Why Do Original Equipt Tires Rate So Low?

[RidingOnRailz]

Originally Posted by CapriRacer

So first off, that formula is faulty. The formula used by the tire manufacturers is quite complex, which is why they publish charts - which is where I got my values.

Here's a version of that chart:

https://www.toyotires.com/media/2125...s_20170203.pdf

Yes, it is from Toyo tires, but the chart is the same regardless of who manufactures the tire. (and for completeness sake, there is some complication that makes it harder to understand. I go into more detail here):

http://www.barrystiretech.com/tirest...izingorgs.html



No, do not use the max pressure for anything other than the max pressure.



No, the 44 psi value is not tied to the 2039# value. The 2039 value is tied to 35 psi.

So if you use the correct 35 psi value, you get 21.46 psi front and 24.88 psi rear.



But if you use the correct values, it says the pressures were OK. Don't forget that the government (NHTSA) looked at all this and had the tires recalled, not the vehicle.



2 thoughts:

First, those are vans, not SUV's

You are pointing out vehicles that use LT metric tires, Look up the base vans that used P type tires (E-150, B1500, etc.) and you'll find a different story.



Those photos were in Dr. Govindjee's report and were published in the NHTSA EA00-023 report used to justify the tire recall. Are you implicating that NHTSA (and Dr. Govindjee's) didn't use photos of the tire in question when they had abundant samples available?

Oh, and I don't think you've yet realized that I am Barry.

So always use 35psi in that formula?? What if we're doing calculations for LT rated tires?

Also, for the OEM size listed for my 2010 Accord - 225/50R17 93V, that size cannot even be found in 93V anymore. Only 94V or higher.

I just put a set of Pirelli P7 pluses on there, 94V: max load 1,477 @ 51psi max cold. Why on earth should I run the formula I ran using only 35psi as the max? Is the 44 or 51(in my case) max cold stamped on those tires 'fake' - or just for marketing?

[Stealthee]

Max is max. You shouldn't run max PSI, pretty much ever. It is simply the maximum pressure the tire could be inflated to. You should run what the vehicle manufacturer recommends.

[RidingOnRailz]

Originally Posted by Stealthee

Max is max. You shouldn't run max
PSI, pretty much ever. It is simply the maximum
pressure the tire could be inflated to. You should
run what the vehicle manufacturer recommends.

Please indicate wherever in this conversation I ever suggested running max cold pressure in ones tires.

The formula I indicated simply uses that max cold pressure as a basis to run the percentage off of, giving you an alternate cold pressure that could be lower, equal to, or higher than what is listed on your vehicle's TIP(Tire Info. Placard) sticker.

"You should run what the vehicle
manufacturer recommends"

You're preaching to the choir with that.

[Stealthee]

Sorry, I guess I misunderstood your question. I have had to argue with people before that insist the max psi is what you should run your tires at. I tried to explain to the one guy I work with how he was wrong and he argued "I used to sell tires I know what I am talking about." I simply replied, "Just because you sold tires doesn't mean you know what's right."

[RidingOnRailz]

Originally Posted by Stealthee

Sorry, I guess I misunderstood your question. I have had to argue with people before that insist the max psi is what you should run your tires at. I tried to explain to the one guy I work with how he was wrong and he argued "I used to sell tires I know what I am talking about." I simply replied, "Just because you sold tires doesn't mean you know what's right."

I do know that running max cold pressure, or even a few PSI below it, makes the steering all squirrelly and twitchy in the few cars I used to run it in, decades ago before I learned to read what was on that TIP sticker, lol!

Most of the cars I've driven since 2005 actually roll and accelerate better down near the pressures that the vehicle mfg. specifies, than at max on the tire. You'd think it would be the opposite, but nope!

[CapriRacer]

Originally Posted by RidingOnRailz

So always use 35psi in that formula?? What if we're doing calculations for LT rated tires? ….

No, don't use that formula at all. Use a load table. That may require you to do some searching on the 'net to find one - and the good news is that it doesn't matter what tire manufacturer publishes the table, they are merely reproducing the table published by the tire standardizing organizations - which apply to every tire manufacturer.

Originally Posted by RidingOnRailz

..... Also, for the OEM size listed for my 2010 Accord - 225/50R17 93V, that size cannot even be found in 93V anymore. Only 94V or higher. ……

In this case you only need to find a table that has 225/50R17 in it. The load index and speed rating don't matter. (OK, there is one hiccup and that is you have to pay attention if the tire is SL (Standard Load) or XL (Extra Load)

Besides, the pressure listed on your vehicle tire placard is applicable to your vehicle regardless - unless you change tire size. (and here you can tell if your vehicle requires an XL tire if the specified pressure is over 35 psi - with a couple of exceptions.)

Originally Posted by RidingOnRailz

…….I just put a set of Pirelli P7 pluses on there, 94V: max load 1,477 @ 51psi max cold. Why on earth should I run the formula I ran using only 35psi as the max? Is the 44 or 51(in my case) max cold stamped on those tires 'fake' - or just for marketing?

