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jadcock (Civil/Environmental) (OP)
13 Jul 06 21:14
We're having a discussion related to this topic on another automotive-related bulletin board.

I understand that tires are speed-rated according to their relative capability to withstand certain sustained speeds.  I suppose heat and pressure build not only as the speed increases, but also as the time duration (at that speed) increases...up to a point.  Yes/no?

An S-rated tire is nominally capable of a sustained speed of 112 mph.  I'm sure that if you went 115 mph, it's not going to blow up.  Or even 120 mph.  But wait -- the next speed rating (T) is at the 118 mph level, so there must be a rather defined line between those two speeds of what a given tire can handle.

That's what our discussion is about...what is the manufacturers', or the industry's, definition of a "sustained speed"?  We're all pretty sure that one could drive an S-rated tire quickly up to 120 mph, then back down below 112 mph without any problem.  Or an H-rated tire, momentarily, up to 140 mph without any problem.

But for how long?  10 seconds?  10 minutes?  Long enough, and you start getting into the territory of the next speed rating.  For instance, if you can drive an S-rated tire at 120 mph for 10 minutes, wouldn't it have a T-rating?  Etc.

I figure there must be some sort of "regulated" assessment method for determining the official speed ratings of a tire.  What is that method?  Does it measure heat/pressure?  Does it measure physical expansion of the tire (centrifugal forces)?  And what is the time duration?  Must it perform at a given level for 30 minutes to achieve that rating?  Or only 5?

I'm sure there's somebody here in the know who can answer this!

Thanks in advance,
Jason
evelrod (Automotive)
13 Jul 06 21:29
Jason, I did some tire tests for a major race tire mfgr some 20 years ago.  I was given to understand that I had a considerable safety margain in hand as I exceeded the rated speeds by fairly large numbers for hours, on occasion.  Never had a failure due to the tire de laminating or 'blowing out'. Did run a tire through the cord once, though.

Rod
NormPeterson (Structural)
13 Jul 06 21:52
Helpful Member!(2)  CapriRacer (Mechanical)
14 Jul 06 8:12
Jadcock,

I'm a tire engineer for a major manufacturer of tires.  I get into these discussions all the time and perhaps I can add my expertise to your discussion.  Please post a URL.

But here's the scoop:

The ECE test requires a tire to be tested at increasing speeds in a stepwise manner at 77F on a smooth wheel and sustain the rating speed without structural failure for 10 minutes.  Yup, 10 minutes!

But as most engineers will tell you, you should "over-design and under-utilize".  This applies to every product including tires.  So vehicle manufacturers will specify a tire that exceeds the maximum speed of the vehicle.

Tire manufacturers want ALL their tires to pass the speed rating, so they'll specify that all their production tires exceed the test by some margin.  Does this mean 20 minutes?  Does this mean 100F (The SAE version!), Does this mean one additional step up in speed?  Depends on the manufacturer.

This "over-design / under-utilize" credo is important as it has a basis in reality.  It's been found that barely meeting the conditions - load capability, speed capability, etc. - results in a certain rate of failure in the real world.  And while we can talk about underinflation, road hazards, etc., the reality is that over-specifying a tire results in reduced failure rates.

But there is one aspect that deserves additional comment.

Pretty much every tire will pass an S rating.  (Please note that I'm limiting the discussion to regular passenger car tires.  Winter tires and light truck tires are a different story - similar, but different.)  And T ratings are not much of a stretch.

So an S rating would - on the surface - seem to be adequate for use in the US where there are speed limits everywhere.

But, in order to pass an H speed rating, a tire more or less has to have a cap ply.  This change has a profound effect on failure rates, way beyond what the increase in speed rating suggests.  The failures rates are so low for tires with cap plies that court room "experts" claim that tire manufacturers are negligent for not using cap plies even in their S rated products.  (My response to that is that these "experts" must also be saying that tires ought to have a minimum of an H speed rating - and if that is true, the "experts" ought to be lobbying NHTSA to specify H rated tires for any tire sold in the US.)

