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On Sun, 05 Mar 2006 18:21:06 GMT, "Stuart A. Bronstein"
<spamtrap@lexregia.com> wrote:
[color=blue]
>"davidj92" <davidj92REMOVE@sigecom.net> wrote in
>news:l9GdndF_XfPAvpbZRVn-gw@sigecom.net:
>[color=green]
>> It seems to me you've found one article that you want to believe over
>> all the other articles available ... [you should] believe the majority
>> opinion ... is true.[/color]
>
>
>Hi David,
>
>Actually I found many articles, some with conflicting ideas.
>
>For example, some say we should replace with softer brake pads, others
>say we should always true brand new rotors, other say never true rotors
>off the vehicle, others say use the parking brake instead of the pedal
>when stopped at a light due to cooling differences, etc.
>
>The majority is often wrong, by the way. The majority will tell you, in
>and of itself, that high octane gas is "better" gas just like the
>majority of chiropracters will tell you your spine needs "adjustment".
>
>There is only one truth, and I'm simply searching for that one truth.
>
>Here's a quick summary to date of some of those articles
>Stu
>
>
>What causes high-speed brake induced shimmy?[/color]


Generally the last 3 articles pretty well nail it, while many of the
others contain truth.

Rotor quality is one BIG variable - and todays rotors are not "aged"
before finishing. The "green" castings relieve after machining - so
often arive out of true from the factory.
Warpage, in itself, seldom causes pedal pulsation OR steering shimmy
unless it is quite sizeable. Thickness variation causes problems even
if very close in tolerance.
Vented rotors often have "callapse" problems, where several "fins"
move in tolerance as the rotor ages, or in use.
Particularly in the "salt belt" todays pads often cause a "glaze" on
the rotors, which traps moisture and pits the rotor away. When it
pits, the oxide layer expands behind the glaze, clausing "blisters"
that cause pulsations.

Machining the rotor temporarily solves the problem, but reduced
thickness and mass makes warpage more likely.

As for surfacing new rotors, SOME need surfacing. On Car machining
will usually give you a truer disk - but since runout is NOT the big
problem, off-car machining on a good lathe will sometimes give better
parallellism than on-car machining, where vibrations can have an
effect.
Pad composition is one thing you CAN change to prevent or greatly
reduce break pulsation problems. Anything with an iron metallic is
going to cause problems in salt-belt areas. Brass metallic is MUCH
better, from early Toyota experience, and carbon metallic or ceramic
pads have proven to be a great improvement in my experience with later
model Ford and GM vehicles.

Carbon Metallic pads on my 90 Aerostar improved rotor life by over
400%, and pad life by 100% over factory parts. Brake effectiveness was
also GREATLY improved - allowing me to actually lock the front wheels
on dry pavement, which was absolutely impossible with the factory
pads.[color=blue]
>
>Vibration felt in the steering wheel only when the brakes are
>applied is not a front end alignment problem, but a brake problem.
>[url]http://www.trustmymechanic.com/besttires.html[/url]
>
>The steering wheel is vibrating because the front brake
>rotors are warped (we call this vibration "shimmy").
>[url]http://www.trustmymechanic.com/brakewarp.html[/url]
>
>Cold judder occurs primarily as a result of a non-uniform
>circumferential rotor (friction ring) thickness, which causes a
>cyclic variation in the brake torque output during braking (1 -
>2). These microscopic variations in the cross-sectional thickness
>of the disc brake rotor, axiomatically referred to as Disc Thickness
>Variations (D.T.V. or R.T.V. (Rotor Thickness Variation)) may arise
>during rotor manufacture as a product of the machining process
>(typical manufactured D.T.V. < 7 m), or, as laboratory and field
>trial testing have demonstrated, may be generated throughout their
>lifetime in-service.
>[url]http://www.eurac-group.com/technote4.htm[/url]
>
>Warping can be caused by excessive heat build up, which softens the
>metal ... however with most ventilated discs ... the sensation of
>warped brakes (wheel shimmy under braking) most often is a matter of
>a brake pad material operating outside of its designed temperature
>range and it has left a thick(er) than normal deposit in one area
>of the disc surface, creating a "sticky" spot that will grab every
>revolution of the disk. In cars with automatic transmissions the
>driver applies brakes when the car is stopped ... the brake pads
>remain in contact with the disc and the discs will cool unevenly
>... leading to warping.
>[url]http://en.wikipedia.org/wiki/Disk_brake[/url]
>
>Wheel shimmy during braking is often caused by thickness variation
>of the rotor disc. If the rotor has runout, a thin spot will develop
>by the continuous touch touch touch as the rotor turns while the
>brakes are not applied. When this thickness variation increases to
>approximately 0.007 inch, the pulsation can be felt by the driver.
>[url]http://en.wikipedia.org/wiki/Disk_brake[/url]
>
>The accepted cause of brake-induced vibrations is disc thickness
>variation. Disc thickness variation generated by off-brake running,
>uneven transfer of lining material to the disc surface, disc
>corrosion, and distortion of the disc under thermal loading. The
>variations in rotor disc thickness cause the brake fluid pressure
>in the caliper to fluctuate, resulting in torque variations.
>[url]http://support.mscsoftware.com/cgi-bin/kb_files/Ford_Brake_Roughness_[/url]
>2001_NAUC.pdf
>
>Pre-loaded wheel bearings have no end play to "absorb" hub and rotor
>run-out. Hence, almost 100% of any axial run-out of the hub and
>brake rotor are transmitted to the brake pads. This axial run-out or
>wobble in the rotor causes the brake pads to wear the rotor unevenly
>over time, producing two sections of the rotor, 180 degrees apart,
>where the rotor thickness becomes thinner than the other two sections.
>This difference in thickness is called Disk Thickness Variation or DTV.
>[url]http://www.rtitech.com/latheinfo.htm[/url]
>
>In general, any run-out greater than 0.002" (50 microns) will
>lead to an increase in DTV of about 0.0004" (10 microns) in about
>3000-5000 miles. In most cars, when DTV reaches 0.0004" (10 microns)
>or only 4 ten-thousandths of an inch, the driver will complain of
>pedal pulsation, steering wheel shimmy, or brake shudder. The most
>important fact to consider here is that the installation of the
>wheel will almost always increase hub/rotor run-out by 0.001-0.0015"
>(25-40 microns), even if the lug nuts are carefully torqued.
>[url]http://www.rtitech.com/latheinfo.htm[/url][/color]

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