Tire Pressure

[22229stingray81]

I have BF Goodrich Radial T/A's size 255 60R 15 on my '81 vette. The door tag says 35 PSI cold on the standard 225 70 tires (mine are the optional larger size). The MAX pressure for those tires is 35 PSI. What pressure do you think I should inflate to? I have them at 33 PSI right now. Thanks!

-Tatortot

 

[SVO]

32-36 PSI cold.

 

[6880 Mike]

Use what you like for ride and handling.

 

[22229stingray81]

My concern is the 35 max PSI the BF Goodrich's have. I realize that's at a particular weight, but if they're 33 cold, then hot they're more like 37 PSI, which again, is above the manufacturer listed maximum pressure. Maybe I'm over-thinking this....

-Tatortot

 

[SVO]

You may be over thinking this but if it truely worries you, then run pure nitrogen instead of air in your tires. Under inflation is a bigger tire killer than over inflation.

 

[22229stingray81]

Roger that. Thanks!



-Tatortot

 

[bill81vette]

I would go with whatever is on the tire.

 

[SVO]

This post left me wondering about a couple of things and I made trips to visit a couple of fellow club members and their cars to clear up the questions I had. O.K., first, I wondered about the statement that the tire had a max tire pressure of 35 psi. Looking at a BFG TA radial side wall, in size P-255R15-60, it states that the tire is rated to carry a 1,642 lb load at a maximum tire pressure of 35 PSI. Which means that 4 tires will support a total weight of 6,500 lbs or so at a cold tire pressure of 35 psi. Your car probably weighs around 3800 lbs, and I would seriously doubt that one could pack another 2,500 lbs of passengers and cargo in a '81 Corvette. So you are way under what the max load the tires will safely carry. Second, Chevrolet recommends a 35 psig cold for the optional P-255 size tire. The '81 owner's manual states that "the tires on your vehicle will preform well at all normal loads when inflated as recommended on the tire placard (located on the left front door of your vehicle)." The owners manual also states that one "should increase the cold tire pressure by 4 psi when driving at speeds 75-85 mph ( where legal to do so)." It also states that "substained speeds above 85 mph is not advised unless your vehicle is equipped with special high speed tires available from many tire dealers." Note that the TA Radial is a "S" speed rated tire and should be held to that 85 mph max substained speed limit. The owner's manual has like 5 pages devoted to tires for you car. If you don't have the owner's manual, reproductions of it is available from most Corvette parts vendors. Also, BFG has an excellent customer service that can answer most questions you might have about their tires.

Anyway, hope this helps.

Bob

 

[22229stingray81]

Bob,

Tremendous help, thank you! As you said, I was unsure about the max PSI to weight relationship, meaning I knew that I had a lot less weight then the specified tire was rated for at the specified PSI, so my thoughts were that all was probably fine. I just wanted to make sure that I wouldn't be rolling down the road and pop all 4 tires some day :L

I'm paranoid, and just wanted a little reassurance. Everything you've said sounds right. Thanks again!

-Tatortot

 

[Vettehead Mikey]

You may be over thinking this but if it truely worries you, then run pure nitrogen instead of air in your tires. Under inflation is a bigger tire killer than over inflation.

If the myth about nitrogen expanding less than air was true, running pure nitrogen would lead to under inflation at highway speeds. Since it is just myth, nitrogen will neither help nor hurt.

 

[imarivertwo]

I think the manufacturer's recomendations may be based on a blend of safety, ride comfort, and handling. I believe that most things are underated for liability's sake, so I have always overinflated my tires. They run cooler, give better MPG, handling is more responsive, etc.
Just my opinion . . . .

 

[toobroketoretire]

I have always run 20 psi front/20 psi rear in my '82 and 24 psi front/20 psi rear in my big block '71. The huge "balloon" tires used on the C3's only require 1/2 the maximum pressure because the tires are only supporting 1/2 the maximum load. The front tires on my big block '71 have 60,000 miles on them and they still have about 3/16" of tread left. And the lower pressures will give a real nice soft ride. Higher pressures will give better handling but at the expense of severe tread wear in the centers because of the over-inflation.

