Uniform Tire Grades - Tire Engineers

wantin150 · Apr 10, 2007

On the way in to work this morning, while driving in the rain, a couple of questions came to mind concerning the way our tires are "graded". I know we have discussed the treadwear value in the past but I don't think that I have recently seen anything on the traction and temp ratings.

I guess what I am really looking for are numbers to associate with the rating. We have "AA" , "A" , "B" , "C" for traction and "A" , "B" , "C" for temp. Are there values related to the test tire for temp/speed/time? Are there stopping distances to meet certain ratings (or friction coefficients), tread compounds, construction?

Obviously there is something to separate these ratings but are they really discernable to the average driver? Why do OEM car companies use a "lesser" graded tire in some applications (i.e. my Tacoma came stock with "B" , "B" traction/temp Firestone HT's)? I know why they use lower treadwear rated tires, just not the other.

All answers greatly appreciated.

Quattro Pete · Apr 10, 2007

I'm not a tire engineer, but here's some basic info for starters:
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=48

Note that the traction ratings refer to wet traction only.

CapriRacer · Apr 10, 2007

QuattroPete gave you a link to what Tire Rack has to say on the subject. Unfortunately, I think they got a few things wrong and those things "colored" their conclusions.

Unfortunately, I am having trouble finding my copy of the test procedures, and while it is important to "get this right", I also feel it is more important that wrong information be challenged as early as possible, so please forgive me if I do this from memory.

Here is what I think they got wrong:

The "Skid" in the traction test. What really gets measured is the peak force, which occurs at about 10% slip - meaning the tire is turning, so I don't think their comment about measuring only tread compound is correct.

They indicated that the Temperature Test on a measure of a tire ability to dissapate heat. In reality the test is step speed test, very similar to the test for speed ratings. My opinion is that when these tests were being debated, there was a very strong "Not Invented Here" resistance to using European Speed Ratings.

I'll post back after I find my copy of the test procedures to let you guys know how good my memory is.

Quattro Pete · Apr 10, 2007

Thanks Capri for clearing things up. TR usually puts a marketing spin on everything, so I'm not surprised of the 'coloring'. Looking forward to those test procedures myself.

rpn453 · Apr 11, 2007

I'm only willing to use traction A or higher. I think the traction B rating on many OEM tires is due to the manufacturer looking for hard, thinly-treaded rubber for good fuel economy. I've driven two vehicles with tires that had that traction B rating and they were so slippery on wet or dry pavement that I consider them unsafe. I'm willing to gamble the cost of a decent set of tires on whether I'll need traction in an emergency situation.

wantin150 · Apr 11, 2007

This is one of the reasons I am asking. The OEM Firestone Wilderness HT's have XXX treadwear (I will replace at 45,000 miles and 3 or 4/32 remaining), a B traction, and a B temp. The dry traction of these tires is pretty decent for a tire on a compact pick-up (2004 Tacoma). New tread depth is either 10 or 11/32. Aftermarket Wilderness HT's are all rated A traction, except the 205-75-15 size which is my OEM. I'm replacing them with a set of Dunlop Radial Rover A/T's (unk, A, B) in a 225-70-15.

I'm getting ready to change the tires on my Matrix from a Goodyear Eagle F1 DSG3 94W (260? AA, A - will have 40,000 at replacement) to a Dunlop Direzza 94W (unk , A, A). I mean really, would the average consumer really notice a difference (granted we are going from a "Max" performance to an "Ultra High" performance tire, but still)? Price difference is $1200 vs about $650.

I briefly read the link posted (I don't know how I missed that one at TR as I frequent that site, so thanks.) and am not really 100% satisfied. I want real world numbers.

GC4lunch · Apr 11, 2007

Quote:

Unfortunately, I am having trouble finding my copy of the test procedures, and while it is important to "get this right", I also feel it is more important that wrong information be challenged as early as possible, so please forgive me if I do this from memory.

