Always use FACTORY tire pressures!

[CapriRacer]

yrwei52,

First, you are going to find - if you haven't already - that the tire industry is chock full of imprecision, sloppiness, and complexity. This discussion deals with some of that.

Second, I hope you realize that when you use the term *P-Metric*, you are referring to the load table for Passenger Car tires as published by the US tire standardizing organization - The Tire and Rim Association (TRA, for short). It's called that because it uses the letter *P* in front of the numbers to designate that the tire is designed using that standard.

The Europeans have a slightly different system. Their standardizing organization is the European Tyre and Rim Technical Organization - commonly referred to as ETRTO. Passenger Car tires designed to that standard do NOT have a letter in front of the numbers. This is sometimes referred to as Eurometric - although I don't think this is in any way official like *P-metric* is.

The Japanese use JATMA - Japanese Automobile Tire Manufacturers Association - as their standardizing organization - and they have a third system.

All 3 of these systems result in very similar results - close enough that, for practical purposes, they are the same. Put another way, the company I used to work for used the same materials regardless of which system they were designing to - so the tires would be otherwise identical except for how the tire size was expressed.

One of the quirky things about these standards is that once they are published, they do NOT change. That is, the relationship between inflation pressure and load carrying capacity is based on a formula and a given set of tire dimensions (which are specified in the standard) and will always result in the same table. One way to look at this is that this table represents the Physics of how tires work.

The P-metric table was first published in the early 1970's, and the table has not changed - except to say that new tire sizes are constantly being added to the published list and obsolete sizes are being removed. If any of those sizes first published survived to the present day, the loads and inflation pressures would be identical - and I assume that there are sizes that survive, but my yearbooks only go back to 1989.

There has been a reference to a change that took place in 1998. In 1996, a new note appeared in the TRA yearbook. It stated it was permissible to use 44 and 51 psi as a max pressure - along with some calculations about the affect speed has on load carrying capacity. That note appears in the latest yearbook unchanged from 1996.

Some background info: When the US Federal government first started regulating tires (about 1968) they had a regulation that said - more or less - that the sidewall had to state the maximum load and pressure. Notice that it doesn't say max pressure or the pressure that corresponds to the max load. It's vague. At the time, the common way of expressing the max load situation was: Max Load XXXX at YY pressure. Please note that this way of expressing things doesn't say what the max usage pressure is - and even back then it was allowable to use more than pressure that stated on the sidewall.

But the Europeans were using speed ratings and the test was conducted at 35 psi for S rated tires, 44 psi for H rated tires, and 51 psi for V and higher rated tires. The note that TRA added was to address this. What should have happened is that every tire manufacturer should have changed to: Max Load XXXX, Max pressure YY. Because of the old way of expressing things, that is not what happened - and the result is this confusing mess we have now. Even Tire Rack thinks that every tire states a max pressure, but some of them don't!!

To add to this confusion, BMW and VW both had vehicle tire placards that had different inflation pressures depending on the situation. The current form of the vehicle tire placard (required as of 2008) does not allow this. A single pressure is specified.

As the photo above of a 2001 VW vehicle tire placard says, there is a matrix where you have: below 100 mph and above 100 mph AND half load and max load. Note that the tire would likely say a max pressure of 44 (or 51) psi, because it is an H speed rated tire. (Please note that a new VW's tire placard would not look like this.)

Note also that the below 100 mph/half load pressures are reasonable - as are the above 100 mph/half load pressures - and certainly, the above 100 mph/max load pressures could be the result of the speed. The only thing that doesn't quite fit is the rear pressure below 100 mph - 41 psi. Note that this tire is NOT an Extra Load tire, and the max pressure listed on the sidewall would NOT be 41 psi. The only explanation I have is that sometimes vehicle manufacturers will specify pressures above 35 psi for Standard Load tires for handling reasons.

OK, now that I have explained that, can I provide documentation to back it up?

Toyo Tires: Guidelines for the Application of Load and Inflation Tables

This directly addresses what you tried to point out about the Toyo A23 225/55R19 99V tire. On page 20, the max load of 1709# corresponds to 36 psi - not 51 psi.

 

[yrwei52]

... This directly addresses what you tried to point out about the Toyo A23 225/55R19 99V tire. On page 20, the max load of 1709# corresponds to 36 psi - not 51 psi.

I thought you're saying the maximum load is always at 35 psi for a standard load tire?

... I have not seen any tires that say Max Load XXXX at Max Pressure YY - but if they exist, I think those are incorrectly stated unless they mean a max of 35 psi for their tires (which is possible). Any tires that say it this way AND say 44 psi or 51 psi max are just flat wrong.