 

[Tom Bryant]

[TABLE="width: 750, align: center"]
<tbody>[TR]
[TD]Some interesting reading from Get Nitrogen[/TD]
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[TR]
[TD][TABLE="width: 746"]
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[TD="width: 123"][/TD]
[TD="width: 125"][/TD]
[TD="width: 125"][/TD]
[TD="width: 125"][/TD]
[TD="width: 125"][/TD]
[TD="width: 123"][/TD]
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</tbody>[/TABLE]
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[TD][TABLE="width: 746"]
<tbody>[TR]
[TD="align: center"]NASCAR[/TD]
[TD="width: 33%, align: center"]SAVINGS CALCULATOR[/TD]
[/TR]
[TR]
[TD] [TABLE="class: columnOneContent, width: 100%"]
<tbody>[TR]
[TD]NASCAR Information
Fundamentally; air, oxygen and nitrogen will all behave exactly the same in terms of pressure change for each 10 degrees of temperature change. However temperature alone is not the whole story.
Ambient air contains moisture, nitrogen does not. If moisture is present it contributes to a greater change in pressure simply because at lower temperatures water condenses to become a liquid. The liquid form of water occupies very little volume and contributes only a negligible pressure to the tire. But at higher temperatures, such as those in a running tire, water evaporates inside the tire and becomes a gas which increases pressure in the tire.
Ambient air contains about 21% oxygen. Oxygen’s smaller molecular size allows it to permeate through the rubber of the tire. By inflating with nitrogen, which is much less permeable than oxygen, the pressure changes due to oxygen loss are greatly reduced.
The racing industry is correct; nitrogen is more predictable. Because nitrogen is dry it has no moisture to contribute extra pressure changes with temperature. Because nitrogen permeates out much slower than oxygen pressure changes due to that leakage are almost eliminated compared with ambient air.
Let’s get a little deeper into the science. Keep in mind that the air in your tires changes about 1psi for every 10 degree temperature change. This means that a significant change in temperature will create a significant change in your tire pressure. Here is a set of Ideal Gas Law calculations showing the effects of a 10F degree temperature change on truck and passenger tires. The two sets of data represent different initial temperatures of 60F and 90F. This demonstrates that the magnitude of the pressure fluctuation differs depending on initial conditions but only slightly.

Calculate pressure change expected for each 10F degrees temperature change:The final pressure is calculated based on the Ideal Gas Law where, for this discussion, P and T change while n, R, and V are fixed or constant.
The Ideal Gas Law equation is P*V = n*R*T where: P = pressure, T = temperature, V = volume, R = the ideal gas constant and n = the amount of gas in the tire in moles.
Using algebra to isolate the variables of interest, P and T, the equation becomes P/T = (n*R/V).
Therefore Pinitial/Tinitial = (n*R/V) = Pfinal/Tfinal since n, R, and V are all constant. That is, we assume no volume (V) change (i.e., no significant stretching of the tire rubber) and we consider the amount of gas in the tire (n) to be constant because the time frame is very short compared to the time it takes for gas to permeate through the tire rubber. The ideal gas constant, R, is by definition constant and therefore cannot change.
As we have shown above Pinitial/Tinitial = Pfinal/Tfinal. This can be rearranged algebraically to Pfinal = [Pinitial * (Tfinal/Tinitial)]. This allows us to calculate Pfinal by multiplying Pinitial by the ratio of Tfinal to Tinitial.
Note: temperatures must be converted to Kelvin units (K), from Fahrenheit units (F), for this calculation. First, assume the tires are filled at 60F, to either 100 psig for a truck tire or 30 psig for a passenger tire:

