Yet another BFG Tire fail.

[ANZAC]

Originally Posted by Ford_6L_E350

No, that's not true and not what I was saying. The tire pressure is set by the vehicle manufacturer to provide adequate load carrying capacity based on the size of tire they specify for the vehicle.

I am *NOT* advocating setting pressure based on actual load. The pressure sets a "load carrying capacity" for that pressure - go look at the tire manufacturers documentation or TRA etc.

Some manufacturers will say "at loads/speeds below X, then use Y PSI, above this use Z". That makes sense, and they make that decision based on a variety of factors.

Ford's approach to the van is (presumably) this is a cargo van, it will probably carry heavy stuff, let's make sure there is adequate tire load carrying capacity ALL THE TIME that matches up to the vehicle/axle load carrying capacity.

So, 80 PSI on 245/75 16 gives you a very specific load carrying capacity (3042lb per rear tire). If you change the tire size to something else, then you need to make sure the pressure you use meets or exceeds that capacity, regardless of actual weight.

Said another way, if you go over the "load carrying capacity" at 80PSI on the stock size 245/75 16, then you have got bigger issues, regardless of the tires or pressure you're running at.

The downsides of overinflation are uneven tire wear, and you're getting closer to the "maximum PSI" which I think increases the risk of blowout, especially if you are in an environment where ambient temperatures can increase rapidly from the temperature you set the pressure at. (normal tire air temps on my Subaru are typically 30-40 deg above ambient after about 10 minutes driving, for example)

On the flip side, if there is some headroom and you're not over the vehicle's weight limits, you are certainly better off making sure the load carrying capacity you're getting from the pressure of the tires for their size is greater than your actual load, as compared to not having sufficient load carrying capacity.

But none of these are absolutes. You can't safely inflate the tires to 120 PSI and say "oh great now I can carry 5000lb per tire". They have both a max PSI and a maximum load carrying capacity (regardless of pressure) that you shouldn't go over, specified by the manufacturer. This is what the E rated tire buys you, a higher maximum carrying capacity.

Note that the TRA sets the lb load carrying capacity per size numbers (I think this is just physics/math for the column of air), the vehicle manufacturer picks the tire, size, pressure to meet their load needs, and the tire manufacturer sets the rating letter and maximum capacity.

If you use that logic, then a D rated tire is just fine because it easily meets the capacity needs of our vehicles (remember my van is not "overweight"). However, common sense says the E rated tire is going to have stronger sidewalls etc and be more durable, so that's a good reason to run them.

More in this thread, with links and stuff, examples on page 2:
viewtopic.php?f=3&t=1582

Come to think of it, all of this would be pretty easy to put in a spreadsheet. Tires, sizes, pressures, axle and vehicle ratings, and then actual axle weights, and flag where you have an issue and what the appropriate pressures/tire ratings should be.

None of this explains E rated blowouts....

[Ford_6L_E350]

Originally Posted by ANZAC

The downsides of overinflation are uneven tire wear, and you're getting closer to the "maximum PSI" which I think increases the risk of blowout, especially if you are in an environment where ambient temperatures can increase rapidly from the temperature you set the pressure at. (normal tire air temps on my Subaru are typically 30-40 deg above ambient after about 10 minutes driving, for example)

None of this explains E rated blowouts....

I can't argue with most of what you say.

But, I will take exception the comments about running close the maximum PSI increasing the danger of a blowout. Running a tire at max pressure should cause no harm to a tire unless that tire is damaged or defective.

On the other hand running underinflated, even for a short time, can damage a tire and cause failure down the road after the tire has been re-inflated to the proper pressure.

And manufacturer's pressure recommendation is always for COLD PRESSURE. Reducing the pressure of a hot tire will make it underinflated. The increase in tire pressure under heat and speed is normal and not a problem. Tire pressure normally rises 4-6psi from cold to hot and should not be reset unless it is lower than the cold pressure.

Some of this explains some tire failures.

Mike

[ANZAC]

Agreed. But I agree to disagree on getting close to max cold pressure. Notice I mentioned swings in ambient temp.

You set tires at 60 deg in the morning at a gas station in the desert, ambient gets to 105, and you tires run at ambient+30. I don't want to be near a max cold pressure tire at 75 deg above where it was set.

[Ford_6L_E350]

Ok, but how much are you going to reduce the tire pressure? How much pressure is due to the ambient temperture change (the only one that counts) and how much due to the heat from the use and roads? The ambient temp change is smaller than the change in tire temps from flexing (load) and the hot road (200+ degrees).

If you really want to reset the tire pressure from XX psi @60 degrees ambient to XX psi @105 degrees ambient, what pressure will you set it at? If you reduce the pressure to XX psi you will be underinflated, because you have not accounted for the pressure rise due to the increase caused by use. Even if the pressure goes above the MAX PSI on the tire sidewall, that is not a problem.

I've never heard of a tire exploding because the pressure was too high unless the pressure was extremely high from overinflation - think 35 psi bicycle tire at 125 psi. Tire failures are far more likely to be caused by underinflation causing damage to the carcass leading to a later blowout.

Tires are designed to run at max load/max pressure. The tire engineers know the pressure will build beyond max (COLD) pressure in use and the tires will tolerate that. They don't tolerate underinflation very well.



