Within normal operating limits, yes it does. Pneumatic tires used to be called balloon tires for a reason. Do this simple experiment, that I was assigned to do in high school physics class. Inflate one of the tires to 20psi, jack up the car and lower it on to a piece of graph paper. Measure the contact patch area. Do the same at 30psi and 40psi. The original experiment was to prove that pounds per square inch means just that. The contact patch area times the air pressure equals the static weight of that corner of the car.

 

Did it vary directly? If not, what is your point?

Also "balloon" tires of yesteryear had crap for casings. A modern radial has a much stronger, stiffer casing. Apples and oranges.

An earlier poster wrote that he lowered tire pressure to 'hook up'. That sounds like drag racing to me, in which case it may very well be that lower pressures produce improved results. Put some corners in there and the story changes.

 

I wouldn't call it ballooning but there is certainly a huge change in contact patch with varied tire pressures. That's all I'm saying.

 

Its one of the worst 'old wives' tales about tires is the contact patch varies directly with inflated pressure; the reason tire contact surface area does not change directly with tire pressure is because the tire structure is not flexible enough to do so as would a party balloon. When the air pressure is changed by 10% for example as 33 to 30 psi, the contact area changes only about 3% and the reason is the ridgidity of the tire structure prevents that from varying directly, as in 1% for 1%.
You can demonstrate that for yourself by dropping a weighted tire onto a flat surface. Cut out the impressions they leave and weigh them to compare the respective areas as we have done.
You can also think about the change if the tire structure were a brick; no matter how great the changes in air pressure would be the contact surface area doesn't change one bit.
The pressure the tire exerts on the road surface changes in relation to the change in the tread structure, not the inflated pressure. Its one of the cfritical attributes in determining the best tire pressure to use on a racecar.

 

 

I, too, have performed that experiment except carefuilly cutting out the contact patch and then weighing them. Trying to measure the area never satistically worked; too much variation in the 'area measurement' to adequately measure. Party balloons will indeed change contact directly with inflation pressure but tires do not; the tire structure prevents that.
In fact, if you have a truck tire you won't get much change at all. And if the structure were a brick there definitely won't be a change at all. The pressure to the ground is the 'vehicle' weight divided by the contact area, whatever it may be. That pressure does greatly affect the interaction between the tread and the 'road'. That pressure is vital to the 'tenticles' the tread inserts into the road surface toi achieve grip. And thats why the ideal pressure is only valid for that one instance.
The rigidity of the balloon has a lot to do with the change in contact area.
And back then balloon tires were a lot closer to balloons than now. Tire structures have come a long way since then.

 

I too was under the impression that tire contact Patch was based on weight and air pressure, is this not true?

 

It is indeed. The difference is the contact patch area is not DIRECTLY related to the tire pressure. As tire pressure changes the contact patch also changes but not directly, For a 10% change in tire pressure the cointact patch area changes about 3%. The amount of change varies with the tire construction, virtually every tire is different. The main affect is the tire inflation pressure changes the rigidity of the tire structure and that appears to be a major affect. Thats why its so diific ult to predict the change in handling as any NASCAR or F1 team can attest.
That help? Its a trend not a plot.

 

"Balloon" was merely a marketing expression from decades ago. Dont get hung up about that. The fact that fabric and steel reinforced tire casings, when mounted to a wheel rim, are very strong means that it does not vary significantly in total volume as you add or subtract air. This is very unlike a party balloon, which changes in volume significantly rather than internal pressure.

When it comes to real world cornering force, acceleration and braking traction, there is of course a lot more going on than contact patch and downforce. Even in completely dry conditions how the tread compound interacts with a given pavement texture matters. Still, your starting point for adjusting oversteer or understeer in a street car without all the other adjustments an F1 team has available is tire pressure variation F/R.

 

Tire pressure is not directly proportional to contact patch, as in 1mm=1psi, but they are related, that is the key. The experiment Mini_Odyssey posted supports this. There are many variables that change this relationship from tire to tire and car to car, surface to surface.