Truck Tire, Initial 100psig, 60F [TABLE="width: 500"]
<tbody>[TR]
[TD]Pinitial (psig)[/TD]
[TD]Pinitial (psia)[/TD]
[TD]Tinitial (F)[/TD]
[TD]Tinitial (K)[/TD]
[TD][/TD]
[TD]Tfinal (F)[/TD]
[TD]Tfinal (K)[/TD]
[TD]Pfinal (psia)[/TD]
[TD]Pfinal (psig)[/TD]
[TD]Pchange (psig)[/TD]
[/TR]
[TR]
[TD]100[/TD]
[TD]114.7[/TD]
[TD]60[/TD]
[TD]288.6[/TD]
[TD][/TD]
[TD]50[/TD]
[TD]283.0[/TD]
[TD]112.5[/TD]
[TD]97.8[/TD]
[TD]-2.2[/TD]
[/TR]
[TR]
[TD]100[/TD]
[TD]114.7[/TD]
[TD]60[/TD]
[TD]288.6[/TD]
[TD][/TD]
[TD]60[/TD]
[TD]288.6[/TD]
[TD]114.7[/TD]
[TD]100.0[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]100[/TD]
[TD]114.7[/TD]
[TD]60[/TD]
[TD]288.6[/TD]
[TD][/TD]
[TD]70[/TD]
[TD]294.1[/TD]
[TD]116.9[/TD]
[TD]102.2[/TD]
[TD]2.2[/TD]
[/TR]
</tbody>[/TABLE]

Passenger Tire, Initial 30psig, 60F [TABLE="width: 500"]
<tbody>[TR]
[TD]Pinitial (psig)[/TD]
[TD]Pinitial (psia)[/TD]
[TD]Tinitial (F)[/TD]
[TD]Tinitial (K)[/TD]
[TD][/TD]
[TD]Tfinal (F)[/TD]
[TD]Tfinal (K)[/TD]
[TD]Pfinal (psia)[/TD]
[TD]Pfinal (psig)[/TD]
[TD]Pchange (psig)[/TD]
[/TR]
[TR]
[TD]30[/TD]
[TD]44.7[/TD]
[TD]60[/TD]
[TD]288.6[/TD]
[TD][/TD]
[TD]50[/TD]
[TD]283.0[/TD]
[TD]43.8[/TD]
[TD]29.1[/TD]
[TD]-0.9[/TD]
[/TR]
[TR]
[TD]30[/TD]
[TD]44.7[/TD]
[TD]60[/TD]
[TD]288.6[/TD]
[TD][/TD]
[TD]60[/TD]
[TD]288.6[/TD]
[TD]44.7[/TD]
[TD]30.0[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]30[/TD]
[TD]44.7[/TD]
[TD]60[/TD]
[TD]288.6[/TD]
[TD][/TD]
[TD]70[/TD]
[TD]294.1[/TD]
[TD]45.6[/TD]
[TD]30.9[/TD]
[TD]0.9[/TD]
[/TR]
</tbody>[/TABLE]


Next, assume the tires are filled at initial temperatures of 90F, instead of 60F:
Truck Tire, Initial 100psig, 90F [TABLE="width: 500"]
<tbody>[TR]
[TD]Pinitial (psig)[/TD]
[TD]Pinitial (psia)[/TD]
[TD]Tinitial (F)[/TD]
[TD]Tinitial (K)[/TD]
[TD][/TD]
[TD]Tfinal (F)[/TD]
[TD]Tfinal (K)[/TD]
[TD]Pfinal (psia)[/TD]
[TD]Pfinal (psig)[/TD]
[TD]Pchange (psig)[/TD]
[/TR]
[TR]
[TD]100[/TD]
[TD]114.7[/TD]
[TD]90[/TD]
[TD]305.2[/TD]
[TD][/TD]
[TD]80[/TD]
[TD]299.7[/TD]
[TD]112.6[/TD]
[TD]97.9[/TD]
[TD]-2.1[/TD]
[/TR]
[TR]
[TD]100[/TD]
[TD]114.7[/TD]
[TD]90[/TD]
[TD]305.2[/TD]
[TD][/TD]
[TD]90[/TD]
[TD]305.2[/TD]
[TD]114.7[/TD]
[TD]100.0[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]100[/TD]
[TD]114.7[/TD]
[TD]90[/TD]
[TD]305.2[/TD]
[TD][/TD]
[TD]100[/TD]
[TD]310.8[/TD]
[TD]116.8[/TD]
[TD]102.1[/TD]
[TD]2.1[/TD]
[/TR]
</tbody>[/TABLE]

