Developing a Proper Suspension Model
 

Although this effort may seem worthless to some 86ers, I'd like to start a thread that compiles known data about the stock vehicle and uses this to develop a virtual model of the car.

Available programs:

• ADAMS - Powerful and complex. Full vehicle dynamics
• Lotus - Simple and effective. Awkward interface.
• WinGeo3 - Simple and effective. Akward interface.
• SolidWorks + Cosmos - Powerful and complex. Takes alot of work to get meaningful results.
• Optimum K - Never used it.
• SuspensionAnalyzer - Never Used it.
• DIY Excel - Hardcore Mode. Requires alot of brains and time.

Similar discussions have been done HERE. I'm comfortable with Lotus. WinGeo3 is standard for alot of FSAE guys. ADAMS is for ballers. Whatever people would be most comfortable with, we can use.

The goal would be to analyze the stock package and look at how various aftermarket options would perform. Since each person has a different goal (autocross, street, rally, drift, etc) their interest will push the project in that direction. I'll be pushing the limits of SCCA street tire.

The Facts (in 'Murika units and Sissy units):
Curb Weight (FR-S, no spare/tools/mats, minimal gas) = 2645 lbs or 1200 kg LINK
Corner Weights:

LF = 823 lbs (701 ideal) or 373.3 kg (318.0 ideal) or 3660.9 N (3118.2 ideal)

RF = 684 lbs (701 ideal) or 310.3 kg (318.0 ideal) or 3042.6 N (3118.2 ideal)

LR = 513 lbs (622 ideal) or 232.7 kg (282.1 ideal) or 2281.9 N (2766.8 ideal)

RR = 625 lbs (622 ideal) or 283.5 kg (282.1 ideal) or 2780.1 N (2766.8 ideal)

Unsprung Weight: (best guess - LINK )

83 lbs or 37.6 kg or 369.2 N per corner

Sprung Weight:

LF = 740 lbs (618 ideal) or 335.7 kg (280.4 ideal) or 3291.7 N (2749 ideal)

RF = 601 lbs (618 ideal) or 272.4 kg (280.4 ideal) or 2673.4 N (2749 ideal)

LR = 430 lbs (539 ideal) or 195.1 kg (244.5 ideal) or 1912.7 N (2397.6 ideal)

RR = 542 lbs (539 ideal) or 245.9 kg (244.5 ideal) or 2410.9 N (2397.6 ideal)

Dampers = SHOWA non-inverted, twin-tube, low-pressure Nitrogen, conventional strut

Shock Dyno:
https://scontent-a.xx.fbcdn.net/hpho...01415143_o.jpg

Suspension Spring Rates: LINK

Front = 131 lbs/in or or 22970 N/m

Rear = 211 lbs/in or 36998 N/m

http://philbedard.com/pics/brzvsfrs.jpg

Spring Dimensions: Link to Bordom.is.me's post

FRONT:

• External Diameter = 4.625"
• Internal Coil Diameter = 3.625"
• Wire Diameter = .5"
• Free Length = 11.625"
• Active Coils = 5 (excluding grinding of bottom coil)

REAR:

• External Diameter = 4.125"
• Internal Coil Diameter = 3.125"
• Wire Diameter = .5"
• Free Length = 10.5"
• Active Coils = 4 (excluding grinding of bottom coil)

Tire Spring Rates: (FSAE guru states 350 N/mm for passenger tires at passenger pressures. I concur.)

30 psi = 6500 lbs/in or 114000 kg/m

45 psi = ~10000 lbs/in or 175000 kg/m

Sway Bar Spring Rate:

Front = 141 lbs/in or 2467 kg/m

Rear = 113 lbs/in or 1978 kg/m

Effective Wheel Rate (in roll, including sway bar):

Front (FR-S) = 373 lbs/in or 6528 kg/m (70% from sway bar)

Front (BRZ) = 410 lbs/in or 7175 kg/m (64% from sway bar)

Rear (FR-S) = 212 lbs/in or 3710 kg/m (37% from sway bar)

Rear (BRZ) = 200 lbs/in or 3500 kg/m (39% from sway bar)

Caster Angle: Link

Front = -5.937 deg

Roll Center Height: Link

Front = 3.140" (stock height) --> ground plane (1" of compression)

Spring Angle:

