PSI, Up/Down???

[CharBaby]

Although the P7's scored very well in the "road imperfection/absorbing ability" in the Tire Rack test with 3 other highly regarded tires(Mich, Yoko, Conti), I just don't think that this absorption made it into every size. It seemed fine when new but, 6000 miles later(even with a tire rotation), I am feeling some differences.

We'll see!

 

[Boss302fan]

I have owned 60+ cars in my lifetime, including many classic/muscle cars. I have found that running 2 PSI over the door placard has just about been perfect for radial tires. Going higher has always increased center tire wear and stiffened up the ride. 2-3 PSI makes a huge difference in ride quality...

 

[CharBaby]

Originally Posted By: Boss302fan
I have owned 60+ cars in my lifetime, including many classic/muscle cars. I have found that running 2 PSI over the door placard has just about been perfect for radial tires. Going higher has always increased center tire wear and stiffened up the ride. 2-3 PSI makes a huge difference in ride quality...



Yeah, I have typically like/enjoyed the 2 PSI over the Mfg Place Card for the FRONT Tires and often, at the recommendation or 1 PSI lower in the REAR.

And of course, I adjust the PSI upward if we're taking the vehicle on the family vacation with people & luggage!

 

[FocusTi]

My door placard calls for 39psi. I keep them at 40.

What I'd love is a guage that's actually accurate. Bought 2 of the 'dial face' guages at Auto Zone, a few months apart, and they read 5psi different from each other, and both read higher (1lb & 6lbs respectively) higher than my slide guage.

 

[KitaCam]

Tire PSI on the Camry is way higher with 17s and performance tires vs. 15s and OEMs...way higher, and treadwear is even from shoulder to shoulder.

 

[friendly_jacek]

Originally Posted By: CapriRacer

Excellent picture and especially the higher pressure at shoulders at the "normal" PSI loaded. This is exactly my experience. In my driving, that is mostly city and cornering at more than everyone's lazy rate, I had insane shoulder wear (front tires) when OEM pressure was used. I bumped the pressure to about 40 PSI many years (and several tire sets) ago and never looked back. Shoulder wear is a history and I have evenly wearing tires.

 

[Hokiefyd]

Originally Posted By: Vikas
If the tire is inflated to 16 psi, how is the 90-100 psi [yellow/red] being shown on the edge of footprint [bottom right picture]? Anybody knows the scientific explanation for it?


That's the contact pressure of the tire on the scale that was used for the test. That's not inflation pressure. That image makes a lot of sense: as you lower the pressure, the outer area of the tread will receive the most wear and the center of the tread will tend to want to lift up off the ground. As you increase the pressure, the tire will become more balloon-like, with increasing contact pressure in the center compared with at the edge.

 

[Quattro Pete]

Originally Posted By: Char Baby
Once again, the tire place card is, F:32/R:32

With the Hankook Optimo H727's, I kept the tires at...F:33/R:31

I can't see how a 1 PSI variation would result in any difference and how your butt dyno would be able to even detect it.

 

[Ken2]

For vehicles with greatly variable weights depending on the load, the placard seems to be written by the lawyers. An empty pickup truck vs. a pickup loaded to its max rear axle allowable weight is one example. We need both drivers with enough sense to know the difference, and placards that show us the difference.

Carrying a passenger on a motorcycle is an example. All the passenger weight is on the rear wheel. My previous Suzuki V-Strom motorcycle called for 33 psi front, 36 rear for solo riding, and 33F/39R for riding with a passenger. The gross axle weight ratings called for inflation of 33F & 41R. My new Suzuki V-Strom is a heavier bike, same size tires, but now they call for 36 front & 41 rear whether the rider is solo or double or loaded to the GAWR. I'm riding solo with 36F/39R, and I like the way it handles. (And, of course, many riders are sure the tire maker's recommended pressure is the 42 on the sidewall, front & rear.) An important consideration especially with motorcycle tires is that traction is so temperature dependent. Tires need to be normally warm for good traction. The main source of tire heat is the flexing which is load & inflation dependent. A tire at 42 in the winter will be both hard riding and hard cold rubber for reduced traction.

 

[Hokiefyd]

We had two Mopar minivans; both called for 36/36 F/R. They had pretty under-sized tires in my opinion (215/70R15 and 215/65R16; both with a load index of 98), and I ran them at 38/34. Wear was good, but you had to adjust during tire rotations. 36/36 is probably a fair compromise, and the most simple for most owners.

Despite being FWD-based, both of our vehicles have a decent weight distribution (56:44 for the Acura and 55:45 for the Honda), and I haven't found that varying the inflation pressures between front and rear makes much of a difference. I run the spec'd pressure in both, 32/32 in the Acura and 30/30 in the Honda.

