B-Line

Eclou,

As much as I appreciate your zestful response, it is a bit out of context.

1) Bobby Ore credentials:
"Professional Racing and Test Driving:

Formula1, Formula II, Formula 3000, Jaguar, BMW, Ferrari, NASCAR-Winston Cup Grand National, Daytona 500, 24 hours of the Le Mans-C2, SCCA, Double-A-Fuel Dragsters, along with Pro Stock and Funny cars.
Driving and Exhibitions:

Under contract with Chevrolet for five years, Automobile dealership product demonstrations and Drive and Ride events throughout the U.S., Britain and Europe for Chevrolet, Oldsmobile, Pontiac, Jaguar, BMW and Mercedes Benz. Oklahoma and New York State Fairs, Monster Truck Shows and Tractor Pulls for SRO Sports, high schools and universities and various charities.
Driving Instructor:

Police Officers Standard Training Certified, Highly advanced counter Terrorism Driving Instructor for the FBI, CIA, NIS, and various government agencies. British Military-MI6 Driving Instructor, Former Secret service drivers, Emergency Vehicle Operations Center training Instructor, Executive Protection, Special Forces, Law Enfocement Instructors, Motion Picture Personnel.


2) Once you start talking about RACE SETUPS which inlcude varing degrees of camber, toe, etc, the tire is no longer running "flat or even" because of the preset suspension setup.

So your retort, while very cute, is quite pointless, unresearched and makes you seem a bit like a smartass without a qualified train of thought.

B

texas911

I'm just going to go with Porsche's recommendation. 33/39.

B-Line

Texas,

I think that's a good idea.

I tried the additional air pressure in the tires and I did not like the ride feel. I have no doubt, my tires would last much longer with more PSI, but I don't own a Porsche to save on tire cost.

Who cares if I need to replace the tires.

The only point I was trying to make was, tire pressure is a subjective opinion. There is no right or wrong answer, it's all a trade off. You can trade ride comfort for longetivity or performance for comfort. So the only right answer, is how does the car feel best for you and can you live with the cost of running the tires at that PSI..

B

Oh, there might be one other side of this argument. PERFORMANCE PSI.. Well, do you want the best performance to be when tires are cold(er) or when the tires are hot(er)? If your on the track, you might want your tires to reach peak performance after your first or second warm up lap, which means, you have to run lower PSI, so the tires can build into ideal PSI.
But if you want optimal street performance, where you usually don't have the opportunity to get the tires real hot and gooey, your better off starting with a higher PSI, closer to the ideal, as you won't really be able to warm em up, the way you do on a track.

eclou

B,

I am certain Bobby is a better driver than myself. Being a better driver does not equate to being more intelligent nor educated. His "max tire pressure" suggestion doesn't hold up to scientific or empiric scrutiny. I have worn out tires prematurely due to over inflation. I have experienced reduced traction due to over inflated tires. If you believe his theory then I'm not going to stop you. My train of thought only follows the laws of physics. If you find the laws of physics to be unqualified and troublesome then I doubt I will convince you otherwise.

anonymousagain

should be easy, but always tire specific (Mfr, size, load):

1) Estimated Vehicle weight; driver and "typical" load (passenger(s), cargo)
2) Front / Rear ratio, to determine total load Front and Rear
3) Front weight/2 = individual front wheel load
4) Rear weight/2 = individual rear wheel load
5) Tire max load/max psi = load per psi
6) Individual wheel load/load per psi

Example: [sorry, edit 'cause F/R were backward]
1) 3700 vehicle weight
2) 51/49 = 1887 F, 1813 R
3) 1813/2 = 907 R
4) 1887/2 = 944 F
5) 1477/51 = 29 R ; 1477/51 = 29 F
6) 907R/29R = 32psi Rear; 944F/29F = 33psi Fromt

To ballpark if you don't know weight or F/R ratio:

1) Fill to max psi
2) Using chalk (sidewalk chalk is easiest), mark a solid line across the tread surface
3) Drive a short distance (around a large parking lot works well)
4) "Read" the chalk wear on the tire
5) Reduce/lower pressure until chalk mark wears evenly across the entire tread surface

Good luck!!

