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Model S Front Suspension

Discussion in 'Model S: Driving Dynamics' started by kennybobby, Nov 29, 2014.

  1. kennybobby

    kennybobby Member

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    Thanks to Edmunds for putting a car on the lift and taking these pictures.

    i thought it was interesting that there are two lower ball joints on the hub or steering knuckle, whereas most cars have just a single ball pivot.

    As the wheel is turned the knuckle rotates about the upper and lower pivot points. The line thru the upper and lower points is the steering axis and the inclination of that axis is related to the wheel caster.

    Edmunds indicates the lower pivot as the intersection of the contol arms.

    It looks to me that for rotation about two ball joints also requires a fore-aft translation of the bottom of the knuckle, effectively changing the wheel base to shorten it on the inner-side of a turn. However to do that requires a moment load about the attachment point of the control arm to the chassis. This is a thrust-like load that flexes and internally displaces the rubber bushings, as can be seen in the pictures in which the wheel is turned left and right.

    There is no control arm to carry the thrust loads although it looks like there is an unused hole that was drilled for one in the bottom knuckle plate.

    i predict that over time a shimmy will become manifest upon braking--a high frequency oscillation of the steering wheel that is quite startling when it occurs.
     

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  2. ecarfan

    ecarfan Well-Known Member

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    Thanks for the post and your analysis. I would note that there are a small number of S owners who have over 60K miles up to 100K and I have yet to hear of anyone experiencing the "shimmy" you describe. Perhaps with increasing numbers of high mileage cars it will show up.
     
  3. linkster

    linkster Member

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    #3 linkster, Nov 29, 2014
    Last edited: Nov 29, 2014
    kenny,

    Did Tesla/Brembo deploy those four rather large brake pad dampner/extension/heat-sink appendages on the front calipers to combat this high-frequency oscillation you speak of?
     
  4. Kbsilver

    Kbsilver Member

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    Suspension design with the double lower ball joints is similar to most BMWs.
     
  5. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    Early BMWs had a problem where if the tire/wheel/hub weren't perfectly balanced the steering wheel would twitch. There was no vibration, just a twitch.
     
  6. kennybobby

    kennybobby Member

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    After as little as 12k miles he experienced fender rubbing in reverse, this could be due to the play in the bushings as they loosen over time and allow greater fore-aft travel of the knuckle:

    Front Tires Rubbing on P85+ when reversing


    The 7-series bmw have steering wheel shimmy at ~55mph and under braking when the front thrust arm bushings get loose, occurs after about 30k miles. This is a recurring topic on bmw forums. i've replaced 4 sets over 120k miles.
     
  7. bxr140

    bxr140 Member

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    Audi also has been doing virtual pivots for quite some time. Like, since the 90s. It's not new tech.
     
  8. kennybobby

    kennybobby Member

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    Additional Pictures

    i would have added these up above with the others to make a more concise and coherent thread, but when i tried to edit i got this message:

    "The administrator has specified that you can only edit messages for 1440 minutes after you have posted. This limit has expired, so you must contact the administrator to make alterations on your message."

    So here are some of the components of the lh front suspension, upper control arm, fore and aft lower arms, spindle and steering knuckle. Notice the sliding bushing in the aft lca where the bottom of the shock attaches, and the webbed rubber in the fore lca bushing at the chassis end--very similar to the bmw version. Also notice that thrust loads are carried by torsion of the rubber bushings--there are no metal thrust carriers except maybe the UCA wishbone holding the pivot although it is bushing mounted too.

    The big steering knuckle casting is made of an aluminum-7% silicon- magnesium alloy.
     

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  9. wycolo

    wycolo Active Member

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    #9 wycolo, Dec 18, 2014
    Last edited: Dec 19, 2014
    [Front Tires Rubbing on P85+ when reversing]

    Yeah, I didn't like the sound (and implication) of that one. Wonder if it is P+ specific. Used to be urethane was always better than 'rubber' (the black stuff) being stiffer. TM might not want to be that stiff actually.

    [edited 18 hours after initial post]
    --
     

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