The procedures are published in Title 49 of the Code of Federal Regulations (CFR) as Part 575.104. The procedures are hard to read in CFR form and are available in a more readable form (though lacking section and subsection numbering) as a .pdf file here:
www.nhtsa.dot.gov/staticfiles/DOT/N... Procedures/Associated Files/TP-UTQG-T-01.pdf The relevant page is page 17 of the .pdf file.

Quote:

Here is what I think they got wrong:

The "Skid" in the traction test. What really gets measured is the peak force, which occurs at about 10% slip - meaning the tire is turning, so I don't think their comment about measuring only tread compound is correct.

The test procedure specifically requires that the wheel be locked, and that the measurement does not commence until a minimum of one-half second after the wheel has been locked:

Quote:

Quote:

"Tow the traction trailer onto the asphalt test surface at a speed of 40 +0, -1 mph. Lock the wheel and record the locked wheel traction coefficient between 0.5 and 1.5 seconds after lockup. Enter SN on Flight Plan and Data Sheet after each skid."

Click to expand...

This part of the test procedure renders the Traction rating of questionable validity for tires mounted on vehicles with an antilock braking system.

GC4lunch · Apr 11, 2007

Quote:

I'm getting ready to change the tires on my Matrix from a Goodyear Eagle F1 DSG3 94W (260? AA, A - will have 40,000 at replacement) to a Dunlop Direzza 94W (unk , A, A). I mean really, would the average consumer really notice a difference (granted we are going from a "Max" performance to an "Ultra High" performance tire, but still)? Price difference is $1200 vs about $650.

The terms "Max" performance and "ultra high" performance are Tire Rack's terms. All tires have strengths and weaknesses, but the strengths of one tire may be the weaknesses of another tire. An average consumer probably would notice differences between the Goodyear Eagle F1 GS-D3 and the Dunlop Direzza, but he (or she) would be noticing the different strengths and weaknesses of very different tires, which are not necessary the factors that cause one or the other to fall within a specific Tire Rack category. For instance, the Tire Rack categories do not directly take into account noise or ride harshness, which can be very different between different tire models.

CapriRacer · Apr 11, 2007

GC4lunch,

Thanks for the link.

I stand corrected. I must have been thinking about GM's test for traction.

Editorial comment: This seem to be another example in the UTQG ratings where NHTSA specifies something that others have done better, but seems to try to re-invent it.

Bamaro · Apr 11, 2007

With the price of gas and environmental as well as geo-political concerns, I wish they would rate tires for rolling resistance.

rpn453 · Apr 12, 2007

Quote:

I'm getting ready to change the tires on my Matrix from a Goodyear Eagle F1 DSG3 94W (260? AA, A - will have 40,000 at replacement) to a Dunlop Direzza 94W (unk , A, A). I mean really, would the average consumer really notice a difference (granted we are going from a "Max" performance to an "Ultra High" performance tire, but still)? Price difference is $1200 vs about $650.

I briefly read the link posted (I don't know how I missed that one at TR as I frequent that site, so thanks.) and am not really 100% satisfied. I want real world numbers.

From the TR chart, a traction A tire could have anywhere from 2% to 36% better wet traction than a traction B tire. A traction AA tire will have at least 2% better wet traction than a traction A tire. A traction AA will have at least 17% better wet traction than a traction B tire. So it is pretty vague.

Of all the tires I've driven on, only those 2 sets of traction B tires have been unacceptable to me for non-winter driving, and would break loose for me under normal aggressive driving. All the traction A tires I've used have been fine, so I wouldn't pay a huge premium to get the AA rating. It is something I prefer though, and I definitely notice the extra grip of my AA Michelin Pilots when I'm taking off quickly.

benjamming · Apr 12, 2007

Quote:

With the price of gas and environmental as well as geo-political concerns, I wish they would rate tires for rolling resistance.

CR rated RR in their Nov-2006 & Nov-2004? tire reports. I have no idea as to the methodology they used to derive their RR ratings.

