Willard:

Think of the water in the puddle as ink. After rolling through the puddle, the tread pattern that is transferred to the ground is the measure.

On belted radials, the length of the contact patch actually changes very little from inflation pressure. Further it has no bearing on center wear, which will be indicated if the tire is inflated to the point that it is picking up the edges.

No need to jack up the car to do this. Just drive through the puddle, continue driving until you leave no tracks, then walk back along the path viewing the contact area.

 

Boy I'm glad I'm reading this after everthing is settled.

wwest, you sound like someone with an axe to grind. I think you have a story to tell involving personal experiences with tires and a wreck. If you do, even though this really isn't the place, I would love to hear it.

However, just because someone once had a nasty problem caused by running through the house with a chef's knife, that doesn't mean you have to stop every kid from running around with utinsels. A plastic spoon just isn't that dangerous.

Lets take an objective look at tire pressure recomendations...first, no car or tire manufacture still has a single recomendation. If you look through the manuals of a car, they usually have additional sections that increase the recomended pressure if you are driving with heavy loads (like towing) or driving at high speeds. Even if you don't see those, if you ask the manufacture they can often provide. However there is a problem, car manufactures don't know what tires you have on your car. They know what tires ship, but those will usually be repalced several times in the life of the car. As such, they probably pick a presure which gives a comfortable ride, that doens't exceed the max sidewall pressure of any standard tire that fits the cars. They also assume you drive you car conservativly and within all legal limits. So Ford/Toyota/etc don't know how you will drive (speed), how much weight you will cary/tow, or what tires are on your car. That one suggestion they put in their books is pretty limited, and some of the manuals now even recomend those pressures only for the stock tires with a reference to the tire manufacture info if you are doing something different.

Tire manufactures have limited room to print on the sides of their tires...frankly the writing is small enough, and I'd rather they printed nothing and gave me a nice smooth tire. That aside, they do have published recomendations/limits based on weight and speed, with the psi increasing as either weight or speed increase. They have publication limits too, there are too many cars to make specific recomendations for every car a given tire might fit, so they come up with charts, and its up to the consumer to figure how how much weight is on each corner of their cars.

In all this uncertainty, there seems to be one underlying truth, going to max sidewall presure listed by the tire manufacture produces the safest ride in most conditions. Notice I qualifty too, I'm not telling anyone to change their pressure to anything. Anyone reading this who can't look at all the data and make up their own mind, should contact both their tire and car manufacture, explain all the details and ask for their best recomendations. Chances are they aren't going to tell you anything outside their safe, lawyer-inspected, published data, but maybe you can get a specific number out of someone.

Let me be brutally honest about my "exploder" feelings...First, I have an explorer sport trac that was from around that time. I'm absolutly certain I could easily tip in over. I drive it very cafefully. Even though I'm usually surrounded by much larger vehicles where I live, I drive like I'm a abrams tank in a preschool playground. I don't like the "other kind" of people who disobey the rules and regulations of our road. And I absolutly hate those people who actively endanger others on the road with swearving, tailgating, running lights/stops...that sort of think
I dislike those people who dare wave American flag or say "I'm a vet", and I protected your American rights. I'm getting into dangerous teritory now. I agree that many people, both veterans and other have helped protect my rights and country. But, unless those vets producted their own everything, guns, ammo, uniform, protective helment, kevlar vest, etc they own us as much as we own them. Frankly we all are part of the solution and I do wish Vets would remember that. On the other hand, most of us didn't put ourselves in the line of fire, so I say thank you. (Now for the really dangerous part) However when vets come back and buy pick old pickup trucks, and drive them in illegally or immorally, and then actually have the gaul to suggest they have some special right because they served, I really want to do something special for these folks. Defending America in a war overseas then endangering American's when driving around their home town...I just don't understand they people.
My appologies, I let this get way off topic from the exploder thing...my own example of an axe to grind as I watched one of these yahoo's tnearly smash a small car off the road this morning. Getting back on the first off-topic, I feel people who drive their cars beyond what is legal, are entirely on their own...legally. When a tailgating truck has to swerve violently to avoid a head on colision and then they roll and get injured, I don't have a lot of concern for them. I remember reading through I lot of the docs back then, and clearly most (>50%) of wrecks involved driver error either through underinflation or, more commonly, bad driving. There were some that didn't fall into that catagory. Those people still made a mistake by doing a class action suit instead of suing indiviually. The problem with class actions is they lump things together...that give them more money and power, but it also mean that if the group of lawsuite isn't reasonable, then even the reasonable ones loose.


That said, I'm going to say what I do. What I do isn't necessary safe or within a given recomendation, but based on what I have read and my personal experiences to date, I'm happy with my choice. I run between max sidewall pressure and 2psi below max sidewall pressure. Generally I try to inspect tires at least montly, and I try to check my cold pressure at the hotest time of the day. If the temperatures are going to go up in the next few weeks, I keep my pressure a 1 to 2 psi under max-sidewall to allow for extra expansion during warmer weather.


Thanks.

 

Sorry, but that doesn't work. Low pressure tires do change shape slightly, but they add contact at the front an rear of the contact patch as apposed to the sides. Driving over a piece a paper wouldn't indicate how much tire was in contact at any given instance, it would only give you a nice tread pattern picture. Sorry.

 

Agreed, but we were discussing proving that max sidewall pressure inflated tires aren't lifting the edges and causing egregious center wear.