Of course you'll notice in page 20 of the document you provided is TRA P-Metric Load & Inflation Table. The size P225/55R19 is a P-Metric tire which has a maximum inflation pressure at 36 psi for the load index 99 at 1709 lbs. Our Toyo A23 225/55R19 99V tire has 51 psi maximum pressure stated on the sidewall with the same load index. "Replacement tires must be of a size, load range, and load capacity (by inflation) that are capable of supporting the load of the vehicles originally installed (O.E.) tires." It clearly indicates the load capacity is closely related to inflation. If you google "tire load and inflation tables", we'll see all kind of such tables from different tire manufactures. All tables indicate load capacity is proportional to tire inflation up to its maximum inflation pressure which is its maximum load capacity. This is the law of physics. I've never seen a table where the load carrying capacity stops growing at 35 psi but continuously lists the same load carrying capacity with higher inflation pressure.

Finally here is a load index chart from Toyo Tires. It clearly states that a load index indicates a load-carrying capacity (maximum load) at maximum inflation pressure!

"The load index is an assigned number that corresponds with the load-carrying capacity of the tire. For example, "96" indicates a load-carrying capacity of 1565 lb. at maximum inflation pressure. The load index for most passenger car tires ranges from 75-100."

 

[CapriRacer]

yrwei52 ,

Nicely done. Youve managed to find some things that seem to contradict what I am saying.

I thought you're saying the maximum load is always at 35 psi for a standard load tire?

Remember I said there is lots of sloppiness and imprecision? Well that Toyo chart is one of those. I was hoping you wouldnt catch it, but you did. Well done!

But the chart doesnt support your position either as it explicitly calls out either 35 or 36 psi for Standard Load tires and not the max pressure.

Ya see, that chart says the tire sizes are all P metrics (has the letter *P* in front) and the loads are from TRA, but that is not true. Only the ones where the load stops at 35 psi are P metric and only those come from the TRA Yearbook. The others are Eurometric and those use 2.5 bar (about 36 psi) for the rating point.

.. Of course you'll notice in page 20 of the document you provided is TRA P-Metric Load & Inflation Table. .

Thats what it says, but it is not true. Some of those tires arent in TRA. My best guess is that they assigned some rookie to this task and no one noticed the mix of tables, nor the addition of the *P* to some of the sizes.

.. The size P225/55R19 is a P-Metric tire which has a maximum inflation pressure at 36 psi for the load index 99 at 1709 lbs. Our Toyo A23 225/55R19 99V tire has 51 psi maximum pressure stated on the sidewall with the same load index.

So doesnt this chart prove that the max pressure is NOT where the max load occurs?

But wait, theres more: Now go to page 29 on that Toyo Document I referred to earlier and find the line for the 99 Load Index. (I agree, the tire in question is a 225/55R19 99V) Notice it is EXACTLY the same as the line we were looking at on page 20. Notice also that it ends at 36 psi and not at the max pressure.

So both charts indicate that no tires have a max load at 44 or 51 psi.

.. I've never seen a table where the load carrying capacity stops growing at 35 psi but continuously lists the same load carrying capacity with higher inflation pressure. .

Nor have I, but I have never seen any load tables (for passenger car tires) that indicate the max load at 44 or 51 psi.

.. Finally here is a load index chart from Toyo Tires. It clearly states that a load index indicates a load-carrying capacity (maximum load) at maximum inflation pressure!
"The load index is an assigned number that corresponds with the load-carrying capacity of the tire. For example, "96" indicates a load-carrying capacity of 1565 lb. at maximum inflation pressure. The load index for most passenger car tires ranges from 75-100."

Yup, like I said, you will find lots of sloppiness around.

So I am going to contend that the statement where it says *at the maximum pressure* is incorrect and then challenge you to find a load table that actually shows inflation pressure vs load carrying capacity where 44 or 51 psi is where the max load occurs similar to the chart on page 20 or the one on page 29.

 

[Chris_Top_Her]

But wait, there’s more

 

[brillo54]

I have a different take on the 35/36 psi discrepancy. On the Tire Rack Tire Specs Explained: Maximum Load page: at the bottom of the first chart showing Max Load Pressure there is this fine print:
*In an effort to internationally harmonize load ratings and ranges, recently introduced and future LL, SL and XL P-Metric sizes will use ISO/Euro-metric maximum load pressures of 36 or 42 psi.

