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

<div class='quotetop'>QUOTE(David H. Hawkins @ Jun 29 2006, 11:44 PM) [snapback]278976[/snapback]</div>

I give you...

Bridgestone RFT (Run Flat Tire), Dunlop DSST (Dunlop Self-Supporting Technology), Firestone RFT (Run Flat Tire), Goodyear EMT (Extended Mobility Technology), Kumho XRP, Michelin ZP (Zero Pressure), Pirelli RFT (Run Flat Technology) and Yokohama Run Flat.

All of which have a semi-constant contact area that is not dependent on air pressure at least in the short term.
The contact area is more complicated than your foumula. Sorry.

That said, I do not dispute that tires with more air pressure role with less effort than the same tires with less pressure. I have experienced it. It is easy to feel on a bicycle.

Me, I'm going to keep my tires somewhere between the Toyota recommended pressure and several pounds below the max pressure printed on the tire.

 

<div class='quotetop'>QUOTE(pocketpenguin @ Jun 30 2006, 11:22 PM) [snapback]279492[/snapback]</div>

I did say "not run flat tires" in my original reply, however even runflat tires obey my basic premise as long as they are not running in the 'flat' condition. under normal use, the car is still supported by the air in the tire. As the tire deflates below proper pressure, weight is TRANSFERED to the sidewalls of the tire. This is described here: basic runflat patent
The second system, as used by Michellin, uses an internal structure, attached to the wheel, to support the tire when it goes flat. During normal driving, this structure provides no support, as you can see in the photo



Notice that as shown above, the runflat support does not even come close to the road in normal operation.
In either case, normal operation uses the air pressure inside the tire to support the car. Now I reallize my formulas are a very simplified stating of the problem, but they DO describe the FUNDAMENTAL relationship between tire pressure and contact patch size.

I have never ridden on runflats, but if the sidewalls were sufficiently stiff to support the car under NORMAL inflation and driving conditions, I expect the ride would become extremely harsh and unpleasant.

 

Dave, I have to agree that your formula is correct if we just consider the tire at rest. The absolute size of the contact patch must follow your formula, based on load weight and inflation pressure.

But nobody cares about the tire when the car is parked.

The behavior of the contact patch under dynamically changing conditions is of greater importance than its absolute size.

When cornering, it's important that tire maintain contact across as much of the tread as possible, without too much "roll" of the tire. Add to this the behavior of the suspension and cornering performance becomes a very complex issue: if the alignment shifts during a hard corner, any tire will lift.

When in a hydroplaning situation---even though the car is being supported by water---the behavior of the patch determines how the tire will expel water to maintain traction.

We accelerate, brake, corner, and encounter rough and wet roads. That's when we care about the behavior of the contact patch. None of that behavior can be modeled by a single equation.

 

<div class='quotetop'>QUOTE(Rick Auricchio @ Jul 2 2006, 04:40 PM) [snapback]280205[/snapback]</div>

I agree with you completely. My original intent was just to indicate an approximate magnitute of the change in contact patch size as related to tire pressure. It is obvious that the dynamic behaviour of a tire is much more complex and important that my simple math could express. Now, having shown my ignorance sufficienly and beaten this poor subject to death, I shall move on an annoy some other poor topic.

 

In the Winter people typically add even more air to compensate for the cold weather. In regards to tires wearing in the middlem due to over inflation, I have seen it many times, but then again I am always amongst car people, or commercial truckers, or other folks who make a living around transportation. My Uncle who recently retired ran a service station for 50 years. I have seen about everything in regards to tire wear.

It is also possible that if the stock tire width for the Prius is slightly more narrow than the rim lip, that braking will cause the tire to slightly cup and as the sidewall compresses during the rear to front weight transfer. This could account for the abnormally high number of complaints about the outside footpront wear pattern even on properly inflated tires. this would be aggrivated by the taller sidewalls on the stock rims and would probably be lessened by a lower profile, or slightly wider tire.

 

<div class='quotetop'>QUOTE(David H. Hawkins @ Jul 2 2006, 02:23 PM) [snapback]280223[/snapback]</div>

Nonsense. We enjoy a spirited discussion! And we all learn from each other.