First, your Pirelli's don't say max load XXXX at 51 psi. They say, max load XXXX, max pressure 51 psi. The max load occurs at 35 psi! And you're not supposed to use more than 51 psi when operated the tire.

And second, don't use that formula. It is based on a false assumption - that the max load is directly proportional to the pressure. It's not. It's PARTIALLY proportional to the pressure. I go into detail here:

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

Use a table.

[RidingOnRailz]

Originally Posted by CapriRacer

No, don't use that formula at all. Use a load table. That may require you to do some searching on the 'net to find one - and the good news is that it doesn't matter what tire manufacturer publishes the table, they are merely reproducing the table published by the tire standardizing organizations - which apply to every tire manufacturer.



In this case you only need to find a table that has 225/50R17 in it. The load index and speed rating don't matter. (OK, there is one hiccup and that is you have to pay attention if the tire is SL (Standard Load) or XL (Extra Load)

Besides, the pressure listed on your vehicle tire placard is applicable to your vehicle regardless - unless you change tire size. (and here you can tell if your vehicle requires an XL tire if the specified pressure is over 35 psi - with a couple of exceptions.)



First, your Pirelli's don't say max load XXXX at 51 psi. They say, max load XXXX, max pressure 51 psi. The max load occurs at 35 psi! And you're not supposed to use more than 51 psi when operated the tire.

And second, don't use that formula. It is based on a false assumption - that the max load is directly proportional to the pressure. It's not. It's PARTIALLY proportional to the pressure. I go into detail here:

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

Use a table.

I looked at the Toyo table in your link. There is a P225/50R17 there, but the load/speed idx = 93V. As far as my Pirellis are concerned, they are 94V, and these are THEIR WORDS - not mine:

" MAX LOAD 670 kg [1477 lbs] AT 350 kPA [51 psi] " (from the tire itself)

According to the Toyo Table, for the closest approximate max load for a size 225/50R17 tire - 1,433lbs - I should inflate to 35psi cold.

Now: I like how the car rides/handles at 32-33psi cold(one would think Honda knows what they're doing!), and I do not like how it feels at mid-30s(psi) or higher pressure - rough ride, twitchy steering. My wife and I, plus a well-stocked toolbox in the trunk, barely equal 350lbs. And that's on weekends. Weekdays, it's just me in the Accord going to my job.

Your assessment?

[CapriRacer]

Originally Posted by RidingOnRailz

I looked at the Toyo table in your link. There is a P225/50R17 there, but the load/speed idx = 93V. As far as my Pirellis are concerned, they are 94V, and these are THEIR WORDS - not mine:

" MAX LOAD 670 kg [1477 lbs] AT 350 kPA [51 psi] " (from the tire itself)

According to the Toyo Table, for the closest approximate max load for a size 225/50R17 tire - 1,433lbs - I should inflate to 35psi cold. …..

First, I only partially understand why some tire manufacturers write this wrong, but they do - and technically it's not exactly wrong, just misleading.

Just like Toyo table indicates, the load carrying capacity maxes out at 35 psi (OK, because it's a European standard, the pressure is 250 kPa (kiloPascals) ~ 36.3 psi - so the load carrying capacity is slightly higher. In the big scheme of things, the difference between what the Toyo table says (which comes from The Tire and Rim Association (TRA) - the American standardizing organization) and the European standardizing organization (ETRTO) is trivial - they are for practical purposes the same.)

Further, the Pirelli tire is misleading because the load carrying capacity of the tire from 250 kPa to 350 kPa is also 670 kg. By writing it the wat they did, many folks have concluded that these tires would be different than if the tire was written to say " at 250 kPa" - and that would be incorrect.

So I understand what Pirelli (and others) are doing, but many folks don't and they draw wrong conclusions from it.

So why do we have American standards and European standards (and we have Japanese standards as well!) that are almost the same, and therefore confusing?

Historically, the world wasn't as connected when these things were set up so it didn't matter. But with the global nature of the tire industry and our ability to communicate, we are encountering these quirks of the past.

There is a program to globally harmonize tires - BUT - they can't undo the past. Just a few weeks ago, I got my 2020 TRA yearbook and noticed that some passenger car tire sizes have a note to that effect.

Originally Posted by RidingOnRailz

…...Now: I like how the car rides/handles at 32-33psi cold(one would think Honda knows what they're doing!), and I do not like how it feels at mid-30s(psi) or higher pressure - rough ride, twitchy steering. My wife and I, plus a well-stocked toolbox in the trunk, barely equal 350lbs. And that's on weekends. Weekdays, it's just me in the Accord going to my job.

Your assessment?

Tire designers can dial in a whole range "feel" to a tire.

When the vehicle engineers were designing your car, they specified the tire size and inflation pressure FIRST, then the ride engineers dialed in the tire feel, the spring rates, the shock damping rates that they felt the consumer wanted. It turns out for you, they were right - but for someone else, it might have been wrong. For example, I prefer a more crisper turn in than is the norm and I am willing to sacrifice the ride to get it. That usually means I use more inflation pressure than what the door sticker says.

