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Old 06-01-2008, 01:42 PM   #51 (permalink)
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I havnt had time to go through all the posts, but studying automotive manufacturing, there is a Factor of Safety1.69 generally through the industry. ie. Wheel Chair With a 200 lb capasity and a Factor of Safety of 2. (Chair Says 200lb max, tested to withstand 400lbs).

Assuming the same safety factor, 35psi max would probably fail at 60~ Psi

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Old 06-01-2008, 02:02 PM   #52 (permalink)
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Below is something I can acroos while researching trailers today. It cam directly from a Michelin RV reference site. My take is their are more dangers to underinflating a tire than overinflating. Although there is no true definition for overinflating other than us saying more than sidewall max. No mention of sudden tire failure with overinflation.


A tire that is underinflated will build up excessive heat that may go beyond the prescribed limits of endurance of the rubber and the radial cords. This
could result in sudden tire failure. A tire that is underinflated will also cause poor vehicle handling, rapid and/or irregular tire wear, and an increase in rolling resistance which results in a decrease in its fuel economy.

Overinflation will reduce the tire’s footprint or contact patch with the road, thus reducing the traction, braking capacity, and handling of the vehicle. A tire that is overinflated for the load that it is carrying will also contribute to a harsh ride, uneven tire wear, and will be more susceptible to impact damage.

Maintaining correct tire inflation pressure for each loaded wheel position on your vehicle is of the utmost importance and must be a part of regular vehicle
maintenance.
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Old 06-02-2008, 12:09 AM   #53 (permalink)
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Quote:
Originally Posted by trikkonceptz View Post
On those statements I have to say that tires do not cause rollovers, drivers do.

...


Unless tires are manufactured with defects, like the Firestone recal, a tire won't fail, it will blow out due to a puncture, but tires do not fail. Obviously some may think I am wrong, but site situations when tires can fail where their maintenance and or abuse is not a factor.
Tire failure (particularly rear tire failure) on high center of gravity vehicles are much more likely to cause rollovers no matter who is driving. Big SUVs and 15 passenger church vans have gotten most of the press from horrific multiple death accidents from rollovers caused by rear tire failure at highway speed. Such vehicles are much more likely to get sideways and 'trip' when rims dig in at an angle after tire failure.

One cannot say that the tire failure is 100% to blame for causing such accidents but one can say the tire failure was the major cause in many cases.


I have seen a fair number of tires fail in service that were properly installed and cared for. Tread separation, belt shift, sidewall tumors, and carcass failure - mostly on cheaper offshore (Chinese and Korean made tires seem to be the worst) product but sometimes on major name brand stuff like Goodyear, Bridgestone, General and Firestone. I have obseved an upswing in failure in the last 3-4 years in particular as those aforementioned cheap offshore tires flood the market, creating downward pricing pressure on the rest of the market that is balancing out with increased material and energy costs.

Tires don't let go nearly as often as they did in the 'good old days' but they *do* still fail. To believe otherwise is naive at best.
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Old 06-02-2008, 01:56 AM   #54 (permalink)
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My question is what is the relationship between contact patch and traction... in physics we are taught that the contact patch is basically irrelevant to friction because as the area increases the force per unit area decreases. Here my a post about it in a different thread:
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Originally Posted by mattW View Post
I just want to try and clarify a few misconceptions about car tires and friction. When you increase the psi of the car's tires the mpg increase is not from less friction with the road, it is from less deformation of the tyre. Rolling resistance and road friction are two separate things. As the tyre comes in contact with the road a section deforms to become flat against the road. As the tyre rolls away the rubber returns to its original shape. This compressing and expanding of the tyre creates heat which wastes energy. This can be reduced by higher pressures. The other part of rolling resistance is adhesion forces where the tyre sticks to the road and is then peeled off. Adhesion is more directly related to grip but is more a function of the tyre compound than the pressure. The reason wider tyres are reported to have more grip is because the wider contact patch gives more surface area for the tyre to cool down in high performance situations, i.e. situations where grip is lost due to temperature. This seems to be the source of the myth that a bigger contact area gives greater friction. The maximum grip of tyres before skidding is proportional the coefficient of static friction (i.e. for rubber on concrete it is 1.0) times the reaction force in newtons. A quote from my university Physics textbook "Physics for Scientists and Engineers" by Serway, Jewett:

"The coefficients of friction are nearly independent of the area of contact between the surfaces. We might expect that placing an object on the side having the most area would increase the friction force. While this provides more points in contact... the weight of the object is spread out over a larger area so the individual points are not pressed so tightly together. These effects approximately compensate for each other, so that the friction force is independent of the area."

