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Old 03-04-2015, 06:05 AM   #43
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Originally Posted by A W Warn View Post
CapriRacer,

Will you expand a little on the definition of the term "underinflated"? What does that really mean?.....
I'm going to be slicing the baloney pretty thin here, so bear with me.

I think everyone will agree that if you are operating a tire at an inflation pressure less than what the tire load tables says, then it is underinflated. Of course, the amount of under inflation is proportional to how much below that value it is being operated.

Then there is the issue of what the vehicle manufacturer specifies - which ought to be a bit more than the load table. In this region, I would use the term *underinflated* when the value is less than specified.

But the physics of the situation is this: There isn't a clear cut line where a tire is so underinflated that it will fail instantly. Sure, a flat tire doesn't last very long (we're talking feet and seconds, here!), and one with only a few psi might last a few miles, but in neither case is it immediate. That's because these types of tire failures are durability issues and it takes a long time for the cumulative affects of underinflation to cause a failure. We engineers use the term *fatigue* - and the more overstressed something is, the quicker it fails due to fatigue.

So one can operate a tire slightly underinflated and think everything is OK - until one day the tire fails.

But if we look at the other end of the spectrum, I am calling anything over the vehicle manufacturer's specification *overinflated* - BUT - a tire can operate just fine a bit over inflated - no wear issues because tire wear is more affected by other factors.

For example perryg114 reports above that the rear tires on his Ranger wear in the center when the pressure is larger than 25 psi. That's because drive tires tend to wear in the center, while steer tires tend to wear in the shoulder - and that affect is greater than the affect inflation pressure has. I would caution perryg114 that he could be running his tires underinflated (that is, below what the load table says) and not realize it.

There is a simple test that works very well in determining if a tire is being operated underinflated - pressure build up. The rule of thumb is that you don't want the pressure to build up more than 10%.

If you have a TPMS that reads the pressure directly and transmits that to the cab, great! But if you don't have one of those, then manual measurements work. While it takes a while for a tire to reach its stable operating temperature, certainly an hour is adequate.

Oh and this isn't a target you should be trying to hit. It's a warning sign that things are wrong. Lower is better - and as I said before, it is just fine to be operating a tire a bit overinflated.
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Old 03-04-2015, 06:56 AM   #44
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I like and agree with what CapriRacer said.

I have seen statements in print from various sources in the tire induftry that operating at 20% blow the specified inflation is considered "flat" and can be used as a reason to void a warranty.
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Old 03-04-2015, 07:05 AM   #45
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I have a Chevrolet 3500HD that came with 225/70R/19.5F tires. Max inflation on sidewall is #95. Tag in door says #60 front and #75 rear. I have run those pressures 90k on the original tires. Wow, I've been running with flat tires for 90k miles.
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Old 03-04-2015, 07:06 AM   #46
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Am I the only one that finds it interesting that, by my count at least, we have two Tire Engineers on this forum, and I've never read either one make a definite statement about a tire, good or bad, but only vague innuendo's of what might result if whatever the topic is about actually does happen?

In every situation there is a worst case possibility.
Steve, You haven't offered information on what your profession is but before you decide to ignore the advice of the tire engineers here I would ask what answer you get when you ask your Dr "At what Blood Pressure will I have a heart attack?" Ever been given a specific number such as 193/99 ?

Ever seen a car advertise the exact fuel economy you can expect?

What is the exact depth of water (in inches) in which you would drown?

I can go on.

Life seldom has simple yes and now answers.
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Old 03-04-2015, 07:06 AM   #47
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I think I am right at the limit of under inflation. There is also the issue of having no traction especially in wet weather with the tires above 25psi. This is a fringe issue and has nothing to do with heavily loaded trailers.

I do think those running 4 E rated tires at 80 psi on a 6000lb trailer is silly and it is just going to beat that poor trailer to death.

Perry
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Old 03-04-2015, 07:08 AM   #48
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Quote:
Originally Posted by SteveH View Post
Am I the only one that finds it interesting that, by my count at least, we have two Tire Engineers on this forum, and I've never read either one make a definite statement about a tire, good or bad, but only vague innuendo's of what might result if whatever the topic is about actually does happen?

In every situation there is a worst case possibility.
I've only seen excuses.
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Old 03-04-2015, 07:12 AM   #49
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Originally Posted by r carl View Post
I have a Chevrolet 3500HD that came with 225/70R/19.5F tires. Max inflation on sidewall is #95. Tag in door says #60 front and #75 rear. I have run those pressures 90k on the original tires. Wow, I've been running with flat tires for 90k miles.
Sorry no you haven't. The placard is based on an assumption of what your load might be.

Now if you provide the actual load you have always run for each tire position we will know what your inflation should be.
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Old 03-04-2015, 09:02 AM   #50
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Quote:
Originally Posted by CapriRacer View Post
I think everyone will agree that if you are operating a tire at an inflation pressure less than what the tire load tables says, then it is underinflated.
For tire dummies like me, would you clarify what is meant by "tire load tables"?
Who's tire load tables? Are you referring to the placard on the vehicle door, a load chart that is provided by the tire manufacturer, or something else?

add edit:
About 3 years ago I purchased a set of tires for my Safari. After they were installed I asked for an inflation/load chart, so that I would know how to inflate the tires properly. The tire dealer, and later the manufacturer's sales rep, told me they did not have one specifically for those tires. The inflation information on the sidewall is the only thing I have to go on.

