All Season Tire DD Thoughts

[Garandman]

I just called my local Sube dealer to find out what stock LGT 17" wheels would cost. $300 each.

Is that with the TPMS sensors?

 

You can still buy BBS RK's for $500 for four from Charles River SAAB and others.

[ilh]

After I sold the RE92s that came with them, my net cost on a set of stock 17" wheels in excellent condition came to $300.

[CapriRacer]

First, let me introduce myself. I am a tire engineer with a major tire manufacturer. My company would prefer that their name not be mentioned in my internet postings, but they can see the benefit of my assisting folks with technical information about tires.

 

I was asked by one of the posters to this forum to look over this discussion because he felt there was some misunderstandings and he wanted my comments. He was right, so I’ve decided to temporarily join the discussion to help put everyone on the right track.

 

Bear with me as this will be a long post, but I need to set the stage:

 

There are 3 major tire standardizing bodies – Tire and Rim Association (TRA), for the US – European Tyre and Rim Technical Organization (ETRTO), for Europe – and Japanese Automotive Tire Manufacturers Association (JATMA). There are others, but they seem to follow these organizations closely. I should also mention that they consult and cooperate with each other as best they can.

 

Among the things these organizations do is standardize the relationship between load carrying capacity and inflation pressure for a given size. We call these relationships the “load curve” and they are published in books as a “load table”.

 

The load table lists the tire size and the load for various inflation pressures. The maximum allowable load for a given size is commonly called the “rated load” and the corresponding inflation pressure is referred to as the “rated inflation pressure”.

 

The reason I mention all this is because of the following statement:

 

Regardless of who manufactures a tire, the load carrying capacity is set by a standardizing organization. While there is some difference, but for practical purposes, tires of the same size have the same load carrying capacity. In other words, they are more or less interchangeable, once you’ve factored in Load Range and things like that.

 

This also means that the vehicle manufacturers use the load tables as a starting point in determining the tire size and inflation pressure for their vehicles. They publish their tire specifications as a label on each vehicle – commonly called the “Placard”.

 

This brings me to the issue of the inflation pressure printed on the sidewall of tires. This is regulated by the US government and the regulation says something like this – “maximum load and inflation pressure”.

 

A few tire manufacturers think this means the sidewall should have the “rated load” and the “rated inflation pressure”. But the vast majority think this means the “rated load” and the “maximum permissible usage pressure”.

 

The standards organizations have followed the Europeans and have set the “maximum permissible usage pressure” for Standard Load passenger car tires as 44 psi (3.0 bar) or 51 psi (3.6 bar). Please note that this is the type of tire this forum would be discussing, so I’m confining my discussion to this type.

 

At this point I should also mention that the rated inflation pressure per the TRA standard for Standard Load Passenger car tires (called P metrics because they start with the letter “P”) is 35 psi. For ETRTO and JATMA (They don’t use a letter in front of the size.) it is 2.5 bar (about 36 psi)

 

So if you look at the sidewall of a tire, it will say 35, 36, 44, or 51 psi, all depending on which tire standard is being followed, how they are interpreting the regulation, and how they are interpreting the maximum permissible inflation pressure standard. But these pressures would be independent of the construction of the tire. This is an important point, so I will print it again:

 

For Standard Load Passenger Car tires, the construction of the tire is independent of what pressure is printed on the sidewall.

 

 

OK, that’s a lot to digest. So I’ll pause at this point, and entertain questions.

[LawGT]

Digested and no questions. My brain needs more info...

[Vimy101]

 

There are 3 major tire standardizing bodies – Tire and Rim Association (TRA), for the US – European Tyre and Rim Technical Organization (ETRTO), for Europe – and Japanese Automotive Tire Manufacturers Association (JATMA). There are others, but they seem to follow these organizations closely. I should also mention that they consult and cooperate with each other as best they can.

 

Among the things these organizations do is standardize the relationship between load carrying capacity and inflation pressure for a given size. We call these relationships the “load curve” and they are published in books as a “load table”.

 

The load table lists the tire size and the load for various inflation pressures. The maximum allowable load for a given size is commonly called the “rated load” and the corresponding inflation pressure is referred to as the “rated inflation pressure”.

 

The reason I mention all this is because of the following statement:

 

Regardless of who manufactures a tire, the load carrying capacity is set by a standardizing organization. While there is some difference, but for practical purposes, tires of the same size have the same load carrying capacity. In other words, they are more or less interchangeable, once you’ve factored in Load Range and things like that.

