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Thread: Should a Strut Bar "Push" or "Pull"

  1. #1
    Registered User Victor Laury's Avatar
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    Default Should a Strut Bar "Push" or "Pull"

    I have strut bars on both my Z car and 510, and everytime I set it up I wonder...

    Is it better to tighten the bar so it is pulling in, as in pulling the towers toward each other or, pushing out, as in pushing the towers further away from each other. It would seem to me that either way will add support to the suspension but, I wonder wich is better?

    This is not a test question.
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    Semi-retired admin Arne's Avatar
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    Victor, does the static position of the mount change when you jack up the car? Say you adjust it to be slack, neither push or pull, then jack up the car. If the bar is now tight, figure out if it's pushing or puling, then set the car back down and adjust the bar the opposite direction.

    I don't know that this method is "correct" but it seems to me that is makes sense. FWIW.
    Arne - Former owner, HLS30-37705, 7/71, 905 Red
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    The whole point of stut bars is to maintain the geometry of both sides making sure any movement is done equally. It shouldn't matter.

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    Registered User Jeff G 78's Avatar
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    I would say that the strut towers are more likely to sag towards each other due to body flex, than away from each other. Therefor, adjusting them to have a tiny bit of pressure outwards should make them the most efficient. You might want to take a measurement of the towers according to the FSM and see if the distance between them is too big, or too small. All of the dimensions are on one page in the body section.
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    Default

    Jeff is partially right. With our old cars and especially with high spring rates the strut towers do flex in when the car goes over bumps. That's kind of what you'd expect to happen. Cornering is different.

    When cornering the load is pushing on the bottom outside of the tire and that then pulls the top of the strut outward. I read this first in an old Porsche Owners Club magazine where they had done some testing with a tattle tale dial indicator and measured the flex before and after installing the strut tower bar on a 944. This was also on a BMW racing site, and for the life of me I can't locate that site anymore. May just not be on the internet anymore.

    When I set mine I always preload a little tension in the bar. Realistically I don't think it matters too much one way or the other though...
    Jon

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    WESTCOASTZRACER ron carter's Avatar
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    A slight amount of tension makes sense. Really you want to triagulate your strut bar, so that there is no movement.

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    Interesting side note - BRE called it a "spreader-bar" back in 73. Gives you an indication of it's purpose and set-up as far as they were concerned.

    FWIW,
    Carl B.

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    Supporting Member =Enigma='s Avatar
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    I don't know what the definitive answer is here, but I would probably start by setting it so that the measurement between the center of the two strut shafts is exactly 913mm, as specified in the BF-3 Body Dimensions diagram.
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    Registered User 240Z-Fan's Avatar
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    I think there are two functions to the strut bar that I can see:

    1) maintain the relationship between the left and right side suspension and therefore avoid bump-steer-like effects due to eaach side flexing independantly (note: this is not the classical bump steer we normally refer to but the effect is the same)

    2) reduce the positve camber on the outside wheel that would be induced during hard cornering. Accomplished by transfering the load to the inside (less loaded) strut tower

    In the case of '1', it doesn't really matter if the bar is preloaded one way or the other

    In the case of '2', preloading it in tension would be more effective at having the inside torsional stiffness help the outside...preloading it in compression would help the outside by virtue of the outside already be in the loaded position during transients.

    Not being a terribly consistent driver, I doubt I could ever tell the difference. I would aim for whatever setting gives me the best hood to fender gap

    Al

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    Last edited by bpilati; 03-12-2014 at 06:24 AM. Reason: note dead link
    Jon

  11. #11
    Supporting Member =Enigma='s Avatar
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    Interesting info. I'm a bit surprised at the conclusion and still trying to make sense of it, but the math appears to work.
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  12. #12
    Registered User 240ZX's Avatar
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    IMO,

    Simply put, a stut tower brace does nothing more than maintain the original (designed) geometric relationship between the stut towers, while under loading.

    Tom

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    Ha ha I like 240z-fan's answer. I agree with his hood gap theory!
    Nicely put. Only wish I'd thought of it...
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  14. #14
    Crumudgeon
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    If you want to install it the correct way (assuming a sqaure chassis):

    1. Level the car side-to-side and fore-aft on a set of slip plates and corner weight scales.
    2. Set the ride height and corner weights.
    3. Using a camber gauge and a level, set each wheel perpendicular to the ground and the chassis.
    4. Check the car level, ride height, and corner weights again. Adjust.
    5. Install the STB and adjust it so its tight but not affecting wheel camber, ride height, and/or corner weights.

    A STB is supposed to maintain chassis dimensions so that suspension geometry remains as consistent as possible. Without any data, just tensioning a STB until it "feels right" is most likely distorting the chassis.

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