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Old 08-31-2008, 03:55 PM   #201 (permalink)
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CapriRacer,

oops, sorry about the broken link, but yes, that is exactly the page
there are others, it was by a source I thought you would respect

"permitted" sounds pretty good to me
I'm sure the two examples above fall way out side the "permitted" range

let me describe my position and perhaps you can help me out

first I have a light truck, an S10 with P tires
I use it some times very hard - my P tire rating has been exceeded occasionally
but mostly though I am looking for a hardy tire with good pressure rating
for day to day economy

second I have a Chrysler mini van - which is a very pragmatic vehicle & I like it
but the front tires are clearly at their limits
this is the third van I have had that eats front tires for lunch
I am 10% over side wall max and they still scrub edges like they are under inflated
this is my wife's primary vehicle and she is by no means a hot rod

it is obvious that Chrysler is a the edge of the "recommended" range on this set up
with so much of the vehicle weight, power and breaking on the front tire
they are unable to take the pounding
but I'm sure Chrysler chose the P tires so I get a smooth ride

I am willing to change rims if that is what it takes

please advise
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Old 09-02-2008, 02:01 PM   #202 (permalink)
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"Rim rating"?

Quote:
Originally Posted by TestDrive View Post
Would the standard passenger car rim you're mounting it on to be rated to 65 psi?
I seriously doubt the rims would ever feel the difference in "tire" pressure. Steel won't budge...aluminum won't move, either. The tire will blow first. IMO, anyway.
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Old 09-25-2008, 11:12 PM   #203 (permalink)
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Metroschultz to the Rescue!

Metroschultz,

Saw a Highlander mini van in US postal service colors
would you happen to know what the wheel and tires are
(I would have checked myself - but we were both rolling in traffic )

it would be interesting to know what the US gumit uses on its mini vans


on another note:
Quote:
I seriously doubt the rims would ever feel the difference in "tire" pressure. Steel won't budge...aluminum won't move, either. The tire will blow first. IMO, anyway.
the answer is probably in favor of the rim being strong enough
aluminum and steel are orders of magnitude stronger in stress and strain
you are likely to pop the bead or side wall before breaking the rim

but never under estimate the power of pressure
44 psi over one square foot is over 6300 lbs of force
not to mention shock loads from bumps and hard turns torquing the hub
especially since the shock absorption is reduced with the higher pressures
a cheap wheel (particularly aluminum) may not be able to take the load

although it is a legitimate issue to put in the risk equation
you are right a good quality rim should not flinch with 10 or 20 more psi
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Old 10-13-2008, 11:30 PM   #204 (permalink)
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oops,
I meant Uplander
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Old 11-07-2008, 03:22 PM   #205 (permalink)
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I'm a hyperinflator.

I'm cheap. I've been frugal since I was 14. I discovered treadwear ratings on tires in about 1980, so I never bought inexpensive tires, but always bought the ones with the highest treadwear ratings. Those tend to be harder compound, and I wonder if that systematically reduces or increases hysteresis losses and/or RR.

Anyway, I have always been a big fan of running my tires at 3-5psi over sidewall. I have been doing it since at least 1977. I had a nice set of toyo radials on my gmc shortbed pickup truck that lasted for 70k miles and still looked pretty good when I sold it. I check the tire pressure 4 times per year and I run snow tires in the winter.

Yes, the ride is a little stiffer. No, I never experienced any unusual tread wear in the center, or anywhere else. Never had any unusual tire failures. I always got excellent tread life by doing this. This would include tires on the following vehicles:

82 GMC pickup
Pontiac Firefly, not sure the year, 4-door
Chrysler Sundance POS, worst car I ever owned, no problems with the tires though.
'99 Subaru Outback
'93 Ford F-250 4wd, gasser
'96 Dodge 3/4 ton diesel
'96 Toyota Corolla
'04 VW Jetta diesel

This is still not a very big sample size, and no real mileage data, but for anecdotal "evidence" it's not bad.

Please carry on.

troy
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Old 11-09-2008, 06:36 PM   #206 (permalink)
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Quote:
Originally Posted by solarguy View Post
,,,,,,

Those tend to be harder compound, and I wonder if that systematically reduces or increases hysteresis losses and/or RR.....

troy
No, Troy. Tires with high treadwear numbers give poor RR - they generate lots of hysteresis.

The triange is treadwear, traction, rolling resistance. These 3 properties are balance in a tread compound and gains in one must result in sacrifices in one or more of the others. So as a general rule tires with good rolling resistance (such as OE tires) generally have poor treadwear or traction (or both.)
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Old 11-26-2008, 05:37 PM   #207 (permalink)
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Hi Capriracer,

This has been a great 11 page thread and I have enjoyed reading it. 5 years ago ish, I took a skid car class in Bend, Oregon. The instructor has passed away, but he had a long list of credentials including time spend on NHTSA boards and other government tire safety committees. He also taught rally driving to pro Japanese drivers for years. He told us that tires are engineered to perform best a little bit below the maximum posted pressure on the tire, and to fill it to about 4 pounds under that level, and keep the differential that OE's specify such as 2 pounds lower in the rear than the front.

