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Old 05-27-2008, 08:15 AM   #1 (permalink)
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Hyperinflating tires

I'm starting a discussion thread here - and I'm hoping to move a discussion that got started when I introduced myself:


So hopefully this will be a bit more visible and a bit more "on topic".

First, let's define some terms:

Underinflation: Using less inflation pressure than is listed on the vehicle placard (assuming the placard tire size is being used.)

Overinflation: Using more inflation pressure than is listed on the vehicle placard (assuming the placard tire size is being used.)

Hyperinflation: Using more inflation pressure than the maximum listed on the tire sidewall.

Vehicle placard: The sticker on the vehicle that lists the original tire size and the proper inflation pressure for that size.


Any other terms we need to define?


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Old 05-27-2008, 08:32 AM   #2 (permalink)
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Old 05-27-2008, 08:50 AM   #3 (permalink)
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credentials... lets see some.
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Old 05-27-2008, 09:17 AM   #4 (permalink)
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I'll agree with your definitions. I've read numerous articles that all warn of the dangers of overinflation, yet NONE of them define what overinflation is. Are they referring to above mfg spec or above max sidewall? There is no way to tell.

I'd have to agree with the others though. If you are going to tell us how it is then your going to need to back it up with some factual evidence.
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Old 05-27-2008, 11:12 AM   #5 (permalink)
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Originally Posted by TheDon View Post
credentials... lets see some.
I've worked for one of the major tire manufacturers for over 30 years. Been in manufacturing, design, testing, was the technical liason to Ford, and my current assignment is in an area where I interact with the outside world. I'm the technical advisor to the 800 number as well as serve on several committes with the various tire technical organizations, such as the Rubber Manufacturers Association, the Tire and Rim Association, and the Tire Industry Association.

My main reason for posting here concerns hyperinflation - inflating tires over the maximum posted on the sidewall of the tire.

I think it might be best if we confine the discussion to passenger car tires and avoid the confusion with Light Truck tires (as defined by the letters "LT" in the tire size) and Truck Tires (over the road, big rig stuff)

And before I start, it should be obvious that I have access to a lot of information. but a lot of it is confined to the normal operating enevlope of a passenger car tire (Inflation pressure in the 30's, speeds below 70 mph, and moderate loads.) Tire manufacturers aren't going to fund studies and testing that doesn't lead to more sales, so the results of some studies won't be published because of that.

Not to mention that a lot of fundamentals have been lost to time - and I think we're going to find this a very real limitation in this area.

Anyway, enough for now, I've got work to do!

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Old 05-27-2008, 11:37 AM   #6 (permalink)
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I will agree with the definitions there.

I personally would never inflate my tires past the limit posted on the tire. I always assume that the manufacturer has tested these tires to a point that the number on the outside stands for some point at which some of the tested tires began to fail after use. I actually go even beyond that and will adjust my tires based on my driving for the day. I have found that my tires will go up by a factor of 'x' based on what kind of driving I'm doing, more so for highway driving then around town driving, so I keep them at the max listed on the sidewall unless I'm going on the highway where the temperature will cause the 'x' factor of expansion to occur in which case I release pressure enough so that it stays within safe limits while they are heated up.
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Old 05-27-2008, 11:51 AM   #7 (permalink)
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I agree that some of us go over the sidewall limit and that it is technically unsafe. But how unsafe is it. Is it like crossing the road unsafe or base jumping unsafe? My point is that although we are exceeding the safe working load, it is by a (probably) small margin and should pose no problem. I'm happy with that personally but obviously its a personal thing. What i really want to know is how the traction varies with pressure, all the way to sidewall.
I'm thinking it goes up because if it dramatically went down, we would be told about it everywhere. Instead, silence. Now i often joke about being able to find anything on the internet but the silence on this subject is phenomenal!
Anyway, it will be good to hear more from you CapriRacer. You sound like a knowlegeable bloke and have a lot to offer here.

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Old 05-27-2008, 12:39 PM   #8 (permalink)
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For me, I think it is unsafe to inflate my car over 39-40psi. The sidewall maximum pressure of my tires (Yokohama Avid TRZ 185/60TR15) is 44psi -- but I find that I start losing wet (and dry) traction if I go above 39psi.

I believe the recommended pressure is only 32psi, and it seems like anything between 36 and 39 is virtually the same rolling resistance; judding by my coasting distances and MPG.
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Old 05-27-2008, 01:40 PM   #9 (permalink)
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Originally Posted by NeilBlanchard View Post
I believe the recommended pressure is only 32psi, and it seems like anything between 36 and 39 is virtually the same rolling resistance; judding by my coasting distances and MPG.
This is well put. If coasting distances (with a degree of confidence) prove that X psi is no better than Y psi, then you have a valid conclusion to keep it lower. Still, in my application, I haven't found any valid sources to persuade me not to run pressures over sidewall max.

The problem we have is the different kinds of tires on all kinds of vehicles.

If people want to play it safe, test in an open parking lot -- similar to Autocross (or even attend one). Try the same lot when it rains -- with different pressures. You can learn quite a bit about vehicle dynamics that way.

For me, there doesn't seem to be any difference with wet vs. dry traction -- but then again, I haven't "officially" tested it -- just parking-lot stuff. Oversteer is more pronounced in wet weather, but I'm driving slower, and IMO, helps the car's handling dynamics (front-heavy: tends to push in corners).

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Old 05-27-2008, 01:57 PM   #10 (permalink)
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Capriracer can confirm this for sure, but I believe the sidewall max psi is for when the tire is cold. The pressure will go up while driving fast but I'd assume that has been considered when the engineers set the max psi rating.
ie, no problems doing 75mph in death valley in the summer at max rated load...

I've been doing some autocross lately with my 95 Neon, and with all season tires, P185/65R14 which are relatively narrow high profile tire common on most cars, I get the best times at about 46psi cold in the front tires, and 35psi cold in the back. Autocross is basically 80 seconds of extreme accident avoidence manuevers at relatively low speeds, up to 40mph in our area. So I would say that running higher psi is definitely safer for city driving. You turn the steering wheel and your car turns quicker and more predictably, there is less deflection from the wheel rim to the contact patch on the tire.

As for absolute braking performance in a straight line, I bet 10psi is best for the rear tires and maybe 25 psi in the fronts. That would give you 4 big fat contact patches. Not good for anything else like turning or going through potholes though....

No tire or tire pressure is close to optimal for all conditions. By inflating your tires up to or slightly beyond the max on the sidewall you might be increasing your stopping distance by 2% on clean dry pavement, you might be reducing your risk of aquaplaning by 2% and you are able to turn 4% better while saving 4% more fuel... Or not, unless you have team of engineers test your tires on your car on your roads you'll never know for sure.

For me, I'm comfortable running 40psi in all tires on both my Neon and Tracker, this is 5psi over the sidewall max for the Neon and the Tracker. I don't drive 100mph for hours in the desert and I can see that tire is very much in contact all the way across the tread so I think this is good comprimise. YMMV.


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