Tire Pressures-What do they really mean?

[Tailwheel]

l had two blow outs last year with my Bridgstone Dueler A/T on my F350. One was very nearly catastrophic and one was not. The nasty one lifted the back of the truck at 65mph and we almost left the highway. Bridgestone was no help at all. I got $60 off on a $148 tire. After number 2 I dropped Bridgestone and now use Michelin. I have been trying to get an answer to the following question.

Is the maximum tire pressure on the tire the maximum runnning pressure under any circumstances or just the maximum cold tire pressure?
Given that you can not run the best pressure all the time what is better a 5 psi low or a 5 psi high pressure?

I have tried for months to get some info from DOT, Bridgestone and Michelin. Last summer I did the a pressure test driving across California. This was in a F350, with Michelin LTX M/S LT285/75R16 and set the cold pressure to 60psi per Ford. Before leaving a beach for the High Desert I set the pressure to 60psi and it was 66 deg F. After 2 hours we stopped at 2500ft and 118 deg F. My tires were sitting at 75psi! The maximum is 70 so what should I have done?

Also, I bought four tire pressure guages (WalMart, Kragen, Napa, and Chief) and they all disagreed with each other from 5-15psi! Anyone got an idea on how to check a pressure guage?

[Grant@Tirerack]

The maximum pressure listed on the tire sidewall is the maximum it will handle under load. You don't want to be exceeding that under normal driving conditions. You also want to follow the pressures listed in your owners manual or on the vehicle sticker. The main issues here are maximum pressure, tire load range, and size. Which year and model F350 is this? Most of the newer Ford F350s I am aware of came with an E load range tires in either an LT215/85-16, LT235/85-16, or an LT265/75-16; all of which were E load range tires (10 ply rated sidewalls) . The Michelin and Bridgestone LT285/75-16 are only D load rated (8 ply rated sidewalls) and do not have nearly the same sidewall renforcement as the heavier duty O.E. tires. They also only have a maximum psi rating of 65. You should never be running higher than the listed maximum on the sidewall of the tire. The E load tires can handle up to 80 psi. In short, all of this tells me you should probably be running an E load tire on this truck. It's clear from what you are finding with the air pressures you are getting that the tires are over loaded and do not have enough sidewall firmness to handle the vehicle weight and stress. If your pressures are running that high at operating temprature, you need to get a higher load rated tire. The problem is the tires are overloaded and if you look, the sidewalls are probably very rounded on the lower part of the tire (between the ground and the wheel). As the tire rotates, that 'bulge' will flatten out as it reaches the top of the tire. That flexing of the sidewall is just like bending a paper clip back and forth. It builds up friction and heat inside the sidewall of the tire. In warm areas, it doesn't take long for an overloaded tire to reach the same temp they used to vulcanize the tire layers together in the mold at the factory. This can lead to the sidewalls breaking down and failure of the tire. A heavier load rated tire (E load) will not flex as much and will not build up as much heat. You can also run higher pressures to help the tire keep its shape. There are no LT 285/ 75-16 tires with a higher load rating than a D. You need to think about getting into another size of tire. You are rolling the dice running that size and the high pressures you are getting are proof of that. Any tire subjected to that will fail eventually. It's simply overloaded. To check the air gauge you can use a known air source (an air tank with a good quality air regulator set to a fixed pressure is the best way). We sell a full line of top of the line air gauges on our website made by Accutire. The deluxe model was Consumer Reports 'best buy' in their review of air gauges. Here is a link to our website:
http://www.tirerack.com/a.jsp?a=DY6&url=/accessories/index.jsp

[CapriRacer]

A couple of additional thoughts:

What is printed on the tire sidewall is the maximum inflation RATING! That means that when the tire is inflated to that pressure (and the pressure is always measured when it is cold) and properly loaded, the tire is designed to operate under those conditions - and the hot pressure will rise to whatever it rises to.

15 psi build up seems to be too high, so it looks like you need to start higher.

