Tire pressure correction calculator?

[MGregoir]

I was just curious if such a beast exists, I figure it doesn't because of the varying moisture content of the air in tires affecting the pressures more than the temperature change itself.

A friend of mine has trouble with the tire pressure monitors in his fleet all the time when the temperature drops, so I was thinking about how to calculate a pressure that is above the TPM low at extreme low temperatures and below the maximum sidewall cold pressure on the high end. This would eliminate a major annoyance with weather changes and level things out.

I know about nitrogen but nobody does nitrogen fills around here and it is kind of a pain to get the hardware for just to fill the tires on hearses.

 

[Shannow]

Within reason, P1*T1 = P2*T2, where T1 and T2 are measured in Kelvins (degrees C plus 273).

It's a close approximation, that ignores any phase changes that may occur with the water, and change in internal volume with pressure.

Will give you an idea of what on the low end will give you on the top end.

 

[CapriRacer]

And for those mathematically challenged:

The rule of thumb is 1 psi drop for every drop of 10F. This works for Passenger car tire presssures - ~30 psi.

But for tires that use higher pressure, well, I've done the math, and the math says a 100 psi tire would be 4 psi for every 10F - But this doesn't feel right to me.

(BTW, I think the formula listed above is wrong: It should be P1/T1 = P2/T2. That would rearrange to P1*T2/T1= P2

- AND -

Not only do you have to use absolute temperature in the P/T equation, but you have to use absolute pressure, too - so add 14.7 to the gauge pressure, do the math then subtract 14.7 back out.

 

[SrDriver]

Below is a web site that gives some good info about tire pressures.

Link Regarding Tires

With crude oil hitting a record over 100 Dollars a barrel yesterday and gas prices headed upward - many predict $4.00 a gallon for regular national average price, things like tire pressures can help save a few bucks.

 

[invaliddata]

To save fuel I recommend running all tires at the maximum pressure indicated on the tire sidewall. Personally I (and others I know) have experienced greater tire life from the increased pressure, and the car will handle better with less deflection in the tires. This is also the way to get the maximum load capacity out of the tires. The main downside of this is a harsher ride, but if it's not bearable one can simply remove some air afterwards.

 

[CapriRacer]

Originally Posted By: invaliddata
To save fuel I recommend running all tires at the maximum pressure indicated on the tire sidewall. Personally I (and others I know) have experienced greater tire life from the increased pressure, and the car will handle better with less deflection in the tires. This is also the way to get the maximum load capacity out of the tires. The main downside of this is a harsher ride, but if it's not bearable one can simply remove some air afterwards.


Sorry, but I think this poor advice - perhaps even dangerous. Here's why:

What is written on the sidewall has nothing factored in regarding what vehicle the tire is being applied to. The weight on the tire is going to create a footprint size that is related to the pressure. This means that a tire that the vehicle manufacturer says should be inflated to 26 psi, could say on the sidewall: 51 psi max - almost twice. This is going to make the footprint very small - and in wet conditions, might make the tire extremely slippery.

Perhaps better advice would be to add 3 to 5 psi above the placard pressure - assuming you are using the placard size.

 

[Cup of Joe]

Originally Posted By: CapriRacer

Sorry, but I think this poor advice - perhaps even dangerous. Here's why:

What is written on the sidewall has nothing factored in regarding what vehicle the tire is being applied to. The weight on the tire is going to create a footprint size that is related to the pressure. This means that a tire that the vehicle manufacturer says should be inflated to 26 psi, could say on the sidewall: 51 psi max - almost twice. This is going to make the footprint very small - and in wet conditions, might make the tire extremely slippery.

Perhaps better advice would be to add 3 to 5 psi above the placard pressure - assuming you are using the placard size.


Interesting, take a look here:

http://www.officer.com/web/online/Editorial-and-Features/Driving-Under-Pressure/19$27281

quick quote from the article:

"What is proper pressure?

The proper tire pressure for the Police Crown Victoria is 44 psi. If you look on the sidewall of the tire, you will see that it lists 44 psi max pressure. Regardless of what vehicle you have, use the maximum pressure listed on the sidewall. Higher pressure results in better performance, decreased tire wear, and it lessens your chance of hydroplaning at a given speed."

Thoughts???

 

[ccs v2.0]

The engineering optional courses must have paid off in police academy courses. Seriously, I wonder what technical background Sgt. Storton gained with his "Master's Degree in Adult Education."

 

[glennc]

Another thing to consider is the front/rear tuning balance of the vehicle. Many cars are tuned so that the front tires slide early, limiting traction. Adding a bit of pressure to the front can help with that (in general adding pressure will give more traction).

