Is the proper air pressure your run in your tires determined by the size of the tire, or the vehicle weight?

For example, if on the ITR I run 32front 30back, and I switch to a new tire size, do I still run the same pressure?

 

Don't forget about temperature. No matter what tire I'm running, that's usually the deciding factor for me...

 

It is determined by both tire sizes and weight of the car along with other consideration like fuel milage, race or street driving, type of surface (if you are racing)...
Basically, on asphalte, you want the contact patch between the tire and the road to be as big as possible while still be in the "tread" portion of the tire.

 

so is there a way to figure out what you should run besise simple trial and error?

 

Just start with the stock recommended pressure....

 

I think your pressures might be too low.

But obviously, this all depends numerous factors.....but, most ITR drivers run mid30psi to low 40psi for 15" tires. Kumho's usually need higher pressures and other better brands may be able to get away with lower pressures.

Here is how we can help you:
1. What brand and size is your tires?
2. What kind of driving to you do? (spirited, just highway cruising, autocross, etc.)
3. Do you want your car to oversteer? feel spirited? or very predicatable?

Believe it or not, little changes (difference of 2psi) in tire pressures do affect the way the car handles at the limit.

 

I believe it is determined by the weight of the vehicle. We measure it in pounds per square inch, right? So if your running a tire at 30 psi it is distributing 30 pounds over every square inch of contact patch. You can see how a contact patch can dramaticly change with small changes in tire pressure.

So with a new tire size, at the same pressure, you have the same size contact patch, just not in the same shape. How this effects handling I don't know.

 

Here we go again.... You forgot the stiffness of the tire!!! The stiffer it is the more it will support the car in addition to the air pressure!!!! Increasing the pressure doesn't decrease the contact area linearly due to the stiffness of the tire.

 

A little trick I was given while at the track is to see just how far does the wear mark from the road goes along the edge of the tread surface.
You start at a certain pressure (recommended pressure for example) then you go and do a few laps and come back in the pits. If you look at the side of the tires, you will see where the tires made contact with the road while in hard cornering. If these marks coincide with the edge of the tread surface this usually mean you are using a tire pressure close to what you need (after that it is driving preferences).
If there is a part of the tread that is not used then the tires are a little overinflated and if the marks go behond the tread surface then the tires are underinflated.

Of course this trick is for road racing. For street use you should be good with recommended pressure.

 

There's no need to maintain exactly the same size contact patch. You need to figure out what pressure works best with any particular model and size of tire. If the pressure changes, so does the size of the contact patch (as you correctly note), and that's okay.

Not true. As long as your tires are fully inflated, the stiffness of the sidewall does not significantly support the weight of the car. The size of the contact patch does indeed depend only on the pressure in the tires and the weight of the car.

 

Doesn't a larger contact patch generaly lead to better handeling?

 

Now when do you consider it as fully inflated? Does the influence of the sidewall suddenly disappear (=totally zero) at a certain pressure? Does that equation applies when the tire is overinflated? When is it overinflated?
And what do you mean by "not significantly support" and then "depend only"? Even if it's not significant it's not zero!!!
We are talking about theoretical model (= ignoring all the side factors) compared to real world situation.

And are you saying that if I keep the weight of the car and pressure the same but I swap the tire with a very soft one (like a balloon) then the contact area will be the same?
Are you saying that your equation is always true?

Link added: http://www.howstuffworks.com/question506.htm


BTW In science, when you find one (=sufficient) example that proves the equation is wrong, that equation is consider invalid. Then it needs to be corrected or updated so that it aplies at every situation.....


[Modified by EuroITR1689, 6:25 PM 4/10/2002]

 

The tires are fully inflated if the air in them is supporting the weight of the car. And that's what happens when your pressure is 33F/35R (as recommended) or anything close to that. As long as the tires aren't flat, the sidewalls are not supporting the car. The air in the tires is.

It's amazing how, whenever a general rule is stated (like the contact patch being based only on pressure and the weight of the vehicle), there's always some idiot who asks about flat tires (like the 7.5 psi in the "How Stuff Works" article) or balloons or some other stupid hypothetical situation.



[Modified by nsxtcjr, 12:51 PM 4/10/2002]

 

Actually, temperature doesn't determine the proper air pressure in the tires. Temperature determines how much air you need to put in the car to achieve the proper air pressure. But you'll want to use that same pressure reading regardless of how warm it is outside.

If you normally drive your car at (for example) 35F/33R, then you want to do that regardless of whether it's warm or cool outside. You'll want the pressure to be 35F/33R when measured cold at the normal outside temperature, whether it's 90 degrees outside or 50 degrees outside.

However, you may need to adjust your pressure if you measure it where the temperature is different from normal. The Tire Rack recommends a rule of thumb of a 1 psi difference for each 10 degree difference in temperature, assuming you're not adding air to the tires or removing any. For example, if you normally drive during the day when it's 90 degrees out, and you're measuring the tires in your 70-degree air-conditioned garage, you want to adjust by two psi to compensate for the change in temperature. So you would want your pressure to read 33F/31R in your garage. When you get outside where it's warmer, the tires will have 2 psi more pressure due to the temperature, so you will be at your desired 35F/33R.