I'm pretty sure the P225/55R19 is relatively new size. Only now are we starting to see more choices for aftermarket tires. So could this size be new enough to be part of the Tire Rack statement?

 

[PatrickGSR94]

So anyway, back to my original post... just because a tire has a higher maximum pressure on the sidewal than stock tires DOES NOT mean tire pressure should be bumped up to achieve the same load handling capability as stock tires.

 

[leeharvey418]

So anyway, back to my original post... just because a tire has a higher maximum pressure on the sidewal than stock tires DOES NOT mean tire pressure should be bumped up to achieve the same load handling capability as stock tires.

Well, no, it doesn't mean that... but what you said in the OP was that people were giving bad advice that running tires with a higher load rating required a higher inflation pressure. I'd agree that that's flat out wrong - a tire with a higher load rating would take less pressure to get the same load capacity. All the same, though, I'd never recommend that anyone go lower than placard pressure without knowing exactly what they're doing. I know from my own experience that when going from a 225/65R17 to a 235/65R17 XL tire, I had to bump up the inflation pressure a couple of psi to maintain handling quality.

I don't think anyone is advocating maxing out tire pressure according to the sidewall, but they are saying that a higher-than-placard pressure can be beneficial without causing excessive tire wear.

...

That looks like a hood when it should be a dead horse.

 

[CapriRacer]

.......... I know from my own experience that when going from a 225/65R17 to a 235/65R17 XL tire, I had to bump up the inflation pressure a couple of psi to maintain handling quality. .........

Ah ...... Mmmmmmm ....... not exactly.

As a tire engineer, I can design a tire that will be sloppy as all get out, and an otherwise identical one that is very very precise - all without affecting the load carrying capacity. It's one of the reasons we do handling tests on propotypes - to see where we are relative to our design goals.

.......... I don't think anyone is advocating maxing out tire pressure according to the sidewall, but they are saying that a higher-than-placard pressure can be beneficial without causing excessive tire wear. ........

Inflation pressure is only a minor player when it comes to evenness of wear. There are other things that have a much bigger impact - like alignment, cornering speed, suspension geometry, etc. That's because modern radial tires have a steel belt that stiffens the tread area and that results in the inflation pressure only having a minor impact on the pressure distribution within the footprint.

Here's an example of one with an excellent footprint.

 

[ColtX-5]

I know from my own experience that when going from a 225/65R17 to a 235/65R17 XL tire, I had to bump up the inflation pressure a couple of psi to maintain handling quality.

I don't think anyone is advocating maxing out tire pressure according to the sidewall, but they are saying that a higher-than-placard pressure can be beneficial without causing excessive tire wear.

Ah ...... Mmmmmmm ....... not exactly.

As a tire engineer, I can design a tire that will be sloppy as all get out, and an otherwise identical one that is very very precise - all without affecting the load carrying capacity. It's one of the reasons we do handling tests on propotypes - to see where we are relative to our design goals.

CapriRacer,

I am also a retired engineer, but not from the tire industry. I have few quibbles with what you've posted, until this. leeharvey described adjusting pressures to tune the handling to his preference. That has also been my experience, to a limited extent, with all the tires I've had and especially the tires I have autocrossed. This experience is the basis of my contention that the door placard pressures are a great starting point for any replacement tire, but are only "right" for the OEM tires.

Interestingly, I had a '92 Explorer 2-door with Firestone tires that I drove like a Mustang (it would bark the tires going into 3rd). The recommended 26 psi lasted about 2 days and from memory I ran 38 in the front and 34 in the rear. I did not have any tire trouble with it and have assumed all along the reason was the higher pressures made the tires run cooler, enough they did not fly apart. (dunno) I could not tolerate the lower steering response at 26psi.

So, I have some questions. Why do sports cars run wide, low profile tires? Other than sidewall stiffness (steering response), how does that improve cornering traction? Reduced slip angle, tread distortion? Along the same line, do drag slicks just look wide or do they have a proportionally long, skinny contact patch?

I had a Frontier with Long Trail T/As on it. Loved them and bought another set. Higher pressure got better fuel mileage with those tires, but it has not been noticeable in any other car/tire. Any guess why?

Fun Discussion!!!!

 

[davesxx01]

Somehow, I just knew the Explorer/Firestone debacle would get brought up in this discussion! As a tech at Ford we always told people to set tires at 32-34! That was a major failure at Ford to suggest 26!

Thanks Barry for coming by this thread and giving your expertise!

 

[CapriRacer]

Colt X-5,

So let's start with this: What changes can be made in the tire arena to make a car handle better? If I leave out an improvement in grip level and focus only on trying to make the car more responsive, the answer boils down to making the car react faster to steering input - and basically that's about sidewall stiffness.