I've looked up every F350 dually for the last 10 years. Most have a different pressure on the fronts that on the rears, but none of those was exactly 60 psi. I think you need to look at the placard and see what the tire size was originally and what the inflation pressure is supposed to be for that size.

(Shh, don't tell anybody, but there are circumstances where you can use more inflation pressure than is written on the sidewall. But those are unusual conditions, but you see my point?)

Hope this helps.

[Tailwheel]

Hi Grant,
You are correct the max pressure is 65. My truck is a 4x4 Crew Cab and I live in a remote desert and drive off road many miles a week. The truck did come with E range tires but they are unsat in the dirt and sand. In truth I have no recommended tire pressure because the tires are not stock. Neither Bridgestone or Michelin have answered my questions about what pressure I should run. The silence is deafening.

I have weighed the truck when loaded: I got 5100lbs rear and 6048lbs front, the LT285/75-16 say they are good to 3306lbs@65psi which should give me a 1500lb margin on the rear and the front is 550lbs to the safe side. What pressure is best for this tire with a 3000lb load? There is no information on that available. I have been very careful about the loads but it is really silly since I don't have any information on how to vary pressures for varying loads.

In a practical sense the max pressure rating is keyed to a max load but that is not all there is to it. If I load my truck to 3300lbs per tire, set the pressure to 65psi cold and go for a ride I will be overpressure in a few miles or less. Rolling will make it hotter and the pressure will go up. So what good is the rating? We can not use it to load the truck or go for a drive without being out of limits and warranty. I know the tire manufacturers have thought this all out. I have gone for 33 years setting my tire pressures cold to whatever the dealer or tire shop said and then going driving thinking I had done right. We are all guessing or ignoring tire pressures and they make tires go pop. Tires should come with an application chart of load, pressure, and temperature and even more important a specified safe pressure range for a load and temperature. No tire ever runs between air checks at constant pressure or volume.

[Grant@Tirerack]

The load rating also takes into account the weight shifting from side to side as you drive. An E load tire will have a heavier sidewall that will resist heat buildup better and allow you to run those higher psi levels you need. You should never exceed that maximum pressure on the sidewall. That's the highest pressure the tire is designed to handle under load. The likely reason Bridgestone and Michellin can't give you an air pressure is because the tire size and load rating are not correct for the truck. Simply put, you need to get into another tire size with an E load rating (likely an LT265/75-16). That would have more load capacity and higher psi rating.
The LT285 D load tires you are running are overloaded, overheating, and exceeding the maximum pressure they are designed to handle. You have already had two tires fail and if you don't get into the correct tire size/load rating, it's going to happen with the Michelins. It's only a matter of time before the sidewall layers start breaking down internally. It's not worth the risk to youself and others on the road.

[Tailwheel]

How can my tires be overloaded when the weight on each tire is less than the max rating for the tire? Your not going to scare me into buying new tires.

My truck is a '96 and the door palcard says 60psi front and 60psi rear.

I am still trying to find written informaiton on tire pressures and loading. I was hoping that an SAE type person might respond with a technical reference.

[Grant@Tirerack]

To begin with, I'm not trying to 'scare' you into buying new tires. I'm simply laying out the facts. Your first post pretty well sums it up for you. You have already had two blow outs using this tire size and are now have tires that are overheating and running in excess of the maximum pressure they are designed to handle. I didn't sell you these tires and I wouldn't have in the first place. It's the wrong size and load rating for the truck. Don't get upset with me for trying to explain this to you and answering your questions. I'm on your side here. You need to get into the face of the shop that is selling you this size.