B-Line

EC, It's interesting what you say but unfortunately, not correct. If you could show me this scientific or empiric scrutiny you so freely quote, I'm happy to read it. But just because you say his theory is wrong and throw in some technical sounding words, does not make you right. And, your experience of ruining a tire by over inflating it, does not mean that inflating a tire to max pressure (suggested on tire) is going to ruin other people's tire. Your arguement, If A then B = C is a NON VALID, FALSE argument.
"I over inflated my tires then my tires were ruined = high PSI ruins tires" is not a valid, scientific arguemnt. Any freshman in college who knows anything about LOGIC/truth tables can tell you that.

Now, you also keep saying OVER inflate. I am saying, MAX inflate as per the tires suggestion. I would assume, if you had a tire with uneven wear, then you over inflated it, yes, that would ruin a tire. But if you have tires that are not out of whack, and you pump em all up, they will wear nice and evenly.

As far as your other argument, reduced traction, this also came up. You do not lose traction, you gain traction. Why? Well, lets talk about these big words you like to throw out like physics and empirial evidence.
The more PSI, the bigger the contact patch between road and wheel. And as we all learned in our first day of high performance driving school, we do things like BRAKE to increase the contact patch to improve grip. So a higher PSI means more contact width = more grip.

Having said that, I do agree with you EC, the SENSATION of driving a car with lots of PSI, is LESS grip.. But that's just the sensation, not the reality. The car feels jumpier, more high strung, more apt to break free. And you have to adjust your driving style to accomodate to the increased PSI. But that does not mean there is less grip and less physics at work.
-- Think of jumping rope. If you move your hands faster, you have to move your feet faster to compensate, otherwise you trip. Same thing with PSI. If you pump up the PSI, you have to make adjustments to your driving style to accomodate it.

I want to further say, I don't like driving a car with high PSI. To me, it's disconcerding, bumpy, squemish... I would prefer to let the tires roll over, wear out faster, and have a smoother, more comfortable ride.

But when talking about correct tire pressure. There is no one exact answer. It all depends on what you are trying to get from the car. If you want longetivity and performance, go with a higher PSI. If you want comfort and security, go lower. They are both formulas that work.

B

eclou

B,

you are now contradicting what you have preached via the "Ore-acle." Now that "Max" psi has become "ideal" psi, you yourself are making more sense than Bobby. A tire's shape depends upon the amount of pressure and load. Pressure changes with temperature - PV=nrT (ideal gas law). Tires change shape with pressure. Tires also change shape with load. Changes in load and changes in pressure both change the shape. Tire pressure should be adjusted to match the load for the "ideal" psi. How can anyone state that airing up a tire to max pressure is always correct? The loads/weights of the car will always vary.

B-Line

EC, good question. The max load formula is more of a guide then a law. We asked Bobby how do we calculate how much PSI to use in the tire. His answer was simple. If the tire says, max PSI is around 51, you probably want to use a pressure of around 45-50, depending on application. PSI will change and you are trying to find a number that is not too high or not too low, rather has a little wiggle.
-- Again, don't quote me here exactly, I am trying to paraphrase.

eclou

B,

you fail to fully grasp the concept that a tire is round. If you take a bowling ball and place it on a table, it only contacts the table in 1 small place - 1 point of contact. This is what a tire inflated beyond "ideal" resembles. If you place a water balloon on a table, it flattens against the table to some extent. Lots of contact area. This is what a tire is supposed to do at "ideal" psi - it can flatten slightly against the road for maximum contact area. It will have FAR more contact area than the bowling ball. Please tell me you can understand this.

