tire pressure and blowouts

[aruba5er]

Last week before leaving on a trip I checked and adjusted all 8 tires. The truck calls for 75 rear, 70 front. It was a warmish day and I'm fiddleing with the DIC ( chev drivers information center) when I got to "front tire pressure #84 rear tp 89# could not believe it. I knew they increased pressure as they warmed up but ? Just saying . I wonder if all those blowouts are caused but a natural increase because of heat. I know my weights and way under any load ratings. Guess thats why I have LT's on the fiver.

[Ohio_Engineer]

Me Again wrote:

atwowheelguy wrote:

That should become the poster child for ST tire problems. Chris

Have seen similar on LT and P type tires made here in states. This is probably factory defect and you should have no problem getting a replacement. You should also file complaint with NHTSA and call the failure "Sidewall Blister, separation" and give the approximate size of the bubble in inches Maybe 3"?

[Ohio_Engineer]

atwowheelguy wrote:
The Combined Gas Law states:

P1xV1/T1 = P2xV2/T2

P absolute pressure in atmospheres {(gauge pressure + 14.7)/14.7}
V volume
T temperature in degrees Kelvin

With an increase in pressure, the tire will stretch a little and the volume will increase. For this exercise, we will assume that the volume remains constant.

70 psig = 84.7 psia = 5.762 atm
84 psig = 98.7 psia = 6.714 atm

Assume T1 = 70F = 294.3K

Calculate the temperature, T2, required to raise the pressure from 70 psig to 84 psig.

T2 = T1 x P2/P1 = 294.3 x 6.714/5.762 = 342.9K = 158 F.

So it would take an 88F temperature rise to increase the pressure from 70 psig to 84 psig.

Now assume a 30F temperature rise from 70F to 100F.
Calculate the pressure rise.
Assume T2 = 100F = 310.9K

P2 = P1 x T2/T1 = 5.762 atm x 310.9/294.3 = 6.087 atm = 89.5 psia = 74.8 psig

So a temperature rise from 70F to 100F would increase the pressure from 70 psig to 75 psig.

Yup you got the math correct. What you don't have is a good measure of the contained air temperature. What is also missing is the pressure increase due to moisture but otherwise good work.

[Me_Again]

atwowheelguy wrote:

That should become the poster child for ST tire problems. Chris

[mpierce]

In the winter, driving semi, I go from maybe -20 in ND, to 80 in Florida. I make sure my tires are where they should be at -20, and LEAVE them alone! I do not worry about the increase in FL. However, if I was to STAY in FL for a few weeks or more, then I would readjust IF I was going to be driving around.

[atwowheelguy]

[tatest]

Blowouts don't occur often as a result of slight overinflation. Actually, blowouts are quite rare, the more usual tire failure in the belted radial era has been tread separation from overheating, whether caused by excessive load or under inflation (two ways to see the same thing) or excessive speed on a damaged tire. After tread separation, the tire body will quickly get damaged enough to deflate, but even then it is not often a blowout, which is an explosive event.

[atwowheelguy]

The Combined Gas Law states:

P1xV1/T1 = P2xV2/T2

P absolute pressure in atmospheres {(gauge pressure + 14.7)/14.7}
V volume
T temperature in degrees Kelvin

With an increase in pressure, the tire will stretch a little and the volume will increase. For this exercise, we will assume that the volume remains constant.

70 psig = 84.7 psia = 5.762 atm
84 psig = 98.7 psia = 6.714 atm

Assume T1 = 70F = 294.3K

Calculate the temperature, T2, required to raise the pressure from 70 psig to 84 psig.

T2 = T1 x P2/P1 = 294.3 x 6.714/5.762 = 342.9K = 158 F.

So it would take an 88F temperature rise to increase the pressure from 70 psig to 84 psig.

Now assume a 30F temperature rise from 70F to 100F.
Calculate the pressure rise.
Assume T2 = 100F = 310.9K

P2 = P1 x T2/T1 = 5.762 atm x 310.9/294.3 = 6.087 atm = 89.5 psia = 74.8 psig

So a temperature rise from 70F to 100F would increase the pressure from 70 psig to 75 psig.

[Ohio_Engineer]

1ofmany wrote:

Dutch_12078 wrote:
Tires rarely blow out due to over pressure. All vehicle tires are designed to operate well above the maximum cold pressure shown on the sidewall. The far more common tire failure cause is under inflation.

This is also my understanding...however, the notion of what "cold pressure" is has been a source of interest. If you live in an area where the temperature varies wildly from day to day (like 30 degrees, or more), check your tire air pressure the day before the temperature is expected to drop those 30, or more, degrees, and then check again the next day after the temperature has dropped. I have seen a 10 psi drop under those circumstances...so, what is the correct "cold temp" reading?