There are three things that can be done to improve sidewall stiffness: Make it shorter (lower profile tires), increase the inflation pressure, and add something inside the tire (that is, change the structure of the tire).

On that last item, the common way is to increase the size of the filler:

Most passenger car tires are designed with a very low filler to produce a soft riding tire, but that also produces a tire with less crisp response. Obviously that applies to many OE tires.

It is also possible to add something in that area to make the tire self supporting - that is, Run Flat.

What I was trying to say was when leeharvey418 changed tire size, he very likely also changed tire brand. That's at least one more variable. What I was taking exception to was the characterization that only load carrying capacity was the cause of the change in handling. (and as a side note: It isn't tire brand itself that makes the difference, it's the change in make and model. Unless the change was simply from one size to another and no change in make and model of tire, there could be a very large change in how the tires perform.)

Now if somebody is saying they are using a different inflation pressure to get a different feel out of a tire - Yup, changing pressure will do that. Personally, I, too, like a crisper handling tires and use a bit more inflation pressure to achieve that. - BUT - as the pressure is increased, the footprint size grows smaller, and that has implications on grip. Go too far and you'll lose considerable grip.

OK, time for some Physics!

Remember your high school physics that said that friction was proportional to the vertical force and NOT the size of the contact area? It's called Amontons' Laws. Unfortunately it does not apply to tires - because the road surface has texture and the rubber in a tire is soft enough to penetrate that texture and you can get additional grip. In fact, the max grip of a tire occurs between 10% and 15% slip. (Very useful for feeling when you are approaching the limit of adhesion)

So if you increase the inflation pressure of a tire, there is a point where you will lose enough grip that you can feel it. It is common for overly inflated tires to have wet traction issues (and not of the hydroplaning kind.)

OK, you asked about fuel economy. First read up what I say about that subject and then we'll talk:

http://www.barrystiretech.com/rrandfe.html

 

[halfast3]

Colt X-5,
...

OK, you asked about fuel economy. First read up what I say about that subject and then we'll talk:

http://barrystiretech.com/rrandfe.html

Intresting discussion. The above link appears to be broken.

 

[CapriRacer]

Interesting discussion. The above link appears to be broken.

Fixed

http://www.barrystiretech.com/rrandfe.html

Followup: Maybe not!

Second followup: Well, I don't know what is wrong, but the manual way to get there is to type in www.barrystiretech.com and find the link to Rolling Resistance and Fuel Economy and its sister page.

Third followup: That's weird. It appears I can paste the url directly onto this page and it will automatically convert it to a link - BUT - if I use the button to create a link (the button with the globe and the chain link), it doesn't create a working link. Mmmmmm!!

 

[leeharvey418]

Ah ...... Mmmmmmm ....... not exactly.

As a tire engineer, I can design a tire that will be sloppy as all get out, and an otherwise identical one that is very very precise - all without affecting the load carrying capacity. It's one of the reasons we do handling tests on propotypes - to see where we are relative to our design goals.

Yes, exactly. I was citing an example from my own experience where it would have seemed that I should have been fine with placard pressure, but where I needed to fine tune the system using the cheapest and easiest design parameter I could - inflation pressure. I wasn't attempting to get higher load capacity out of my tires, but I was looking to increase overall system stiffness, and especially torsional stiffness about the steering axis. Sure, I could have gone with a different tire, but when I had just committed to new tires in the first place, that would be kind of silly.

If I'm working on chassis development for a new vehicle and I need to go back to a tire supplier and ask them to adjust a particular design parameter to achieve a design goal, then that's one thing. For the purposes of this thread, though, we're talking about getting replacement tires to do what we want them to do. Handling tests on prototypes aren't exactly an option for the average customer.

 

[ColtX-5]

Colt X-5,
..........................

Remember your high school physics that said that friction was proportional to the vertical force and NOT the size of the contact area? It's called Amontons' Laws. Unfortunately it does not apply to tires - because the road surface has texture and the rubber in a tire is soft enough to penetrate that texture and you can get additional grip. ....................................

OK, you asked about fuel economy. First read up what I say about that subject and then we'll talk:

http://www.barrystiretech.com/rrandfe.html

I'm glad you fixed the friction comment. As to the fuel economy question, I guess pressure can get swamped by other factors of which I was unaware. (grammar save )

So, back to my handling questions, the short 45 ratio sidewalls on my MX-5 tires exist because it's easier to make short stiff. That obviously enhances steering response, but must also help control tread distortion and slip angle, both improving traction.