Mike

[ANZAC]

I do not advocate underinflation. I am saying if you overinflate beyond recommended pressure, be careful and don't go past the max cold pressure.

[jage]

Wait- max cold, say 65psi

I set my tires to Max Cold in the a.m. at 60º F
Run around in the desert sun till noon, now 100º
My tires are now ~69psi maybe 70psi

The tires are over max cold pressure, right?

But they are not "cold"... I seriously doubt the max cold pressure is calculated by the manufacturer for the tire at a level where are reasonable (even 40º) shift would seriously damage your tires or put you at risk to the point where you need to adjust up and down during the day for it...

[BroncoHauler]

WAIT, what about the effects of elevation changes?

[ANZAC]

Originally Posted by jage

Wait- max cold, say 65psi

I set my tires to Max Cold in the a.m. at 60º F
Run around in the desert sun till noon, now 100º
My tires are now ~69psi maybe 70psi

The tires are over max cold pressure, right?

But they are not "cold"... I seriously doubt the max cold pressure is calculated by the manufacturer for the tire at a level where are reasonable (even 40º) shift would seriously damage your tires or put you at risk to the point where you need to adjust up and down during the day for it...

Cold is when you haven't driven on the tires and put any heat above ambient in to them. Cold is really "ambient". I've had tires go from 40 to 130 F absolute on a given day due to ambient changes and driving on them - and with commensurate increases in pressure. Yes, the heat from driving them will increase the pressure and that is expected. I am talking about the risk of ambient temp increases *combined* with "over" inflating due to perceived load concerns. The interesting truth is on temp/pressure increases from driving, the numbers generally seem to be 30-40 deg higher than ambient, max (at least on car tires, not sure about trucks), and the tires are definitely designed for that.

The point I'm making is that there have been a lot of BFG failures, why add any extra stress to the equation by inflating above recommended pressure for the load capacity. And I definitely am not advocating underinflating based on where you think ambient might go. If people want to drive around with >80 or 100 PSI in their tires in the desert heat at 80 MPH because it will support their over-GVR van, well.. good luck with that.

Also, over-inflation increases the risk of road damage from potholes and "bruise" damage because the tire is not able to absorb the shock loads.

Has anyone seen any reports of BFG issues on other forums, eg Jeep?

[coyotearms]

I keep on getting sucked into this discussion! This is not a reply to any one post, and I hope I don't step on anyone's toes. Every tire has a marking on it showing the maximum permissible cold inflation pressure. I doubt if anyone would argue that label means pressure before you start driving. If you get up at noon in the middle of the day in death valley, and your tires say maximum inflation pressure is 80 psi, I think you are o.k. to put in 80 psi. If you live in Tahoe and it is December at 4 am when you go out to drive, you can put 80 psi in them. It is probably a good idea, however, to adjust your COLD pressure for the climate you are in, So the next morning after you have driven from Lake Tahoe to Death Valley the previous day, check your cold pressure. The additional pressure build up under these normal conditions is taken care of in the tire design.

I sold my Mustang GT in order to have only two vehicles at a time after I bought my E350. It had BFG g-Force™ T/A® KDWS 235x50ZR18 97W tires on it. Z rating indicates capable of speeds over 148 mph and the W limits that to 168 mph. I have driven that car between 130 and 140 mph. No one drives that fast and does not worry about blowouts. So I dug into the data on the tire and found something interesting. At very high speeds you must INCREASE the tire pressure and DECREASE the load rating of such tires.
What is meant by that is if you know you are going to go crazy at speed when you first get into your can in the morning, you have to adjust your COLD pressure upward and at the same time possibly needing to take out the dead body you were storing in the trunk.

[Edit of 10/20/2011: I took out some conjectural reasons here why I thought the manufacturer requires this. Let's just say it is an example of just how complicated the interactive effects of tire flexing, tire temperature and air pressure are and some situations the opposite of the "normal" rules of increasing pressure to carry more load.]

I think that is were the confusion is coming from in some of these posts, regarding interpretation of max pressure and what happens when you drive. But as long as you 1) never set the maximum pressure COLD higher than the maximum for the tire, 2) follow the load ratings and 3) regularly check cold pressure, you're doing all you can do in a vehicle like a heavy van. But damage is damage, and if a tire has been abused and/or patched, or if it is a poorly made tire, you may not be safe even after all that. What is food for thought is that moving from something like the Mustang to something like an SMB I wonder if you can ever feel good at speed knowing you have 2000 miles of aired-down time on bad roads with sharp rocks!!! When my replacement tires on the SMB had less than 6000 mi on them, and we did not have too much time to get from Seattle to Tucson for the Overland Exposition I later noticed on the GPS
I don't even know where it happened, and it rained a lot on the way---opps!

So claims that a tire is really great tire after 4000 miles on them, or trying to sort out anecdotal data for causes of failure is futile. There is a great amount of information on this topic that should cool everyone's jets some at http://www.tirerack.com/tires/tirete....jsp?techid=35.

I am the first to admit the attached file and the tirerack link is just about the limit of my knowledge on tires. . .

[Silver350]

Isn't it 1 P.S.I. for every 10 degrees in temperature? So a 50 degree swing will be 5 P.S.I., right? You'll see more than that by just driving.