Passenger Tire, Initial 30psig, 90F [TABLE="width: 500"]
<tbody>[TR]
[TD]Pinitial (psig)[/TD]
[TD]Pinitial (psia)[/TD]
[TD]Tinitial (F)[/TD]
[TD]Tinitial (K)[/TD]
[TD][/TD]
[TD]Tfinal (F)[/TD]
[TD]Tfinal (K)[/TD]
[TD]Pfinal (psia)[/TD]
[TD]Pfinal (psig)[/TD]
[TD]Pchange (psig)[/TD]
[/TR]
[TR]
[TD]30[/TD]
[TD]44.7[/TD]
[TD]90[/TD]
[TD]305.2[/TD]
[TD][/TD]
[TD]80[/TD]
[TD]299.7[/TD]
[TD]43.9[/TD]
[TD]29.2[/TD]
[TD]-0.8[/TD]
[/TR]
[TR]
[TD]30[/TD]
[TD]44.7[/TD]
[TD]90[/TD]
[TD]305.2[/TD]
[TD][/TD]
[TD]90[/TD]
[TD][/TD]
[TD][/TD]
[TD]30.0[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]30[/TD]
[TD]44.7[/TD]
[TD]90[/TD]
[TD]305.2[/TD]
[TD][/TD]
[TD]100[/TD]
[TD]310.8[/TD]
[TD]45.5[/TD]
[TD]30.8[/TD]
[TD]0.8[/TD]
[/TR]
</tbody>[/TABLE]
[/TD]
[/TR]
</tbody>[/TABLE]
[/TD]
[/TR]
</tbody>[/TABLE]
[/TD]
[/TR]
</tbody>[/TABLE]

 

[Tom Bryant]

[h=1]Interestingly, this article about air pressure states the exact 1 psig change in pressure for every 10 degrees temperature change as nitrogen.

Tire inflation pressure and temperature[/h]As discussed in some of our other articles on inflation pressure, tires must be properly inflated to achieve maximum handling, traction, and durability as designed by the tire manufacturer. It is the air pressure that supports the weight of your vehicle, not the tire itself. Tire pressure should be monitored frequently for safe driving and optimal tire performance.Tire pressure can change with fluctuations in temperature. So, how should you correctly monitor tire pressure as the outside temperature changes?First, it's important to remember that gas expands when heated and contracts when the temperature declines. In North America, the daily temperatures rise and fall between day and night, as well as seasonally. As the days get shorter and colder during fall and winter, it's especially important to check your tire pressure.Second, it's important to know that the recommended tire pressure for your vehicle (as specified in the owner's manual and the tire placard for the vehicle) are both based on cold inflation pressure. This means that the tire pressure should be checked in the morning before the tire has been run, before the ambient temperature rises during the day, and before the tire is exposed to direct sunlight.A good estimate to use when comparing tire pressure to air temperature is for every 10 degrees F, tire pressure will adjust by 1 psi. For example, if the outside air temperature increases 10 degrees, the tire pressure will increase by 1 psi. Conversely, if the air temperature falls 10 degrees, the tire pressure will decrease by 1 psi.In most parts of North America, the difference between average summer temperatures and average winter temperatures is about 50 degrees F. This means that your tires will fluctuate approximately 5 psi (assuming no other air loss) between the coldest and warmest times of the year. A drop of 5 psi during colder months will affect traction, handling, and durability. This is why it's important to remember to check your inflation pressure, especially during colder times of the year.In most parts of North America, the average daily air temperature fluctuates by approximately 20 degrees F. Using our rule-of-thumb formula from above, this means that the inflation pressure can fluctuate by approximately 2 psi during the day. This fluctuation can be even more pronounced if the tire is subject to direct sunlight.In addition to changes from sunlight, the tires temperature is also affected by driving. Tire pressures can increase by up to 5 psi in the first 20-30 minutes of driving, before finally stabilizing.If you live in a cold climate and have a heated garage, the opposite problem can occur. As soon as you leave the confines of your heated garage and go out into the colder weather, your tires can lose inflation pressure. Again, for every 10 degrees F dropped, the tires will lose approximately 1 psi each.In all three scenarios, the same problem arises, regardless if you check the air pressure when the tires are warm from driving, when there are ambient temperatures, or if the tires are in direct sunlight. If the inflation pressure is correct when a tire is warm, then it will likely be anywhere from 2-5 psi under-inflated when it is cold.Bottom line -- you should always check your tire inflation pressure when your tires are cold, and inflate them to the recommended pressure at that time.

