Calculating suspension geometry - Working BACKWARD from Roll Center

Discussion in 'Car Modding' started by KingKenny04, Oct 15, 2016.

  1. KingKenny04

    KingKenny04 Registered

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    I've got a very strange situation that I need some help with. I'm trying to map out the suspension geometry for a car. I have quite a few bits of real world data for this car (I won't say which just yet), but I'm missing some very key data points that would help me plot a suspension (I'm using the ISI physics sheet).

    I have a page that shows the Cartesian coordinates of the chassis-side mounting points for the damper, upper and lower wishbones, and steering arm, all in relation to a reference point on the chassis. The problem is I do not have any measurements that show me where this reference point is on the chassis i.e. how far forward of the midpoint of the wheelbase, how far left or right of chassis centerline this point is, and how high this point is. I also do not have coordinates for the upper and lower wishbone mounts and steering arm mount on the wheel upright, nor do I have dimensions or angles for the wishbones. So I know where the chassis side points of the suspension are in relation to each other, but not where they are exactly on the car.

    Here's what I DO have:

    -Wheelbase
    -Track width
    -Wheel dimensions
    -Tire dimensions
    -Wheel offset
    -Damper to Wheel ratio
    -Design ride heights
    -Damper length at design ride heights
    -Static Camber at Design level
    -Camber change per suspension movement
    -Roll Center height at design level
    -Roll Center change per suspension movement
    -Center of Gravity height
    -Anti-dive Angle
    -Anti-squat angle

    I also have one value each for front and rear called "Ratio (Wheel/Spring)". I'm not sure if these are meant to be motion ratios (they are numbers less than one, I can't say what number exactly for confidentiality reasons). If they are, I also have dimensions for various springs for this car.

    Is there any way I can use all of this information along with the coordinates I have to map out the suspension geometry in relation to the car?
     
    Last edited by a moderator: Oct 15, 2016
  2. lordpantsington

    lordpantsington Registered

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    Yes, Start drafting things in a CAD program. The Anti %'s will help you locate the inner pickup points. From there Roll Centers will help approximate Outer locations, but it is going to be difficult to reverse engineer the locations based purely on the kinematic curves. It will also be helpful to have the lengths of various control arms.

    Wheel movement vs Spring movement, is exactly what motion ratio is. Often you'll find them expressed either way (spring vs wheel/wheel vs spring) you'll have to figure out which is appropriate and apply as necessary.
     
  3. KingKenny04

    KingKenny04 Registered

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    Can you explain how I can use the Anti %'s to find the inner pickup points? I have a pretty good idea of how to map out everything in AutoCAD, I'm just not sure how to factor in anti-dive and anti-squat.

    EDIT: I've been reading a lot about anti-dive and I'm still a bit lost on how to use it to locate my chassis mount points. I've seen two formulas:

    Anti-dive % = Brake Bias[tan(anti-dive angle)(wheelbase/CoG height)]

    This doesn't really help as I already have all of these values

    Anti-dive % = (Side view instant center height)/(height of 100% anti-dive line at instant center distance behind the front axle)

    I guess side-view instant center is what I'm sort of trying to find in the first place, so this formula doesn't help either. It also doesn't help that when I try to find calculations for finding side-view instant center, they all require me to already know my mount points, which is what I'm trying to find in the first place. I'm a little lost here. I feel like there's a formula here that will help me and I'm just not seeing it.
     
    Last edited by a moderator: Oct 15, 2016
  4. lordpantsington

    lordpantsington Registered

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    Google image search will help (as well as reading vehicle dynamics books). The first image on this page is a good illustration: http://www.team.net/TR8/tr8cca/wedgemath/04_anti_dive/anti_dive.html Anti's give you a projected line where your pick up points might be. You are going to need more info to specifically locate them, but at the very least you know they sit on that line.
     
  5. Marek Lesniak

    Marek Lesniak Car Team Staff Member

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    @KingKenny04
    I kind of 'been there' already. Having a proper suspension analysis program will help a lot. My approach is, to use pictures of a car to sort of estimate locations of outer joints (like, +/-2cm) and from there, try to move around outer points to see, in which direction you need to go with them sideways and upwards, to get closer to the curves.
    What's the PITA, is at some point you might end up getting closer and closer to match the roll center curve but in the same time, you will be getting further away from the camber gain curve.

    There are some general relations, which might be helpful when you tweak outer coordinates:
    - moving a wheel carrier upwards (both upper and lower outer joints by the same distance) moves camber gain curve down,
    - keeping lower outer ball joint height location and changing height of the upper outer ball joint, you alter one side of the camber curve,
    - for a given outer lower joint location, moving upper outer joint upwards/downwards, you alter camber gain spread,

    Good luck!
     
  6. KingKenny04

    KingKenny04 Registered

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    Thanks for all the help so far everyone.

    I tried punching some stuff into AutoCAD last night and came up with some weird figures. The data I have states that this car has 87% anti-dive and an 11.8 degree anti-dive angle. I used some of the formulas I found along with the wheelbase, cg height, and brake bias stated in the documents I have to calculate the anti-dive figures myself, and confirmed that those two numbers for anti-dive are correct.

