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Thread: Spring Preload?

  1. #1

    Spring Preload?

    All, I have a complex question about spring preload and its' effects, hoping someone has some solid info.

    The front strut on a BMW is an interesting spot. You want to lower the car to improve the CG, while at the same time keeping the spring perch above the tire to maximize inboard tire space. The effect of this is that my struts have a very short total travel. With the damper part of the strut fully extended, I have around 2.0" of distance between the strut body and the bumpstop. (total travel)

    Preloading the spring is the only way to adjust ride height or bump/droop travel. So depending on how high I set the spring perch on the threaded strut body, the spring will compress, and the car will sit higher or lower at rest. (static ride height) Depending on where the car sits, I could have 1" of each bump and droop travel, .5" bump & 1.5" droop, or 1.5" bump and .5" droop, you get the idea. I get one or the other, depending on where the car is sitting, which is determined by spring preload.

    My corner weights are 885 & 829 for the front, and I'm using 5.5" 650lb hyperco springs. I have a solid inch or so I can move the spring perch up before I get to the top of the strut body.

    So without any preload, (I can turn the springs on the perches by hand, but no up/down play) at full droop (wheel off the ground) I have 13.75" of ride height. (center of wheel to fender) At rest it's at 12.5" ride height, so I have about .75" bump travel and 1.25" droop travel. (Meaning the springs compress about 1.25" with the car sitting static)

    So my actual question is, if I preload the spring, and raise the car, giving me more bump travel, does the car actually have farther to roll, and will it roll the same amount, or less? Am I making the 650lb spring act like say a 750lb spring, or am I actually reducing the effective spring rate, and just changing the roll starting point?

    So for an example/exercise, what is the difference between a 6" 650lb spring compressed to 5.5", and a 5.5" 650lb spring with no preload?

    The goal is to lower the car as much as possible, (advantageous CG) without either getting into the bumpstops or getting coil bind. Control arm geometry, from all reports, does not seem to be a big problem on the front of the E46.

    It seems to me that if I want to avoid getting into the bumpstops, I should preload the crap out of the springs, (1"+?) or use a bigger front bar or stiffer springs or all of the above. But if preloading just sets the "starting point" for the roll, and the car will roll to the same overall position, it won't matter where I set the preload.

    This is sort of a followup to the troubles I had last season with warm-weather push when I had 550lb springs up front, I was getting into the bumpstops whenever grip was high. at 650lb springs and stiffer bar settings, the cable ties say I'm getting right up to the bumpstops, but not as bad, and the car is definitely behaving better. I want to make sure this won't be a problem when I go to Milwaukee or Lincoln,
    BMW 330i #313 ///MAC

    Quote Originally Posted by Rex Jr
    Lord, grant me the courage to trail-brake where I can, to accept that the timing light is correct, and the wisdom to know where to apex...

  2. #2
    Matt, I've not read the entire post in detail but I'll take a shot with the couple minutes I have left on line.

    The preload will not change the rate. If you want to avoid the bump stops while reducing the travel available you'll have to increase roll stiffness with either a stiffer FSB or higher rate springs. The preload will only impact the static or nominal position of the suspension and the corner weights.
    Jay McKoskey
    1991 Miata #63/163

  3. #3
    When/why would more preload than enough to seat the spring be desirable?

    Also my understanding is if a 500lb/in spring is "preloaded" 1" it would then take greater than 500lbs of force to compress it any further. Correct or incorrect?
    1999 miata |2009 Xterra | 2008 WR450 SM

    Quote Originally Posted by Hank Hill on Christian rock
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  4. #4
    I started a similar thread a couple years ago which basically resulted in a lot of "it doesn't matter on a car" responses

    suspension preload thread link

    also, I agree with Ted
    IE - on a motorcycle if you are taking a passenger the manufacture suggests increasing the preload to prevent the suspension from bottoming out
    Last edited by seannabend; 06-07-2010 at 12:45 PM.
    [] '95 BMW M3 [] '02 VW GTI 337 [] '98 Subi 2.5RS [] '03 Suzuki SV650 [] '77 BMW R60/7 []

  5. #5
    hmm, I think I may have answered my own question, via the power of the interwebs.

    http://forum.miata.net/vb/archive/in.../t-309892.html

    What these folks (and other sources I've found) say is that preload only affects ride height (and by extension the relationship of bump to droop traval) up to the point where you have actually preloaded more than your corner weight. (in my case that would be around 1.25")
    BMW 330i #313 ///MAC

    Quote Originally Posted by Rex Jr
    Lord, grant me the courage to trail-brake where I can, to accept that the timing light is correct, and the wisdom to know where to apex...