I'll take one more crack at trying to explain this. The tires are overloaded because the weight on each tire changes when the vehicle/suspension moves. The load rating required is always going to be higher than the 'static' weight of the vehicle sitting still. You can't just go by the axle weight and divide by two to get the required load rating for a vehicle. It's sort of like the old Dennis The Menace cartoon where he is standing on the bathroom sink talking to Joey and says "Hey, ya wanna see me weigh a million pounds?" as he is getting ready to jump onto the bathroom scale on the floor.... When you go around a corner, the weight distribution changes on each corner of the vehicle. If you turn left, the tires on the right side of the truck are going to have more force and stress on them than the inner tires. That 'static' weight on each corner will increase on the outer tires. The same is true when the vehicle comes to a stop. The force shifts to the tires on the front axles. The opposite happens when you hit the gas. All of that and more is taken into account when the engineers at Ford figured out which tire size, load rating and air pressure to use on the truck. They are the only ones who do that testing and come up with the required combination (not the tire companies). The the 2003 industry tire guide (published by Bennett Garfiled, The 2003 Yokohama Tire Fitment Guide, and even the 2003 Bridgestone/Firestone Tips Tire Inflation Pressure Specifications guide all show that Ford now lists the following tire size and pressures for the 1996 model year F350 models:

F350 4x4 LT235/85-16 E load 65psi front-80psi rear
F350 (DRW) LT215/85-16 D load 58psi front - 65psi rear

You can see this for yourself in print if you like. If you go to any local tire shop they can show you a copy of the industry tire guide where that is clearly listed. Those sizes/ load ratings and pressures are provided by Ford to the tire industry for use on this vehicle. The tires the local shop sold you can not even be inflated to the correct psi listed by Ford for the non Dualie truck. I would ask the shop why they would sell you that tire size to begin with. It would also be a very interesting question for Ford to ask why the current size and load they reccomend do not match up with what was printed on the truck placard back in 1996?
I would check directly with Ford to see why that has changed since they printed that placard and put it on the truck at the factory. I think they are the only ones who can answer that one. Please post their response here if you can. I would be very interested to see what they have to say about that one I'd also like to hear what the local shop says about selling a tire that can't even be inflated to the reccomended psi listed by Ford for the truck. Something is not right in the mix here and the fact that your tires are running that high of a pressure is all the proof you should need. The fact that you are asking these questions here tells me that you already know they are not a safe size to be running on this truck.

[CapriRacer]

Looking for an SAE type person? You got him!

I just looked up a 1996 F350 dually and it shows LT215/85R16's on 6" wheels inflated to 58 front and 65 rear.

LT285/75R16's take a minimum of 7 1/2" wheels. Hopefully, you've changed wheels and added spacers so that the minimum spacing is correct. (BTW, minimum dual spacing for LT285/75R16's is 13.07")

But if you didn't, these duals are in danger of kissing and that causes all kinds of weird types of things, including some unusual, but very identifiable, failure modes.

Plus, my book says this is a 10,000# GVW truck. You've measured 11,150#!!!

All these things could explain a lot, including the heat buildup, and the fact the BR & M won't answer your question - they know there is something wrong.

From a theoretical point of view, the narrow wheel would decrease the load carrying capacity since the air chamber is now smaller. And of course this is all theory, since the wheel is off the charts.

Now the deal with inflation pressure is this: It gets set cold and what it goes to, it goes to. The fact that a tire is stamped on the sidewall at X pressure, doesn't mean that X is the maximum pressure it should ever experience. All these pressures we discuss are COLD, with the exception of the running (or hot) pressure.

And there is a chart that covers load vs inflation pressure. It called a load table and it's published by The Tire and Rim Association for tires designed to US based standards, which LT metric tires are). These load tables don't get published on the web much because of how many load tables there are. (My 2001 copy is an inch thick and cost $55). There is a book published by the European Tire and Rim Technical Organization, but that's for tires built to European standards. They have similar tires, but their sizing coding is different, and besides, we don't need to add that to the confusion.

Hope this helps.

[Grant@Tirerack]

That would be true for the dual rear setup but based on the tire size he is running, I don't think this is a (DRW).

[CapriRacer]

Good point, Grant. I just assumed with the load he was carrying, it had to be a dually. He certainly needs one with the GVW he's quoted.