 

This also means that the vehicle manufacturers use the load tables as a starting point in determining the tire size and inflation pressure for their vehicles. They publish their tire specifications as a label on each vehicle – commonly called the “Placard”.

 

This brings me to the issue of the inflation pressure printed on the sidewall of tires. This is regulated by the US government and the regulation says something like this – “maximum load and inflation pressure”.

 

A few tire manufacturers think this means the sidewall should have the “rated load” and the “rated inflation pressure”. But the vast majority think this means the “rated load” and the “maximum permissible usage pressure”.

 

The standards organizations have followed the Europeans and have set the “maximum permissible usage pressure” for Standard Load passenger car tires as 44 psi (3.0 bar) or 51 psi (3.6 bar). Please note that this is the type of tire this forum would be discussing, so I’m confining my discussion to this type.

 

At this point I should also mention that the rated inflation pressure per the TRA standard for Standard Load Passenger car tires (called P metrics because they start with the letter “P”) is 35 psi. For ETRTO and JATMA (They don’t use a letter in front of the size.) it is 2.5 bar (about 36 psi)

 

So if you look at the sidewall of a tire, it will say 35, 36, 44, or 51 psi, all depending on which tire standard is being followed, how they are interpreting the regulation, and how they are interpreting the maximum permissible inflation pressure standard. But these pressures would be independent of the construction of the tire. This is an important point, so I will print it again:

 

For Standard Load Passenger Car tires, the construction of the tire is independent of what pressure is printed on the sidewall.

 

 

OK, that’s a lot to digest. So I’ll pause at this point, and entertain questions.

 

 

Thanks for sharing your expertise. Great info. I read your post 4 times and have a question. A 35 max psi tire and 51 max tire are basically constructed the same if I understand you correctly the only difference being the manufacturers association standard being used? That is a considerable difference if load ranges we are considering remain the same. No doubt I am missing something.

[CapriRacer]

Thanks for sharing your expertise. Great info. I read your post 4 times and have a question. A 35 max psi tire and 51 max tire are basically constructed the same if I understand you correctly the only difference being the manufacturers association standard being used? That is a considerable difference if load ranges we are considering remain the same. No doubt I am missing something.

 

In this case, it is the way the regulation is being interpreted.

 

But to anticipate your other question - the differerence between a tire labeled 44 psi and 51 psi is the way in which the standard is being interpreted. This is an area where I don't understand why other tire manufacturers label some of their tires 51 psi. Perhaps it has something to do with the construction, but my gut tells me this is not the case. I think construction changes are needed for higher speed ratings and not the pressure change.

[krzyss]

 

My hero ;-)

 

Krzys

[Vimy101]

In this case, it is the way the regulation is being interpreted.

 

But to anticipate your other question - the differerence between a tire labeled 44 psi and 51 psi is the way in which the standard is being interpreted. This is an area where I don't understand why other tire manufacturers label some of their tires 51 psi. Perhaps it has something to do with the construction, but my gut tells me this is not the case. I think construction changes are needed for higher speed ratings and not the pressure change.

 

So it would be safe to say that any tire within its load range can be safely run at its indicated cold max psi and fall within safety standards? Surely there must be some construction differences that would account for the differing max psi otherwise rolling resistance would differ vastly between brands. I know from your info that it depends upon how the load standards are interpretted but from a vehicle OEM standpoint they must want efficient tires that help them more easily meet CAFE standards. Thanks again for the excellent insights.

[HansGT]

It is amazing to think that you are still more than 10 lbs below the OEM design specs.

 

Remember your PSI will go up as the tires get warm. So you don't want to pump a cold tire to it's max then go drive. And 51 max is just that...."max", not the standard.

[Vimy101]

Remember your PSI will go up as the tires get warm. So you don't want to pump a cold tire to it's max then go drive. And 51 max is just that...."max"' date=' not the standard.[/quote']

 

The max pressure is max cold inflation temperature and takes into account temp increase from operation. I should add that the "cold" pressure is ambient temperature before the tires have been run.

[CapriRacer]

So it would be safe to say that any tire within its load range can be safely run at its indicated cold max psi and fall within safety standards? Surely there must be some construction differences that would account for the differing max psi otherwise rolling resistance would differ vastly between brands. I know from your info that it depends upon how the load standards are interpretted but from a vehicle OEM standpoint they must want efficient tires that help them more easily meet CAFE standards. Thanks again for the excellent insights.

 

 

I want to answer your questions very carefully, so I'm going to do this in chunks:

 

"…..So it would be safe to say that any tire within its load range can be safely run at its indicated cold max psi and fall within safety standards?…."

 

This indicates to me that you haven't quite got the concept.

