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On Tue, 28 Feb 2006 08:46:10 GMT, "Stuart A. Bronstein"
<spamtrap@lexregia.com> wrote:
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>Can you help me diagnose what is causing heavy shimmy upon braking?
>
>My '98 4Runner 2WD with about 50K miles is vibrating heavily when braking
>at highway speeds. It does not vibrate under any other conditions.
>
>The cause is perplexing me.[/color]

99% of these are rotor problems.
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>Here is all that I know.
>
>- The 2WD 4Runner is not equipped with ABS braking[/color]

Irrelevant. When ABS kicks in you know it.
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>- The four wheel alignment was done about two years ago[/color]

Irrelevant
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>- New tires about a year ago were dynamically balanced when mounted[/color]

Irrelevant if problem occurs only on braking.
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>- The front rotors are smooth with only a single circular scratch in one[/color]

Scratches are irrelevant.
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>- That one rotor scratch is so thin a pen tip won't even catch on it
>- The rotors are 0.860 & 0.855 inches thick (min=0.787 inches)[/color]

So they could be turned if you choose that route.
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>- The pads are about 1/4 inch thick (minimum = 0.039 inches)[/color]

Pad thickness is irrelevant to pulsation problems.
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>- Disk runout was 0.002 & 0.0025 inches (min=0.0028 inches)[/color]

Surely you meant MAX=.0028 inches. What is the thickness variation?
That's more critical than runout.
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>- The rear brake drums were not checked[/color]

Check the drums unless the problem includes steering wheel wobble
induced while braking in which case it is the front brakes for sure.
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>- I did not perform any other tests as I don't know of any other tests
>
>One wierd thing happened while checking disk runout. There was a repeatable
>half of a thousanth of an inch "blip" when I rotated the rotors. It was
>always in the same spot on the passenger side rotor.[/color]

Now you are looking at the problem! Get the rotors turned or replace
them.
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> I could hear a
>scratching sound for a second when I saw the blip on the dial guage. Funny
>thing was I could not see any "dent" or imperfection in the rotor at that
>point.[/color]

Its not easy to see half a thousandth. But a half thousandth
thickness variation will induce brake pulsation.
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> This rotor wasn't even the scratched rotor so it looked nearly brand
>new (even after 50K miles).
>
>Other than that blip, everything seemed normal.
>
>At first I checked runout without removing the pads and without putting the
>lug nuts on but the runout was changing until I realized the rotors were
>only held on by the lug nuts so I removed the pads and jammed a screwdriver
>into the rotor fins so I could torque down (83 foot pounds) the lug nuts
>with a washer to prevent cracking the disks.
>
>Do you have any idea what test to run to determine the cause of vibration?[/color]

You could check the rears by applying the parking brake at speed. Its
sort of tricky to do safely. Just get the rotors turned or replace
them and be done with it. If you replace them it will be a lot longer
before the pulsation comes back. Quality rotors such as Mountain or
Brembo brand or Toyota original equipment will go MUCH longer before
developing annoying pulsation than cheap units made in China. Quality
rotors should NOT be resurfaced out of the box. If somebody tells you
"See they have runout and need to be machined!" what is really
happening is they have a worn spindle or cones on their lathe and they
will machine runout into what was a quality piece. Shipping and
handling does NOT knock rotors out of round as was tested by Brake and
Front End magazine several years ago.

Don
[url]www.donsautomotive.com[/url]
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>Stu[/color]