linked.


See the index on the left side of his blog and read through his posts on reserve load capacity. I'll trust what Roger Marble says over a tire dealer any day.

JimK-NY
Explorer II
Explorer II
myredracer wrote:
......This is important and you want at least 15 percent capacity and more is better. Running on psi lower than the max. sidewall rating will reduce the reserve capacity and can result in premature tire failure.

......

According to my Firestone dealer, LT tires are load rated so that they are considered safe at 100 percent of the load rating for the life of the tire.

Where did you come up with the 15 percent number? I did not see it on the webpage you linked.

myredracer
Explorer II
Explorer II
Set the cold inflation pressure to the max. sidewall psi after all tires have been in the shade and not in use for at least 2 hours (3 is better). "Cold" is the ambient temp. in the immediate area of the tires. Day of trip or day before, won't really matter. Sometimes it's not possible to have your camper in the shade later in the day so either the evening before or early morning of the trip is fine to set the CIP.

Remember, always follow the 3 golden rules for ST type tires (if that's what you have): never tow (1) under-inflated, (2) overloaded & (3) over the max speed rating of the tire (usually 65 mph for ST). Sadly, neither the dealer or RV manufacturer tell you this and there really ought to be a sticker on the exterior right above the wheels with the pertinent info.

If you haven't yet, it's good practice to go to a scale and find what the weight on the axles/tires is, and side to side can also be a good idea for a trailer with a heavy slide on one side. The weight will give you the amount of "reserve load capacity" you have in the tires. This is important and you want at least 15 percent capacity and more is better. Running on psi lower than the max. sidewall rating will reduce the reserve capacity and can result in premature tire failure.

Anything and everything you ever want to know about RV tires can be found on RVtiresafety.net by a retired tire engineer. Lotsa great bedtime reading for camping. ๐Ÿ™‚

jadatis
Explorer
Explorer
Stated already that the rubber of tire must stay below a sertain temperature, but what is that temperature?

I dont know either, but the man from Vredestein told me that the tires are "backed" at 170 degr C /338 degr F?. This sets the sulfurbridges at that rate that the rubber is flexible but not hard.
My suspicion is that this hardening already begins at lower temp.
So as long as I did not hear it for shure , I will keep it at 130 gr C /266 degr F?.

Then if we calculate with this, and say that higher temp differences give more heatransfer, its lineair.
So example : outside ( O) temp 65degr F, Inside tire temp ( I) 115 degrF, hottest part of rubber ( R) 265 degr F.
Then temp difference R/O 200 degr F and R/I 150 degr F.

Now new situation O= 90 degr F, I = 140 degr F, R must stay below 265 degr F.
Then temp differences , R/O=175 degr F , R/I= 125 degr F.

Then differences between situations.
R/O 175/200=85% of heat-energy possible to transport in new situation then in old.
R/I 125/150=83% transport possible per second in new then old.

Now I state that for instance half of deflection gives half of heatproduction a cycle, so also linear.
So roughly the tire must produce 84% of heatenergy in new situation the old , and so must have only 84% of the deflection in new then old situation. Then heatproduction is in balance with Cooling down of rubber, so rubber reaches in new situation also 265 degr F.

Now its gets even more complicated, howmuch lesser deflection does the tire get in new situation versus old. Lesser deflection gives lesser heatprocuction, and when constant temps in and out tire and rubber, the heating up and cooling down is in balance.
We could calculate the loadcapacity for the new pressure , wich would give again a ratio between old and new surface on the ground, and you could determine the new deflection for that.

Will spare you the calculation, because I have to find it out myself first.

But this all made me suspect that within a sertain range of outside temps,, and same speed, the heating up and cooling down of rubber is in balance so hottest part of rubber stays below 265 degr F.

You will understand that if outside temp is 265 degr F , the rubber can never cool down that much that it stay below 256 degr F, because temp differences are zero then .
Luckyly we dont drive at these extreme outside temps.
But mayby and outside temp of 110 to 120 still keeps the heating up and cooling down in balance ( because of the lesser deflection by the higher tirepressure) so rubbber stays below 265 degr.
And again if I determined this 265 degr F right to be the temp at wich hardening begins.

jadatis
Explorer
Explorer
When 65 outside,a inside tiretemp would be expected of 110/115degr, when driving about 50/55 m/h.
Is 45/50 d temp-rising.
80/90>140=50/60 rising. So mayby a higher cold pressure would be advisable.

