The science/physics/math/safety behind overinflated tires...

[Three60guy]

Here is something I came across concerning this thread.

Any comments, thoughts?

Autozone Reference Library - Tires

[tochatihu]

I have been reading Prius posts here and elsewhere for a very long time, but never yet one that spoke of center tread wearing more rapidly than the edges. There have been plenty of 'edge wear' posts, of sourse.

The belt construction of radial tires make it very difficult for the center of the tread to bulge out. Perhaps one could achieve this, but not at any pressures at or below what is says on the tire sidewall.

[hdrygas]

In the Prius Milage Record run of >100 mpg the "well worn" tires were at 60 psi. Please check me if I am wrong but that is what I remember. " In the absence of Data you can have any opinion you wish". That was outside normal driving but it is documented.

[theorist]

<div class='quotetop'>QUOTE(Alphawolf @ Jun 27 2006, 08:51 PM) [snapback]277729[/snapback]</div>

I'm just wondering if we are "robbing Peter to pay Paul" so to speak...
[/b]

Of course there are tradeoffs and we give up somethings to gain others.

From higher pressures we _generally_ give up some:

• ride comfort
• braking traction on dry roads
• acceleration traction on dry roads
• less rattles developing
• safety from blowouts from hitting potholes or other road hazards?
• less wear on the suspension

From higher pressures we generally gain:

• slightly lower fuel consumption and emissions
• less hydroplaning
• better traction in shallow snow and slush
• more responsive steering
• safety from blowouts from tires overheating at high speeds and/or under heavy loads
• safety from rim damage hitting potholes

I like to think of each of these objectives as concave (down) functions of tire pressure, each having its own unique optimal tire pressure. Clearly riding on flat tires (zero pressure) or tire tread glued to 24" flat rims (or tires magically inflated to infinite pressure without blowouts) wouldn't serve any purpose.

Finding the tire pressure that is optimal overall depends on what weight we give to the different objectives or criteria. As our preferences vary from person to person, so will the optimal tire pressure.

As for me, I'm currently running Continental ContiProContact tires at 50/47 psi recently. (The maximum tire pressure for these is 51 psi.) These are very soft grand touring tires with grippy tread but soft sidewalls and the ride is still comfortable. I'm keeping them inflated this high partly to reduce hydroplaning, as we've been having plenty of rain (and even more forecasts of rain) in Boston. I plan to lower them when the rain goes away (in a week or two God willing) and look forward to seeing the effect on fuel consumption, ride, handling, noise, traction, etc.

[ginostef]

<div class='quotetop'>QUOTE(theorist @ Jun 28 2006, 12:27 PM) [snapback]278080[/snapback]</div>

Of course there are tradeoffs and we give up somethings to gain others.

From higher pressures we _generally_ give up some:

• ride comfort
• braking traction on dry roads
• acceleration traction on dry roads
• less rattles developing
• safety from blowouts from hitting potholes or other road hazards?
• less wear on the suspension

From higher pressures we generally gain:

• slightly lower fuel consumption and emissions
• less hydroplaning
• better traction in shallow snow and slush
• more responsive steering
• safety from blowouts from tires overheating at high speeds and/or under heavy loads
• safety from rim damage hitting potholes

I like to think of each of these objectives as concave (down) functions of tire pressure, each having its own unique optimal tire pressure. Clearly riding on flat tires (zero pressure) or tire tread glued to 24" flat rims (or tires magically inflated to infinite pressure without blowouts) wouldn't serve any purpose.

Finding the tire pressure that is optimal overall depends on what weight we give to the different objectives or criteria. As our preferences vary from person to person, so will the optimal tire pressure.

As for me, I'm currently running Continental ContiProContact tires at 50/47 psi recently. (The maximum tire pressure for these is 51 psi). These are very soft grand touring tires with grippy tread but soft sidewalls and the ride is still comfortable. I'm keeping them inflated this high partly to reduce hydroplaning, as we've been having plenty of rain (and even more forecasts of rain) in Boston. I plan to lower them when the rain goes away (in a week or two God willing) and look forward to seeing the effect on fuel consumption, ride, handling, noise, traction, etc.
[/b]

Just a couple of notes to add to this thread about overinflation.

1) Toyota specifies the recommended minimum tire pressure I believe, - not maximum is that right? In either case, it is set to compromise ride quality with tire wear and we all know it is whacked up because all of our tires wear on the outside (either suspension characteristics or underinflation).

2) All tire manufacturers state the maximum inflation pressure as COLD TEMP inflation. So - at rest, cold, your tires can be set safely to 44psi if that is the max pressure on the sidewall. A rise in pressure with heat and road friction is expected in those numbers. That's why you don't fill the tires when warm if you can avoid it. There is no overinflation going on here.

3) yes there is some road feel fed back into the harder tires, but I found this totally goes away with the Comfortreds. The Integrities are a cheap tire and ride like one. Those who have upgraded have seen dramatic improvements in wet and dry operation, as well as comfort.

There is no question in my mind that the tires have to run harder to get thebest mileage. I run mine at 42/40 and have had literally no problems. My Integrities ran 35K miles that way and had edge wear, not center wear. Remember that the wear relates to the car weight on the wheel as well. The heavier the car, the more a high pressure tire will wear along the center line - the Prius will never push a good tire that far.

Some drivers might - but the car won't!

Gene

[DaveinOlyWA]

i have run 42/40 on my car and my last oil change my tires were measured at 7 mm inside, outside and center on all 4 tires. i have my oil changed at Toyota of Olympia. the first time i took it there, they changed tire pressures to 35 psi. the 2nd time i took it there, i requested they not change the tire pressure, they have not touched it since.

[tnthub]

Tires pressure ratings are always done "cold". Tire pressure rating are determined by the tire manufacturer.

[akim]

What are people's thoughts of driving with maximum (e.g. 42 psi) inflated tires in the snow?

I would think that during the winter months, one would want to lower tire pressures to 'Toyota recommended' ranges for better traction in ice and snow. Safety over mpg?

[gschoen]

It's much more dangerous to have underinflated tires. It's difficult to overinflate your tires to an unsafe level, if you follow the max PSI on the tire sidewall.

The tire manufacturer lists the cold pressure, they allow for thermal expansion and a big safety margin, so running at max PSI still has a big safety margin.

Some people inflate beyond the sidewall max pressure. I don't think that's the best idea, but it works for them.

Car makers list lower pressures for a better ride and other factors, they are concerned about the total performance of the car, and not just that of the tires (which do better at higher pressure)

[ghostofjk]

<div class='quotetop'>QUOTE(Alphawolf @ Jun 27 2006, 05:51 PM) [snapback]277729[/snapback]</div>

And, it is known that over inflated tires wear out quicker in the middle of the footprint. [/b]

Well, as long as we're reconsidering "things that seem obvious in our minds", let's challenge this one.

Yes, I, too, have seen the pictures in tire shops of various kinds of "irregular" tire wear: bald on the edges, bald in the middle, scalloped, etc. But I've had 37 cars, have inflated the tires above mfr. recommendations on all of them (even pre-radial, bias-ply tires) and have NEVER seen extra wear in the middle on a tire.

On my '02 Prius, I ran the OEM Bridgestone Potenzas (by general acclamation, even worse than the Integrities) at 42/40 for 53,000 mi. and when I replaced them they easily had another 5K in them. I had rotated them every 5K as per the manual. The tread wear was even from shoulder to shoulder.