What does "Maximum Inflation Pressure" really mean?

[szh]

Hi, all.

I find the explanations for the term "Maximum Inflation Pressure" (e.g., as on the Tire Rack web site) do not satisfy me. Some of their tires listed nowadays have a 44psi "maximum", others have 51psi and I am now seeing tires with entirely different values (like 39psi and 50psi). Why is there such a variance for normal tires sold for normal cars in the US today?

I know this is supposedly the maximum cold (i.e., not sun-warmed either) tire pressure that the tire should be used at. And it can get up to 6 psi warmer in use, etc. So what does this term really mean in real life? And how does one figure out the correct inflation pressure to use on a particular car or set of rims?

In my current 15" SZ50EP's on a Q45, I used a trial-and-error method, over many miles of driving, to settle on 36psi to get even reasonably wear across the tread and still have good handling. And I am still not sure that this is the ideal or correct pressure! Do the experts here have better ideas and recommendations? I'd like to avoid repeating the process with the 17" tires I need to get next.

Z

[greg_atlanta]

Lower max psi = stronger sidewall

Higher max psi = weaker sidewall (needs more air to keep its shape)

51 psi tires get damaged more easily, but they satisfy the need to have big wheels and low profile tires with a slightly softer ride than a real performance tire.

[Q45tech]

The question is what weight will the tire support at what temperatire/speed and still pass the explosion test brand new?

If one assumes that the hot temperature is 30-50F warm then the safe psi is 3-5-8 psi greater than the max cold psi.

The next question is what is the REAL LOAD vs the MAXIMUM LOAD.......the smaller the real load, the smaller will be the slipangle.

I always assume the graph will be linear at least above 30 psi. 31/44= 70% of max load x 1521= 1071.......pretty close to the static weight on an unloaded Q front sitting still.But when you add 2 [180lb] front passengers it goes to 1251 pounds or 82.3% of max load ..............36 psi? When you do an ABS full brake 70% [4300x 0.7/2=1505 pounds of weight is on each tire for 3-5 seconds..........would require 43.5 psi]

Obviously 44 lbs cold is too much and 29 psi is too little.......what is the average load on the tire with the typical passenger load.

The real question is how much safety factor your current tires have compared to the oem Michelin Sport XGTV. How real is the 1521 pounds at 44 psi...................how many minutes will the tire take to explode and at what temperature and camber.

"More recent data provided by Goodyear indicate that fuel efficiency is reduced by one percent for every 2.96 psi of under-inflation. (26) ........from http://www.nhtsa.dot.gov/cars/...N_23_

http://www.aiam.org/main_pre_det.asp?PR ... ko...c.pdf

[szh]

Thanks, Dennis, for your comments!

Since the original XGTV is not available any more, and the replacement Pilot XGTV does not behave well at all in wet weather, I had to look at other tires. In a 15" rim, the Firestone SZ50EP 225/60-15 Z rated, was an excellent alternative for what I wanted: good wet weather behavior and overall generall goodness!

Now that I have 17" rims on the way, I am probably going to go with the 245/45-17 SZ50EP's since they are 95 load rated. But there are a few other choices that seem good too. The new Yokohama AVS db S2 looks very interesting. AZHitman also suggested the Yokohama ES100, I think (although I have not yet seen his comments on that tire).

I have seen higher load rated tires in 17", but I am not sure about the tire quality - the reviews at Tire Rack seem to favor the SZ50EP and the AVS db S2 for now.

Yes, I definitely want to avoid over-loading tires! So, it looks to me that, for a 44psi max inflation pressure tire, my choice of 35 to 36psi is about as reasonable as I can make it - a good trade-off between tread wear and loading.

BTW, what pressure did you run your SZ50's at?

Z

[SmithSR]

WOW, a nice change from the hate war going on in 240sx chat. I've spent many years in a tire shop, and the basic number is still 32. every car is different, so with bigger cars, 35 or thereabouts is normal. every tire model is different, but the basic rule is to get maximum footprint without sacrificing tire longevity. too much air wears the center of the tread. too little, over time, will wear both shoulders somewhat. since the majority of Americans dont think twice about tires or tire pressures, a good start is to at least maintain manufacturer's recommendations. a good tire shop can explain the basics better, and IMHO, it's better to talk to a person than to read info off tire rack website...

[Q45tech]

Wet weather performance is a function of tread softness and inflation pressure [directional tread design has very little to do with it except in one very narrow test [where the water depth exceeds the tread depth by 10%]........by 20% extra the advantage goes away.

So owners must specify the water depth and water temperature to select the best tire......for that situation.

Inflating tires with air is what causes all the damage [internal reaction with rubber and steel cords]........take time to study why professionals use dry Nitrogen.

Doubt that there is any better Summer/Wet tire in 15" than the SZ50EP............

You calculate the cold pressure to cover the static load and assume that the heated [running pressure] will self adjust to cover the turning/speed load added.On a Q front that is usually defined as 1150-1200 pounds [per tire] on a 1521/44 psi tire 34.5-35.0 psi cold is the number so that it can grow to 40 psi in the heat of the day running 70 mph...............strange is that the imprinted number on the tire placard [for high speed driving].

Inflation growth depends on the moisture content of the inflation air used in the tire.

Never seen a Q front tire worn out in the middle from overinflation........impossible if you stay at 44 psi cold or under.........too much weight.

[maxnix]

too much air wears the center of the tread

Is this really true for a radial constructed tire not inflated beyond its maximum load carrying cold pressure?

I believe it is a left-over remnant from the days of bias-ply construction when the sidewals were stiff and the tread did indeed change profile with varying pressure. I think that radial construction precludes most of the flexing of the tread due to pressure differentials within a radial tire's specified operational ranges.

[Q45tech]

Probably correct soft single ply polyester sidewalls just deform long before the tread does.

Now there are still some dual/triple ply sidewall radials made [the government hates this because of increased road friction....not a green mpg tire].

Fortunately Michelin found stronger polyester materials that can come close to a dual ply sidewall.........not 100% but close yet almost all the failed Michelin I've seen had blown sidewalls.........similar to all brand 51 psi failure mode.http://www.michelin.com/corpor...6.jsph ... SS152.html

[SmithSR]

radials do indeed wear prematurely from over-inflation. this wear is not nearly as noticeable as with old school bias tires, but i have seen documentation of instrumented tests that uphold the theory. the concept may be a mute point when considering a heavy car weight. tire construction is still improving at an incredible rate, and i'm sure one day soon they'll have to rewrite the rule book on tire service.

P.S. Q45tech, your knowledge is overwhelming. write a book and i'll buy it.