All Season Tire DD Thoughts

[Vimy101]

^ More info! More info!

[CapriRacer]

Speed ratings were developed in Europe to deal with the speeds that some vehicles were able to attain on unlimited highways – eg. Autobahn.

The early speed ratings were abbreviated by the operating conditions - S (Standard), H (High Speed), and V (Very High Speed), and later expanded in more or less alphabetical order. Of course, H is misplaced. Another quirk to the ratings is that Z is an open ended, vehicle specific rating, but W and Y were added after Z was created, so there is this peculiar kind of overlap.

The ratings are based on an ECE test. I think ECE stands for Economic Commission for European, but part of what they do is set testing standards for a wide variety of products.

The test consists of a tire mounted in a fixture at the rated inflation pressure and applied to a wheel at the equivalent of the rated load (see earlier postings on rated load and rated inflation) in a room at 77F. To pass the test (and receive the rating) the tire must sustain the speed without failure for 10 minutes. There is a warmup procedure that is tied to the speed rating desired.

Please note: This is a “testing to standard” kind of thing. It is known ahead of time what the intended usage is (the rating desired) and the test is conducted with the goal of “reaching the standard”. Needless to say, tire manufacturers generally run the test beyond the “standard” to assure ALL tires would pass the test. How and what their testing procedures are varies with each tire manufacturer, and I’ve heard of some very interesting extensions beyond the rated test condition.

A number of important things come out of this. I’ve put a plus or a minus in front of each to indicate how the speed rating test relates to real world conditions.

+ Rated Load – normally a tire is not used at the rated load, so the actual speed capability would be greater than indicated.

- Rated Inflation – High inflation pressures improve the speed capability, so using less than the rated would reduce the speed capability, even if the load is appropriate for the pressure.

- 77F – Any time the ambient temperature gets hotter than this, the speed capability is less. BTW there is an SAE speed rating test that is run the same way, but at 100F.

- 10 minutes – not a lot of time at the speed, but it would take a long time to develop any sort of long term durability issue with the tire.

- Recently produced tire – While the test doesn’t specify a recent production, that is in fact what gets tested. Tires do degrade over time and speed capability is certainly affected negatively.

The net effect of this is that there are many more negatives than positives, and so this should be taken into account when choosing tires.

But one of the other aspects that sometimes gets forgotten is something a professor of mine used to say - "Overdesign, Underutilize." This means that while a product should be designed to exceed the standard (in this case the speed rating), it should also be used under that standard. (In this case, the vehicle manufacturer should specify a speed rating more than what the expected maximum speed will be.)

To add some further depth to this, one of the lessons out of the Firestone situation was that tires with more capability – load and / or speed – had reduced failure rates, especially for tires with road hazard related conditions. Put in layman’s term – you are safer using bigger tires and higher speed ratings.

Ok, that's enough for now.

[Vimy101]

Thanks for the write up. Please keep the info coming as it is very important for high performance car enthusiasts to be aware of this stuff.

[outahere]

..........Recently produced tire – While the test doesn’t specify a recent production, that is in fact what gets tested. Tires do degrade over time and speed capability is certainly affected negatively.........

 

Has the tire industry reached a consensus on what is the safe, service lifetime, in years, of a passenger car tire that is subjected to low use?

 

For example, how long before the compact spare tire for my Subaru becomes too old to safely use? 5 years? 10 years?

[Vimy101]

I would say that tires that have seen 3 years of service regardless of mileage won't perform up to design specs. I'm hoping that when I get my winter wheel/tire set up gojng that I'll always be able to carry a full size OEM spec circumference spare. I haven't tried it but surely a stock wheel will fit in the spare well. Anyone tried it?

 

Looking forward to the expert's input.*

 

*The word expert has negative connotations in today's USA for some reason but I use the term in its purest sense meaning someone who has studied (another bad term these days) and has knowledge that is elite (another presently negative term) compared to the average Walmart shopper and most other car owners.

 

(I appreciate tires like women love shoes. Same difference.)

[CapriRacer]

Has the tire industry reached a consensus on what is the safe, service lifetime, in years, of a passenger car tire that is subjected to low use?