GC4lunch · Apr 12, 2007

Quote:

From the TR chart, a traction A tire could have anywhere from 2% to 36% better wet traction than a traction B tire. A traction AA tire will have at least 2% better wet traction than a traction A tire. A traction AA will have at least 17% better wet traction than a traction B tire. So it is pretty vague.

Why rely upon a "vague" Tire Rack chart? Go to the original source, the NHTSA regulations.
Here they are.

Quote:

(ii) Traction. Each tire shall be graded for traction performance with the word "TRACTION," followed by the symbols AA, A, B, or C, when the tire is tested in accordance with the conditions and procedures specified in paragraph (f) of this section.
(A) The tire shall be graded C when the adjusted traction coefficient is either:
(1) 0.38 or less when tested in accordance with paragraph (f)(2) of this section on the asphalt surface specified in paragraph (f)(1)(i) of this section, or
(2) 0.26 or less when tested in accordance with paragraph (f)(2) of this section on the concrete surface specified in paragraph (f)(1)(i) of this section.
(B) The tire may be graded B only when its adjusted traction coefficient is both:
(1) More than 0.38 when tested in accordance with paragraph (f)(2) of this section on the asphalt surface specified in paragraph (f)(1)(i) of this section, and
(2) More than 0.26 when tested in accordance with paragraph (f)(2) of this section on the concrete surface specified in paragraph (f)(1)(i) of this section.
(C) The tire may be graded A only when its adjusted traction coefficient is both:
(1) More than 0.47 when tested in accordance with paragraph (f)(2) of this section on the asphalt surface specified in paragraph (f)(1)(i) of this section, and
(2) More than 0.35 when tested in accordance with paragraph (f)(2) of this section on the concrete surface specified in paragraph (f)(1)(i) of this section.
(D) The tire may be graded AA only when its adjusted traction coefficient is both:
(1) More than 0.54[mu] when tested in accordance with paragraph (f)(2) of this section on the asphalt surface specified in paragraph (f)(1)(i) of this section; and
(2) More than 0.38[mu] when tested in accordance with paragraph (f)(2) of this section on the concrete surface specified in paragraph (f)(1)(i) of this section.

As I noted in an earlier message, NHTSA's mandatory UTQG test procedure to determine the Traction grade (which was adopted many years ago) involves locking the wheel on which the tire is mounted, and therefore the test procedure returns results of dubious worth in relation to how well the tire will brake when mounted on a vehicle with an antilock braking system (ABS), as most new cars are these days.

Moreover, as I have noted in an another thread, the NHTSA UTQG test procedure inadvertently covers up the great weakness of all-season tires -- that they retain water on their tread (necessary for snow traction), creating (in wet, but nonsnowy conditions) a lubricating film of water between the tire and the pavement at the tire's contact patch. Because the wheel is locked and the contact patch is dragged 29 to 30 feet across the pavement before measurements are commenced, the retained film of water on the tread will be scrubbed off at the patch of contact between the tire and the pavement; in contrast, on a vehicle with ABS, the wheel and tire will continue to rotate under hard braking, constantly presenting a fresh contact patch which, on an all season tire, will have a film of water adhering to it.

Quote:

All the traction A tires I've used have been fine, so I wouldn't pay a huge premium to get the AA rating. It is something I prefer though, and I definitely notice the extra grip of my AA Michelin Pilots when I'm taking off quickly.

At certain traffic lights in our neighborhood here in Portland (Oregon) where we come to a stop facing uphill, when we were driving on the Traction A Michelin Pilot HX MXM4 all season tires that came on our Mazda6 from the factory, the front tires spun every time (even with careful feathering of the accelerator) when we accelerated uphill from a full stop when the pavement was wet. In contrast, the Traction A Dunlop SP Sport 01 three-season tires mounted on our Nissan Maxima (which has higher torque and a lower first gear ratio than the Mazda6, both of which make the Maxima more of a jackrabbit from the stoplight than the Mazda6) never spin in the same situations at the same stoplights, nor do the Goodyear Eagle F1 GS-D3 three-season tires that we replaced the all-season Michelin tires with on the Mazda6 (though the latter are AA rated).

rpn453 · Apr 12, 2007

Quote:

Why rely upon a "vague" Tire Rack chart? Go to the original source, the NHTSA regulations.

