Center of Gravity in Relation to Instant Center - '09 GT500

Discussion in '2007+ Mustang GT500 Tech' started by Scott88, Sep 13, 2020.

  1. Scott88

    Scott88 Junior Member

    18
    0
    Does anyone have a reference to find the center of gravity for the '09 GT500s? After measuring the contact points for the rear suspension, it looks like my instant center is 84" in front of the front tire (I have 20" staggered rims - so the slope of my LCAs is surprisingly negative). I purchased Maximum Motorsports LCA relocation brackets and noticed they have three settings. The lowest drops the arms by 3.875", which will bring the instant center behind the front wheels and much closer to the neutral line.

    The problem is I can only guess the neutral line. I don't have four scales to weigh the corners to calculate the center of gravity - so I just guessed by measuring the top of the waterpump (24" high) and my clutch (89" in front of my rear axle).

    I'm going after good handling - not hard launches - so I don't want the anti-squat to be over 100%. However, if I'm wrong on the estimated center of gravity, my neutral pitch line will be wrong and my anti-squat ratio will be incorrect.

    Obviously, any reference out there would be for stock. I can adjust from that. The only Mods I have that weigh anything are larger/lighter wheels, BMR subframe connectors, and Shelby rear strut tower brace.

    Below is the graph after measurements. The only piece estimated is the green line (neutral line).
    IC1.png IC2.png
     
  2. Juice

    Juice forum member

    1,560
    343
    The CG on mine is just in front of the door handles. When I "hit" the CG perfectly, both front and rear tires come off the ground at the same time when jacking up the side for service Nascar style.

    Sorry, that's the best I got. Way too much science there for me... lol
     
    Scott88 likes this.
  3. Norm Peterson

    Norm Peterson corner barstool sitter

    Age:
    72
    3,261
    158
    Car and Driver put a 2013 GT through some K&C testing a few years back and arrived at 21" for the CG height, at about 58.6" forward of the rear axle line. Your GT500 is a bit heavier with the weight a little higher up and a bit more forward. Unless you've installed tires that are taller than OE (not just bigger wheel diameters), you might want to work with a CG height more like 22" and located about 60" forward of the rear axle line. That's just my guess, but I have run through the anti-squat math a few times and I have seen CG height and weight distribution numbers for various cars.

    I don't know what you measured your car's rear axle pivot point coordinates to be, but measurements I made on my own car and that K&C data basically agreed on the stock anti-squat being about 30%.

    One thing I can tell you is that shooting for 100% anti-squat by tinkering only with the LCA inclinations is very likely to make the car drive way too "loose" in the back. Aka tailhappy and oversteerish, where making a violent evasive maneuver or even a sudden lane change could suddenly put a lot whole more excitement into your drive.

    Another thing is that there's no requirement for eliminating squat on a car intended for handling (or for street driving in general, for that matter), and unless you're going to get into other rear suspension mods as well it's really best to just let some squat happen. What you can do is take some squat out by lowering the axle-side LCA pivots a little, and take some more squat out with stiffer rear springs.

    FWIW, the amount of squat you're getting does not define how much rearward load transfer is happening. All it is, is a visual indication that some amount of load transfer is happening, with a suspension that has not been made rigid either geometrically (anti-squat) or elastically (spring stiffnesses).

    If you post up your pivot point coordinates and tire sizes, I can throw them into a spreadsheet that shows more than just the static-height snapshot. And sanity-check them against what I have that puts the antisquat at around 30%. I would ask if those numbers included trunk loads (including any heavy Shaker 1000 or aftermarket sound system weights back there).

    Throw in your car's spring stiffnesses plus driver/passenger/cargo weights and I can do a little more, like find out how much things change when you get in the car to drive it.


    Norm
     
    Last edited: Sep 14, 2020
  4. Scott88

    Scott88 Junior Member

    18
    0
    This is really helpful. I completely agree about the goal. I'm not trying to get to 100%, just looking for an improvement over the current setup which has a LCA at a slightly negative slope. Before I did the math, I assumed the relocation bracket might bring the anti-squat from 30% to about 70%. Now, after doing the math, it looks like it could swing from 11% to over 100% - depending on what setting I choose on the bracket.

    Passenger LCA
    -Rear connection point: (0, 8.6875)
    -Front connection point: (19.5, 8.3125)
    y=-0.0192x + 8.6875

    UCA:
    -Rear connection point: (0, 20.5)
    -Front connection point: (8.5, 19.8125)
    y = -0.0809x + 20.5

    Rear tire contact point = (0, 0).

    I have 305/35R20s on the back and 255/35R20s on the front. Shaker 1000 & and CS strut tower brace in the trunk (no spare tire). Driver weight is 200 lbs; passenger 150 lbs. I can't find the spring stiffness: they are stock '09 GT500 springs.

    Thanks again Norm for the detailed reply. Really helpful!
     
  5. Scott88

    Scott88 Junior Member

    18
    0
    By the way: if I plug in your guestimation for the center of gravity (60, 22), I get 7% anti-squat now, and 70% anti-squat if I choose the lowest setting of the relocation bracket.

    Without knowing too much on the subject, 70% was my rough goal. Seems like you're recommending 30% as a good goal for a car aimed at handling. Is this true and if so, I'd be interested to learn why you'd recommend 30%.
     
  6. Norm Peterson

    Norm Peterson corner barstool sitter

    Age:
    72
    3,261
    158
    For handling, I look a lot more to axle roll steer, which is also a function of LCA inclination. In a little more detail, roll steer comes from LCA side view inclination, LCA plan view skew, and PHB mid-length height above grade.