 

Let me put it this way:

 

All standard load passenger car tires are rated at 35 or 36 psi. All standard load passenger car tires can use 44 or 51 psi under certain circumstances. It doesn't matter what is printed on the sidewall.

 

"….. Surely there must be some construction differences that would account for the differing max psi otherwise rolling resistance would differ vastly between brands….."

 

Ah….no. See above

 

BTW rolling resistance has a lot to do with how much mass there is in the tread area. More tread = more rolling resistance. Compound plays a role here as well!

 

"…..I know from your info that it depends upon how the load standards are interpretted but from a vehicle OEM standpoint they must want efficient tires that help them more easily meet CAFE standards…….."

 

Some vehicle manufacturers are absolutely extreme about rolling resistance, some don't care at all.

 

I just came back from a tire seminar where a presenter summarized what he told NHTSA (National Highway and Traffic Safety Administration) about tires and their role in fuel economy. His research found that about 7% of fuel is used in rolling the vehicle, but about 30% was used by engine losses. Some vehicle manufacturers spend a lot of research effort in areas where they think they can have significant improvements. Some don't!!

[Vimy101]

 

Let me put it this way:

 

All standard load passenger car tires are rated at 35 or 36 psi. All standard load passenger car tires can use 44 or 51 psi under certain circumstances. It doesn't matter what is printed on the sidewall.

 

What? No wonder you had to answer carefully. So you are saying that a tire rated for 35 or 44 psi can safely be run at 51 psi? What are the circumstances?

 

"….. Surely there must be some construction differences that would account for the differing max psi otherwise rolling resistance would differ vastly between brands….."

 

Ah….no. See above

 

Surely energy would be lost with a flexing sidewall vs one stiffened by a higher inflation pressure. Are the sidewalls of the carcass that strong that increased pressure would have little effect in keeping the tire round at 35 vs 51 psi?

 

 

Some vehicle manufacturers are absolutely extreme about rolling resistance, some don't care at all.

 

I just came back from a tire seminar where a presenter summarized what he told NHTSA (National Highway and Traffic Safety Administration) about tires and their role in fuel economy. His research found that about 7% of fuel is used in rolling the vehicle, but about 30% was used by engine losses. Some vehicle manufacturers spend a lot of research effort in areas where they think they can have significant improvements. Some don't!!

 

7% is not an insignificant number and you better believe that most on this forum are familiar with with driveline and engine losses. I would bet that some auto OEMs would be just fine with some other outfit footing the bill for research into rolling resistance that they can exploit later. Would it be safe to say that the larger OEM concerns are more interested in it than the smaller?

 

However, I find it astounding that you say the numbers posted on the sidewall are more or less meaningless. For instance, if all passenger car tires are rated for 35 psi but someone with a 51 psi tire runs it at that level and exceeds one of the "circumstances" which leads to tire failure and injury grounds for a law suit would result.

 

Thanks for the info. Please keep it coming as I believe tires are the most important component in motoring.

[CapriRacer]

What? No wonder you had to answer carefully. So you are saying that a tire rated for 35 or 44 psi can safely be run at 51 psi? What are the circumstances?

 

Again, I have to be careful. 51 psi is such an extreme usage pressure that it is almost never used.

 

The way it works is the vehicle manufacturers specify the tire size and pressure. Almost always the pressure is 35 psi or less - which is exactly in line with the load tables. But occssionally they will specify a pressure higher than that - and while I can't tell why they did that (I wasn't at the meeting where they made that decision), it appears to be for vehicle handling reasons.

 

But I still think you haven't gotten it yet. The tires aren't "rated" at 44 psi or 51 psi. They are rated at 35 or 36 psi, but there are circumstances where you can use a higher pressure. The constructions aren't different just because the sidewall says 44 psi or 51 psi.

 

Surely energy would be lost with a flexing sidewall vs one

stiffened by a higher inflation pressure.

 

I need to correct a couple of misunderstandings you have.

 

The vast majority of energy loss is because the tread flexing, The sidewall is just barely involved. So sidewall stiffness isn't really a factor in rolling resistance.

 

And as I said above, the constructions aren't different just because the sidewall has a different pressure written on it. Please note that I am discussing ONLY Standard Load passenger car tires here. What happens with other types of tires is different.

 

Are the sidewalls of the carcass that strong that increased pressure would have little effect in keeping the tire round at 35 vs 51 psi?

 

For practical purposes the construction required for tires to do the other things they have to do results in one that is much stronger than is needed just for inflation pressure.

 

For instance, the static burst pressure for a standard load passenger car tires is way over 100 psi!