I had an '87 Mustang. It came with Goodyear Gatorbacks. Later I put Michelin XGTs on it, even later Dunlop D40M2s (?), all the same size. All performed fairly close to each other, but could not have been different in design. The Goodyears had surprisingly stiff sidewalls and flexible tread. The XGTs were the opposite with loose, soft sidewalls and rigid tread (plus higher absolute cornering grip, if less precise getting there). The Dunlops steered as crisply as the Goodyears and seemed as grippy as the Michelins with neither the stiff sidewalls nor stiff tread of the other 2, but in between, to the hand. I liked the Dunlops best even though they had an odd relationship between steering angle and traction at their limits.

So, I can see many ways to skin a cat, most internal, but an equal number of question about how tires work. The biggest being why a short, wide contact patch, common in a low profile tire, seems to give higher cornering grip/traction.

 

[leeharvey418]

So, I can see many ways to skin a cat, most internal, but an equal number of question about how tires work. The biggest being why a short, wide contact patch, common in a low profile tire, seems to give higher cornering grip/traction.

It's not the end-all of your questions, but it's a very healthy start: http://books.sae.org/r-146/

 

[CapriRacer]

........ The biggest being why a short, wide contact patch, common in a low profile tire, seems to give higher cornering grip/traction.

First, I hope you realize that most low profile tires have grippy tread compounds compared to high profile tires.

Second, low profile tires have larger contact patches than comparable high profile tires. I wish there was readily available data to back that up, but it makes intuitive sense in that the tread width can be increased arbitrarily without changing the length of the footprint.

 

[ColtX-5]

First, I hope you realize that most low profile tires have grippy tread compounds compared to high profile tires.

Second, low profile tires have larger contact patches than comparable high profile tires. I wish there was readily available data to back that up, but it makes intuitive sense in that the tread width can be increased arbitrarily without changing the length of the footprint.

I do. But, what happened to the contact area being proportional to internal pressure and vertical load applied to the tire?

 

[CapriRacer]

I do. But, what happened to the contact area being proportional to internal pressure and vertical load applied to the tire?

Sorry, that's a myth. It's true that more load = larger footprint - and - more pressure = smaller footprint - But they aren't proportional.

 

[PatrickGSR94]

CapriRacer,

I am also a retired engineer, but not from the tire industry. I have few quibbles with what you've posted, until this. leeharvey described adjusting pressures to tune the handling to his preference. That has also been my experience, to a limited extent, with all the tires I've had and especially the tires I have autocrossed. This experience is the basis of my contention that the door placard pressures are a great starting point for any replacement tire, but are only "right" for the OEM tires.

Interestingly, I had a '92 Explorer 2-door with Firestone tires that I drove like a Mustang (it would bark the tires going into 3rd). The recommended 26 psi lasted about 2 days and from memory I ran 38 in the front and 34 in the rear. I did not have any tire trouble with it and have assumed all along the reason was the higher pressures made the tires run cooler, enough they did not fly apart. (dunno) I could not tolerate the lower steering response at 26psi.

So, I have some questions. Why do sports cars run wide, low profile tires? Other than sidewall stiffness (steering response), how does that improve cornering traction? Reduced slip angle, tread distortion? Along the same line, do drag slicks just look wide or do they have a proportionally long, skinny contact patch?

I had a Frontier with Long Trail T/As on it. Loved them and bought another set. Higher pressure got better fuel mileage with those tires, but it has not been noticeable in any other car/tire. Any guess why?

Fun Discussion!!!!

I don't see how you could run almost 50% higher tire pressure and not have tire wear problems... unless of course Ford's recommended 26 psi was totally screwed up, which I suspect it was.

When I was young and stupid, I threw some +2 wheels and tires on my first car, a 1989 Camry. It came with 185/70-14, and I went up to 205/50-16 tires. They looked monstrously wide in comparison to the old tires. Anyway, the recommended pressure for stock tires was 30 psi. But being stupid I tried to get rid of the noticeable "bulge" in the tire sidewall near the ground on the front tires, because I thought it made the tires look underinflated. I ran them for a few months at somewhere between 35 and 40 psi, and in just a few thousand miles the center of those 205/50-15 tires were worn down to the wear bars.

Of course there's always room for experimentation and adjustment, especially for drag or autocross purposes (I ran 15-20 PSI front when I drag raced my Integra, and near 40 during autoX), but for everyday driving duties I have always used the factory pressure settings, even on the 205/45-16 tires I run on my Integra today.