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Bad shimmy under braking is a clasic sign of warped brake rotors.


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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
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