    I drew up a side view diagram with the correct tire size, wheelbase, and anti-dive line marked.

    I went on a separate sheet and used the coordinates I had for the chassis mounts for the wishbones to plot each of those points on a side view plane (front to rear and height). I then drew lines through the mount points to get the instant center. The problem is when I copied that diagram over to the side view diagram, I have to move the chassis mounts WAAAAY down on the tire to get the instant center to intersect the anti dive line. Im not sure what Im missing here.

    Good news is I have photos of the wishbones that seem fairly centered and flat, so I can use the coordinates to calculate the lengths of their open ends, then put those photos into AutoCAD to approximate arm lengths, so thats something.

    Can anyone suggest suspension analysis software I should check out?
     
  7. Jokeri

    Jokeri Registered

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  8. KingKenny04

    KingKenny04 Registered

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    I actually tried using that last night and it threw me some memory error. Is there some patch I need for it to work with Windows 10?
     
  9. Slamfunk3

    Slamfunk3 Registered

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    Have you tried the chassis spreadsheet from the official site? I don't think it calculates all the things you need but i know it does do anti-squat and lift, as wll as some other suspension geometry bits. Might be worth a shot.

    Carfactory is a great tool but ever since some win7 update it hasn't been working. I seem to recall a thread on here about it but i don't think it ever had a successful outcome.
    TK
     
  10. KingKenny04

    KingKenny04 Registered

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    The ISI spreadsheet is what I'm using for the final calculations. My issue is I already have anti-dive and anti-squat pertenages and angles with the relative brake bias and cg height, as well as roll center height. I dont have wishbone dimensions or suspension geometry
     
  11. lordpantsington

    lordpantsington Registered

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    Were your wheels loaded in the photo with the wishbones looking nearly flat?
     
  12. KingKenny04

    KingKenny04 Registered

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    The photos I have of the wishbones are just the wishbones themselves on a flat surface. They arent attached to anything.

    EDIT: Okay so I took a stab at trying to use carfactory. From what I've read it seems like it might solve some problems for me. I set up a virtual machine running Windows 7, its installed but for some reason it keeps throwing me an error saying that access to some server was blocked, and it won't let me use the program unless it can get through to that server. I'm assuming that because this is such an old program that server simply does not exist anymore? Does anyone know how to get carfactory running offline?
     
    Last edited by a moderator: Oct 16, 2016
  13. Jokeri

    Jokeri Registered

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    Carfactory has to have access to the internet
     
  14. KingKenny04

    KingKenny04 Registered

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    I know, the VM I was running was connected to the internet and I had all windows security features with it shut off.

    I guess I'll just have to find an iso and do a full install on an old drive, see if that works.
     
  15. Slamfunk3

    Slamfunk3 Registered

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    I don't know which version of win7 you have but if possible try not to do any updates to it. I believe one of the patches was responsible for making Carfactory no longer work.
    TK
     
  16. lordpantsington

    lordpantsington Registered

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    Do any of you suggesting K!CF have enough data to double check the validity of its output? (specifically roll center.)
     
  17. KingKenny04

    KingKenny04 Registered

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    Ive got Windows 10. I tried to get around that error by running it on a vm with windows 7 and it started, but it couldnt connect to the carfactory server. I tried doing a windows xp vm but it wont let me download .net 2.0. Just no luck with this thing haha. Im gonna try to install xp on a separate hard drive I have and see if that will work.
     
  18. KingKenny04

    KingKenny04 Registered

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    Quick update on this: I managed to work with carfactory by using teamviewer through to a friend's computer. Punched in all the data I had, carfactory spit out a really strange geometry that stretched from the outside of the tire all the way to the centerline of the car.

    So I sort of gave up on carfactory. However, I managed to discover a way of making this work using the ISI spreadsheet. I chose a point somewhere that looked "right" on the susp page diagrams to be the point for my front lower aft wishbone mount, and then used the coordinates I mentioned earlier in this thread to place the other three wishbone mounts, damper mount, and steering arm mount. I then moved all six of these points together up and down, side to side, and forward and back until the spreadsheet's antidive, motion ratio, roll center height, and pushrod length all matched the figures I had in the technical manual. So I've been able to create a front suspension that matches the real world values I have from the technical manual, while maintaining the correct geometry as illustrated in the homologation document I have.

    EDIT: Okay so I've run into an issue. The ISI Spreadsheet lists a value for "Anti Lift" at the rear, but the documents I have only list Anti Dive and Anti Squat. HOWEVER, the documents I have list BOTH an anti dive and anti squat value for the rear suspension. Is there any way for me to use these to calculate anti lift?
     
    Last edited by a moderator: Oct 25, 2016
  19. borbor

    borbor Registered

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    If you know how to come up with the % anti-dive front, you can get the % anti lift rear. It's literally the same formula except you're replacing theta f with theta r and % front braking with % rear braking.
     
  20. lordpantsington

    lordpantsington Registered

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    So now that your locations seem correct, can you load your suspension.pm back into K! for analysis and see how that compares to your data for Roll Center? If it is roughly in the ballpark, I'd like to know. Thanks!
     

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