 

[toobroketoretire]

Measuring Sidewall Bulging

Two years ago I decided to run some tests to see just how much my 255X60R-15's sidewalls bulged at lower pressures. Because it is the sidewall flexing that leads to blowouts at high speeds. I made a giant caliper then inflated one of my rear tires to 35 psi and measured it's total width across the sidewall bulges. It measured something like 11-1/4". Then I dropped the pressure to 30 psi, 25 psi, 20 psi, 15 psi, 10 psi, and finally 5 psi. What I found was the tire's total bulging only increased 1/16' for each 5 psi reduction and the sidewalls BARELY bulged until the pressure dropped below 5 psi at which time the width rapidly increased. So unless the pressure was below 5 psi it was impossible to determine how much pressure it had by just looking at it.

 

[kpic]

Then I dropped the pressure to 30 psi, 25 psi, 20 psi, 15 psi, 10 psi, and finally 5 psi. What I found was the tire's total bulging only increased 1/16' for each 5 psi reduction and the sidewalls BARELY bulged until the pressure dropped below 5 psi at which time the width rapidly increased. So unless the pressure was below 5 psi it was impossible to determine how much pressure it had by just looking at it.

1/16 of a foot??

Anyhow, is that why you under inflate your tires??

 

[toobroketoretire]

Blown Tires On Ford Explorers

If I recall correctly Ford had recommended 25 psi in the Explorer tires and when a tire burst and the Explorer rolled over investigators found the other 3 tires had an average of 19 psi. But how many psi did the blown tire have just before it blew? Under 5 psi? There's no way of knowing but after doing the "bulge testing" test two years ago on my 255X60R15 I suspect the blown tires had a LOT less than 19 psi. Something to ponder anyway.

 

[kpic]

So what does a Ford Explorer have to do with a Corvette?? Other than Ford had problems.

Curious minds want to know....

 

[toobroketoretire]

Tire Bulging Test

The tire bulging test I ran two years was really revealing as it showed a tire hardly bulges until it's almost out of air. Each 5 psi drop in pressure only caused the tire to bulge a total of 1/16" or 1/32" on each side. It maintained it's original appearance until the pressure dropped below 5 psi and THEN it began to widen enough to notice. So when you see a car going down the road with a low tire it's REALLY low and it's those tires that will blow out when run at higher and sustained freeway speeds.

 

[kpic]

Tire manufacturers do a lot of research with tire pressures. True, up or down a few PSI may or may not be good. However, your under inflation pressures will show abnormal tire wear. It is what it is, time to accept reality.
You measured under inflated pressures?
A. Who cares?
B. Who would waste their time on something so flat stupid?

Point is, each and everyone of us needs to accept there are people who are smarter than us. Simply because they do it 8 hours a day.
Your problem is you're are dead set and determined you can reinvent the wheel and same as anyone else you can't. Time to deal with your shortages..

There is an old saying, a legend in one's own mind. You are the most perfect application of that saying, I've ever experienced.

BTW, I've read Comet is less abrasive than Bon Ami; so try it to seat your new rings. Then again, Ajax the foaming cleanser might be the "trick" setup.