The test that I have proposed will 'out' a truly over-inflated tire.

MM

 

I didn't realize we were speaking specificly about edges at that point. Thank you for explaining.

 

Back in the day, when driving at the track (PIR road course) the practice was to run sidewall max pressure in dry conditions and reduce down to 20-25-ish psi for wet. In dry conditions the risk was peeling the bead off the rim during hard cornering. In the wet, traction was reduced enough to obviate that risk, and the softer fill maximized what traction there was.

Benton 16jun08

 

Yes, sure..

I've been stupid, UTTERLY STUPID.

FACTOID...

If raising the tire pressure results in improving FE than one MUST conclude that the increased pressure resulted in lowering the tire's, tires, contact "patch", CSA, with the roadbed.

Less contact with the roadbed, less traction.

Perfectly safe....???

NOT...!!!

But where is the "line"...??

Printed right there on the door post, the inside of the glovebox, or in your owners manual, by the people in a position to know BEST.

That's why what Ford did with the EXPLODER was so very, very wrong.

 

Less contact doesn't necessaily mean less traction.
Example: Ice skaters. They have excellent traction on tiny little bits of metal. Try to do the same thing on a smooth pair of loafers and you aren't going to go anywhere but the ground.

Same thing can be true with tires in snow or rain...cut the contact patch in half, and you put twice as much weight per square inch of the area which is in contact. You punch through the wet and snow and get to the ground which is really the only thing you can count on for solid acceleration and braking. If you look at high traction tire you find something interesting in their "agressive" tread...there is less material per square inch..they give somewhere for water and snow to go so the little bit that is there can get to something solid. If they can get to something solid, they still act like big paddles that dig in and push again the snow in front or behind. Once the compression of snow or mud reach a limit, they tire gets indirect traction.

Now back to the "people who know best", why do you think the car manufactures know more about tires than tire manufactures? Why do you think they give guidelines and charts instead of a single number which represent perfection.

I find it curious you waffle between "they know best" and psedo-science, if you have data which shows that tires inflated to max-sidewall instead of auto manufactures, please post it or a link to it. I can find lots of people bickering about it, but when you compare apples to apples, max-sidewall always comes out ahead.

While I found the previous curious, I find the following absurd, why would you suggest both...
1. that Ford, not the tire manufacture, was at fault with its inappropriately low numbers
2. and that car manufactures know more that tire manufactures
Isn't the first a perfect example that Ford knew less?

 

STUPID, probably not. Misinformed? Undoubtedly.

I don't know why you contine to throw the Explorer into a discussion about hypermiling Highlander Hybrids. This is known as "kitchen sinking" an argument. But since you did: They had problems with underinflation. The problem was exacerbated because people (like you apparently) believed what was written on the door post...

Traction and contact patch are not related in the way that you want them to be. That is one of the important points you need to grasp.

That would be why lap times improve as inflation pressure increases up to and past the max sidewall psi. At some point, way past that max sidewall, lap times fall off again.

<<
First Law of Friction

We all think we know how friction works. But pick up a physics book such as Fundamentals of Physics by Halliday, Resnick, and Walker, or a book such as Friction by Bowden and Tabor, and read the two laws of friction and you'll get a surprise. The first law is easy to believe: The friction between two surfaces is proportional to the force pressing one to the other. This force could be the weight of a motorcycle pressing the tire into the pavement, or the clamping force pressing two pieces of wood together. "Proportional" just means that if you double the pressing force you double the friction.

The second law is where all the trouble starts. To understand it, suppose you set up an experiment. You put a brick on a table and investigate how much force it takes to start the brick sliding. You screw an eyebolt into the brick, run a line from the eyebolt to a pulley on the edge of the table, and then attach weights to the end of the line. You add weight until the brick starts to slide.
Second Law of Friction

Now here's the interesting part, and the surprising part. You would notice that the orientation of the brick doesn't make any difference. That is, the friction is the same whether the brick is on its large face, the smaller side, or the small end. The friction is independent of the contact area.

Don't believe it? You're not alone. Bowden and Tabor, in their book Friction , tell about one of the first modern investigators of friction, Guillaume Amontons. In 1699 Amontons published a paper on friction in which he reported on the two laws. As Bowden and Tabor put it, "The second law, that friction is independent of the size of the bodies, was viewed by the [French Royal Academy of Sciences] with astonishment and skepticism. They instructed their senior academician De la Hire (1640-1718) to repeat Amontons' experiments and check their validity. This he did and confirmed Amontons' conclusions. Amontons' laws of friction have remained with us to this day as a very good working approximation."

>>

 

Sorry, I have a good old book on race cars. I wish I could find some similar quotes on the internet. The short version was talking about lateral G's associated with turns. Because we are pulled to earth at one G, it was assumed that car's couldn't turn at more that 1 lateral G or they would break traction. An absolute rule that couldn't be broken...of coarse it can. Today's modern slicks are almost like glue and easily allow more than one lateral G...and that's withou applying any downforce tricks. The last thing I read was early written in early/mid 90s are refered to a car from late 80's that achived a record 1.4 to 1.6 (don't remember exact number) of sustained lateral G's. And they were careful to demonstrate that there were no wings or vacuum effects creating "additional artificial gravity". It really is fun to read about some misconceptions associated with basic physics, although this is one of those that if I had been asked in high school if was likely to be true....I might have said yes.

Thanks MMooney, I'll definatly go check out the texts on this.