[RidingOnRailz]

Originally Posted by CapriRacer

First, I only partially understand why some tire manufacturers write this wrong, but they do - and technically it's not exactly wrong, just misleading.

Just like Toyo table indicates, the load carrying capacity maxes out at 35 psi (OK, because it's a European standard, the pressure is 250 kPa (kiloPascals) ~ 36.3 psi - so the load carrying capacity is slightly higher. In the big scheme of things, the difference between what the Toyo table says (which comes from The Tire and Rim Association (TRA) - the American standardizing organization) and the European standardizing organization (ETRTO) is trivial - they are for practical purposes the same.)

Further, the Pirelli tire is misleading because the load carrying capacity of the tire from 250 kPa to 350 kPa is also 670 kg. By writing it the wat they did, many folks have concluded that these tires would be different than if the tire was written to say " at 250 kPa" - and that would be incorrect.

So I understand what Pirelli (and others) are doing, but many folks don't and they draw wrong conclusions from it.

So why do we have American standards and European standards (and we have Japanese standards as well!) that are almost the same, and therefore confusing?

Historically, the world wasn't as connected when these things were set up so it didn't matter. But with the global nature of the tire industry and our ability to communicate, we are encountering these quirks of the past.

There is a program to globally harmonize tires - BUT - they can't undo the past. Just a few weeks ago, I got my 2020 TRA yearbook and noticed that some passenger car tire sizes have a note to that effect.



Tire designers can dial in a whole range "feel" to a tire.

When the vehicle engineers were designing your car, they specified the tire size and inflation pressure FIRST, then the ride engineers dialed in the tire feel, the spring rates, the shock damping rates that they felt the consumer wanted. It turns out for you, they were right - but for someone else, it might have been wrong. For example, I prefer a more crisper turn in than is the norm and I am willing to sacrifice the ride to get it. That usually means I use more inflation pressure than what the door sticker says.

"For example, I prefer a more crisper turn
in than is the norm and I am willing to sacrifice
the ride to get it. That usually means I use more
inflation pressure than what the door sticker
says."

So you're of the 'more-is-better' crowd! Well you're hardly alone.

I mentioned it somewhere a while ago, but drivers(at least U.S. ones) fall into three camps as far as tire pressures are concerned:

40/20/40%, respectively: Underinflated, at or close to Tire Placard, and lastly, above Tire Placard and up to Max on the Tire.

You probably couldn't tolerate my TV picture settings either: nothing higher than the mid-point, and all calibrated for accuracy, not flashy picture.

Setting things the 'way they're supposed to be' is a lonely proposition, for sure!

[CapriRacer]

I would characterize my approach to inflation pressure as "slightly over is better than slightly under", but surveyors would classify my cars as "properly inflated".

And the impression I am getting is that since the advent of TPMS, the percent of cars with underinflated tires is around 25% - and perhaps less. Of course that depends on what "underinflated" means - and I use 10% as that value.

[RidingOnRailz]

Originally Posted by CapriRacer

I would characterize my approach to inflation pressure as "slightly over is better than slightly under", but surveyors would classify my cars as "properly inflated".

And the impression I am getting is that since the advent of TPMS, the percent of cars with underinflated tires is around 25% - and perhaps less. Of course that depends on what "underinflated" means - and I use 10% as that value.

Inflating an OEM size tire to above the vehicle's Tire Placard "properly inflated"?

[CapriRacer]

Originally Posted by RidingOnRailz

Inflating an OEM size tire to above the vehicle's Tire Placard "properly inflated"?

Is "properly inflated" within 3%? If not, what tolerance do you propose?

[fredjacksonsan]

This is a great discussion. I usually go a few psi up from door sticker as it seems to give better wear.

"Twitchy" is a good description of how overinflated tires feel.

Thanks for the info on the H rated Explorer tires, CapriRacer...that was part of the story of which I was not aware.

[RidingOnRailz]

Originally Posted by fredjacksonsan

This is a great discussion. I usually go a few psi up from door sticker as it seems to give better wear.

"Twitchy" is a good description of how overinflated tires feel.

Thanks for the info on the H rated Explorer tires, CapriRacer...that was part of the story of which I was not aware.

Most people do overinflate, to some degree or other. 'More-is-better' is contagious!

Personally, I run several gauges to eliminate a 'high-reader'(a gauge that reads high leads to under-inflation).

Unless I'm running a different size/duty tire, I inflate to what's supposed to be in there, or maybe +1psi during fall into winter.

[CapriRacer]

Originally Posted by RidingOnRailz

Most people do overinflate, to some degree or other. 'More-is-better' is contagious!

Personally, I run several gauges to eliminate a 'high-reader'(a gauge that reads high leads to under-inflation).

Unless I'm running a different size/duty tire, I inflate to what's supposed to be in there, or maybe +1psi during fall into winter.

The problem here is that the temperature varies from day to day - and even within a day. How do you set the pressures for when you are going to be driving the car? My approach is a few psi over. That way I am never too low and I naturally prefer the crisper turn in.