Strange steering wheel shimmy | FerrariChat

Strange steering wheel shimmy

Discussion in 'Technical Q&A' started by JohnMH, Dec 15, 2017.

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  1. JohnMH

    JohnMH Formula 3

    Jan 28, 2004
    1,883
    Bologna
    In prior posts in the TR / BB section I asked some steering rack questions about my 1991 Testarossa. I ended up replacing the rack as I could not source an inner tie rod to replace one with free play in it, in an effort to eliminate a steering shimmy which is felt whenever I reach 140 km/h.

    In summary, the shimmy is felt in the steering wheel (not the seat). Its frequency or severity is not affected by being on the throttle or the brake at that speed. It gets worse if I try to go faster than 140.

    To try to isolate the issue I have:

    Checked all 4 rims for straightness (the left front was slightly bent - fixed now)
    Installed 4 new michelins and had them balanced (twice) I typically run 2.5 bar front, 2.8 bar rear.
    Replaced a steering rack with a new rack
    Had an alignment done (twice)
    Cleaned and repacked all 4 CV boots (the rubber boots were split, but the hub bearings seemed fine)
    Rebuilt all 4 calipers

    All suspension and bushings were replaced on all 4 corners about 6 years and 10,000 km ago (all seem sound, no play or visible damage detected). All 6 Konis rebuild by Koni at that time (no visible leaks). Front steering ball joints look good and show no play when the car is in the air and the wheel is pushed and pulled.

    I have even checked to see if any sheet metal was loose under the front of the car which may cause a wind related issue. All seems tight there.

    With most wheel related issues I thought I would notice a rhythm in the shake which speeds up as the speed of the car increases. Not this time. The car is as smooth as glass until I hit the 140 barrier.

    The obvious choice is limiting it to 139, but what would be the fun in that?

    Any ideas?
     
  2. Mitch Alsup

    Mitch Alsup F1 Veteran

    Nov 4, 2003
    9,774
    Raise one wheel at a time.
    Place dial indicator on the outside rim and zero
    Rotate wheel and read out-of-roundness
    Place dial indicator on inside rim and zero
    Rotate wheel and read out-of roundness
    Repeat 3 times on other wheels.

    Something is non-concentric with the axle, it might be the wheels or it might be something between the wheels and the axle flange itself--something like the rotor hub, possibly even the axle hub where everything else bolts up to.

    Let's say a wheel displays out-of-roundness.
    Remove wheel
    Place dial indicator on rotor
    Rotate rotor and read out-of-roundness
    Remove caliper and rotor,
    Place dial indicator on the face of the axle flange and zero
    Rotate axle flange and read out-of-roundness.
     
  3. waymar

    waymar Formula 3

    Sep 2, 2008
    1,354
    Northeast, PA - USA
    Full Name:
    Wayne Martin
    Wheel bearing adjustment, toe adjustment...... sometimes doesn’t effect all speeds.
     

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