Question Regarding Air Pressure

[ARCOgraphite]

Originally Posted by Kira
Much as above. Everyone I know goes higher than placard.


NASTINESS WARNING: People simply using the placard as gospel without considering specifics and unknowns is like people who obey "lifetime transmission fluid" claims by car manufacturers.
Involvement is necessary.


I always gauge wear and adjust up or down to achieve even wear across the tire.

But In our broken roads around here, the impact harshness goes WAY up if I'm more than 3psi over placard. which would be over 37psi in most instances. I've knocked Strut top Bearing Races out of a few cars running 35-37psi ( Mazda 626, Mitsubish 3000GT)
I also do some hard 50- 60 mph slalom testing after adjustment on the local highway (when low traffic) to look for propensity to spin. I would say that many PASSENGER cars would get very tail happy when you approach 40 psi in the rear tires. Im usually down 1 or two PSI over the fronts in a FWD machine.

And now, there is more tuning to terminal understeer now than I have ever seen before. '

And we have those Ninny Nannies - ASC, ASR now too.

Most people where I worked never looked at their tire pressure or oil level.
And that was a cutting edge data and comm tech contractor.

 

[zeng]

Originally Posted by CapriRacer


And lastly, it is a fallacy that a tire's wear is EXACTLY right if the pressure is EXACTLY right. Not only are there things that have more influence, not all tires have the best footprint even if properly inflated.

+1

 

[MrMoody]

Here's a couple of honest questions.

If one radial tire can carry its maximum load at 35 psi, why does another of the same size and load rating require 50 psi to do it?

And more importantly, does this make a difference in the proper inflation, more so for a given load, which on most cars with the usual 1 passenger is somewhere around half of the maximum for the tire?

IMO, on older cars (pre TPMS) the placard pressures are the minimum acceptable with the car loaded at its rated maximum, with concern only for comfort & tire failure and none for tire life or handling. Newer cars seem to have gone up somewhat by comparison despite the tires being larger and higher load rated.

 

[CapriRacer]

Originally Posted by MrMoody
Here's a couple of honest questions.

If one radial tire can carry its maximum load at 35 psi, why does another of the same size and load rating require 50 psi to do it? ….


Sorry, not true. I can think of 2 possible reasons for the confusion (there may be more!)

1) In spite of the fact that different P type tires have different max pressures, the load curves are the same. That statement kind of gets confused in that there are 3 different tire standardizing organizations and they use different formulae. I explain that in more detail here:

Barry's Tire Tech: Load Tables

2) There is a difference between P type tires and LT type tires and even though the dimensions are the same, there are considered diferent sizes because the letters are part of the "size" - technically called a "Size Nomenclature".

Originally Posted by MrMoody
…...And more importantly, does this make a difference in the proper inflation, more so for a given load, which on most cars with the usual 1 passenger is somewhere around half of the maximum for the tire? ……..


I hope the above allows you to see that this second question is kind of a non-sequitur - as in the proper inflation (considering only load carrying capacity) is the same regardless of which tire standardizing organization is issuing the load curve and regardless of who makes the tire.

I am sure this post is going to generate some additional questions: Ask away!

 

[OVERKILL]

Originally Posted by MrMoody


IMO, on older cars (pre TPMS) the placard pressures are the minimum acceptable with the car loaded at its rated maximum, with concern only for comfort & tire failure and none for tire life or handling. Newer cars seem to have gone up somewhat by comparison despite the tires being larger and higher load rated.


If that were the case the language would be inflate to a minimum of that pressure, but that's not the language used at all, it's a set number with nothing indicating that exceeding it is encouraged or desirable. Now, we could get all fancy like European marques and indicate different pressures for different loadings and speeds, this is the placard that was on my E39 M5:

The "US Spec" pressures were the "fully laden" pressures with no other options. That is, the MAXIMUM pressure indicated on the placard was to allow for the vehicle operating with a maximum number of occupants and their gear. There is no mention of an option or suggestion to go above those pressures.

Contrarily for "rest of world" use, if you were NOT fully loading the car, the recommended pressure was LESS, at 34 front, 38 rear versus 44 front and 48 rear. The LOWER pressures, which aligned far better with my use of the vehicle, resulted in notably better traction and far better ride.