Can you comment on this at all? I would really like an engineer’s idea of what they do to their cars at home. The new car I have now says 33 front / 32 rear. But the new tires I have put on are 51max. This is such a large gap, that i am running them at 40/39. I'd like to run them higher 46/45. How true is this statement the driving instructor said? Your credentials are very impressive btw. Thank you in advance.
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Old 11-28-2008, 07:36 AM   #208 (permalink)
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Driving on the street is considerably different than driving for sport or in competition. While tires used in sport driving or racing are subjected to high stresses, they are not expected to last many thousands of miles. This means that fatigue resistance isn’t as high a priority when designing tires for this purpose. Further, a street tire used in a sport application doesn’t have a long lifespan and fatigue resistance isn’t a factor.

Put another way, the experiences people have concerning tires in racing and sport driving have limited application for street usage, so care must be taken when transferring those experiences. You’ve highlighted one aspect of this - In racing, you want a car that responds quickly to steering input – higher pressure does that.

Street cars that are used in sport driving or racing have a limited amount of things that can be done to them – sometimes because the rules forbid such changes. Changing springs, shocks, and sway bar can dramatically change the way a car handles. But if the rules say you can’t change those, then tire pressure is one of the easiest – and perhaps the only – change you can make. Plus, it is pretty costly to experiment with different suspension pieces, where changing pressure is pretty free of cost.

When I was running street tires on my road racing car, I used as high as 42 psi on the front and 38 on the rear (and each corner was a different pressure) on a car where the stock pressures were 27 front / 31 rear. This was the result of repeated experimentation and the result was at least partially compromised by how much money I was willing to spend, the physical limitations of the car’s alignment adjustment, and the rulebook limitations.

So what do tire engineers do with tire pressures on their street cars? What about vehicle engineers?

While I can’t really speak for others, the impression I am left with is that, by and large, they use the placard pressures. However, I don’t think they, as a group, are much different than the general population and you would find similar trends of neglect – just maybe less so.

But personally, I like a car that is more responsive and I am willing to put up with the decreased ride quality. I am also concerned about unexpected impact failures – not all the roads are pothole free. So I use 3 to 5 psi above the vehicle placard.

Further, my official position is that the car manufacturers spend a lot of time testing cars at the placard pressure and unless one is willing to spent the time, has the expertise, and actually performs a huge amount of testing, then it might be best to default to the car manufacturer’s recommendation.

And taking it a step beyond that, knowing how tires are designed, and how the pressure listed on the sidewall of a tire is derived, I can not in good conscience stand idly by and not say anything when folks report using pressures above the sidewall maximum. Needless to say a lot folks take offense at this position – and they shouldn’t. It is merely a technical concern about safety. Unfortunately, there is not a lot of good reliable data available on the subject – one way or the other – about those aspects high pressure usage that concern safety – handling, tire failures, etc.

I am also concerned about misinformation – and there is a lot of it out there. “Old wife’s tales”, myths, rumors, and just plain misinterpretation of available information are things I want to get corrected. It’s not good when folks suffer under an incorrect notion – particularly one that could lead to tragic results.

I have been taken to task for not having good reliable information – particularly PUBLISHED information that can be readily accessed – and rightly so. To that end I have been searching for such information and pointing it out when I find it – regardless of whether it supports high inflation pressures or not. Data is neutral – it is neither good nor bad (except when the data itself is faulty or the process to get it is faulty.)

I am particularly concerned about “noobies” – folks who are new to the subject. I feel it is important that both the good and the bad be presented – AND how well founded that information is. It is clear that increased tire pressure improves fuel economy. It is also clear that increased pressure improves the wear rate, improves hydroplaning resistance, improves steering response, and hurts the ride quality. What is less clear is the effect increased pressure has on evenness of wear – although it seems that the improved wear rate more than offsets any other wear issue.

What kind of gets lost in the shuffle is that the rate of these improvements (and harmful effects) diminish as the pressure increases. Put another way, the difference between zero to 5 psi above the placard is more than the change from 5 to 10 above the placard – and it keeps diminishing as the pressure is increased.

But there are issues where there is little data - impact resistance, traction (snow, wet, dry), durability – and while opinions abound, it is difficult to sort out fact from conjecture. So you will see heated discussions on these issues – and that’s good. Debate will help sort this out – at least until someone actually does a closely controlled study and publishes their research.
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Old 12-06-2008, 06:10 AM   #209 (permalink)
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Thank you for your very informational response. :-)
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Old 12-10-2008, 02:04 PM   #210 (permalink)
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I'm a newb here but have driven lots of miles in many different vehicles and tires. I have read most of this thread with interest. Historically I have run most tires on most vehicles at placard pressure for max load. When it is expressed as such or 2-3psi over placard on cars designed for comfort. With a few exceptions when running at max sidewall I have noticed in most cases a loss of wet weather traction which is important to me and not much increase in mileage. I have not seen much here to make me change that opinion.
What I have gotten is that tires are very hard to quantify. If you put a steel I beam in a test machine and bend it x# of cycle until failure you can measure that easily. change how far you bend it and you can up or down the cycles it will handle. But tires are composite structures made of many different compounds and structures depending on the intended purpose. The individual componets are easy to quantify. But the complete unit has so many stresses. Flex load of sidewalls in compression both plain compression and rolling compression as well a extension stress from inflation. The tread flex on several axises. All these things come into play and therefore the manufactures test to make sure that under MOST NORMAL condition that the tire will perform OK for a REASONBLE period of time. Beyond that hard data is hard to come up with. And like most of us have experienced most tire failures are from defective manufacturing processes along with bad maintance practices on the part of the owners. I think I'll keep doing what I've been doing.

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