 

[CharBaby]

Originally Posted By: Quattro Pete
Originally Posted By: Char Baby
Once again, the tire place card is, F:32/R:32

With the Hankook Optimo H727's, I kept the tires at...F:33/R:31

I can't see how a 1 PSI variation would result in any difference and how your butt dyno would be able to even detect it.


I may have been determined by the gauge I was using at that time! But, this was the best ride/handling balance w/o the harshness. Also, those were different tires!

 

[Quattro Pete]

Originally Posted By: Char Baby
I may have been determined by the gauge I was using at that time! But, this was the best ride/handling balance w/o the harshness. Also, those were different tires!

Well, yeah, so the tire model will make a much bigger difference in ride/handling than a small change in PSI.

 

[CharBaby]

Thank you all for your comments

I know what I will do now!

 

[Vikas]

Originally Posted By: Hokiefyd
Originally Posted By: Vikas
If the tire is inflated to 16 psi, how is the 90-100 psi [yellow/red] being shown on the edge of footprint [bottom right picture]? Anybody knows the scientific explanation for it?


That's the contact pressure of the tire on the scale that was used for the test. That's not inflation pressure. That image makes a lot of sense: as you lower the pressure, the outer area of the tread will receive the most wear and the center of the tread will tend to want to lift up off the ground. As you increase the pressure, the tire will become more balloon-like, with increasing contact pressure in the center compared with at the edge.
I understand that. But the contact pressure times the contact area will equate to the total load supported by that tire. Actually, if you integrate the pressure over area, you will get the total supported weight. I suppose I am making lot of simplifying assumptions here but at least for a range of pressures that should be true. If the inflation pressure is way too high, it would seem that the contact patch would be thinner as the weight supported is always going to be contact pressure times area and if you increase the inflation pressure, contact area has to decrease. If the inflation pressure is too low, it would make sense that shoulder of the tire i.e. rubber would be supporting the weight of the car rather than the air in the tire and thus it would make sense that the shoulder would be exerting lot more pressure on the pavement. What was surprising is that even at the recommended inflation pressure, the shoulder edges were exerting lot more pressure in that picture!

Now, in ideal condition contact pressure and inflation pressure will be the same because that part of the tire is not accelerating up or down and thus on that patch of the rubber contact pressure exerted by the pavement has to be same as the inflation pressure exerted by the tire on the pavement. Newton's second law and all that

I believe the real answer to my apparent dilemma is that tire shoulder have NO air supporting it and thus none of my explanation applies there!

 

[CapriRacer]

Originally Posted By: Vikas
I understand that. But the contact pressure times the contact area will equate to the total load supported by that tire.....


Sorry, but that is NOT true. There is no correlation between the size on the contact patch and the load and the inflation pressure - except to say that more load and less pressure results in a larger footprint. I go into more detail here:

Barry's Tire Tech - Air or Tire?

I cite 3 places where this has been disproven. There are more, but those are the 3 that people might be able to find on the internet.

I'm not going to quote the rest of the post because it just follows up that incorrect statement and draws some conclusions that are wrong because of it being incorrect - except to say that this statement:

Originally Posted By: Vikas
.......I believe the real answer to my apparent dilemma is that tire shoulder have NO air supporting it and thus none of my explanation applies there!


is now explained.

 

[CharBaby]

Originally Posted By: Quattro Pete
Originally Posted By: Char Baby
I may have been determined by the gauge I was using at that time! But, this was the best ride/handling balance w/o the harshness. Also, those were different tires!

Well, yeah, so the tire model will make a much bigger difference in ride/handling than a small change in PSI.


Without question it's the tire model(Pirelli P7 Cinturato A/S+).
I wasn't saying that the lower PSI in the tires created a Cadillac ride, just saying it went from toooo harsh, to a more tolerable level compared to the previous tires, without going too low in PSI.

In a car like this Mazda, I do notice a 2-3 PSI difference(Butt Dyno) more so than in many other cars such as my wife's Lexus! Especially with the P7's more than the Hankook H727's.

Also, these small PSI differences may not be apparent in a BMW, Audi, Benz but, they are here!

I have to say that we do like the P7's as they have taken care of all the issue that we had with the other tires(which weren't many). They have just picked up an issue of their own unfortunately! That being the harsh(er) ride over road imperfections with OE recommended tire PSI or above.

Unfortunately, the body structure of this car isn't that of a luxury car so, more is heard and felt! And there could be other issues as well and the fact that the tires are "H", not "T" rated so, they seem less forgiving.

 

[CharBaby]

Thanks once again all!