JimK-NY
Explorer II
Explorer II
Thanks,
I don't know about reaching 212 degrees, but I do know on a highway run during 80 or 90 degree weather, the psi on my tires will increase corresponding to a temp of a little over 140 degrees.

jadatis
Explorer
Explorer
Once had contact with someone o a valve importer in Holland ( where I live) And He gave documents about maximum pressures of valves given , where for cold of 18 degr C/65 degr F, and standards for higher pressure for warm above that max cold pressure the valves must stand . He also wrote me that the tire inside air can rise to boiling point of water even ( 100 degr C/212 degr F) , for instance by descending from mountains annd using the brakes to often.
Then the heat of brakes is transported by the rimms to the tire inside.

Once had me explained in a long telephone conversation with a man from Vredestein ( now part of Apollo) about tire pressure advice is for giving the rubber not higher temp then the point at wich to many sulfur bridges form. The rubber needs a sertain amount of sulfur bridges to make it flexible, so your nailprint wont keep standing in the rubber, but to many make it hard ireversible, and every next bending of it by the deflections , then tears the damaged structure a bit further until at a sertain moment blowing tire, or treath coming loose .
Mind that every segment of the tire deflects and flexes back about 10 times a second when driving about 50m/h. This produces heat, wich has to be cooled down, or better transported to the outside.

Dont know anymore if he also mentioned the 65 degr F , but combined it myself.

Some write its 20 degr C /68 degr F . but this 3 degr F would not give such a difference , so who cares who's exactly right.

And because for valves also 65 degr is used, I assume for tires too.

So no hard evidence but circumstantial.

JimK-NY
Explorer II
Explorer II
Thanks for the data. I have looked everywhere and could not find the recommendation for using 65 degrees as a baseline. Where did you find this recommendation?

jadatis
Explorer
Explorer
Might be for 2 reasons why you want to know this.

First already given for the probable other outside temperature tomorrow then tonight.
Then you must know that advice pressure is given for 65 degr F, and between 55 and 75 the difference is nihil.
But if higher temp , for instance 100 degr F , the pressure rises cold measured that much that you cant deny it.
But at that temp , the tire needs this higher pressure to give lesser heatproduction ( because of lesser deflection) , the cooling down is also less then because of the smaller temperature difference between hottest part of rubber of tire, and outside and inside tire temp.

Second reason , I think you mean, is that loaded the tire gets higher presssure. And that is not true.
Its even so , that if you fill the tire off the ground, so with zero weight on it, to for instance 80 psi, and you get it ont the ground fully loaded, you still measure 80 psi. I once checked it with my own car , by measuring several times yacked up and on the ground, and its true.

So fill the tire to best determined advice pressure for the weight on it and speed used, calculated back to 65 degr F.
And when you check, also check the outside temperature at that check.
If between 55 and 75 degr F outside, dont bother to recalculate, but for higher then 75 degr F recalculate ( for wich I give here a list). When below 55 degr F , you can fill to advice pressure, for giving same deflection of tires, and so better fuel saving, but for savety not needed, yust look in the list I give, to see if the pressure dropp is in line with the temperature dropp.

Cold pressure is when inside tire temp= outside tire temp, that simple.

When driven the bending of rubber heats up the tire inside, so you dont measure the cold pressure. Also when sun is shining on the black tire, it heats up the tire inside, so tire inside is hotter then tire outside, and you again dont measure the cold presssure.

Here the list and use it like this.
determine best advice pressures for front and back and eventual Tag axle. Then look this up in my list at the line 65 degr F , is fatt printed. Then when adjusting also note the outside tire temp so ambiรซnt temp. and look in the row what pressure it has to be for that temp, and fill or adjust to that pressure.

Mostly you will find that if you check and note the ambiรซnt temp, that the pressure is in line with the temperature. so you dont have to adjust, so do nothing.

You can also take over , only your advice pressure for intervalls of for instance 20 degr from 65 degr F. and store that somewhere.

RCMAN46
Explorer
Explorer
If you have a TPMS I recommend a couple days before the trip.

Then check the pressure with the TPMS at the same time of day a couple times before the trip.

On a trailer all tires should read the same if there is no direct sun exposure.

This will find any slow leaks such as a nail that was not previously known.

JimK-NY
Explorer II
Explorer II
We know air pressure changes with temperature. The problem remains how to set the pressure at one temperature when the tire is expected to work at different temperatures. Constantly changing pressures as the temperatures change does not seem feasible. One day I might be up in the mountains where the morning temp is 40 degrees. I might drive down to a warmer location where the afternoon temp is in the 90s and the morning temp was 80.

I have decided to set my pressures for 70 degrees and leave them there. That means that at 80 degrees the pressure will start a degree or two over the maximum psi. On those cold mountain mornings at 40 degrees, the tire pressure will be about 4 degrees lower than the maximum rating.