 

7% is not an insignificant number and you better believe that most on this forum are familiar with with driveline and engine losses. I would bet that some auto OEMs would be just fine with some other outfit footing the bill for research into rolling resistance that they can exploit later. Would it be safe to say that the larger OEM concerns are more interested in it than the smaller?

 

No, it seems to be divided by region. The American manufacturers are big on RR, the Japanese and Europeans are not.

 

However, I find it astounding that you say the numbers posted on the sidewall are more or less meaningless.

 

Astounding or not, it is true - at least for standard load passenger car tires!

 

For instance, if all passenger car tires are rated for 35 psi but someone with a 51 psi tire runs it at that level and exceeds one of the "circumstances" which leads to tire failure and injury grounds for a law suit would result.

 

Tire manufacturers are targets for litigation. A tire get a puncture, it goes flat, the vehicle rolls over, the driver gets killed, the family sues, the lawyers make money. Simple!

 

But it could be argued that no reasonable person would inflate their tires to 51 psi when the placard clearly states 35 psi. After all, the tire manufacturer doesn't know what vehicle the tire is going on. The pressure on the sidewall is a maximum and it even says so.

[Vimy101]

Fascinating stuff. Well, at least I think so. It seems quite odd that tire OEMs would have such divergent marking on their tires but as you informed me/us it does depend upon how certain things are interpretted. As to side wall flex, it does make sense that the side wall on radial tires doesn't make a huge difference on rolling resistance as what may look like a properly inflated tire could very well be >20% under the placard pressure.

 

What is your take on the Firestone/Ford Explorer thing?

 

Thanks again for sharing your expertise.

[tantal]

Don't forget: rolling resistance is proportional to grip, and grip is what you want for good handling.

 

No wonder US automakers want low rolling resistance tires - they make cars to go in a straight line.

 

Pathetic.

[CapriRacer]

First, let me correct part of my previous posting that was incorrect.

 

In response, I said that the pressure listed on the sidewall of a tire is meaningless. That is not true.

 

The pressure listed on the sidewall of a Standard Load Passenger car tire is meaningless with regards to the construction.

 

There is 3 items of importance in this statement:

 

1) Standard Load - Extra Load tires use higher inflation pressures in order to carry more load and the construction of an Extra Load tire is stronger than a Standard Load tire. The important point here is that it is primarily the load that requires the upgraded construction.

 

2) Passenger Car tires - The situation for other types of tires is different and they follow the old way do expressing the load / pressure thing. I suspect this has to do with consumers and litigation.

 

3) You may find some differences in construction and you might note a trend towards tires with higher pressures having stronger constructions, but it isn't the pressure that is driving this.

 

Now to the question:

 

What is your take on the Firestone/Ford Explorer thing?

 

Firestone published a report written by an outside consultant that pinpointed the shortcomings in the tire design. The report also discussed the rubber processing, but that part was way outside my expertise and I had to ask someone else about the implications of that part of the report.

 

The bottomline was that there was something about that particular tire coming out of that particular plant.

 

When Firestone brought up the inflation pressure spec of the Explorer, I got out my calculator and verified what Firestone was saying. They said - and this is important if you are going try to assign the blame to Ford - that the Ford pressure specification resulted in no extra capacity if the vehicle was loaded to the capacity of the vehicle.

 

This means that if the vehicle was overloaded or the tires were underinflated, the theoretical load carrying capacity of the tires could be exceeded.

 

But there are other discussion points brought up about Ford's culpability.

 

Cost pressure: It's true that Ford's purchasing group are tough negotiators. (They wouldn't be doing their job if they weren't!)

 

Speed rating: Usually this is expressed by referring to the UTQG temperature rating - C.

 

The rollover tendency of the Explorer.

 

So if Ford is guilty of something, it is: (I'm going to confine this to tire related stuff.)

 

A) poor engineering practice by specifying a low inflation pressure

B) poor engineering practice by specifiying a low speed rating

 

In the final analysis, the real problem was the tread pattern design that resulted in a tire that wasn't built the way the tire construction designers intended, aggravated by an old and compromising rubber processing system, aggravated by Ford's poor engineering practices.

[Vimy101]

Doing more reading, the max psi is the maximum cold pressure a tire is designed for to operate at maximum load.

[Vimy101]

In this case, it is the way the regulation is being interpreted.

 

But to anticipate your other question - the differerence between a tire labeled 44 psi and 51 psi is the way in which the standard is being interpreted. This is an area where I don't understand why other tire manufacturers label some of their tires 51 psi. Perhaps it has something to do with the construction, but my gut tells me this is not the case. I think construction changes are needed for higher speed ratings and not the pressure change.

