What do you keep your psi at?

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"If your car comes with a Cooper All-Season that has a load rating of 89 and you switch it out for another tire, lets say ultra-high performance all-season with a load rating of 95, that's the same size."

I'm not CapriRacer, and don't have his knowledge, but I have never seen a "better" tire with a higher load rating in the same size. AFAIK, you will find LT (load range ???) tires that handle more load, but high performance ... as in racing or cornering ... they aren't. Can you give an example of one?

"How about sidewall stiffness? I didn't see much mentioned in the article about that. Test: Seat two tires on identical rims. One with load rating of 89, one of 105(big jump) and a known stiff sidewall. Decrease the pressure on both as much as possible without breaking the bead. Now measure the depth of the cup in that tire. There's no way in [censored] they're the same."

I believe that if both tires are recommended for use with the rim width in question, you will get a good contact patch over the range of recommended pressures. If WELL below recommended, will it cup? I'd frankly not expect it if rim width and tire size are within mutual specifications.

Now extrapolate this out to running pressure.
 
Originally Posted By: Brien
.......So, is that article, or are you, suggestings that tread compound and tread design don't have any effect on how the pressure affects contact area?.......


To be precise: Tread compound and tread design (that is to say the pattern of grooves and sipes) have very a negligible effect on how the pressure affects contact area.

And just to take this a step further: The question oilgal is asking concerned the placard and (If I read the question right) whether different manufacturers of tires would need to use different pressures. This question does not involve the shape or the size of the footprint. This is about load carrying capacity.

Quote:
......How about actual tire construction?.......


Within the context of the question - No. Tire construction is going to be within a small range of values for a given size and the actual construction is built around the load curve - not the other way around.

Quote:
.....If your car comes with a Cooper All-Season that has a load rating of 89 and you switch it out for another tire, lets say ultra-high performance all-season with a load rating of 95, that's the same size. That both tires should be filled to exactly the same pressure to have the same contact area?......


It isn't contact area that the placard pressure is all about. It's load carrying capacity.

But to "sort of" answer your question: Regardless of the tire manufacturer, the load curves involved are close enough that the pressure on the placard is appropriate.

Quote:
......From personal experience using the temperature method this doesn't seem to hold true. Sometimes the recommended pressure is right, sometimes its high or low. It's not always right. More times then not, when making a drastic change in tire type, but not size, the pressure had to be altered and tested using the Temperature method.......


I'm not sure what point you are trying to make, but be aware that higher speed rated tires will probably run cooler - and that's a good thing - but that doesn't say anything about the durability of the materials within nor the laod carrying capacity issue - which is one of the primary reasons for the pressure as specified on the placard.

Quote:
........How about sidewall stiffness? I didn't see much mentioned in the article about that. Test: SSeat two tires on identical rims. One with load rating of 89, one of 105(big jump) and a known stiff sidewall. Decrease the pressure on both as much as possible without breaking the bead. Now measure the depth of the cup in that tire. There's no way in [censored] they're the same.

Now extrapolate this out to running pressure.......


First, tires of a given size will be within 2 Load Indices of one another with 2 exceptions. (I hope everyone stays within the confines of passenger car tires, because this gets a lot more complex when we start to talk about LT tires and worse - truck tires .)

1st exception: Standard Load (SL) to Extra Load (XL). That was addressed in the article. Fundamentally, the same load curve applies to both except SL tires end at a lower pressure (and therefore, a lower load)

2nd exception: Low Load tires (LL). This applies only to P metric tires and only for low profile tires (45 series and lower). No vehicle manufacturer specifies LL tires, so the general rule is to only use tires that have at least the same Load Index as specified on the vehicle placard.

But it is very rare to encounter an LL tire, and it will never appear on a placard.

I understand what you are trying to say, but the example you picked doesn't exist.

In summary, you have to be careful here: Many of us (me included) like our tires to handle a certain way - and that sometimes means we will use inflation pressures higher than what is specified on the vehicle placard.

However, as a good starting point - and strictly from the point of view of load carrying capacity - the vehicle placard is an appropriate figure.

Please do not confuse running temperature - or handling characteristics - or softness of ride - or footprint size and/or shape - or evenness of wear - with the load carrying capacity of the tire. Every tire is designed first around the load curves for its size - and the rest comes later.

Hope this helps.
 