Improving handling does not always mean bump the tire pressure up as high as you safely can, there are other factors in play. Your suspension has to be able to compensate for that extra rebound. You are also finding a sweet spot between lateral and forward grip... losing some pressure gains you some contact patch (forward) but adds more tire deformation and hurts your lateral.

Another reason track guys run higher pressure is tire wear... try hot lapping low pressure on track on non-R tires and tell me what happens to your tread

At least this has been my experiences, which are far from scientific, just pieced together through common sense thinking and personal experience.

 

Not so much related to cornering force or fuel economy is winter traction. Narrow tires and high pressures are generally better in ice and snow. There could be rare cases where you want to "float" on snow, similar to running on sand. Wide tires at very low pressures work best for that, but a Fit is not an off road vehicle. The ground clearance is low enough that deep snow is not a good idea. The Fit stays garaged and the Tundra or Suburban are used at least until the roads are plowed.

 

Someone said "adding more air to the tire does NOT reduce rolling resistance" lol
Whatever it does, it takes less power (= more mpg) to turn them!
Try riding a bicycle, with 45psi in the tires. Then put in 75psi, and tell me there isn't any difference.

 

As a avid distance cyclist, both mountain AND road, my avg speed on a road bike is faster with 95psi vs 120psi, my avg speed on a mountain bike is faster as well but that's because i get way more traction running 18psi rather then 65psi the tires are rated for. Reason being, the tires are designed to be pneumatic, when they are inflated to max the tire deflects off road imperfections rather then to absorb. Deflections cause you to lose speed as they become like little walls/curbs youre trying to run over rather then rolling over them. Running significantly low air does cause rolling resistance but running more then needed doesn't mean best efficiency.

 

In Exxon/Mobile tests done for the state of CA, states, "At 25% pressure loss, tire rolling resistance coefficient increased by ~16.4%"
According to that, tires with 40 psi, should have a rolling resistance coefficient 16.4% less than a tire with 30 pis.

 

It doesn't work that way. You can't compare an underinflated tire to a properly inflated tire and then assume the same change in the other direction by overinflating.

 

Looking at your sinature, now I get it. Your a comedian!! You had me going there! lol

Btw, they found a 30psi tire (not underinflated) had more rolling resistance, than a tire with 32psi.

 

That's not what you wrote. There are clearly limits to how much benefit, and to what limit, you can get in mileage by increasing tire pressure. Underinflated tires waste gas. Overinflating helps somewhat (at the expense of traction, which is why the car makers don't increase the recommended pressure to claim higher mileage) but only to a point.

Has nothing to do with what I do for a living.

 

Glad to hear that you now agree that "overinflated" tires gives better mpg. By overinflated, I assume you mean any pressure above the door sticker, and not over Max pressure listed on tire. Anyone inflating over tire sidewall max rating is looking for trouble.
The amount of mpg increase, with a few #'s extra, might not be much, but it will be an increase.
I just increased my new Fits psi to 38, which I don't think should cause any handling problems, and from what I have read, from reliable sources, that the tires should not wear out the centers. They say, too low a pressure will wear out the edges, but higher pressures will not wear out the centers.

My main point is that adding more air DOES reduce rolling resistance.
If you have any published info to the contrary, I would love a link to it.

 

Now agree? I never disagreed. But there are obviously reasons why Honda said to inflate to 33 lbs and not 36 or 38 or 40 lbs. They do everything they can to increase mileage by tiny amounts yet they aren't saying to inflate the tires beyond 33 (in the Sport model, don't know what it is for the Base). There's gotta be a reason.

Did Exxon also look at stopping distances for overinflated tires? I give not crashing a higher priority than a few bucks a year in gas costs.

 

Sorry about the misunderstanding. It was Mini_Odyssey that said extra psi doesn't reduce rolling resistance.
Increased psi a lot must reduce handling, which I'm sure why Honda set their figure. There a lot of people who buy cars, and have no idea how to drive safely.
You have to know a vehicles limitations.
Assuming that modern tires will not wear the centers, with additional psi (to a limit of course) I wonder how it would affect tire life, compared to Hondas recommendation.