Capriracer perhaps - Very high pressure question

[JHZR2]

I have a question regarding exceeding sidewall pressure ratings.

A lot of hypermiler folks set their tire pressures at well above door and sidewall ratings. 50 PSI or a p-metric tire is not uncommon, and Ive seen mention of 70 psi. The thinking is that a tire will not pop until at least 250 psi, and so there is still plenty of safety margin.

So, besides higher shock loads to the vehicle body and potentially higher centerline wear, what are the issues regarding increasing tire pressure above sidewall numbers? What else can happen? Apparently a lot of folks are doing this without issue, and Id certainly assume that this is better than driving with too low a pressure...

However Id appreciate any technical insight as to what the pros and cons are with doing this from a tire quality, longevity and performance (and any other technical tire) standpoints.

Thanks!

JMH

 

[alreadygone]

NOT very technical, but a couple of observations.

Couple years ago customer's wife in central TX visiting family. Her father took Jeep Cherokee and aired tires to 60+psi! middles completely worn out when she got back to AR, probably 600mi.

YEARS ago customer with show grade Chev PU (even though daily driver) ran 3 different sets of our (Kelly built "Super Star) P275/60R15"s over 70,000 miles per set!!! This was when average was maybe 30,000. This is an aftermarket size and he was running them on aftermarket 15X81/2 inch wheels at 45psi, sidewall called for 32psi.

I'd not worry about safety as much as hitting the "sweet spot".

Bob

 

[Master ACiD]

i had a jetta a few years ago and i addicently put 70psi into the tyres, and drove it around for a month like that. my gauge was bad. i thought i was putting 44psi into them, but it was atleast 70psi because 70 was the maximum of the new gauge i bought and it was maxed out.
in the month of time, i didnt hurt the tyres, but the ride was stiff as 2 guys in a prison shower.

 

[CapriRacer]

It's been my experience that some of the folks pursuing fuel economy....well....they are a bit "out there"....AND anything I might say is going to be ignored. "True Believers" are like that!!

But the truth is that - yes - the actual bursting pressure for passenger car tires is much, much higher than is printed on the sidewall.

BUT, there is a reason why the maximum pressure is printed on the sidewall.

As inflation pressure is increased the tire gets stiffer. This means that when an object is encountered by the tire - it could be a simple expansion joint - the force against the tire goes up.

The physics is that the energy absorption ability of the tire (or anything for that matter) is proportional to the amount of deformation that takes place. The greater the deformation, the more energy that is absorbed, and the lower the force that is generated.

Up to a point - at some force level the cords in the tire will break - EXPLOSIVELY!!

So tires that run high inflation pressures are more prone to impact failures than at low pressures. The problem is that most folks do not experience an impact failure, so they don't know what it is like and how bad the results can be.

And some of those folks think there was a defect in the tire - further diluting the what people think is going on.

It is unfortunate, but some of those who have experienced an impact failure aren't around to share their experience.

 

[Quattro Pete]

Quote:

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As inflation pressure is increased the tire gets stiffer. This means that when an object is encountered by the tire - it could be a simple expansion joint - the force against the tire goes up.

The physics is that the energy absorption ability of the tire (or anything for that matter) is proportional to the amount of deformation that takes place. The greater the deformation, the more energy that is absorbed, and the lower the force that is generated.

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I guess it's like with a balloon. Blow it up just a little, and try to poke it by sticking a full finger in it. The balloon will flex and accomodate your finger without bursting. But try doing the same when the balloon is fully blown...

 

[Tom_T]

Agree totally with above posts. I also think you have to keep in mind diminishing returns as the pressure goes up.

As a side note, I keep my tires inflated to the max listed on the sidewall, I do not seek out potholes, I do not buy cheap tired, and I still get about 3 "belt separations" to each tire I actually wear out.

 

[XS650]

Quote:

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Agree totally with above posts. I also think you have to keep in mind diminishing returns as the pressure goes up.

As a side note, I keep my tires inflated to the max listed on the sidewall, I do not seek out potholes, I do not buy cheap tired, and I still get about 3 "belt separations" to each tire I actually wear out.

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I keep my tire pressure 2 or 3 psi above the car manufacturers recommended pressure, well below the sidewall pressure, and can't remember the last time I had a belt separation.

We might be seeing a bit of a pattern here.

 

[Tom_T]

XS650 take me to school on this, really! We'll use the wife's and 2 kids cars as examples. They don't care about ride quality, they gass 'em & go. The placards state 36psi and I've kept the pressure at 44psi for what I thought were safety reasons. Logic being if they have to hit the brakes and manuver I want to give them the best possible chance they have to avoid an accident. Fuel milage and wear are not the top priorites here. Their safety is. It scares the you-know-what out of me when I get the "Dad, my car is making a thumping noise when I drive" as I realize what could have happened.

Am I really overinflating these tires and endangering those that I am trying to protect? Not trying to be overly dramatic here but this exactly how I fell about it. I've had this happen over the years with Toyo, Dunlop, Michelin and Goodyear.

Not trying to hijack the thread, this it somewhat relevant.

Thanks.

 

[CapriRacer]

Quote:

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......XS650 take me to school on this.............

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I'll do all the thin'in' around here, Babaloo!! .....er...teachin'....er.......opps - Sometimes I get carried away.