What I meant by "vague" is that, since they allow quite a range of friction coefficients for each rating, the differences between different traction ratings could be large or small, depending on the tires. It sounds like you've observed that directly.

I realize the tests are flawed, but I think they do give some indication of traction.

GC4lunch · Apr 12, 2007

Quote:

Quote:

Why rely upon a "vague" Tire Rack chart? Go to the original source, the NHTSA regulations.

Click to expand...

What I meant by "vague" is that, since they allow quite a range of friction coefficients for each rating, the differences between different traction ratings could be large or small, depending on the tires. It sounds like you've observed that directly.

What I have observed directly is that the Traction test procedures fail to measure how a tire operates on a vehicle equipped with ABS. I have no doubt that the Traction A-rated Michelin Pilot HX MXM4 and the Traction A-rated Dunlop SP Sport 01 tires could have measured identically on the NHTSA test; maybe they did. But tests that consider only how a tire brakes when the wheel is locked and after it has been dragged 29+ feet across the pavement while locked with the same part of the tread as the contact patch during that time do not measure how a tire will brake on a car that has ABS. It is an independent variable.

Quote:

I realize the tests are flawed, but I think they do give some indication of traction.

I will grant you the "some" part.

wantin150 · Apr 12, 2007

Thank you all for all of the great responses. Couple more (hopefully quick)questions:

I noticed in the information posted by GC4Lunch that the final coefficient to determine rating is an "adjusted traction coefficient". I take it to mean that the road surface friction coefficient is also taken into account? This may be getting into more math than I want to, but how is the adjustment calculated?

GC4lunch · Apr 12, 2007

Quote:

I noticed in the information posted by GC4Lunch that the final coefficient to determine rating is an "adjusted traction coefficient". I take it to mean that the road surface friction coefficient is also taken into account? This may be getting into more math than I want to, but how is the adjustment calculated?

Quote:

(vii) Average the 20 measurements taken on the asphalt surface to find the standard tire traction coefficient for the asphalt surface. Average the 20 measurements taken on the concrete surface to find the standard tire traction coefficient for the concrete surface. The standard tire traction coefficient so determined may be used in the computation of adjusted traction coefficients for more than one candidate tire.
_
(viii) Prepare two candidate tires of the same construction type, manufacturer, line, and size designation in accordance with paragraph (f)(2)(i) of this section, mount them on the test apparatus, and test one of them according to the procedures of paragraph (f)(2)(ii) through (v) of this section, except load each tire to 85% of the test load specified in Sec. 575.104(h). For CT tires, the test inflation of candidate tires shall be 230 kPa. Candidate tire measurements may be taken either before or after the standard tire measurements used to compute the standard tire traction coefficient. Take all standard tire and candidate tire measurements used in computation of a candidate tire's adjusted traction coefficient within a single three hour period. Average the 10 measurements taken on the asphalt surface to find the candidate tire traction coefficient for the asphalt surface. Average the 10 measurements taken on the concrete surface to find the candidate tire traction coefficient for the concrete surface.
_
(ix) Compute a candidate tire's adjusted traction coefficient for asphalt ([mu]a) by the following formula:

([mu]a) = Measured candidate tire coefficient for asphalt+0.50 -Measured standard tire coefficient for asphalt
_
(x) Compute a candidate tire's adjusted traction coefficient for concrete ([mu]c) by the following formula:

[mu]c=Measured candidate tire coefficient for concrete +0.35 [mu]Measured standard tire coefficient for concrete

Uniform Tire Grades - Tire Engineers

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