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Old 11-07-2008, 12:24 PM   #21
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In my former life, I worked for a large regional airline. I am a licensed A&P mechanic. Now I am a computer support guy, since I did not like living in the big city (Dallas). Here is what the FAA had to say regarding nitrogen in tires and it tells you why it is used in airplane tires:

http://www.airweb.faa.gov/Regulatory...pandSection=-4

For car tires, I don't see any benefit for anyone who regularly check their inflation pressures.

I'll see your $.02 and raise you a nickel.
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Old 11-07-2008, 01:53 PM   #22
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Originally Posted by theholycow View Post
That's worth a lot more than $.02. Too bad people don't understand, most people only ever think of overinflating a bicycle tire and figure that's how it works. Folks on this forum tend to be the exception.

When you worked as an adjuster, did you see any failures attributable to overinflation? If so, what can you tell us about them?
Oh yeah, plenty of downsides to an over inflated tire. The main issue is impact damage. Your tires take a serious beating over rough services. Since the over inflated tire "gives" less you can cause a tread separation or sidewall buldges. Both are impact related failures that can happen with any tire at any psi, over inflated just makes them less forgiving. Unlike under inflation, over inflation (within reason) doesn't directly cause tire failure, but it can contribute to a failure.

Most people here already know that your taking a chance of unevenly wearing tires at high pressures. That will vary from tire to tire and the weight of your vehicle. Proper rotations will limit it and is required for any mileage claims. I would think that lower speed rated tires would have more of a tendency to unevenly wear. I say that because the higher the speed rating, the better a tire holds its shape at speed, under braking and cornering loads. These traits also help the tire maintain its normal profile even at max pressure. Lower speed rated tires are not as "stiff" and the profile is easier to distort with improper psi.

Over all it should be perfectly safe to run your tires at max pressure all the time. Many performance cars list two sets of tire pressures, so have most of my motorcycles. One number is for normal driving and the other is for high speed driving. The high speed driving has always called for higher pressures. I can't give the true scientific reason behind it, but it makes me believe that it is good to be high, your air pressures at least
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Old 11-07-2008, 01:55 PM   #23
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Originally Posted by moorecomp View Post
I'll see your $.02 and raise you a nickel.
I'm broke, can I borrow that nickel? lol
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Old 11-07-2008, 03:15 PM   #24
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Quote:
Originally Posted by T.P. View Post
Oh yeah, plenty of downsides to an over inflated tire. The main issue is impact damage. Your tires take a serious beating over rough services. Since the over inflated tire "gives" less you can cause a tread separation or sidewall buldges.
D'oh! I hate when this happens...someone presents some good data that conflicts with my own good data. In my experience, many times I've gotten impact damage when the tire sidewall gets pinched between the road and the rim and gets cut by the rim, all while at the vehicle manufacturer's recommended inflation. It's only happened to me once at max inflation (it was a really bad hit).

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I'm broke, can I borrow that nickel? lol
Can we split it?
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Old 11-09-2008, 06:35 AM   #25
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Originally Posted by theholycow View Post
D'oh! I hate when this happens...someone presents some good data that conflicts with my own good data. In my experience, many times I've gotten impact damage when the tire sidewall gets pinched between the road and the rim and gets cut by the rim, all while at the vehicle manufacturer's recommended inflation. It's only happened to me once at max inflation (it was a really bad hit).



Can we split it?
Oh, heck yeah, that kind of damage can happen at any pressure. Overall, my personal opinion is it is better/safer to run a bit more psi then recommended. As you have discovered, you really need to smack it to instantly/visually damage it when at max pressure. To be honest, I wasn't even thinking of how easy low pressure impact breaks happen as well.

Think of it this way, a impact at high inflation can cause damage to the tire that is not visible to the eye, but it would take a lot of hit's before a failure occurred. While a low pressure impact is more likely to result in an instant failure.

Sure, I will split the nickle, once moorecomp pay's up
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Old 11-09-2008, 08:41 AM   #26
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I'll second 'overinflation is better than underinflation'

I work in a shop and 2/3 of our business is tires. I have yet to see properly inflated or overinflated tires with broken belts (bulges) except in extreme situations (hitting a cat converter on the highway in my case. farm truck on gravel rocks while overloaded, and the like) wheras I've seen many people come in asking to have a tire repaired that's leaking and has a broken belt that's only down to 25 or 30 psi (no other sidewall damage) and several tires with run-flat operation (sidewall damage) with MANY broken belts. I've only seen tread seperation on VERY abused tires (again, rocks/gravel), alignment wear, or underinflation (wears down the edges of the tire to the belts, allows elements to get into the inner layers of the tire and seperates it)

I've never seen overinflation wear except on 1/4 and 1/2 ton pickup rear tires that are at least 35 psi and never rotated.

as for higher speed rated tires, it's not that they don't deform but that the rubber is a sticker compound so you can keep traction at those speeds. If you take them to those speeds or track them at lower speeds, they'll wear out faster than a non-speed rated tire (I've read stuff in articles about this online... driver of X high end car complained to tire mfr that 50k mile tires were only lasting 10k miles. driver said they never exceeded the speed rating (180 mph) and the tire company basically laughed in their face because the tread wear warranty is at "normal" speeds

the higher PSI from mfrs is because the less inflated tire will heat up more from more rapid flexing as it rolls faster. higher psi=less flexing=less heat=fewer blowouts. higher pressures also reduce sidewall flex... I dunno about you but I don't want my sidewalls rolling under while trying to maneuver at any speed, especially the excessive kind. it doesn't work well, I've tried it (yes, on the track)
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Old 11-09-2008, 10:18 AM   #27
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as for higher speed rated tires, it's not that they don't deform but that the rubber is a sticker compound so you can keep traction at those speeds. If you take them to those speeds or track them at lower speeds, they'll wear out faster than a non-speed rated tire (I've read stuff in articles about this online... driver of X high end car complained to tire mfr that 50k mile tires were only lasting 10k miles. driver said they never exceeded the speed rating (180 mph) and the tire company basically laughed in their face because the tread wear warranty is at "normal" speeds
Speed rating and rubber compound are two different things. Here is an example: The Bridgestone Potenza G 019 is considered a high performance all season tire. It is available in many V rated sizes. It even has a 50k mi tread life warrenty. Then look at the Bridgestone Potenza RE-01R, it is considered an extreme performance summer tire. It is also available in many V rated sizes, but is a much stickier tire. Because it is a softer compound, it has no tread life warranty at all.

Speed ratings are directly related to how a tire is constructed. When a tire is speed rated tested, it is ran against a drum at different increments of speed. Once it has reached the highest speed it is rated for they run it for 10 more minutes to ensure the conformity of the tire. The tire rack web site has a very good explanation, way better then I could ever do.

A better indicator of rubber compound can be found in the UTQG ratings assigned to the tire. Lets look at the G 019 and the RE-01R again. They both have a AA traction rating. The harder, longer lasting (50k warranty) G 019's have a treadwear grade of 400-460 depending on size. On the other hand, the sticky, no mileage warranty RE-01R has a grading of 140-180.

The benefits of a speed ratings can be felt at speeds WAY below what they are rated for. Lets say a car comes with H (130mph) rated tires. Let's also say you never do 130mph. You still exert similar forces on the tire with cornering and braking loads.

The case of driver X you mentioned is something I have seen more then a few times. Most of the time it is a simple cause, excessive/abrupt acceleration and braking. You can toast a set of tires with smoky burnout's in no time, without ever exceeding the speed limit, much less the tires speed rating...
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