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Oops, sorry, I didn't realize we were only talking about load rating.

What I'm getting at is maximizing the contact area for different tires and designs. I haven't found a one pressure fits all for that.
 
Originally Posted By: Brien
So, is that article, or are you, suggestings that tread compound and tread design don't have any effect on how the pressure affects contact area?

How about actual tire construction?

If your car comes with a Cooper All-Season that has a load rating of 89 and you switch it out for another tire, lets say ultra-high performance all-season with a load rating of 95, that's the same size. That both tires should be filled to exactly the same pressure to have the same contact area?

From personal experience using the temperature method this doesn't seem to hold true. Sometimes the recommended pressure is right, sometimes its high or low. It's not always right. More times then not, when making a drastic change in tire type, but not size, the pressure had to be altered and tested using the Temperature method.

How about sidewall stiffness? I didn't see much mentioned in the article about that. Test: SSeat two tires on identical rims. One with load rating of 89, one of 105(big jump) and a known stiff sidewall. Decrease the pressure on both as much as possible without breaking the bead. Now measure the depth of the cup in that tire. There's no way in [censored] they're the same.

Now extrapolate this out to running pressure.



I just happened to spend some time today, with three very good mechanics, and I asked them about this. All three of them agree with basically what you wrote above.

One of them also cited the recent controversy, between Ford and Firestone, regarding the Ford Exploder tire tread separation issue. The crux of which was basically this. Ford blamed Firestone, claiming they sold Ford a bunch of bum tires. Firestone countered, with their claim, that they had told Ford that their door sticker psi instructions were wrong for the tire in question.

I had reset my tires to 35 psi this morning, after reading this thread, and I have to admit, my van handles much better, after raising pressure again this afternoon.
 
Originally Posted By: CapriRacer
Brien said:
And just to take this a step further: The question oilgal is asking concerned the placard and (If I read the question right) whether different manufacturers of tires would need to use different pressures. This question does not involve the shape or the size of the footprint. This is about load carrying capacity.



That is exactly what I was asking. Thank you.
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I have tires of the same OE height, though a bit wider, and being V rated they have a much higher load bearing capacity, than those which came on the van. They are also a bit "stickier", being made of a softer compound, and they have a softer sidewall which means they must be inflated to a higher psi than the stock tires. They will wrap and yield very poor corning characteristics, if not inflated properly, and they will also cup causing them to wear faster.
 
Originally Posted By: Oilgal

That is exactly what I was asking. Thank you.
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I have tires of the same OE height, though a bit wider, and being V rated they have a much higher load bearing capacity, than those which came on the van. They are also a bit "stickier", being made of a softer compound, and they have a softer sidewall which means they must be inflated to a higher psi than the stock tires. They will wrap and yield very poor corning characteristics, if not inflated properly, and they will also cup causing them to wear faster.


Let me take issue with two statements:

Quote:
......and being V rated they have a much higher load bearing capacity.......


In theory, load carrying capacity (Load Index) and speed rating are separate issues. If you survey a given tire size - like on a Tire Rack web site - that regardless of the speed rating, the load carrying capacity is independent - and pretty much the same for a given size. The point of that article linked above was to explain what differnces you will find in that survey, and why they aren't really differences at all.

But the interaction between speed limitation and load carrying capacity is really complex. If you were to conduct a series of tests - we tire engineers would use a "step-load" test - you'd find that tires would fail at lower loads for higher speeds.

Nevertheless, the way these things are "rated" - the process is independent.

Second issue:

Quote:
.....and they have a softer sidewall which means they must be inflated to a higher psi than the stock tires......


As a general rule, V rated tires have stiffer sidewalls than S rated tires. I going to suggest that the poor handling characteristics you are alluding to are more a statement about the vehicle (It is a van after all!). The cupping wear you are experiencing may be the result of a misalignment condition - and dare I say it? - "spirited driving".

Vans aren't designed for spirited driving and the steering geometery (I'm talking akerman, here) is set up for something more sedate. Push the envelope and the akerman has to be changed to accommodate. If not, you'll get cupping wear. It is not uncommon for street cars converted to racecars - and racing on "street" tires - to develop this kind of wear. A shim here and a purposely bent part there, and you can turn a street car into reasonably well handling racecar without these sorts of problems.


Plus......

Quote:
....I have tires of the same OE height, though a bit wider....