 

<div class='quotetop'>QUOTE(David H. Hawkins @ Jun 29 2006, 10:29 PM) [snapback]278923[/snapback]</div>

Your first formula is correct, but the later statement is mistaken. The first point is that sliding friction depends on coefficient of friction, normal force and velocity, but not on surface area. The normal force is the total weight on the tire. So the sliding friction is actually independent of the inflation pressure. Thus there is no "increased friction". If your point was that this might effect maximum braking force, the answer is no, assuming uniform road surface characteristics. If instead you were attempting to analyze fuel economy then you're barking up the wrong tree. Rolling resistance does not come from sliding friction; the tire does not slide along the road, it is stationary or moving perpendicular to the road surface. Rolling resistance is mainly caused by hysteresis in the rubber flexure of the tire. Higher pressure = less flexing thus lower rolling resistance.

All of the above analysis is first order, but higher order effects are not very significant.

 

can anyone tell me if pirelli p3000 tires would help in mpg?



<div class='quotetop'>QUOTE(theorist @ Jun 28 2006, 11:27 AM) [snapback]278080[/snapback]</div>

 

<div class='quotetop'>QUOTE(frankk67 @ Aug 28 2007, 10:20 PM) [snapback]503557[/snapback]</div>

Consumer reports rating are not so good.
Rolling resistance is bad.

 

really?

all the reviews seem to rave about them being quiet and long lasting, i figured if they were 80k tires with a synthetic compound and an A/A rating on traction/temp. then if i added extra air, it might help increase mpg.

so then what tires will increase mpg? what about other sizes like a 65 or wider tire?

<div class='quotetop'>QUOTE(ohioflash @ Aug 28 2007, 09:32 PM) [snapback]503562[/snapback]</div>

 

<div class='quotetop'>QUOTE(frankk67 @ Aug 28 2007, 07:20 PM) [snapback]503557[/snapback]</div>

I have them on my Prius. They lowered the mileage by about 1-2 mpg. I think closer to 1. I have some uncertainty on that, since there was also a mileage drop at the same time due to hotter weather and use of the air conditioner, and possibly a very small apparent (faux) mileage decrease due to slightly larger tires! But I think I have apportioned the deltas about right.

On the other hand the Pirellis feel so much better -- much more sure-footed. I was happy the pay the mileage penalty. (The before and after tires were both kept at 42/40.)

 

<div class='quotetop'>QUOTE(frankk67 @ Aug 28 2007, 07:20 PM) [snapback]503557[/snapback]</div>

I have them on my Prius. They lowered the mileage by about 1-2 mpg. I think closer to 1. I have some uncertainty on that, since there was also a mileage drop at the same time due to hotter weather and use of the air conditioner, and possibly a very small apparent (faux) mileage decrease due to slightly larger tires! But I think I have apportioned the deltas about right.

On the other hand the Pirellis feel so much better -- much more sure-footed. I was happy the pay the mileage penalty. (The before and after tires were both kept at 42/40.)

 

I always run the tire pressures as stated in the manual. I've done this with all the cars I've owned. Some engineer decided that is the optimal pressure for safety, tire wear, comfort and economy. I didn't design the car and neither did I design the tires. I'll rather trust the engineer who did design them.To think I know better is just stupid...

I'm worried that we're all sacrificing the safety of our families and the lifespan of our tires to save a few bucks at the pump. How 'bout an overzealous insurance company who refuses or reduces an accident payout 'cause they say your tires were not inflated according to manufacturer specs....Huh? Ever thought of that?

Time for TWEELS!! They'll make this whole argument moot.

 

<div class='quotetop'>QUOTE(Albertus @ Sep 14 2007, 05:32 PM) [snapback]512716[/snapback]</div>

This has been covered SO MANY TIMES and even in this thread! "Overinflation" up to the maximum sidewall cold inflation pressure is perfectly safe and for many reasons beneficial. Your family is safe, nobody's insurance company is going to deny an accident payout for having 42psi in the tires. The dramatic BS is getting incredibly old.

 

Yes. And I don't know if anyone has mentioned this in this thread or other threads, but Volvo and I believe other car manufacturers are in the practice of recommending two levels of pressure -- a lower psi inflation for comfort and a higher psi inflation (but within the tire's maximum psi rating) for fuel economy and wear.

 

<div class='quotetop'>QUOTE(Boo @ Sep 14 2007, 05:33 PM) [snapback]512835[/snapback]</div>

You are absolutely correct which points out just why Toyota recommends the lower tire pressure on the Prius.

Anyone who reads this whole thread will get a fine education on the subject. There really isn't much more to say.