I'm not trying to sound like an expert or anything I just wanted to clear up some myths being thrown around.
Is there any reason why what I said isn't true? I don't know anything from my own testing, that was just repeating what I learnt at University.
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Old 06-02-2008, 02:21 AM   #55 (permalink)
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A Conclusion?

OK, here's a question:

During testing, how high has a tire's pressure limit been tested?

Let's say we put 60 psi/warm in a 44 psi-rated passenger tire. In a controlled lab facility, has anyone witnessed a tire failure? How about 60, 80, 100? What is the maximum average pressure a "gas station pump" can inflate a tire?

So far, a valve stem failure and potential road debris can likely cause an overinflated tire to fail. Further questioning:

What is the statistical different in overinflation vs. recommended for road debris failure?

A valve-stem failure would not likely cause a catastrophic blow-out. The pressure would be released a ordered fashion, as not to heat the tire to a catastrophic blow-out, but rather to a uninflated condition -- which is very noticeable to the driver.

I had a catastrophic blow-out on a brand-new '96 Civic. Highway speeds at 70 mph. The tire disintegrated into 2 separate rings with sidewall separation. Stabilizing the vehicle was a challenge and nearly resulted in a loss of control. Tire experts examined the carcass and blamed a weak manufacturing process. Dunlop's lawyer argued underinflation. Long story short, I was screwed out of a brand new tire and had to buy a new one out of pocket. Good luck getting any "Warranty" replacements on failures.

I vowed never to buy a Dunlop product again -- and recommend the same for any reader.

The questions in the first portion of this thread are requested for consideration. Otherwise....

I propose a collective experiment.

If there is interest, I plan to draft an experiment using the Scientific Method. Those who utilize overinflation could volunteer to offer data based on pressure, number of miles driven, tread wear (inside, middle, outside -- and last vehicle alignment), pictures of the tire, and reports of any failures. A long-term extension study could be offered to track the longevity of these pressure. I propose weekly check-ins with consistent testing of pressures (warm) and ambient temps. A control group of normal-pressure participants would be requested.

Statistical analyses could generate some conclusion on this subset of data. Although it's not a final conclusion, a basis for additional, repeatable testing could be derived.

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Old 06-02-2008, 08:09 AM   #56 (permalink)
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RH77, count me in. My stock summer tires (44 psi max sidewall) have been ran at 50 psi since new (3500 miles so far).
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Old 06-02-2008, 09:38 AM   #57 (permalink)
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Just adding a note on valve stem failure. While I've yet to hear of anyone suffering tire failure from hyperinflation, one EcoModder member has suffered valve stem failure:

Quote:
I had two tires split at the valve stem. One actually started to leak so badly that I had to change the tire.

My max PSI is rated at 44PSI on the sidewall, and I was running 55 PSI, but on the day of the tire failure, I filled them to around 55 or so ........... and found out later how inaccurate that tire guages can be. I was actually at around 65- 70 + PSI.
( So it's a good idea to check the pressure with more than one guage. )
http://ecomodder.com/forum/showthrea...plit#post22167
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Old 06-02-2008, 09:50 AM   #58 (permalink)
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If you do lose one from overinflation, the damage can be severe. I've seen some tire training videos that are pretty impressive. The safety cages over some tire machines that are used when seating beads are beefy steel things, and there are plenty of videos out there documenting tire safety cage testing that give you an idea of the amount of force involved when a tire fails from overinflation.

Think a 400 lb tire machine vaulting into the air, and a steel containment device being blown to bits.
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Old 06-02-2008, 10:46 AM   #59 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
Just adding a note on valve stem failure. While I've yet to hear of anyone suffering tire failure from hyperinflation, one EcoModder member has suffered valve stem failure:

http://ecomodder.com/forum/showthrea...plit#post22167
Good point. But I still haven't heard of a "catastrophic blow-out" with such a failure. If the leak is slow enough, an underinflation condition could result I suppose -- which comes back to frequent pressure checks.

My focus in this thread, is the argument of a dangerous tire failure from the indicated pressures.

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Old 06-02-2008, 11:48 AM   #60 (permalink)
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About treadwear: Wayne at Cleanmpg has the original tires on his Accord, 100,000 miles later. Completely even treadwear and still 4-5/32 tread depth. He's had them at about 60 psi since new. They are NOT worn down in the middle.

I know it's just one example, and not representative, but there it is.

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