Yes "cold temp" is confusing to some. What it really means is not warmed up from running for previous 2 hours or exposure to direct sunlight in previous 2 hours.

Basically the tire load & inflation tables that are used by all companies specify that tire inflation should be set when the tire is "cold" before starting a trip. Don't try and guess the future temperature.

Tire temperature will change by about 2% for each change in temperature of 10F. Note the temperature is difficult to learn with external TPMS which will read low temperature due to external cooling so the real "tire Temp" is maybe 10 to 20F higher than what your TPMS indicates.

[Grit_dog]

OP, it's very possible you had accurate pressures since tire air pressure changes 1-2psi avg for every 10degF.
Up der in Cheeseland, u fill dem tires up nice n plump when it's 0 deg out a couple months ago and they're good tires dontcha know, don't leak a drop of air, it's totally believable that they could be 10psi higher on that nice sunny May afternoon when you check them again.
Not likely the tpms is that far off. like my prev post, due to liability reasons, they need to make sure critical features are safe for the general public. They have. A vested interest in protecting their assets from the 10million sue happy consumers they serve every day.

[Grit_dog]

Dutch_12078 wrote:
Tires rarely blow out due to over pressure. All vehicle tires are designed to operate well above the maximum cold pressure shown on the sidewall. The far more common tire failure cause is under inflation.

This^
Just like tire/wheel ratings, gvw, how many slices of toast is advisable to put in your toaster, mfgs have a healthy factor of safety built into most every critical component on a vehicle, so they have some control over things they can't control that could kill you if they break.
My last half ton company truck,means using it pretty hard and couldn't convince my district manager why in the world I'd need a HD pickup for heavy civil construction ops.
So I did what any good performance oriented superintendent would do. I improvised.
Timbrens shimmed up hard to the axle tube almost. 60psi in the p rated rear tires, good trailer brake controller and now she was ready to tow/haul what I needed to!
The rear half of that truck got beat like a rented mule. Couldn't pop a tire to save my life...and it wasn't from a lack of trying.

That said, tire pressure specs are meant to be taken with a small amount of common sense and attention to detail. They're meant for in increase in pressure from "cold" to the increase due to altitude that day or heat generated from running down the road. Not meant for 80 psi at 30 below zero then transport to Death Valley in the summer to see how high it goes.

[Dutch_12078]

1ofmany wrote:

Dutch_12078 wrote:
Tires rarely blow out due to over pressure. All vehicle tires are designed to operate well above the maximum cold pressure shown on the sidewall. The far more common tire failure cause is under inflation.

This is also my understanding...however, the notion of what "cold pressure" is has been a source of interest. If you live in an area where the temperature varies wildly from day to day (like 30 degrees, or more), check your tire air pressure the day before the temperature is expected to drop those 30, or more, degrees, and then check again the next day after the temperature has dropped. I have seen a 10 psi drop under those circumstances...so, what is the correct "cold temp" reading?

I agree, the cold setting can change as the ambient temperature changes, but the fact remains that modern tires in good shape are more than capable of handling the typical changes. My take would be the tires should be inflated to the proper pressure at the start of the travel day if temps are fluctuating that widely. Tires rarely need to inflated to the max though, unless the load range used is close to its maximum weight rating.

[1ofmany]

Dutch_12078 wrote:
Tires rarely blow out due to over pressure. All vehicle tires are designed to operate well above the maximum cold pressure shown on the sidewall. The far more common tire failure cause is under inflation.

This is also my understanding...however, the notion of what "cold pressure" is has been a source of interest. If you live in an area where the temperature varies wildly from day to day (like 30 degrees, or more), check your tire air pressure the day before the temperature is expected to drop those 30, or more, degrees, and then check again the next day after the temperature has dropped. I have seen a 10 psi drop under those circumstances...so, what is the correct "cold temp" reading?

[msgtord]

the bear II wrote:
Don't trust the DIC
I use several fleet vehicles, different makes and models, anytime I manually check pressure or gas mileage the DIC is off to the point where I wouldn't trust the information. And yes my tire pressure gauge is very accurate.

This also goes for the speedometers... some are off by > or < 5MPH.

Yep. Best to use it as a guide, or a notification to hand check the pressure.

It wasn't until I started using speed measuring devices that I realized the variation in speedometers. Allot of folks are realizing it now that more GPS units are being used.

[imgoin4it]

X2 on not trusting the DIC. I have had several discussions about the descrepancy between my Guage and the DIC with my dealers service dept folks. The descrepancy is not always the same which really frustrates me. Dealer folks say there will be different differences depending on altitude etc and then pretty much admit they may never match. I think they did match when truck was new but do not anymore. I trust my gauges.