Assuming for the moment that it is not a dually, then the wheels would be 7" wide - still too narrow for an LT285/75R16.

The other issues here are the load capacity of the wheels, the load capacity of the wheel studs, and the load capacity of the rest of the suspension. In short, there is a severe overload situation here and tires are just one component. I'm looking at the same books you are, and the front axle load is probably 800# over the GAWR.

Pretty scary!

To try to clarify things for those who are following this thread - The load tables are set up for smooth road surfaces and legal highway speeds. When tires are operated in some mode other than that, the situation has too many variables to be able to have a table for each situation. As I said earlier, my TRA yearbook is an inch thick. Who knows how thick it would be if all possible variables were covered.

But I think the 15psi buildup is a good indicator that the cold inflation pressure has to be higher.

Hope this helps.

[Grant@Tirerack]

I'd still like to know why the vehicle placard has a totally different pressure listed (even for a DRW) than what is listed for the truck now. Something is not right in the mix here....

[CapriRacer]

I have found some errors in Tire Guides.

Most of them have been on Ford trucks. May be this is one of them!

[Grant@Tirerack]

Just for kicks, I went back and checked the 1996 tire guides and the listed sizes, load ranges, and pressures were the same. I would have to think if it were not correct, they would have changed the listings.

[Tailwheel]

Thanks CapriRacer,
I'll try to look at a copy of the load tables.
Grant,
OK I got the dynamic load thing, the truck is bouncing along and the tires are seeing more weight, maybe 1.3gees (just a guess). I had expected the manufacturers to account for that in a safety factor when stating the maximum load. The load I quoted is a worst case load for me and not what I had when either blowout occurred.
Someone asked about my placard:
The truck is an F-350 Crew cab, 4x4, automatic, with limited slip rear, & not dually. My door placard: Front GAWR 4600lbs, Rear GAWR 6048lbs, & GVWR 9200lbs. It calls out 60psi front and rear for the stock tires. Thinking about my max gross and the way the truck looked and handled I wonder if the scale was wrong. The tires were not squashed (i think they were at 70psi) and the springs had a couple inches of travel. It steered fine.

I am sorry this has gotten so confused. My questions are about a normally loaded truck and have nothing to do with my blowouts or the max loading I cited as a case for the sum of the weight on wheels being less than the sum of max load.

Is a maximum tire pressure rating a true maximum or just a cold or starting out pressure maximum? I heard CapriRacer say they are cold numbers.
Is the recommended pressure good for all the possible loadings of a vehicle, up to max gross weight? I heard CapriRacer say there is a published table that addresses this.

I'm not going to worry about why my placard differs from other numbers. They are more legal values than real limits and just don't effect my use. Now, don't fall out of your chair.

[CapriRacer]

Great info, Tailwind. Let's review it.

Front GAWR - 4600 #
Rear GAWR - 6048 #
GVW - 9200 #

My experience with Ford Trucks is that the GAWR's EXACTLY match the load carrying capacity of the tires at the inflation pressure specified. While I don't know for sure what your placard says for the original tire size, it is likely LT235/85R16 and the loads would be as follows:

LT235/85R16
Front / Rear @ 60 psi - 2485 #
Front / Rear Axles = 4970 #

Something's wrong here!

Possibilities - the placard is incorrect, the vehicle has been remanufactured by a conversion company and is therefore not really a "Ford" (that usually means there is another placard and sometimes the original placard is covered over to eliminate confusion), there is something other than the tire that is the limiting factor (like the wheel!), the GAWR has some reserve capacity built in, or maybe something else,

OK, let's move on:

Actual loads:

Front - 6048 # (31% over GAWR)
Rear - 5100 # (16% under GAWR)
GVW - 11,148 (21% over GVW)

I think you can see the problem - 1) too much load for the vehicle, 2) the load is too far forward.