 

[MrMoody]

Originally Posted by OVERKILL
If that were the case the language would be inflate to a minimum of that pressure, but that's not the language used at all,

In at least some cases, it is exactly the language used. On the GTO, the placard states, "SEE OWNER'S MANUAL FOR ADDITIONAL INFORMATION"
In the manual, an exact quote is "The recommended cold tire inflation pressure, shown on the label, is the minimum amount of air pressure needed to support your vehicle's maximum load carrying capacity."
There is also the following chart:

I run the GTO at the recommended pressures because it seems to handle, ride and wear properly like that.
On the other hand, the Grand Prix has more weight on the front axle and smaller tires, yet only recommends 30psi. I'm sure they are safe like that, but don't seem optimal. The tires feel, look and wear as though they are underinflated, so I run the fronts at 38. It handles better and is not harsh by any means. Is this due to some difference between the P7s and the OEM tires? Seems doubtful since they are the same size.

 

[OVERKILL]

Originally Posted by MrMoody
Originally Posted by OVERKILL
If that were the case the language would be inflate to a minimum of that pressure, but that's not the language used at all,

In at least some cases, it is exactly the language used. On the GTO, the placard states, "SEE OWNER'S MANUAL FOR ADDITIONAL INFORMATION"
In the manual, an exact quote is "The recommended cold tire inflation pressure, shown on the label, is the minimum amount of air pressure needed to support your vehicle's maximum load carrying capacity."
There is also the following chart:

I run the GTO at the recommended pressures because it seems to handle, ride and wear properly like that.
On the other hand, the Grand Prix has more weight on the front axle and smaller tires, yet only recommends 30psi. I'm sure they are safe like that, but don't seem optimal. The tires feel, look and wear as though they are underinflated, so I run the fronts at 38. It handles better and is not harsh by any means. Is this due to some difference between the P7s and the OEM tires? Seems doubtful since they are the same size.


Interesting info, thanks for the share, I hadn't seen that before. How many vehicles have you seen this on?

 

[MrMoody]

It may be a GM thing, I'll have to look around. Our previous '15 Buick Encore has similar wording in the manual, "The recommended pressure is the minimum air pressure needed to support the vehicle's maximum load carrying capacity." But it doesn't have the reduced load chart. The Hyundai has neither, nor does the Grand Prix.

 

[zeng]

+1
Indicated (placard) pressure being minumum makes lots of sense.

 

[OVERKILL]

Originally Posted by zeng
+1
Indicated (placard) pressure being minumum makes lots of sense.


You'll note BMW's verbiage calls out "Cold tire inflation pressure" as a spec, not a recommendation. So it's interesting to see the difference in the language depending on the manufacturer.

 

[OVERKILL]

Originally Posted by MrMoody
It may be a GM thing, I'll have to look around. Our previous '15 Buick Encore has similar wording in the manual, "The recommended pressure is the minimum air pressure needed to support the vehicle's maximum load carrying capacity." But it doesn't have the reduced load chart. The Hyundai has neither, nor does the Grand Prix.


Interesting. Language from the E39 M5 Manual:
Originally Posted by BMW
The inflation pressures are indicated on a label attached to the B-pillar behind the driver's door (visible with door open). All pressure specifications are indicated in psi (kilopascal) for tires at ambient temperature


FCA for the Jeep:
Originally Posted by FCA
Each tire, including the spare (if provided), should be checked monthly when cold and inflated to the inflation pressure recommended by the vehicle manufacturer on the vehicle placard or tire inflation pressure label.


Note that BMW remarks as to pressure as a specification, whilst FCA as a "recommendation".

Ford also went with the specification language, this was from our Expedition warranty guide:
Originally Posted by Ford
Purchase a good quality air-pressure gauge and check the inflation pressure of your tires once a month. Underinflated tires wear out faster, waste fuel, and are unsafe. Keep your tires inflated to the pressure specified by the vehicle manufacturer.

 

[Garak]

For reference, the G37 manual has mentions all over the place, and just mentions filling to the cold pressure, with more warnings about doing it cold than anything else, warnings against incorrect tire pressure and under inflation specifically, and then little talks about Infiniti's quirky TPMS.

I was actually surprised the cold pressures are listed right in the manual, in addition to the placard, since I've seen many manuals refer to the placard and have nothing in the book. I guess for the G37 it's easy, since there were only three tire options, all with the same inflation pressures, as opposed to a truck with 16 different tire options and a dozen different gross weights and drive train configuration combinations.

So, no minimum or maximum business. I wish I had some of the older GM manuals around.