But the point I want to make is that the step between T and H is enormous from a tire durability point of view.  And I don't recommend anyone use anything less than an H rated tire.  Considering that the risk of a tire failure includes fatality, the cost / benefit seems to be there.

  
jadcock (Civil/Environmental) (OP)
15 Jul 06 9:04
Thanks everyone for your responses.

The difference between an S (112) and a T (118) rating is rather small.  Thinking a little further, if any given tire bears an S speed rating, does that necessarily indicate that it failed at the T speed?  Or is this just an example of a tire manufacturer being very conservative with their ratings?

CapriRacer, I'm interested in the cap plies.  I used to own a set of H-rated tires (Michelin Pilot XGT H4).  They had four tread plies, 2 steel, 1 polyester, 1 polyamide (and the one polyester sidewall ply).  I replaced those with a set of S-rated tires (Michelin X Radial DT) since my car only requires S-rated tires.  The X Radial DT tires, in the same P225/60R16 size, show 5 tread plies, 2 steel, 2 polyester, 1 polyamide (with the two polyester sidewall plies).  I felt very comfortable with those tires because they show an increase in tread plies, although I certainly recognize that something is different because they only achieve an S rating.

And what's interesting is, those same X Radial DT tires in a different size, like we have on our Grand Caravan (P215/70R15) only have 3 tread plies (2 steel and 1 polyester), with the one polyester sidewall ply.

What is the definition of a "cap ply" in the context of a tire?  And can I assume that the four plies in my old H-rated tires must have been "thicker" or "better" in some way than the five plies in my new S-rated tires?  It was odd to see more plies in a "lesser" tire, but I'm sure I'm generalizing tire design to a point where my logic is incorrect.

Michelin advertises most of their H+ rated tires as having a BAZ ply, or Banded At Zero (degrees to tire rotation).  Is this Michelin advertising that cap ply required of all H+ rated tires?  It seemed to me that that one polyamide ply in my old Pilots were probably that BAZ layer, but then when I saw the X Radials as having a polyamide ply, I started questioning my assumption.

Tire design fascinates me.

Thanks again,
Jason
CapriRacer (Mechanical)
16 Jul 06 10:05
Jadcock,

Be careful going down the path you're traveling.  While it is an interesting direction for engineers, it tends to lead folks towards a greater risk of tire failure for the sake of saving money.

But let me answer your questions:

"....Thinking a little further, if any given tire bears an S speed rating, does that necessarily indicate that it failed at the T speed?...."

No.   

"....Or is this just an example of a tire manufacturer being very conservative with their ratings?...."

Actually you have this backwards.  A tire manufacturer designs a product to fit a certain market segment, then tests to see if the design meets the requirements.

For example, a generic all season tire would need to be S rated with a 50,000 miles tread wear capability and have a good ride.  So the design would be geared towards those parameters, and then tests would be performed to determine if the design goals were met (with sufficient reserve to satisfy the manufacturers internal standards).

Taking your example:  "....The X Radial DT tires, in the same P225/60R16 size, show 5 tread plies, 2 steel, 2 polyester, 1 polyamide (with the two polyester sidewall plies.  I felt very comfortable with those tires because they show an increase in tread plies, although I certainly recognize that something is different because they only achieve an S rating...."

No, they GAVE it an S rating, but I'll bet it will pass an H rating, and I'll even take odds on a V rating.

"....And what's interesting is, those same X Radial DT tires in a different size, like we have on our Grand Caravan (P215/70R15) only have 3 tread plies (2 steel and 1 polyester), with the one polyester sidewall ply...."

Looking at it from the point of view of market segment, this starts to make sense.  The reason Michelin uses a 5 ply tread is that they have determined they need that beyond what is needed to satisfy the speed rating.  Based on my experience, a P225/60R16 is a tough animal to get to perform in the real world with a reasonable level of failure, so it doesn't surprise me that Michelin would use a cap ply for that size, and not for the other.

"….What is the definition of a "cap ply" in the context of a tire?.…"  

A cap ply is a additional layer of a fabric and rubber composite that is applied over the belts and oriented in more or less the circumferential direction.  What it does is restrict the growth of the tire in the circumferential direction - like centrifugal forces.  But I also think it adds bulk and reduces the standing wave, which may actually be more important in the context of tire failure.