 

For example, how long before the compact spare tire for my Subaru becomes too old to safely use? 5 years? 10 years?

 

Yes, they have reached a consensus that there are too many variables to try to "draw a line in the sand". But they have reached a consensus that inspecting a tire is not a reliable method of determining if a tire is good. Inspection will reveal things that ought to trigger a tire's removal, but a potentially dangerous tire might not show symptoms, such as dry rot.

 

In essence, the problem of a time limit is that they feel the need to give good reliable information and recommendations, but there isn't what is called an "inflection point" - a point where the curve changes direction - so the line drawn would be arbitary. So I will give you my take on the subject.

 

The biggest factor here is ambient temperature. Several states can be grouped together as the worst of the bunch (based on return data) - AZ, CA, NV, TX, NM, and FL. Notice that FL seems the odd man out, but what is probably going on here is that FL is warm all year, where the other states are hot, but only during the summer.

 

So for these states, a properly maintained tire seems to cross a point at about 6 years, but I have seen some tires where it is clear they should have been removed at 4 years. I'll talk about that at the end of the post.

 

The return data also seems to point to certain states as having very little temperature affect - like MN. But there appears to be a 10 year limitation there, but it is much harder to discern as many tires are worn out before this time.

But you have to remember we are talking about fatigue failures here - which means it takes some tie to develop the failure. Put in context, this means that a thoroughly old, but unsed tire (like a spare) will still perform for some length of time before the failure renders the tire unusable.

 

Another important factor is maintenance - and for this topic that means inflation pressure. Underinflated tires generate more heat and magnify the affect ambient temperature plays. This is particularly important when it comes to the effect speed has on durability. Running an underinflated tire at higher speeds magnifies the stresses, so you have both heat and stress being amplified.

 

So a guy living in Phoenix driving 85 mph on a 4 year old tire underflated by 5 psi has many, many times the risk of a guy living in Cleveland driving 55 mph on a properly inflated 8 year old tire.

[ilh]

Is this dry rot in the grooves on this tire being sold in the marketplace of these forums?

 

http://i15.photobucket.com/albums/a389/rallyGC8/FOR%20SALE/proxes5.jpg

 

I cannot imagine those cracks are a good thing.

 

--Lee

[LawGT]

I have no idea who is selling the tires shown above, but they should not be used. That is dry rot in the grooves. Tire absolutely not safe.

[CapriRacer]

Because tread compounds can be quite stiff, they are sometimes prone to cracking. While I don't like what I see in the photos, I'd want to examine the rest of the tire to see what it tells me about the overall age. Can you post a photo of the DOT number?

[Vimy101]

^The tread wear and oxidation are readily apparent, IMHO. Those tires are cooked. Stick a fork in them.

 

I will have more questions for you as from the info I've gleaned from your posts indicates that max inflation provides max load which was the thrust of my original postion. Not to be confused with max handling vs max load.

[CapriRacer]

....from the info I've gleaned from your posts indicates that max inflation provides max load............

 

Unfortunately, that isn't quite right, so before you go on and think yourself into a corner:

 

The maximum load for a given pressure increases as the pressure increases, up to some value (for both load and pressure). This particular combination of load and pressure is called the "rating" point - although we usually refer to the rated load and rated inflation pressure separately and not as a point with 2 dimensions.

 

But, a tire can be safely inflated beyond the rated pressure and in some circumstances, it is more appropriate than not.

[Vimy101]

The maximum load for a given pressure increases as the pressure increases, up to some value (for both load and pressure).

 

Isn't that what I said?

 

I realize that there would have to be some kind of proportionality for pressure required for a given load to achieve a nominal rating point. Could you provide an example of a hypothetical situtation where one might inflate tires to max pressure for a particular load to achieve a proper rating?

 

Once again, thanks for your time and info.

[CapriRacer]

Isn't that what I said?

 

I realize that there would have to be some kind of proportionality for pressure required for a given load to achieve a nominal rating point. Could you provide an example of a hypothetical situtation where one might inflate tires to max pressure for a particular load to achieve a proper rating?