 

Absolutely.

Let's say you have a car that weighs 2640 pounds, and you have all four tires inflated to 33 psi. The sum of the area of the four contact patches will be 80 square inches. It will be 80 square inches regardless of whether the tires are soft or hard, whether they're 185 mm wide or 275 mm wide - as long as the air in the tires is holding up the car.

If you believe otherwise, please tell me how you can divide 2640 by 33 and get an answer other than 80.

 

This is completely wrong. The sidewall of the tire does not support the car - do you really think that that a little piece of rubber is capable of supporting a possible 1000 lbs per corner?

The area of the contact patch is based solely on the weight of the car and the tire pressure.

That is why magazine reviewers who talk about wider tires putting more rubber on the road are full of it.


[Modified by norice, 10:50 AM 4/10/2002]

 

You cannot remove from the equation the different rigidity of different tire walls to calculate the appropriate air pressure needed. If so how can you explain the "run flat tires" standard on the Corvette and other models . It is capable to run with no air pressure because of extra reinforcement in the tire walls. ( Tires are not made of soft rubber only). The more rigid the tire is,the less importance air pressure as in determining the size of the contact patch. Of course a tire that is to rigid will not give you a big enought contact surface and you will be sliding all over the place. It is all a question of balance and different need for a tire. Your way of caculating is correct if you have a tire that as no strenght a all in the side walls.

 

Looks like every idiot on the board wants to talk about flat tires. Maybe because they're driving around on four flat tires?

If the tires are flat - or close to flat - the air in the tires is not holding up the car, so the size of the "contact patch" is not relevant.

I suggest you all re-read the link to "How Stuff Works" that EuroITR1689 posted above - particularly the part where it says how you can calculate the size of the contact patch when the tires are at 30 psi, but not when they're flat at 7.5 psi.

As long as the air in the tires is holding up the car, the sidewalls are not. And the area of the contact patch can be found by dividing the weight of the car by the pressure in the tires. Even on the Corvette.



[Modified by nsxtcjr, 1:40 PM 4/10/2002]

 

A special molded insert is what gives these tires their run flat ability - not the sidewall stifness.

 

So what you are saying is that the materials used to build a tire has no efect what so ever on the surface contact on the ground ?. It is just weight divided by air pressure? They could use any material on this planet and the result would be the same? Yeah that sound right to me... I'm sorry for being such an idiot....

 

As long as it's a flexible material holding the inflated gas which holds up the car, yes.

Funny how the same idiots who ask about flat tires also ask stupid questions like, "What if the tires were made only of steel blah blah blah"?

Apology accepted.

 

Jeeze, man. Go get a cup of coffee. This misconception is not unusual. I would guess that less than 5% of car heads even know that the sidewall does not support a car but that the air does.

 

I know sidewalls don't carry the weight of the car in common tires. I just don't like people giving an answer as if it was an absolute truth with not possibility of variance. And then calling everybody an idiot who don't say: "thank you master for sharing your great knowledge, I will take this as the word of god".
That being said let's releive some pressure and let the happy patch grow in surface... (this is an happy patch)

 

Yeah, I bumped up my front tires by 3psi for an autocross and it felt soo much crisper.

For my mustang(around 2850-2900lbs)
1. Falkin Azenis 205/55/16
2. Spirited, and maybe an autocross
3. spirited I guess. Dont want it to break lose in a corner though.

For my ITR
1. Stock RE010
2. Both spirited, and autocrosses.
3. Same as above.
Also, for the ITR, I'm trying to burn these old tires up because I have a new set taking up room that I need put on.

 

Can't you keep this nice and "scientific"? Is this your way to convince people you are right? Well, it doesn't really work.

Your theory is actually very very easy to verify.
Just go to the garage and measure the contact area of all 4 tires. Then reduce the the pressure with 10%, 15%, 20% (or increase it). If your theory is right, the contact area should increase with 10%, 15% and 20% accordingly.
Then try the same with a different car with stiffer or softer tires. I have already tried this, so I know the answer.

Actually, you are already setting up boundaries for when your theory is valid. That is exactly what I tried to say. I tried to say that your theory is not ALWAYS valid. It looked like you were trying to convince people that your theory is ALWAYS true.

Now you are saying it only apllies when the air is the only thing that supports the car. Of course that is true, that is the ideal case. The question is: when is it the case that the air is the only thing that is supporting the car? What happens if I keep the weight and tire pressure constant and but I swapped the tires with a much stiffer one? Is the influence of the sidewall still the same (according to you: zero, right?)?

Now try to convince people with arguments and proofs instead of calling people idiots..... Normally I don't drink, but I'm going to get some





[Modified by EuroITR1689, 1:39 PM 4/11/2002]