Announcement

Collapse
No announcement yet.

Coil Spring Preload: What is it and what can we do with it?

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Coil Spring Preload: What is it and what can we do with it?

    Hey all,

    I’ve recently been looking at the unsprung masses for the Prado for critical damping models, and thought it would be interesting to show how you can measure the unsprung and sprung mass ratio from a ride height calculation. To do this you’ll need to know your coil spring preload, your coil spring rate, and your measured vehicle weight accurately.

    The photo below shows what preload is;



    Coil preload is just how much the coil is compressed in the assembled strut, shown in the middle. The total reduction of coil length from free height to working height in the vehicle is also shown.

    Once you calculate the coil working height, you can then easily determine the ride height from knowledge of the suspension geometry and the relationship between coil working height and strut open length. This relationship also shows the large 107mm range of motion that the coil has, shown below;



    Here are some of the important features of preload and how it can be used to calculate the final ride height;

    1. Definition: Preload is the Force on the coil spring in the assembled state of a coilover strut/spring assembly. In this state, preload has nothing to do with a car, you can pick up an assembled coilover strut/coil in your hand and hold it, and it is in a preloaded state.

    2. A preloaded coil spring will not reduce working height any further unless the applied Force is greater than the preload Force. If there is 150kg preload, then applying a Force of 150kg will not do anything to the coil, you’ll need more than 150kg of Force to start compressing the coil height further.

    3. To determine the final coil working height, preload Force must be subtracted from the effective vehicle weight Force at the strut position, in the IFS case determined by the motion ratio of the lower lever arm. Weighbridge vehicle weights are measured at the end of the lever arm at the tire contact patch, and are less than the equivalent weight at the strut position (lever arm mechanics dictate this). The same principle applies to wheel rate, the equivalent spring rate at the end of the lever arm is less than the actual spring rate at the strut/coil position.

    The figure below shows the number crunching needed to arrive at your ride height;



    Note that I’ve varied the unsprung mass until I obtain my measured ride height of around 792mm, measured droop of 97mm, and measured coil working height of around 298mm. The calculations suggest around 122kg for the corner unsprung mass in the Prado 120 IFS. This figure for unsprung mass can be correlated with physical measurements of the weight of each component in the suspension;

    http://www.pradopoint.com/showthread...rado-120/page2

    Note that some scaling of the weight of individual components is assumed, such as half the CV weight etc. When I finally measure the weight of everything in the IFS, I can come back and compare this figure and see how well the scaling works.

    4. Changing preload will change your vehicle ride height. Increase the preload, increase the ride height etc. This is achieved by increasing or decreasing the coil seat height position (defined by center ring eye to underneath lowest point of coil). The obvious example is the Bilstein circlip groove.

    Knowledge of the effects of preload yields several relationships of interest. The first and most simple is how preload scales with coil seat position;



    The amount of preload compression compared to the final total coil compression at working height is significant, and it can be around 27% of the total at the OEM coil seat position.

    The next most interesting relationship is how much the extra lift changes your droop and your bump:droop ratio. The next figure shows the classic lift vs droop relationship, and demonstrates you’ll lose droop with lift in a roughly 1:1 ratio. Three different Bilstein circlip positions are shown as an example;



    Perhaps the most neglected but interesting relationship is the bump:droop ratio and how it can change as a function of strut open length and vehicle lift. The figure below shows the bump and droop travel on the shaft as your ride height and shaft position changes over 3 different Bilstein circlip positions;



    While the bump travel doesn’t change as a function of strut open length for a fixed ride height, as the lower arm-bumpstop contact remains constant at a shaft length of 468mm, you’ll easily notice how rapidly you run out of droop travel as you lift. The real sweet spot is where the bump:droop ratio is close to 1:1, for 575mm open length giving ca. 97mm droop at around 792mm ride height. This is why I have always recommended to limit your front ride height to in between 790-800mm, as the bump:droop ratio quickly increases for lifts beyond 800mm and you’ll rapidly lose droop.

