The "load rating" of tires is based on their "inflation pressure" when "cold" and this applies in spades when the ambient temperature drops below 0*F so don't forget to check your tire pressure this winter and add enough air to achieve the correct "cold inflation pressure" given in this link... http://hmcclub.homestead.com/Goodyear_Tire_Inflation___Load_Charts.pdf ...especially if you're hauling a load!

As stated in the reference link for Light Truck Tires... The inflations shown in the load tables are "minimum cold pressures" for the various loads listed. Higher pressures should be used as follows: ...A) When required by the speed/load Table 1 on Page L-2 ...B) When higher pressures are desirable to obtain improved operating performance. ...The combined increases of A and B should not exceed 10 PSI above the inflation specified for the maximum load of the tire.

Even if you're not hauling a load the stock 235/85R16 front tires on an "empty dually" are still carrying 75% of their "maximum" rated load and this requires a "minimum" cold inflation pressure of 55 psig which should be increased to 65 psig for sustained highway speeds.

If the last time you checked your tire pressure it was P1 psig and the temperature was T1*F and the temperature is now T2*F your current tire pressure is... P2={(T2+460)/(T1+460)}(P1) psig.

For example if the last time you hauled a load was in the fall when T1=60*F and you filled your tires to P1=80 psig the pressure P2 when T2=-30*F is now only... P2={(T2+460)/(T1+460)}(P1)={(-30+460)/(60+460)}(80)={(430)/(520)}(80)={0.827}(80)=66 psig and the load carrying capacity of a 235/85R16 tire has been reduced from 3,042 lbf per tire at 80 psig to only 2,623 lbf per tire at 66 psig.

 

Great info and something that can easily slip the mind. Thanks!

 

Thanks for the reminder Gene, but if it drops to 0 deg F here, my truck won't be going anywhere.

And as long as we're talking tire pressure, when was the last time anyone checked the pressure in their spare? Supposed to hit 70 this weekend, so I think I'll check mine before it gets too cold again.

 

As a former tire tech, I can tell you exactly when I check my tire pressure on my spare, right after i use it and it goes back under the truck haha.

Excellent link from goodyear.

 

My approach to maintaining spare tires is to fill them to an inflation pressure that's 10 psi higher than the "maximum load cold inflation pressure" stamped on the side of the tire's casing because if you need to use the spare it's easier to bleed some air from it than it is to add some.

Even when the tire is in use it's perfectly acceptable and in fact sometimes recommended to air LT tires to a cold inflation pressure that's up to 10 psi higher than the "maximum load cold inflation pressure" stamped on the side of the tire's casing and this "fact" has been the source of much confusion on some of the RV forums I participate in.

For example a 235/85R16 LRE tire requires a "minimum" cold inflation pressure of 80 psig to carry its "maximum" rated load of 3,042 lbf per tire but If you want this tire to "safely" carry its "maxim load" you need to inflate it to "more" than the 80 psi "maximum load cold inflation pressure" stamped on the side of the tire's casing to allow for deceases in cold inflation pressure due to deceases in "ambient temperature" or increases in "ambient atmospheric pressure"!

One reason so many RVers experience tire failures is that they operate their tires at or even above the tire's maximum load rating yet they only fill them to the "maximum load cold inflation pressure" stamped on the side of the tire's casing and then after the ambient temperature drops a few degrees their tires are "under inflated" for the load they're carrying!

 

Thats what I do with my spare too, air it up a little more then need be.

Its tricky to figure out the ambient temperature sometimes. Have to take the temperature from the air, then you should always consider the heat caused by the tires carrying the load, plus you have to try and anticipate what the weather is going to do later in the day when you are actually driving!

Tires should also be bought so your not at max weight capacity of the tire at all times anyways. That way the differences in a few psi wont be a safety issue

 

No, no, and "heck" no this is a recipe for instant tire failure! Never and I mean never adjust your tire pressure unless the "tire temperature" is "equal" to the "ambient temperature"! The load rating is based on the "cold inflation pressure" and this means the tire has to reach an equilibrium with the ambient temperature.

The "best" time to adjust your "tire pressure" is at "sunrise" after the truck has set overnight and "before" the sun comes up and has a chance to heat one side of the truck which can cause those tires to be inflated to a 10 psi lower "cold inflation pressure" than the ones on the shaded side of the truck!