"....And can I assume that the four plies in my old H-rated tires must have been "thicker" or "better" in some way than the five plies in my new S-rated tires?...."

Nope!!!   Much more complex than that.

"....It was odd to see more plies in a "lesser" tire, but I'm sure I'm generalizing tire design to a point where my logic is incorrect..."

Yes, but I hope you see where you went down the wrong path.


"....Michelin advertises most of their H+ rated tires as having a BAZ ply, or Banded At Zero (degrees to tire rotation).  Is this Michelin advertising that cap ply required of all H+ rated tires?...."

No, I think they are advertising spiral cap plies, as opposed to a "spliced" cap ply or an "angled" cap ply (but I don't know of anyone who is using an "angled" cap ply, so this advertsing seems absurd to me.)

The caution I want to give you is that the speed rating is the result of a test and just because it passes the test doesn't mean the tire will perform.  There is a fairly good correlation between the test and high speed capability, but as time has gone on, it's been discovered that the test isn't quite adequate for such places where the ambient temperatures are high or where the roads aren't very smooth.  Not to mention cases where the tire is damaged in some way.

But the marketplace is relatively immune to these types of discoveries - Price seems to be a stronger driver than performance.  Recent events bear this out.  So I've taken the posture that an H rated design is the minimally acceptable design and that's what ought to be required for sale in the US.  When I am in a position where I can influence legislation, I will do so.
 
patprimmer (Publican)
16 Jul 06 19:44
Although I have no first hand knowledge in the tyre industry (I was on the fringe selling yarns suitable for tyre cord and polymer to manufacturers of tyre cord) I would suspect that if a tyre company had two identical sized tyres, and already had a higher speed rated version, and a small market developed for lower speed rated cheaper version, they might use the same carcase with a different rubber or tread to save the cost of setting up the lesser carcase. It will be a sum of set up time vs material savings vs profit margin vs total turnover change.

I imagine that off road tyres might have heavier construction than their speed rating suggests as this will also impact on load carrying and puncture resistance. There are many qualities affected by carcase construction, not just speed rating.

Plies is a count of the number of layers of fabric. The weight and construction of the fabric can vary in many ways. Like CapriRacer says, it is complex and can involve direction of lay of yarns, the design of the weave, number of threads in a yarn, number of fibres in a thread, thickness of fibres, stretch induced into fibres yarns and threads during processing, twist direction and degree,
type of material in the fibre, surface treatments for adhesion to rubber, various rubber compounds used in the build up etc etc etc.

Regards

eng-tips, by professional engineers for professional engineers
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jadcock (Civil/Environmental) (OP)
17 Jul 06 12:43
"No, they GAVE it an S rating, but I'll bet it will pass an H rating, and I'll even take odds on a V rating."

Wow -- important distinction there!  What I'm reading is that the official speed rating on a tire really isn't a good indicator of the true or real world performance capabilities of a tire?

"Based on my experience, a P225/60R16 is a tough animal to get to perform in the real world with a reasonable level of failure, so it doesn't surprise me that Michelin would use a cap ply for that size, and not for the other."

What about that size is distinctive?  Is it just a "weird" size in terms of height or width to make work?  Or is it because that size of tire appears on all types of vehicles from luxury cars to "sports cars"?

"The caution I want to give you is that the speed rating is the result of a test and just because it passes the test doesn't mean the tire will perform...So I've taken the posture that an H rated design is the minimally acceptable design and that's what ought to be required for sale in the US."

If the rating on the sidewall is somewhat arbitrary (for example, the X Radial tire in P225/60R16 might really pass an H or V test, but they assigned it an S), is it appropriate to restrict your selection set to only those tires which bear that somewhat arbitrary rating?  I agree 100% that safety should be a top priority for tire selection, but it seems that following the speed rating scale by itself may not give you an indication of the true capabilities of a tire.  Or did I mis-understand you?

Thanks again for your thoughtful and comprehensive replies!  Like I said before, the world of tire design is really fascinating to me.
CapriRacer (Mechanical)
17 Jul 06 15:52
Jadcock,

"....What I'm reading is that the official speed rating on a tire really isn't a good indicator of the true or real world performance capabilities of a tire?...."