    5. Changing preload will not change the coil working height. This point seems a bit counter intuitive at first, but it occurs quite simply because any increase in preload is matched by a reduction in additional vehicle load on the coil.

    6. i) An increased preload compared to OEM coil seat height will result in a “slightly” different ride feeling at ride height. This is because the coil motion now works over a different part of the Force-Displacement part of the valving curve of the strut. eg, on bottom circlip, with 575mm struts, shaft position at ride height is 522.5mm, move to second circlip, shaft position is at 529.5mm. You have not only changed the bump:droop shaft travel ratio, but you are also operating the coil starting with same working height at a different stroke position, ie, a different part of the Force-Displacement curve of the strut, and you will therefore experience a different Force. How much this corresponds to seat of the pants feeling is not much, but the Force will be a slightly different magnitude.

    6. ii) An increased preload will also give you a harder bang when the car comes back down if you lift a wheel at full droop. This is because the coil has more preload, so more Force is required to overcome the preload and get the coil moving. ie, you feel more Force.

    So after all that reading you can appreciate that an understanding of coil spring preload can give you a great insight into your vehicle ride height and how those ride height changes affect your suspension geometry.

    To make these calculations accurate means accurate figures for your coil spring rates (load-deflection measurements), coil lengths, and your vehicle weights, so in the future I’ll probably corner weight my Prado to check how reliable the ride height calculations are.

    Best

    Mark
    Whitey
    Shockie Maker of the Month Award
    Last edited by Whitey; 20-09-2017, 02:53 PM. Reason: Updated plot
    2006 GXL petrol auto. ARB deluxe bar x3 HID IPF's, ARB alloy roofrack, ARB awning, BFG A/T, Safari snorkel, Piranha breathers, Pacemaker extractors, custom Ironman 45710FE 436-569mm with Dobinsons 350, custom Ironman 45682FE 383-618mm with Dobinsons 487, Firestone kevlar 60psi airbags, 30mm extended Roadsafe links, AMTS bashplate and recovery points, ABR Flyer with Powersonic AGM.

  • #2
    Nice work Batman. The weight distribution F and R and side to side will be interesting. Full and empty tanks?
    My 150 build - http://www.pradopoint.com/showthread.php?27423-A-Random-approach-to-a-Bluestorm-150-GXL-D4D-automatic

    Comment


    • #3
      Originally posted by mjrandom View Post
      Nice work Batman. The weight distribution F and R and side to side will be interesting. Full and empty tanks?
      Hey,

      I was planning on starting empty, but was also planning on with/without the swaybars, as I can see at least on my front swaybar that there's definitely a little pull on one side at ride height to create a few mm difference. I wound my coil seat up a bit higher on one side to counteract it.

      Standard lop sided Prado!

      ...what is the Batman reference to??

      Best

      Mark
      2006 GXL petrol auto. ARB deluxe bar x3 HID IPF's, ARB alloy roofrack, ARB awning, BFG A/T, Safari snorkel, Piranha breathers, Pacemaker extractors, custom Ironman 45710FE 436-569mm with Dobinsons 350, custom Ironman 45682FE 383-618mm with Dobinsons 487, Firestone kevlar 60psi airbags, 30mm extended Roadsafe links, AMTS bashplate and recovery points, ABR Flyer with Powersonic AGM.

      Comment

      canli bahis siteleri bahis siteleri ecebet.net
      mencisport.com
      antalya escort
      tsyd.org deneme bonusu veren siteler
      deneme bonusu veren siteler
      gaziantep escort
      gaziantep escort
      asyabahis maltcasino olabahis olabahis
      erotik film izle Rus escort gaziantep rus escort
      atasehir escort tuzla escort
      sikis sex hatti
      en iyi casino siteleri
      deneme bonusu veren siteler
      casibom
      deneme bonusu veren siteler
      deneme bonusu veren siteler
      betticket istanbulbahis
      Working...
      X