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Old 12-13-2009, 08:08 PM   #1 (permalink)
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"Inflation pressure does not affect grip": Autospeed article.

Except in aquaplaning where higher inflation reduces it.

Something I suspected but didn't have the info to back it up.

AutoSpeed - Tyres, Grip and All That.

Myth 2: A larger contact patch = more grip

Okay, most people will come to the conclusion that if you have "more rubber on the road" you will have increased grip. Sorry to say this folks, but to very close to 100% accuracy, the size of the contact patch is irrelevant.

The actual grip that a tyre can generate is dictated by the coefficient of friction of the rubber compound used in the tyre.
One issue to consider is that, for wet weather driving, despite what you may have heard, it is better to increase your tyre pressure, not reduce it. The reason is that there is a relationship between tyre pressure and the speed at which there is the onset of aquaplaning. In the Imperial system, the equation is 9 times the square root of the tyre pressure. So, if your tyres are at 25 psi, if you drive into a puddle that is deeper than your tread depth, you will aquaplane at 45 mph (72 km/h), whereas if your tyre pressure was 36psi, you would aquaplane at 54 mph (87 km/h). The advantages are obvious.
The tyre pressure has no direct bearing on the level of grip (apart from aquaplaning), but it does have a bearing on the heating and cooling characteristics of the tyre.
ADDED: I'm putting all my links here for easy access:

According to these tests Higher inflation pressure reduced stopping distance and increased coefficient of friction...Scroll down for stopping distances.

FEA chapter III. tire pressure survey and test results

From NHTSA Passenger van tire inflation study.

Good tire care improves vehicle
handling as well as fuel efficiency
and tire life. Proper tire maintenance
can prevent such events as tread
separations and tire blowouts which
may cause loss of control of a vehicle,
when not handled properly, and result
in a rollover. Low tire pressure can
also increase stopping distances and
the chance of hydroplaning on wet

Found some interesting pages from a racing tire book. I have not had time to read the whole thing.

From excerpt#1

Adhesion is a property of rubber that causes it to stick to other materials, as we see with adhesive tape. Adhesion is generally thought to be the result of momentary molecular bonding between the two surfaces. If bond strength is the same at all the bond sites the force that resists sliding is proportional to the total of all the minute areas of contact. If the two surfaces were perfectly smooth the true area of contact would be the same as the observed area of contact, but this is not the case. Real surfaces are actually very rough on the molecular scale and contact is limited to the highest protuberances on each of the two surfaces. The true area of contact depends on the surface profiles, properties of the materials, and the contact pressure.

Tire Technology, excerpt from The Racing & High-Performance Tire

From excerpt #2

At higher slip angles portions of the tire patch are sliding, and you get less increase in lateral force with an increase of slip angle. This is called the transition region. As the curve tops out, more of the contact patch is sliding and the tire produces less lateral force. After the peak of the curve, lateral force can fall off 30% within a few degrees of extra slip angle. At these high slip angles most of the contact patch is sliding, producing a lot of heat and wear.

Tire Technology, excerpt no. 2 from The Racing & High-Performance Tire

Tire Technology, excerpt no. 3 from The Racing & High-Performance Tire

SAE paper digested for us by Barry.

Barry's Tire Tech

Peak traction goes up and sliding down a tiny bit. ABS would like this. Also they found cornering response and cornering traction improves. The only downside was bruise resistance and puncture blow out got worse with higher pressure (not surprising.)

This is not advice. I'm just sharing waht I have found.

My personal opinion is that tire inflation to max sidewall is generally better for the following reasons:

1) Lower heat build up. Reduced risk of accident or rollover resulting from loss of control from heat related tire failure.
2) Reduced wear
3) Slightly increased peak traction, slightly reduced sliding traction. Slightly reduced stopping distances (may have to do with increased traction)
4) Reduction in hydroplaning (onset of hydroplaning happens at a higher speed)
5) Reduces slipping in snow* (depending on conditions, sometimes lower pressure is better)
6) Increases tire responsiveness in cornering.
7) Reduces body lean in cornering, improves steering feedback (subjective)
8) Reduces fuel consumption
9) Reduces steering effort

Downsides to max sidewall:

1) Slightly harsher ride quality.
2) May increase rollover tendency in already rollover prone SUVs
3) Harder on suspension components in rough road conditions
4) Increases risk of tire bruising or failure from impact with certain objects.


Last edited by orange4boy; 12-21-2009 at 07:50 PM..
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Old 12-13-2009, 08:19 PM   #2 (permalink)
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Old 12-13-2009, 08:20 PM   #3 (permalink)
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Old 12-13-2009, 08:59 PM   #4 (permalink)
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In fact, according to these tests Higher inflation pressure reduced stopping distance and increased coefficient of friction...Scroll down for stopping distances.

FEA chapter III. tire pressure survey and test results

Unfortunately, higher pressures were not tested.

These data indicate that stopping distance is longer with lower psi for every case except for two cases with the full depth tread with ABS on the Dodge Caravan.
(Poor Cara. Sorry Christ)

FEA chapter III. tire pressure survey and test results

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Old 12-14-2009, 01:32 AM   #5 (permalink)
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Orange4boy, thanks for a new weapon in the EcoModder arsenal
eˇcoˇmodˇding: the art of turning vehicles into what they should be

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Old 12-14-2009, 01:44 AM   #6 (permalink)
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tire company in cahoots with oil company to keep making wasteful rollers?

I would run at 90 psi If they would wear evenly. screw ride comfort built into tires, thats what the suspension is for.
Because we are big dumb redneck Americans and we only want V8s that thunder and use lots of gas pushing our empty Super Duties down the highway at 100 MPH in the fast lane while warming our butts in heated leather seats and chowing down on double quarter pounders and texting on our cell phones. We cant understand non-V8 engines because that ain't whut NASCAR uses...
Not this American.
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Old 12-14-2009, 05:25 AM   #7 (permalink)
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I wonder why the tire companies consistently disagree with this...

Edit: Something to note, this article seems to have no empirical testing, which I think is important in this kind of situation. Issues related to tires and fuel economy can often have the books cooked via mental exercises, which, while seemingly sound, are crucially lacking various pieces of the puzzle. Colour me skeptical.
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Old 12-14-2009, 07:20 AM   #8 (permalink)
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Originally Posted by orange4boy View Post
1) Unfortunately, had they done a little research, they would have found this:

Fact or Fiction? Tire contact patch and air pressure.

That directly contradicts Item #1 - and has data to back it up. So it isn't a myth!

2) In item #2, they go with the classic friction formula F = ľN. Unfortunately, that does not apply to tires.

Tires get a lot of their grip by penetrating the macro texture of the pavement. When pushed to an extreme, the rubber is torn off the tire - with subsequent increases in force. As a result, it is possible to get more than 1g forces.

If you read up about drag racing, you''ll find reports of well in excess of 1 g acceleration. You'll also find some skid pad testing over 1 g of regular production cars - albeit high performance cars like Corvettes.

So, the whole article seems built on an incorrect series of assumptions.
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Old 12-14-2009, 07:23 AM   #9 (permalink)
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You know, that is the exact reason why the standard for TPMS makes no provision for an upper pressure warning. It was determined it does not constitute a significant safety hazard.

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Old 12-14-2009, 10:16 AM   #10 (permalink)
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Possibly not relevant to on-road discussions, but with regards to off-highway driving on extremely rough surfaces anyone who says inflation pressure doesn't affect grip can stop talking immediately if not sooner.

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contact patch, inflation, max sidewall, tires, traction

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