And it does depend on the tire. With my summer tires I was running 3-4 psi higher in the front tires (32/36 or so) to get the handling I liked in a 50/50 weight-distribution car. With my winter tires I have to run equal pressures or I get snap oversteer in wet conditions, which is scary.

Most drivers after performance will tend to run higher pressures, and the above linked article is in line with that.

Drivers after economy will also tend to run higher pressure, as it will help rolling resistance and usually also tire life.

What is best, as usual, depends on more than just one thing. You could end up with unsafe handling if you increase the pressures indiscriminately, and if you aren't familiar with your vehicles limits in different conditions then you could be in for an unpredictable surprise.

I do believe that just following the door placard compels you to adopt compromises that were made by people other than you on a one-size-fits-all basis and that with a bit of care you can improve the results you get given what is important to you.

 

[CapriRacer]

Originally Posted By: TaterandNoodles


Interesting, take a look here:

http://www.officer.com/web/online/Editorial-and-Features/Driving-Under-Pressure/19$27281

quick quote from the article:

"What is proper pressure?

The proper tire pressure for the Police Crown Victoria is 44 psi. If you look on the sidewall of the tire, you will see that it lists 44 psi max pressure. Regardless of what vehicle you have, use the maximum pressure listed on the sidewall. Higher pressure results in better performance, decreased tire wear, and it lessens your chance of hydroplaning at a given speed."

Thoughts???


Yes, do they arrest people for impersonating an engineer?

 

[mechtech2]

Plain air is about 80% nitrogen anyway.
It's the moisture in the air that causes the largest swing in pressure.

 

[Russell]

I saw a tire pressure calculator somewhere that estimated tire pressure for each axle. It took in account, axle load capacity, number of tires per axle, maximum allowable tire pressure and load capacity of tire. wish i knew where i saw it.

 

[Dominic]

Essentially the best way to figure out your tire psi range is as follows:
1. Measure tread width
2. Deflate tire to 5psi below door sticker and roll through water, measure the track laid down and look for dry spots.
3. Inflate 2psi at a time and roll through water, measure the track until the width begins to decrease (this is your max pressure).

The idea is you want all parts of the tire, center and corners, fully touching the ground when cold. As the tire spins and warms the pressure will go up but from my experience it really isn't more than 2-4psi even on a hot day @ 90mph.

If you run a tire underinflation it could come off the rim, and you won't be getting the same amount of force on the ground at all widths since the center of the tire will not be pushed down into the ground as hard as the corners (causing uneven wear).

If you run a tire over inflated you reduce rolling resistance because you're effectively stiffening and narrowing the tread contact patch. This makes it cut through water better yes, but it doesn't lead to better tire wear either because you wear out the centers. This is not as important on small width tires (195-205), but its very important on a big 275mm wide tire or larger.

I usually run 3-7psi above the door placard. On my Subaru I am currently running 40psi because anything less leads to worse handling and ride. That said, on my camaro I only ran 32psi because I had very stiff good quality performance tires that would wear out the centers if I went above 34 (and it was a heavier car too).

Tire pressure should NEVER exceed the sidewall marking of the tire, but for best wear the sidewall is not accurate. Most tire sidewalls are 44psi or 51psi based on the construction of the tire (performance tires are rated at 51 mostly).

In the end I don't use the placard that comes with the vehicle, instead I tend to stay in the 35-40psi range (less than maximum, but more than the car calls for) to get my best wear and performance.

 

[CapriRacer]

Originally Posted By: Dominic
.........Essentially the best way to figure out your tire psi range is as follows:
1. Measure tread width
2. Deflate tire to 5psi below door sticker and roll through water, measure the track laid down and look for dry spots.
3. Inflate 2psi at a time and roll through water, measure the track until the width begins to decrease (this is your max pressure).

The idea is you want all parts of the tire, center and corners, fully touching the ground when cold.........


Here's the problem I have with this - and the similar "Chalk Test".

http://www.geocities.com/barrystiretech/footprint.jpg

I got this from a book published by the National Highway and Traffic Safety Administration (NHTSA) titles "The Pneumatic Tire". It's a reference book over 700 pages long with different experts summarizing the available information on tires - with an absolutely HUGE set of source documents.

As you can see from the color footprint pressure graphs (Aren't the collors just terrific for visualizing the presssure!), under all 6 conditions - from fairly light load, high pressure to fairly low pressure and moderate load - the shopulder of the tire is in contact with the ground. From, this I conclude, that trying to use the footprint as a gauge on the proper pressure is far from foolproof.

I urge caution for folks who want to use this method. In particular, I urge everyone not to use anything below what the palcard says (if you are using the same size) for street usage. What you might want to use for racing is an entirely different matter.