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"Stuart A. Bronstein" <spamtrap@lexregia.com> wrote in message
news:m5UMf.17396$2O6.6452@newssvr12.news.prodigy.com...[color=blue]
> 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.
> Here is all that I know.
>
> - The 2WD 4Runner is not equipped with ABS braking
> - The four wheel alignment was done about two years ago
> - New tires about a year ago were dynamically balanced when mounted
> - The front rotors are smooth with only a single circular scratch in one
> - 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)
> - The pads are about 1/4 inch thick (minimum = 0.039 inches)
> - Disk runout was 0.002 & 0.0025 inches (min=0.0028 inches)
> - The rear brake drums were not checked
> - 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. 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. 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?
>
> Stu
>[/color]
You can narrow down which end, if a brake problem, by using
emergency
brake--BE CAREFUL--to slow vehicle. If shimmy still shows, it's the rear;
else, it's the front. And if you find a caring soul who turns rotors, he
can, by trial-and-error, fasten the rotor on the lathe so as to minimize the
material needed to remove the runout you do have. (While rotors are being
turned, it won't hurt to 'plane' your disc pads on a flat concrete surface.
I've seen a few times where turning rotors didn't eliminate all the
pulsating; whereas, going back and planing the pads corrected it.) HTH, s