Yes it's quite true that the flexing of the tire produces heat and at every rest stop I shoot the tread temperature on all 12 of my tires with my IR gun and the tire temperature after several hours on the road is typically 100*F or more than it was before starting out!

If the initial ambient temperature was 40*F when I adjusted the "cold inflation pressure" to 80 psig then after several hours on the road the tire temperature is 140*F and the "hot inflation pressure" is... P2={(T2+460)/(T1+460)}(P1)={(140+460)/(40+460)}(80)={(600)/(500)}(80)={1.2}(80)=96 psig ...and this is now the correct "minimum hot inflation pressure" for carrying the tire's maximum load.

If I adjusted the tire pressure at a rest stop my tires would be "under inflated" by 16 psi and I'd never make it to the next rest stop!

 

I guess maybe I didnt elaborate.

When your checking tire pressure, you dont always go to the max of what the cold inflation pressure is, unless your planning at your truck being at max load for the day.

Say you want your tires at 50 psi for when you drive. You can estimate what the temperature would do later in the day, and adjust accordingly. Yes for sure the best time in the day is to check tires first thing in the morning. You can figure the psi will change with the weather by about 1 psi ever 10 degrees or so. (good rule of thumb)

That is the farthest thing from a recipe for tire failure, and if a person wants to keep ontop of their tires and get maximum performance (duration, wear, handling, etc.)the last thing you will do is pump your tires up to max pressure each time you head out on a trip. Best thing to do before each trip is adjust accordingly for the trip, if your empty, or loaded, or half loaded, etc.

Im assuming you are just talking about your specific case, and you being at max load of your tires all or most of the time, so for you, yes that advice would not be relevant.

Not everyone drives at max capacity of their tires though, and its false information to those people to tell them to adjust their tires so high, if that is not needed.

 

I guess you edited your post while I was responding to it but my heck no applies to the above as well!

Well if you look at the specifications for most motorhomes, 5th wheels, and pickups you'll see this is seldom possible but for the first time in over 10 years I've finally got tires on both my 5er and my truck that are "overrated" for the load they carry!

 

As I previously explained and as is stated in the link I gave if you plan to operate at maximum load you need to inflate the tires to at least 5 psi higher than the "maximum load cold inflation pressure" stamped on the side of the tire's casing and as stated in the link up to 10 psi higher is recommended if the wheels are appropriately rated!

No one should follow this dangerous advice because it could kill you and your loved ones!!!

 

nitrogen is a great option if its available in your area. nitrogen dosent compress in the cold so if u have a tire full of just nitrogen their should be alot less hassel in the winter with pressure loss.

 

This cant kill anyone now your being silly. We are talking about a measurement of 2 psi, perhaps, in each tire that adjusts with the ambient temperatures. And thats only if your wanting to keep your tires at a certain accurate temperature while driving.


You have me curious now, how did you measure the temperature on the inside of your tires when you did your calculations?

 

First of all plain old "air" is already 80% "Nitrogen" to begin with and pure 100% "Nitrogen" obeys the same "gas law equation" as plain old "air" does so that pure 100% "Nitrogen" will "compress" exactly the same as "air" does and a tire filled with pure 100% "Nitrogen" will experience exactly the same "reduction" in winter time "inflation pressure" as a tire filled with plain old air does!

As I explained in several technical posts on this thread... https://www.ford-trucks.com/forums/6...-pressure.html ...pure 100% "Nitrogen" is a good choice for aircraft tires to reduce the risk of fire if a tire blows and ruptures a hydraulic line... however when it comes to truck and RV tires "Nitrogen" is basically an overpriced "scam" because "dry" air works as well as "dry" Nitrogen but filling your tires with a lot of water vapor is detrimental which is why your air compressor should include a "dryer"!

 

Tire failures have killed many and the leading cause of these failures is "under inflation"!

Where did you read that I measured the temperature inside my tires?

This is the advice that everyone should follow! Don't even "check" your tire pressure when the tires are hot because doing so will let enough air escape to reduce their pressure below the correct value that was set when the tires were cold!

 

Interesting idea. Not to hijack your thread, but on the subject of tire pressures, do you (or anybody else reading this thread) have any experience with the in cab pressure monitoring systems?