It's possible that the rim width is affecting that way the van handles. It is not uncommon for vehicle manufacturers to use narrow rims - not outside of the allowable, but on the narrow side of it - and it might be that the tire size currently being used is outside that tolerance. Even if it isn't, that might partially explain the excessive tuck-in and why it feels like it needs more pressure.







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So capri, would you care to give a little bit of info on the relationship between Pressure and performance(contact area)? Obviously we would have to assume that whatever the pressure the tire meets the required safety rating for that vehicle.

Like, lets say one tire meets the required load at 25psi and another at 35psi. What exactly is changed? Given that as many factors as possible are the same, or very close to it(tread, compound, etc..), what would the actual difference be?

For the Above example lets say that both tires are stated to have a max of 44 cold psi.

I don't mean to seem like i'm arguing, i'm just trying to learn all that I can. From what I've read, and/or experienced, PSI usually changes handling between different tires of the same size. If I can get more info why this isn't true that'd be great and help me alot.
 
Originally Posted By: Brien
So capri, would you care to give a little bit of info on the relationship between Pressure and performance(contact area)?......


Sure, that why I do this.

Quote:
.....Obviously we would have to assume that whatever the pressure the tire meets the required safety rating for that vehicle.

Like, lets say one tire meets the required load at 25psi and another at 35psi. What exactly is changed? Given that as many factors as possible are the same, or very close to it(tread, compound, etc..), what would the actual difference be?

For the Above example lets say that both tires are stated to have a max of 44 cold psi.

I don't mean to seem like i'm arguing, i'm just trying to learn all that I can. From what I've read, and/or experienced, PSI usually changes handling between different tires of the same size. If I can get more info why this isn't true that'd be great and help me alot.


First, for there to be a difference in the pressure between 25 and 35 psi as indicated in the question, there has to be a significant difference in the tire's size.

For example: a P215/75R15 SL (LI = 100) at 35 psi, carries the same load as a P235/75R15 SL (LI = 105) at 25 psi.

Put another way, you won't have a situation where you have a P215/75R15 SL - with or without the letters - that has a Load Index much different than 100. (BTW, I looked this up and ETRTO shows a LI = 100 for a 215/75R15 and JATMA doesn't list any 75 series tires.)

This is an important point that sometimes gets lost in the heat of the discussion.

OK, since one of your initial premises is wrong, it's going to be difficult to properly reformat your question - but let me try.

I think what your next question should have been is: Why is it that tires in the same size respond differently at the same pressure?

Well, for one, they were designed to.

There's a lot of things that go into making a tire handle. Not only should a tire grip (meaning a grippy tread compound), but it ought to respond quickly to steering input. The quicker a tire responds, the better it feels.

Steering response is more or less a function of the stiffness of the sidewall. Increasing the height of "the filler", does that - but so does increasing the inflation pressure - and I'll bet that is what you are alluding to.

The problem is that whatever could be done to the construction of the tire to improve the steering response, hurts the ride harshness. So when the tire engineer designs a tire, this has to be a deliberate decision on his part.

Needless to say, a tire designed to handle is going to have both a grippy compound and stiff sidewall.

BUT

Sometimes, a tire will be designed knowing that a harsh ride is not a desireable feature, but a high speed rating is required (Vehicle manufacturers makes life for us tire engineers sooooo very difficult). This means a series of trials to find the right combination.

At this point I should also mention that you can gain a bit of good steering feel by using a wider belt width. And that means a wider tread width (which is sexy) but this tends to decrease a tire's resistance to alignment related wear.

Or how about a different belt angle? That also helps handling / hurts ride. This one also has the side benefit of affecting that wonderful "slapping" noise you sometimes hear when going over expansion joints.

Of course you can change that by changing the tread pattern - which of course changes the wet traction properties.

And then you adjust for that by .......

You get the idea.

Notice I haven't once refered to contact area? There's a whole different line of thinking going down that path.

So, yes, in a given tire size, you will get a wide range of differences - all of which are compromises. And, yes, if you use a different criteria other than load carrying capacity, you can get a different "answer" as to what the "proper" pressure should be.

For example: If I use fuel economy as the only criteria, I'm going to want to use whatever the maximum pressure is written on the sidewall - and may be a little bit more!

Do I think that is the "right" answer? No, and I would point out the increased susceptibility to impact damage and the small footprint (increased stopping distance), not to mention the harsh ride.