 

The more DD I perform the more I understand why you have to be circumspect with the information you are able to impart to us. This tells me that tires are arguably the most critical system on any car nominally operable car. My DD also indicates that tires have to be able to be built in such a manner to be able to withstand a very high static shock where, as you said, tire pressures can spike.

 

Reading some pro trucker forums were very enlightening in addtion to various gov't and industry sites. The truckers mentioned rim/wheel psi ratings as well.

[CapriRacer]

Doing more reading, the max psi is the maximum cold pressure a tire is designed for to operate at maximum load.

 

Ah....Mmmmm.....not exactly.

 

There are 2 configurations for what is written on the sidewall:

 

1) Max load XXXX at YY pressure. This tells you where the maximum load occurs relative to the pressure.

 

2) Max load XXXX, Max pressure YY. This tells you what the max load is and what the max pressure is, but doesn't tell you about the relationship.

[outahere]

...............1) Standard Load - Extra Load tires use higher inflation pressures in order to carry more load and the construction of an Extra Load tire is stronger than a Standard Load tire. The important point here is that it is primarily the load that requires the upgraded construction..........

 

All this tire load rating stuff gets a bit confusing. My Michelin Pilot Exaltos (215/45x17, 91W, XL) have, stamped on the sidewall, Max Load 1356 lb and Max Pressure 50 psi. These tires are extra load (XL), which I understand is referenced to 41psi, not 35 or 36psi.

 

Is the tire rated to carry a load of 1356 lb at 35psi, 36psi, 41psi, or 50psi?

 

The load designation for this tire is "91", which corresponds to a load of 1356 lbs for a standard load tire. But since this tire is an "extra load" tire, why is it not rated at higher than 1356 lb?

[CapriRacer]

Originally Posted by Outahere

All this tire load rating stuff gets a bit confusing. My Michelin Pilot Exaltos (215/45x17, 91W, XL) have, stamped on the sidewall, Max Load 1356 lb and Max Pressure 50 psi. These tires are extra load (XL), which I understand is referenced to 41psi, not 35 or 36psi.

Is the tire rated to carry a load of 1356 lb at 35psi, 36psi, 41psi, or 50psi?

The load designation for this tire is "91", which corresponds to a load of 1356 lbs for a standard load tire. But since this tire is an "extra load" tire, why is it not rated at higher than 1356 lb?

 

This is a good opportunity to talk about Extra Load tires.

 

These are exactly like Standard Load tires except that they can carry a larger load than the same size standard load tire but at a higher inflation pressure - 41 psi instead of 35 or 36. As a result they are built a bit stronger. Perhaps this is why many folks think that increased inflation pressure should result in stronger tires.

 

Load index's (or is it indices?) are load references, and not tied to an inflation pressure. So tires can have the same load index and vastly different inflation pressures. This is more visible in truck tires, where tires are a wide variety of Load Ranges for a given size.

 

So your 214/45R17 XL's are rated at 1356 # at 41 psi. A Standard Load 215/45R17 would be rated at 87 Load index or 545 kg at 2.5 bar (1199 # at 36 psi, assuming Michelin is following ETRTO and not JATMA)

[Vimy101]

Let's make the move to metric.

[outahere]

This is a good opportunity to talk about Extra Load tires.

 

These are exactly like Standard Load tires except that they can carry a larger load than the same size standard load tire but at a higher inflation pressure - 41 psi instead of 35 or 36. As a result they are built a bit stronger. Perhaps this is why many folks think that increased inflation pressure should result in stronger tires.

 

Load index's (or is it indices?) are load references, and not tied to an inflation pressure. So tires can have the same load index and vastly different inflation pressures. This is more visible in truck tires, where tires are a wide variety of Load Ranges for a given size.

 

So your 214/45R17 XL's are rated at 1356 # at 41 psi. A Standard Load 215/45R17 would be rated at 87 Load index or 545 kg at 2.5 bar (1199 # at 36 psi, assuming Michelin is following ETRTO and not JATMA)

 

Thanks for the explanation!

[CapriRacer]

OK, guys!

 

When I did my first posting, there were 2 areas that needed to be addressed. The most important one was the relationship between the pressure printed on the sidewall of a tire and the strength of that tire. Hopefully, these posts have revealed that you can't tell about the strength of a passenger car tire just by looking at the pressure printed on the sidewall.

 

The second, but still important, item that needed to be discussed was speed ratings.

 

Anyone up for that?

[ilh]

Yes, please.