Be careful, here.  One point does not make a trend.  Don't forget, the speed rating is first assigned, then the test is run to assure it meets that speed rating.  

"....What about that size is distinctive?  Is it just a "weird" size in terms of height or width to make work?  Or is it because that size of tire appears on all types of vehicles from luxury cars to "sports cars"?...."

I wish I knew as it would have save a lot of folks a lot of headaches.  I just know AFTER the fact.

"....If the rating on the sidewall is somewhat arbitrary (for example, the X Radial tire in P225/60R16 might really pass an H or V test, but they assigned it an S), is it appropriate to restrict your selection set to only those tires which bear that somewhat arbitrary rating?  I agree 100% that safety should be a top priority for tire selection, but it seems that following the speed rating scale by itself may not give you an indication of the true capabilities of a tire.  Or did I mis-understand you?...."

Yes, you did misunderstand me.

The rating is what the tire manufacturer assigns to it.  This means it is appropriate to use where the vehicle manufacturer says this is the specified speed rating for tires on that vehicle.

This is similar to SAE grading of bolts.  There are standards that have to be met, and the bolts would be tested against that standard and would be appropriate to use those bolts in places where an SAE standard grade bolt is specified.

This is not a grading of the performance level - it is a compliance with standards sort of thing.


jadcock (Civil/Environmental) (OP)
17 Jul 06 16:35
Thanks a lot for your continuing responses.

I had come to the table understanding, as you can probably tell, that the speed rating grade is/was strictly a performance assessment of the highest speed (in this case) of which the tire is capable.

This is how the UTQG ratings are assessed, is that correct, especially the traction/temperature ratings?  A government lab tests the tires, measures the result, and assigns a grade that correlates to the measured results?  The UTQG ratings are an "after the fact" kind of rating?

But I understand now how the speed ratings work -- they're more of a way (effectively) that different tires for different purposes are grouped.  If a manufacturer sets out to design an all season passenger car tire, they'll call it an S or T and make sure it's designed as such, even if the true performance may in fact be much higher.  Is my understanding correct...or at least more correct than when I first started?  :)

I see that some of Michelin's X Radial sizes bear a T rating, apparently for those vehicles which require that from the factory, as you mentioned above.  Since the P225/60R16 size only comes in an S rating in this tire model, could one correlate that there aren't really any (or many) OE applications in that size that require a T rating?  Or if there were, you'd think Michelin would have called it a T vice an S.

Thanks again.
CapriRacer (Mechanical)
18 Jul 06 7:27
Jadcock,

Now we're cooking!!!

"....This is how the UTQG ratings are assessed, is that correct, especially the traction/temperature ratings?  A government lab tests the tires, measures the result, and assigns a grade that correlates to the measured results?  The UTQG ratings are an "after the fact" kind of rating?...."

Kind of, but more that direction than not, especially compared to speed ratings.  There are some undervaluations in UTQG ratings in order to make a tire line fit into a marketing scheme.

"....But I understand now how the speed ratings work -- they're more of a way (effectively) that different tires for different purposes are grouped.  If a manufacturer sets out to design an all season passenger car tire, they'll call it an S or T and make sure it's designed as such, even if the true performance may in fact be much higher.  Is my understanding correct...or at least more correct than when I first started?  :)...."

Yup, that's it exactly.

"....I see that some of Michelin's X Radial sizes bear a T rating, apparently for those vehicles which require that from the factory, as you mentioned above.  Since the P225/60R16 size only comes in an S rating in this tire model, could one correlate that there aren't really any (or many) OE applications in that size that require a T rating?  Or if there were, you'd think Michelin would have called it a T vice an S....."

I suspect that this is the same problem we discussed earlier - that there is something about this size that makes it difficult relative to speed ratings.

To embellish this a bit - getting a T speed rating isn't much of a stretch and a T speed rating certainly doesn't restrict a tire to a particular market segment the way a V speed rating does.  So I suspect the S speed rating in a line of T rated products indicates there is something behind the lower rating and not just trying to avoid a certain type of vehicle fitment (or lack of fitments.)
jadcock (Civil/Environmental) (OP)
18 Jul 06 8:32
Thanks again CapriRacer.