Let's discuss the "blowout". Tires have "memory" and anytime a tire is overloaded, the possibility of a starting a failure increases. Once the failure starts, the failure may take many miles before it becomes visible to the naked eye, and many more after that before the tire suffers a catastrophic event. What load is on the tire at the time of failure is irrelevant.

(BTW, I don't like the term "blow out". Too vague! Belt separation, impact break, run flat, etc are much more to my liking.)

"The tires were not squashed" Radial tires are exceedingly difficult to judge whether they are underinflated or not - the pressures would have to be WAY off to be able to see it, even for an expert. Even travel on the springs, and good steering feel, don't eliminate that possibility there is an overload situation. And of course, it is the worst case that has to be considered when we discuss all this loading.

Is the maximum pressure rating a true maxmium, etc. etc? What is printed on the sidewall is the maximum COLD inflation pressure RATING for the tire. What this means is that under normal circumstances that pressure should not be exceeded when setting the cold inflation pressures.

When folks talk about tire inflation pressures, they are always discussing COLD pressures (pressures at the ambient condition), with the exception being the inflation pressure after the vehicle has been operated - we tire geeks call that "warm" or "hot" pressure. Because the warm or hot pressures are dependent on the speed, the load, the road surface temperature, etc, these warm pressures are too variable to use for anything EXCEPT as an indication of whether the cold inflation pressure is adequate. This pressure buildup is only an indication, as the actual operating temperature is going to vary some between tire manufacturer, so it's hard to be precise about what the limits are.

There are some circumstances where it is permissible to inflate tires over what's printed on the sidewall. Knowing what I know about tires, I don't think this "maximum" is as absolute as it is sometimes portrayed. But I also don't want people to think they can willy-nilly inflate the tires over the rated inflation, either. Caution and knowledge are important when the "rules" are violated.

That bring us to the "rules" you are violating - the rim width is important because that controls the shape of the tire, which affects the way the tire carries the load. Using too narrow of a rim causes the tread to arch, so when a load is placed on the tire, a lot of flexing takes place in the tread area, which generates extra heat - right where you don't want it!

Vehicle manufacturers publish the GAWR and GVW figures to describe the loading conditions the vehicle was designed for. The vehicle manufacturers do all their testing based on these figures. Sure they perform tests where the vehicle is overloaded, but that's to 1) take into account things the test doesn't account for, 2) to provide for production variation - you know - differences from individual vehicle to individual vehicle. What this means is - all the components on the vehicle are designed for what's published on the placard. Aside from the load situation, you also have some other things that adversely affect the vehicle - unimproved roads, offroad usage, and ambient temperature. Now, that's just the ones I know about!

These are all things that are ringing alarm bells in my head!

So what should you do?

1) See what you can do to get the maximum loading situation under control. It is an exceedingly bad idea to exceed these limits, especially considering the adverse factors you've got.

2) Plan to deal with the rim width situation. The maximum tire size suitable for a 7" wheel is LT265/75R16 Load Range E (that puts the tire within the maximum GAWR ). Or you should plan to get 7 1/2" wheels if you plan on staying with the LT285/75R16's.

3) Account for the adverse conditions you're forced to deal with. That means adding inflation pressure to your present (and future) tires.

In an earlier post you asked how to test a pressure gauge. Without access to a known accurate gauge, it is difficult. But you can take advantage of the fact that most gauges are very close to the actual value. So use the "Odd man out" rule. That is you can make an educated guess which gauges are not reading properly, and the rest are grouping themselves around the real value, so choose the gauge that is closest to the median value. In my case, I have access to a test fixture that is traceable to the National Bureau of Standards (they don't call it that now!). I've attached a tag of masking tape to every tire gauge I have, with a note as to what the gauge is reading compared to the real value. You could use the tag to identify the average value , putting each gauge in context.

The last thing is the load tables: I've searched the web a bit and found some load tables, but didn't find one for an LT285/75R16. may be a bit more searching will reveal one.


Hope this helps.