Anyway, here's the scoop:

What is written on the sidewall of a tire is governed by a government regulation that is a little unclear.

Everyone agrees on what the regulation means when it refers to the maximum load carrying capacity. But the regulation also requires that the maximum inflation pressure be printed as well.

Most folks think it means exactly that - the maximum allowable usage pressure. So most tires are branded with a pressure well above where the maximum load carrying capacity occurs. For Standard Load Passenger car tires, the maximum load carrying capacity occurs at 35 or 36 psi.) However, the tire standards allow higher pressures (44 or 51 psi) for certain situations - and it is these pressures that usually get printed on the sidewall.

HOWEVER, there is one school of thought (notably in the Michelin group) that says that S and T rated tires are supposed to be inflated to no more than the 35 or 36 psi value and that the 44 psi value only applies to H and higher speed ratings. This is not true, but for the purposes of complying with a regulation, it is a good arguing point. Nevertheless, there is disagreement on what pressure should be printed.

What this means is that what is printed on the vehicle placard works regardless of the tire manufacturer, and what is printed on the sidewall is different depending on the tire manufacturer - so the vehicle placard is consistent within itself. That's a good reference point for inflation pressure setting - not what's on the sidewall!

Having said that, many car manufacturers specify a pressure that is a little on the soft side. Using a bit higher pressure - 3 to 5 psi more - will have some real benefits, among them better tire wear, durability, crisper steering, with the obvious loss in ride comfort. But I want to emphasize that this isn't always the case and a little reality check goes a long way here.

Using 44 psi instead of 36 psi as specified on the placard is going to decrease the tire's footprint and on dry pavement - and in some wet pavement situations - this is going to decrease the traction of the tire. I would not recommend you continmue using pressures more than 5 psi above the placard.

But let's address the separation issue you've brought up.

Every time I see a rank ordering of states by the amount of stire separationbs, FL is always up there. So your concern about separations is valid. But at the same time, I have seen many folks misdiagnose wear issues and other non-related issues as separations.

The rate you quoted (3 out of 4) is so high - way, way, way, way higher than anything I have ever heard, that I suspect there is an incredibly large amount of misdiagnosis going on.

So unless you are overloading the tires, or have a bad pressure gauge, these are probably not separated, but irregular wear that is causing the noise that makes you think it is a separation.

Hope this helps.

 

[Tom_T]

Thanks so much for setting me straight! I replace tires in pairs, and by doing that when I have a problem with one tire I end up replacing 2 so my estimation of separations vs wearout may be skewed high. I diagnose separation when I can feel a lump in the tread area, that's after I feel the car hump up backing out of the driveway. I have seen them the size of an egg, well half an egg. I don't think I am confusing this with cupping as they are always an outward bulge and don't cause a noise so much as a feel. Each car (4) has it's own gauge and while they might not be the best I have cross checked them and they are consistent.

I don't know how tire failures are reported but friends and extended family seem to experience the same problems that I have, although I can't attest to their maintenance habits.

I'll drop the pressure, thanks.

 

[White 03]

Capriracer said:
For Standard Load Passenger car tires, the maximum load carrying capacity occurs at 35 or 36 psi.) However, the tire standards allow higher pressures (44 or 51 psi) for certain situations

More education please;

the max load capacity does not occuer at the max pressure. Does this also apply with truck tires? If I remember right I have seen charts with load capacity for my PU tires that went up as the pressure went up. In my case LT265/75r16 goes up to 3415 pounds at 80 PSI and at 65PSI it would be 3260 and continue down with pressure decrease. To me this would indicate the amount of air, size and pressure, made the load capacity. Am I doing something wrong here? Or is there something different with the passenger tires compared to LT tires?

If this is off topic please delete.

 

[CapriRacer]

Quote:

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......

More education please;
The max load capacity does not occuer at the max pressure. Does this also apply with truck tires?........

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It applies to all tires, but it becomes quite obvious in passenger car tires - and obvious enough that the tire standards writers had to recognize this.

Quote:

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.......If I remember right I have seen charts with load capacity for my PU tires that went up as the pressure went up. In my case LT265/75r16 goes up to 3415 pounds at 80 PSI and at 65PSI it would be 3260 and continue down with pressure decrease. To me this would indicate the amount of air, size and pressure, made the load capacity. Am I doing something wrong here? Or is there something different with the passenger tires compared to LT tires?........

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No, that part still applies, but even in LT tires, it is permissible to use pressures above the point where the maximum load carrying capacity occurs.

HOWEVER, you have to be careful how much pressure you add. In the case of passenger car tires, this has been defined to a maximum of 44 (+8) or 51 (+16) psi. But this has been defined for LT as well - and the maximum amount of additional pressure is 10 psi.

The point I was trying to make was that some folks are using pressures way, way above this and that is not good.

 

[gregmp]

Capriracer is dead on about the physics of this topic.

In some situations 2-6 psi above placard may help for even wear, gas mileage and handling.
It will hurt overall grip and ride.

 

[wantin150]

I don't think additional pressure (within "our" accepted 2-6 psi over placard) affects grip in a negative way at all. Higher pressures usually mean stiffer sidewalls which usually means less flex while cornering. Less flex means better grip. The more contact patch you have, the better off you are (with the possible exception of snow).

I don't want to get into the whole "airing down" thing for drag racing as it isn't applicable here.