 

Once again, thanks for your time and info.

 

I'm a little confused, and I think it is because of the words "rated" and "max".

 

But let me give you a practical example:

 

My daughter's Mustang calls for 35 psi for a P205/65R15. The maximum load carrying capacity at that pressure is 1400 # - and that happens to be the rated load and pressure. Needless to say, the Mustang doesn't weigh 5600 # - there's a lot more to selecting a tire size and pressure than just what the load table says.

 

I have a friend who races a Mustang just like my daughter's (except his has a roll cage, and other safety equipment - not to mention a much more skillful driver!). His cold pressures are in the 40 to 42 psi range. BTW, the sidewall says 44 psi max.

 

So let's start with that.

[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)

 

So, if I inflate my XL tires above 41 psi, to their max pressure of 50 psi, I do not gain any more load capacity? Is that a correct interpretation?

[CapriRacer]

So, if I inflate my XL tires above 41 psi, to their max pressure of 50 psi, I do not gain any more load capacity? Is that a correct interpretation?

 

Yes, that is correct, but perhaps a better way to state that would be: If the vehicle placard says I should use 41 psi, then it is OK to use more inflation pressure, but I don't gain any extra load capacity.

[Vimy101]

I'm a little confused, and I think it is because of the words "rated" and "max".

 

But let me give you a practical example:

 

My daughter's Mustang calls for 35 psi for a P205/65R15. The maximum load carrying capacity at that pressure is 1400 # - and that happens to be the rated load and pressure. Needless to say, the Mustang doesn't weigh 5600 # - there's a lot more to selecting a tire size and pressure than just what the load table says.

 

I have a friend who races a Mustang just like my daughter's (except his has a roll cage, and other safety equipment - not to mention a much more skillful driver!). His cold pressures are in the 40 to 42 psi range. BTW, the sidewall says 44 psi max.

 

So let's start with that.

 

The OEM placard factors in hard breaking with a car that is fully loaded with 5 adults, tankful of gas and luggage in the trunk so your daughters 1400 pound each load front tires might be tested to their limit especially if she hit a nasty pot hole at the same time with the ambient temperature being 100F. Correct?

[CapriRacer]

The OEM placard factors in hard breaking with a car that is fully loaded with 5 adults, tankful of gas and luggage in the trunk so your daughters 1400 pound each load front tires might be tested to their limit especially if she hit a nasty pot hole at the same time with the ambient temperature being 100F. Correct?

 

First, I am not a vehicle engineer, so I can only give you my impression of how vehicle engineers do what they do.

 

Obviously, vehicle engineers consider the maximum loading when selecting a tire size and inflation pressure. There are probably some other "fudge factors" they also use to get them to a point where they have confidence that it is similar to what they've done before.

 

Once there, they test the vehicle on calibrated ride roads - segments of a test track where they have built specific humps and obstacles that emulate various conditions encountered. The one ride road I was on had angle iron set at various heights that a vehicle could be driven over. These were both "step up" and "step down". Needless to say, they do these tests in varying ambient temperatures and have to wait for certain conditions to appear.

 

I have seen test fixtures similar to flight simulators where they can simulate the road input and measure how the suspension and vehicle structure react. The only thing missing is that the tire "rolls" over obstacles and that has to be modeled to get the same reaction at the wheel end.

 

But the one thing that hasn't seem to change over the years is Ride and Handling Engineers. These guys have calibrated butts and their job is to drive the vehicle and "tweak" the springs, shocks, sway bars, AND tires so they all work together and produce the ride and handling qualities desired.

 

So in some respects, you're right, but I suspect there is a some "underutilization" going on - and this would be just ordinary good engineering.

[krzyss]

I think I promised some time ago that I list my 2003 Passat pressure list.

Here it comes:

half load===max load

..F.....R...........F....R

28...28........30...39---up to 100mph

30...28........33...42---more than 100mph

 

If I am not mistaken VW changed the way they list pressure to just single pair in 2004.

 

Krzys

[goneskiian]

Great stuff! Thank you all for your input. Especially Vimy101 and CapriRacer.

 

Cheers!

-Ian