But that would not prevent someone from asserting that it works for them.

So when asked what is the "right" pressure - I always point to the placard. Usually this question is asked by folks who don't have a clue - don't want a clue - and want one simple reliable answer they will remember forever. That number at least has the benefit of having a guy carefully think it over - and publish the result - and be responsible for the result. And if it is wrong - well there have been a few recalls to replace the placard - meaning they eventually get it right.

Now if you don't like that pressure - that's fine. Use what you would like, just be aware of the other things that are being affected. And I would point out that using something less than the placard pressure is not a very good idea - because of the load carrying capacity issue.

(OK, I think that leads us to the discussion about empty pickup trucks!)
 
Originally Posted By: CapriRacer


Needless to say, a tire designed to handle is going to have both a grippy compound and stiff sidewall.



You make a lot of good points, but this is an assumption that does not ring true. Sport tire designers have to make compromises as well, and some sport tires do not have stiff sidewalls, and require a good deal more pressure to prevent rollover, while others are much stiffer. The stickiness of the compound varies as well, and it is not always engineered the way one would think it would be.

A case in point are the OEM tires that came with my Impala. These had a sticky compound for an all-season tire in 1994, and a very stiff sidewall. They were also problematic in that they caused the car to dart back and forth on uneven pavement as well as wander on grooved pavement. I may have been able to do an alignment change to cure this, but my alignment specs were toward the handling side of things, more than the stability side. These specs were within the OEM tolerances though.

I replaced these with Kumho 712s, which had a much softer sidewall, but a much stickier compound. The original tires broke loose very smoothly, and allowed the car to be held in a drift easily. The 712s broke cleanly, and made it much harder to drift, but gave a lot more traction before the break. After coming from the BFGs, the lack of darting and wander was almost spooky.

I later went to the Hankook K104. This tire had roughly the same stick as the 712, but a much firmer sidewall. It was like going back to the OEM tire, but with more stick. They had the same darting and wander issues that the OEMs had as well.

I went back with 712s because they had better high-speed hydroplane characteristics, as well as the aformentioned lack of wander and darting.

All three of these tires had the same size and load rating, but all three had very different character, and different pressure requirements to prevent roll-over.

I have also had "economy" tires on other cars that had stiff sidewalls, and when I replaced them with a "Sport" tire, the sport tire had the softer sidewall.
 
I'm sorry. I have been really sleep deprived lately, and I should have realized that what you are saying does not apply to my situation. My minivan is not at all stock anymore. The suspensions and just about everything else has been modified.

My van has no ride or tire wear issues at all, and it handles far better than a stock minivan. You would have to drive it to believe it.

The reason I got involved in this thread, is because deviating from the door sticker psi guide, to better match my suspension modifications and tires, is what finally corrected the tire wear and handling problems I was having. It took some time to figure all of that out.

I guess I just want to say this.

I think that unless you are running the same exact tires, and struts and/or shocks, or those very much like that which the OE car maker used, to determine the door sticker psi guide, then your reasoning doesn't really apply.

Just my opinion based upon my experience.
 
my 95 BMW 525i door sticker states 33 front and 41 rear with full load. Lighter load out of Us or canada 26 front 33 rear. I have been running about 33 front/rear all around as I usually drive by myself.
 
Our Suzuki SX4, came with Bridgestone's - Turanza EL400-02. 1356lbs. max load, and 44psi max inflation.
I run them somewhere in-between 39psi & 43psi. I change the pressure every few weeks, and of course, keep all 4 tires equal on this iAWD.
 
Originally Posted By: sprintman
Door stickers are for best ride. For best handling/wear higher pressures are required. Also radials should never be run less than 32psi cold
Why is that? Our Escape specs them at 30 PSI, and the Lancer is either 26 or 29 PSI for the rear. I've run the Escape's tires at as low as 21 PSI for the wintertime, with 29 PSI being standard.
 
I was running 32 all around, since the door placard specs 30. For a while I tried 33-34, and found the ride acceptable.

Now, however, with new automatic leveling shocks in the rear, the ride is noticeably firmer (though not harsh). I was thinking I'd bring the rear tires down to 31 and leave the fronts at 33-34. I understand higher psi in front can improve handling on FWD cars, and the softer pressure in back could mitigate some of the firmness from the new shocks. Any problems with this?
 
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