Date: Fri, 29 Oct 1999 14:28:10 -0400
Reply-To: David Beierl <dbeierl@IBM.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@IBM.NET>
Subject: Uniform Tire Quality Grading, was: Michelin MXT Specs.
In-Reply-To: <3819D084.E9D628BE@bama.ua.edu>
Content-Type: text/plain; charset="iso-8859-1"; format=flowed
Hi Joel --
At 12:51 10/29/99 , Joel Walker wrote:
>nope. not exactly. each manufacturer has its own 'standard
>tire' ... which is why the temp and wear ratings are
>garbage. you cannot compare tires between manufacturers,
>only within one manufacturer ... and all it tells you is
>that tire A MIGHT wear longer than tire B.
>
>you have to read the small print VERY carefully ... even on
>the government stuff. they'd like you to think its a
>quantitative rating, but it isn't. :(
Taking your advice, I read the small print (Title 49, Code of Federal
Regulations, Part 575.104). To sum up:
Traction ratings are measured against the "ASTM E501 standard tire for
skid resistance tests" which is tested along with the "candidate tire" and
the results used to adjust for local test conditions. The actual rating is
a measurement of absolute coefficient of friction in locked-wheel
conditions on concrete and asphalt surfaces wetted according to ASTM specs.
Wear rating tests are conducted on a specific 400-mile course in Texas, and
use the "NHTSA Course Monitoring Tire" to adjust for day-to-day variations
in test conditions on that course. This tire must be bought from NHTSA at
the course location. The actual rating is the absolute expected life of
the tire under test conditions, expressed in increments of 20% and rounded
down to the nearest 20%, where 30,000 miles equals 100%. The test is 6,400
miles in duration. After each 400 miles the tires are rotated on the
vehicles in a specified pattern. After each 800 miles, 1) the vehicles
(but *not* the drivers) are rotated in the convoy, 2) vehicle alignment is
checked and if necessary returned to the original setting, usually midpoint
of the mfr. spec. 3) wear is measured to the nearest .001 inch. Drivers
travel at posted speed limits and there is a specified procedure for
stopping at lights and stop signs, based on signs posted at specific
distances from each stopping point.
Temperature ratings are conducted by rotating a loaded tire against a large
steel wheel for specified rpm and durations. Ambient temp is 95 degrees
F. They begin by 250 rpm for two hours, then let the tire cool back to
ambient. After that it's 375 rpm for half an hour, then up 25 rpm for half
an hour and continue up to 575 rpm for half an hour. At each stage it's
evaluated for failure as follows:
"tire is considered to have successfully completed
a test stage in accordance with this
paragraph if, at the end of the test
stage, it exhibits no visual evidence of
tread, sidewall, ply, cord, innerliner, or
bead separation, chunking, broken
cords, cracking or open splices as de-fined
in § 571.109 of this chapter, and
the tire pressure is not less than the
pressure specified in paragraph (g)(1) of
this section."
If it fails 500 rpm that's a C
If it passes 500 rpm that's a B
If it passes 575 rpm that's an A.
So anyway, based on all that I think that the tire grades are in fact an
objective measurement of relative performance of tires *under the specified
test conditions.* Thoughts, anyone?
david
David Beierl - Providence, RI
http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"
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