Your comment about the P225/60R16 size being "problematic" is intriguing to me.  It's such a common size.  You didn't elaborate much previous, and if you don't want to, that's fine, but you're saying that there's something, physically, about that size that makes tire design "difficult"?

The previous tires I owned were the Michelin Pilots I mentioned earlier, in the P225/60R16 size.  They also sell some Z rated versions of the Pilots.  Maybe those things are SUPER overengineered.

I bought the X Radials because I wanted a better and quieter ride than the already very good Pilot tires...and some more treadlife.  I had one Pilot that blew a hole in the sidewall (actually, closer to where the sidewall meets the tread) and had to replace the entire set.  I put a whole new set of Pilots on the car, because the rest were aging as well.  My second set of Pilots were trouble-free, but at 40,000 miles, they were down to about 4/32" -- too thin for me.  That's pretty good wear for a 400/A/A tire on a heavy FWD car (Cadillac Seville), but I wanted a more compliant tire this time around.  I really like the X Radials -- and they seem to have about 80% of the handling capability as the Pilots, with a much more isolated ride.

The X Radials are 740/A/B tires, with an 80k mile warranty.  I don't expect to make that mileage, and in honesty, the tires will probably dry-rot before the tread wears down at the rate I'm putting the miles on lately.  The other tire I considered is the Michelin Energy MXV4 Plus.  I can get them in both H and V ratings in my size.  They were a pretty penny, though, and I didn't see the value added.  Perhaps if we had this conversation back in January when I bought my new tires, I'd have the Energys...or most probably the Pilots again due to the price.

Thanks again...!
CapriRacer (Mechanical)
18 Jul 06 9:41
For those that have stayed with the thread this long, here are some additional comments from my side:

It is not uncommon for folks to want longer wearing tires - Saving money and all that.  This is particularly true for OEM tires.

OEM tires:  Tires designed for Original Equipment are frequently "compromised" compared to Replacement market tires.  This compromise is usually in the area of Rolling Resistance.  Unfortunately, there is a tradeoff to obtain good rolling resistance values - tread wear and / or traction (especially wet traction).  So folks who are disappointed in the wear characteristics of their OEM tires (especially if they don't rotate their tires on their FWD car), will seek tires with better UTQG ratings - and that usually means S or T speed rated tires.  

The test that is used for speed rating worked well when it was invented in Europe.  However many folks take the rating as an absolute.  It is not!  That is, a tire with an S rating doesn't mean that it will perform under ALL conditions below 112 mph for the entire life of the tire.  Temperature playes a HUGE role in tire performance.  This is one of the lessons from the recent spate of tire failures.  

Since we can't undo the speed rating system, there aren't many alternatives to improving what the average consumer experiences.

One of those is government regulation.  There are places in this world where tires are required to have a minimum of an H speed rating (or something that eliminates everything lower than an H.)  The US populace has a real aversion to this type of regulation and seems prefer the market to do the regulating.

Nevertheless, the US government looked at quite a few issues and came up with following:

Tire Pressure Monitoring Systems - a technology that was emerging.  Inflation pressure was certainly one of the items proven to be a problem in the real world and the cost of these systems was relatively affordable.

More stringent minimum tests - In some respects this leads us towards higher speed ratings

Age related tests - Unfortunately nothing has emerged that satisfies the need to identify minimally acceptable aging.  The direction the regulators is headed is towards a regulation based on time, but this is problematic as well.
   
In the meantime, tire manufacturers have worked with the vehicle manufacturers to address issues in that arena.  One outgrowth is that vehicle manufacturers are starting to specify larger load capacities and higher speed ratings.

But the problem in the replacement market remains.  It's difficult to convince folks to buy more expensive tires, particularly when failure rates are so low (The average guy probably will never experience a tire failure so he can't imagine what the fuss is all about.)  Plus it is a commonly held belief that tire failures are caused by manufacturing defects - which is contradicted by comparing the failure rates of S rated tires to H rated tires.

Bottomline: Please don't take speed ratings as an absolute.  Please don't downgrade to a lower speed rating.  If you do downgrade, don't downgrade below an H rated tire.  

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