Since the rims on my Safari have PSI 65 MAX cast into them, I cannot inflate the tires to the 80 PSI maximum that is marked on the sidewall. I'm running at 60 PSI with a load ranging from 1500 to 1800 Lbs on each tire. Are these tires underinflated?
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Old 03-04-2015, 09:26 AM   #51
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Originally Posted by Tireman9 View Post
Sorry no you haven't. The placard is based on an assumption of what your load might be.

Now if you provide the actual load you have always run for each tire position we will know what your inflation should be.
My service truck weight is 12,600# all the time. FA is 4800# and RA is 7800#. GVWR is 15,000#.
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Old 03-04-2015, 10:13 AM   #52
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The tire discount I go to has free tire pressure checks, well last time I went tech inflated tires to sticker on truck that is more than side wall on tires. Argued with me had to inflate by law. got manager we went over what was proper agreed with me then inflated pressure that I spec. Tech is no longer emp.
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Old 03-05-2015, 08:22 AM   #53
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Originally Posted by A W Warn View Post
For tire dummies like me, would you clarify what is meant by "tire load tables"?........


http://www.barrystiretech.com/2005traloadtable.jpg

This is a tire load table. Basically, it is an inflation pressure vs max load table sorted by tire size and type. Somewhere in the heading will be notes about where the table is applicable.

Quote:
Originally Posted by A W Warn View Post
.......Who's tire load tables? Are you referring to the placard on the vehicle door, a load chart that is provided by the tire manufacturer, or something else?.......
Well, the load tables are issued by a tire standardizing organization, such as The Tire and Rim Association (TRA, for short) the US based organization - and those tables are reprinted by the tire manufacturers. All the tire manufacturers use the same tables published by the tire standardizing organizations. To make matters a bit more confusing, there are sometimes some differences between tire standardizing organizations, but those differences are minor.

Quote:
Originally Posted by A W Warn View Post
.......
add edit:
About 3 years ago I purchased a set of tires for my Safari. After they were installed I asked for an inflation/load chart, so that I would know how to inflate the tires properly. The tire dealer, and later the manufacturer's sales rep, told me they did not have one specifically for those tires. The inflation information on the sidewall is the only thing I have to go on.

Since the rims on my Safari have PSI 65 MAX cast into them, I cannot inflate the tires to the 80 PSI maximum that is marked on the sidewall. I'm running at 60 PSI with a load ranging from 1500 to 1800 Lbs on each tire. Are these tires underinflated?
First, your vehicle should have a vehicle tire placard that lists the original tire size and the proper inflation pressure for that size. Vehicle tire placards can be found on doors, doorposts, glove boxes, fuel filler doors, trunk lids, etc.

There should also be a certification label that will list the GVW (Gross Vehicle Weight), and the GAWR's (Gross Axle Weight Ratings). Trailer manufacturers haven't always done a good job with the certification label as many trailers exceed those values.

Second, what tire size are we talking about? There will be a load table that will list a minimum inflation pressure for a given load. Typically, the vehicle manufacturer will specify MORE inflation pressure than the minimum.

And lastly, if I want to answer this question directly "..... Are these tires underinflated?....", I need to know 3 things: The tire size, the actual load on each tire, and the inflation pressure. I'm short one item.
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Old 03-05-2015, 08:33 AM   #54
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Originally Posted by r carl View Post
I have a Chevrolet 3500HD that came with 225/70R/19.5F tires. Max inflation on sidewall is #95. Tag in door says #60 front and #75 rear. I have run those pressures 90k on the original tires. Wow, I've been running with flat tires for 90k miles.
Quote:
Originally Posted by r carl View Post
My service truck weight is 12,600# all the time. FA is 4800# and RA is 7800#. GVWR is 15,000#.
OK, let's do the math:

A 225/70R19.5 inflated to 60 psi front /75 psi rear has a load carrying capacity of 2615# front (single) / 2860 rear (dual), which means the front axle GAWR can be no higher than 5230# and the rear GAWR no higher than 11,440#. Yup, both are below those values.

So does that answer your question?
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Old 03-05-2015, 09:13 AM   #55
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Quote:
Originally Posted by CapriRacer View Post
............Second, what tire size are we talking about?

..........And lastly, if I want to answer this question directly "..... Are these tires underinflated?....", I need to know 3 things: The tire size, the actual load on each tire, and the inflation pressure. I'm short one item.
For my trailer tires use these (lowest load to highest load)

For tire # 1 of 4:
Tire size = ST225/75-R15 load range E
Tire load = 1,600 lbs
Tire inflation = 60psi

For tire # 2 of 4:
Tire size = ST225/75-R15 load range E
Tire load = 1,800 lbs
Tire inflation = 60psi
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Old 03-05-2015, 09:58 AM   #56
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OK, let's do the math:

A 225/70R19.5 inflated to 60 psi front /75 psi rear has a load carrying capacity of 2615# front (single) / 2860 rear (dual), which means the front axle GAWR can be no higher than 5230# and the rear GAWR no higher than 11,440#. Yup, both are below those values.

So does that answer your question?
You think they are under inflated?
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