B-Line

I also want to add EC, and this is what you NEED to understand,

TIRE MANUFACTURERS keep the recommended PSI ratings low for two reasons.
1) Lower PSI = More wear = More replacement tires
2) The ride is more comfortable.

But the tires are capable of MUCH MUCH higher PSI's, which reduces wear and increases performance.

B

B-Line

with less PSI, the middle of the tire (inside the tire) "caves in" so to speak, buckles..
With more PSI, the middle of the tire (inside the tire) "pushes out" creating more contact.

eclou

B

Tire manufacturers have something called a "warranty" for mileage. If their tires wear out too soon they pro-rate the tire. If the tires always wear out too soon people will buy another brand

Underinflated tires are not very comfortable when you lose traction or when you pinch flat the tire.

Higher PSI does reduce wear only when the maximum contact area is achieved. This is ideal PSI . Inflate beyond this and you will wear out the center of the tread prematurely, and you will have less contact. Look to the right side of the picture

B-Line

EC,

Based on your diagram, your argument makes sense, but a "properly inflated tire" can last 70,000-80,000 miles (according to ORE-acle) on a 5000 lb. Expedition for example.

So, if the tire manufacturer is offering a 30,000 mile warranty, under what they say is ideal, in reality, the tire is underinflated because you should be able to get 70k-80K or more out of them.
In other words, your diagram is right, your theory is right, but your numbers and the numbers the tire manufacturer are giving you are WRONG. A properly inflated (but less comfortable tire) should be way higher PSI. To the extreme that "properly inflated" equals PSI in the 45 to 50 range, depending on tire, car, etc.

What the manufacturer is giving you is a balance between proper PSI for WEAR and what it thinks is the proper PSI for COMFORT. They are not mutually exclusive.

So Tire company A says, the right pressure for long wear =50 but that's a rough ride and no one will buy the tire. So, lets tell them to set it at 35, it will still last 35k miles, etc. and will have a good balance between comfort, sport, longetivity..

But you can get those same tires to give better performance and longer life.

eclou

B,

The Expedition tires, such as Wrangler RT/S and others, don't wear out because the rubber compound is hard as nails, and the rolling diameter is close to 31", vs 24-26" for a passenger tire. The truck tires will undergo far less revolutions than a passenger car tire. At 35psi, my Wrangler RT/S tires still had more than half tread after 60k miles on my Z71 truck. They had absolutely no grip though. Some migrant worker is running them on his truck now. He will probably get another 60k miles out of them. I never ran 50 psi. If I did, the tires would have been overinflated and worn out earlier. Maximum life of the tire will be at the pressure that gives the maximum contact patch at that given load.

B-Line

EC,

We can go back and forth all day long on this. Bottom line is, tire pressure is somewhat subjective. Where are you? What are the conditions? What do you want to achieve with the car?
There are performance settings, comfort settings and settings that help extend the tire life.
They are mutually exclusive.

Again, I will state, that I did not like driving with a high PSI. I lean more towards the factory setting. But if someone where to ask me what my tire pressures are for track use, I also have to ask, HOT or COLD? and then further - When do you want to feel peak performance with the tire, in the beginning, middle or end of the session.

I used to run Kumho's, heat cycled and shaved for my lap days at Sebring. The guys on driving the Hoosier's would disappear at the beginning of the session. They heat up and gripped quickly. My Kumho's, took laps and laps to get strong. By the end of the session, I caught up to and passed the guys on Hoosiers that were slipping all over the track.

Point is, picking a PSI or a tire compound for that matter, there is no PANACEA. No one right answer. It's about making good adjustments based on a sound understanding of what you want from the car.
If I'm driving the canyons of Malibu, I'm going to set my PSI different than I would if I were going to Laguna Seca. In Malibu, the tires don't get nearly as hot, but I need grip at a moments notice. At Laguna, the tires will run much, much hotter, but I also need them to last a lot longer.

B