    Basically, you don't want the axle trying to steer itself too far 'wide' in a turn as the car develops roll, and the faster you're going the more you're going to prefer that the rear axle remain pointed close to straight ahead with the wheels parallel to the car's centerline.

    I'll see what happens with your numbers in a bit. I'm going to have to make a few assumptions and take a guess at your car's PHB attachment point heights unless you have that information handy.


    Norm
     
  7. Norm Peterson

    Norm Peterson corner barstool sitter

    Age:
    72
    3,261
    158
    Initial thoughts . . . I can resolve most of the vertical coordinate differences between your measurements and mine as being due to the difference in tire diameters (rolling radii, actually). But there's some difference at the UCA's axle-side height that cannot be explained that way. Are there any differences between the GT500 axle housing and the non-GT500 GT axle housing that I'm not aware of? I know the GT500's differential is referred to as being a "high torque" version of the 8.8 so I can at least picture how there might be other axle differences. I'd remeasure my car except that the rear of it is on blocks for a differential job that ran into a snag.

    I only have 18.38" for the side view projected lengths of the LCAs. This shouldn't be a big factor here.


    Norm
     
    Last edited: Sep 14, 2020
  8. Norm Peterson

    Norm Peterson corner barstool sitter

    Age:
    72
    3,261
    158
    About what you have now is this . . . positive values for axle roll steer means the axle is steering itself inward (not trying to steer itself around you on the outside).
    Scott no LCA relo.png


    Moving the LCAs downward by 3.875" makes it look like this . . .
    Scott -3.875 LCA relo.png

    Only 1.5" LCA relocating starts bringing roll steer back toward some thing that's probably manageable without hosing antisquat completely, but it's probably more than you'd want for a road course track day. Or a "canyon run", for that matter . . .
    Scott -1.5 LCA relo.png

    It appears you'd hit some sort of inflection point in the antisquat geometry at around 1" LCA relo, but I don't think the car's antisquat behavior would be as bad as the plot looks. The actual variation over a couple inch ride height change isn't that much, staying within a couple percent.
    Scott -1.0 LCA relo.png



    Norm
     
    Last edited: Sep 14, 2020
    GlassTop09 likes this.
  9. Midlife Crises

    Midlife Crises Member

    502
    257
    I am enjoying this thread and wish I understood more of how these settings all interact.
     
  10. Scott88

    Scott88 Junior Member

    18
    0
    Great stuff! I had to take an hour and look up what Axle Rollsteer was and its causes. It kind of makes sense, but still am not fully grasping how PHB travel/height/length affects it. I'll keep chewing on that.

    I took another look at the axle contact point for the UCA and noticed it was 1.625" behind the axle. So the chassis contact point was correct, but the axle contact point is actually (-1.625, 20.5). The PHB contact points are: axle/driver side (-5, 12.1875); chassis/passenger side (-6.75, 13.4375). I'm still trying to find what the stock springs are: I noticed you entered 175 and 150.

    So since I'm after better handling and not better launches, I should pay more attention to the axle rollsteer graph than the anti-squat graph. The top setting (shortest change) on the MM relocation bracket is 2" lower. So that puts the axle roll steer somewhere between the second and third screenshots above. Rollsteer between -0.19 and -0.06: both negative, and slightly more prone to oversteer than my current setup.
     
  11. Norm Peterson

    Norm Peterson corner barstool sitter

    Age:
    72
    3,261
    158
    Roll steer can be defined by a construction line between two points. One of these points for a PHB-located axle is the midpoint of the PHB, which is going to vary at half the amount of ride height change. The other point is defined by the LCAs, specifically by the point in space at which their axes converge in plan view (if they're perfectly parallel in plan view, that "convergence point" goes to infinity and the roll steer construction line truly does become parallel to the LCAs in side view.

    The roll center is actually where this construction line crosses the plane of the axle, which generally isn't at quite the same height as the midpoint of the PHB.

    3-link roll steer.jpg

    Source: Race Car Vehicle Dynamics


    Norm
     
    GlassTop09 likes this.
  12. Norm Peterson

    Norm Peterson corner barstool sitter

    Age:
    72
    3,261
    158
    Thanks. I just ran with rates somewhat firmer than was used for the 'regular' GT, more in line with the later-year S197 Boss. Strictly a guess on my part.


    That's been my philosophy. Particularly if your interest in handling runs more toward driving that ranges from brisk street driving and "canyon running" up to HPDE on a real road course. In those situations, there is no need to improve launch performance and once you're up to speed you normally don't need that kind of help from the geometry.


    With care and preferably a drill press I would think you could add your own holes. Ideally along an arc such that pinion angle would not change if you're running non-adjustable LCAs, or not having to adjust them if you do.

    Depending on whose LCAs you're using and on how the relo brackets are attached, there may be a limit on how high up you could locate these DIY holes (potential interference between the tops of your relocated LCAs and the main relo bracket attachment bolt). Though there's probably a work-around or two for that situation as well.


    Norm
     
    Last edited: Sep 15, 2020
  13. Scott88

    Scott88 Junior Member

    18
    0
    More good stuff. I think I'll confirm the assumed numbers (CoG, Spring rates, etc) before I drill holes. This is a lot to chew on; I thought the LCA were parallel with the car's centerline. Thanks.
     
  14. Norm Peterson

    Norm Peterson corner barstool sitter

    Age:
    72
    3,261
    158
    They're close enough to parallel (OE) that you can't rely on your eyeballs. Nor should you assume anything here; you just have to make the measurements. Aftermarket LCAs may be closer to parallel but might not be quite there either. It's been a long time since I was under there with measuring equipment to know whether precise parallelism is even possible.


    Norm
     
    Scott88 likes this.
  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.