  6. #6
    Quote Originally Posted by Teddy.B View Post
    When/why would more preload than enough to seat the spring be desirable?
    It's the only way to change ride height and to change the relationship of bump and droop strut travel on my struts.

    Also my understanding is if a 500lb/in spring is "preloaded" 1" it would then take greater than 500lbs of force to compress it any further. Correct or incorrect?
    Yes, so assuming your corner weight is 750lbs, (easy math) without preload the spring would compress 1.5" when sitting on the ground, and with 1" of preload, it will compress another .5".
    BMW 330i #313 ///MAC

    Quote Originally Posted by Rex Jr
    Lord, grant me the courage to trail-brake where I can, to accept that the timing light is correct, and the wisdom to know where to apex...

  7. #7
    Fourth Gear marks_lude's Avatar
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    I don't under stand how preload is possible? how could you add enough force to the spring to compress (preload) it another inch? that would require more weight on that corner.
    Quote Originally Posted by murph View Post
    It's the only way to change ride height and to change the relationship of bump and droop strut travel on my struts.
    You can change ride height, by moving the lower perch. If thats not possible (due to tire clearance) then you can look for different length springs.
    --------- Matt Marks -------- 32 STR ---------

  8. #8
    Fourth Gear marks_lude's Avatar
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    Also, as Teddy said, you could get preload by setting up the suspension so that it is mechanically stopped before the spring can fully extend (therefore, remaining "loaded"). This would require something like the shock topping out, or the bushing binding at full suspension extension (droop). Likely not a good solution, and as Teddy mentioned, it would require more force than the preload to get any suspension movement.
    --------- Matt Marks -------- 32 STR ---------

  9. #9
    On most high-end BMW struts, this is the only way to change ride height. I'm sure more sophisticated suspensions use better methods, but this is what BMW does. The bottom of the strut is fixed. The top spring perch is fixed. The body of the strut is threaded, and the bottom spring perch can be moved up and down. If you really want to you can lower it to the level that the spring would come loose when at full droop, but that's a really bad idea considering how short my total strut travel is and how common it is for BMWs to lift an inside front wheel.

    Indeed getting a full inch out of a 650lb/in spring would take an awful lot of cranking on the adjuster that likely is not a good idea. I may go for about 1/2", or for some different sized springs, trying to talk to Vorshlag about that.
    BMW 330i #313 ///MAC

    Quote Originally Posted by Rex Jr
    Lord, grant me the courage to trail-brake where I can, to accept that the timing light is correct, and the wisdom to know where to apex...

  10. #10
    Fourth Gear marks_lude's Avatar
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    Quote Originally Posted by murph View Post
    The bottom of the strut is fixed (to the suspension?). The top spring perch is fixed (to the chassis). The body of the strut is threaded, and the bottom spring perch can be moved up and down.
    As long as the two sides of the shock can move relative to each other (which they have to in order to absorb a bump), you can't add preload to the spring over the static ride weight of that corne,r without some sort of mechanical limitation to suspension travel (full droop, shock topping out). if there was more force on it, the spring would simply elongate and the shock would extend. there's nothing to "hold" the force. It's like if you push down on a fender and hold the car down by adding weight. That additional load can only exist when you are adding weight to that corner. As soon as you stop pushing down, the shock/spring elongate from the added force and the car raises back up to static ride height.

    I'm not sure what you mean by fixed, but the two sides of the spring (top hat and lower perch) should be able to move freely relative to each other.
    Last edited by marks_lude; 06-07-2010 at 05:00 PM.
    --------- Matt Marks -------- 32 STR ---------

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