Front = 15.5 deg

Rear = 4 deg

Spring Motion Ratio:

Front = 1.050 (2.5" compression) --> 0.997 (1.5" compression) --> 0.95 (OEM height) Link

Rear = 0.768 (2" compression) --> 0.763 (OEM height) --> 0.758 (2" rebound) LINK

Sway Bar Motion Ratio:

Front = 0.92

Rear = 0.59

Natural Frequency (in ride):

Front (FR-S) = 1.24 Hz

Front (BRZ) = 1.43 Hz

Rear (FR-S) = 1.54 Hz

Rear (BRZ) = 1.47 Hz

Natural Frequency (in roll):

Front (FR-S) = 2.27 Hz

Front (BRZ) = 2.38 Hz

Rear (FR-S) = 1.95 Hz

Rear (BRZ) = 1.89 Hz

Tire Info:

215/45-17 Michelin Primacy HP LINK

Section Width = 8.46 in or 0.215 m

Contact Width = 7.5 in or 0.1905 m via TireRack measurement

Contact Length (at 60 mph and stock pressure) = ~5.71 in or 0.145 m

Overall Diameter = 24.7 in or 0.6274 m

Load Rating = 87W SL (168 mph or or 270 kph)

UTQG = 240 A A (hmm... A?)

Max Load (at rated pressure) = 1201 lbs or 545 kg

Max Rated Pressure = 51 psi

Tread Depth = 9.5/32 in

Material = Green X - Low Rolling Resistance

Wheel Info:

Wheel Diameter = 17 in or 0.432 m

Wheel Width = 7 in or 0.178 m

Wheel Offset = +48 mm or 1.89 in or 0.048 m

Track Width: LINK

Front = 59.8 in or 1.51m

Rear = 60.6 in or 1.54m

Roll Bar Diameter: LINK

Front = 0.71 in or 0.018m

Rear = 0.55 in or 0.014m

Center of Gravity Height = 18.1 in or 0.46 m LINK

Roll Center Height:

Front = 2.1 in

Rear = 3.8 in

Wheelbase = 101.2 in or 2.5705 m LINK

Suspension Travel:

Bump Travel = 2.5 in or 0.1 m

Rebound Travel = 3.5 in or 0.15 m

Max Roll Angle = +/- 2.3 deg

Max Pitch Angle = 0.9 deg (braking)

Roll Resistance on Front: 61%

Braking on Front = 60%

Anti-dive Under Braking: 5% (front)

Anti-lift Under Braking: 196% (rear)

Anti-squat Under Acceleration: 64% (rear)

Steering Ratio: 14.3 (on center) - 14.4 (at 90 deg)

Distance (axial) between lower strut bolts = 2.4" (or 60.5 mm)

Thickness of lower flange = 1 in (or 25.4 mm)

Distance (radial) from strut center to lower bolt = 2.4 in (or 60.7 mm)

Distance (axial) from AST's lower bolt to hat mount = 15.4 in (or 392 mm)

Upper strut thread:
- AST = M12x1.25-25, with 1 mm of thread relief, 5 mm long (for camber plate)
- KW = M14x1.5-30, with 0 mm of thread relief, 10 mm long (for camber plate)

Length of AST's spring = 7.1 in (or 181 mm)

Distance (axial) from lower bolt to sway bar mount = 4.5 in (or 115 mm)

Distance (radial) from strut axis to sway bar mount = 1.97 in (or 50 mm)

Diameter of sway bar mount hole = 0.4 in (or 10.2 mm)

Here's a cool MATLAB function that helps you simulate shock dyno graphs:
function y=critdamp(cwlbs,srppi,spmr,shmr,lsd,knee,hsd)
%cwlbs=corner weight, lbs minus unsprung for more accuracy
%srppi=spring rate, lbs per inch
%spmr=spring motion ratio
%shmr=shock motion ratio
%lsd=low speed damping, percentage of critical
%hsd=high speed damping, percentage of critical
%knee=location of knee, in inch per second

lbf2n=4.448; % 1 lbf = 4.448 newtons
m2i=39.37; % 1 meter = 39.37 inch
p2kg=0.4536; % 1 lb=0.453 kg
if (spmr>1)+(shmr>1)
disp('Motion ratios must be less than 1, but I''ll convert it for you')
spmr=1/spmr;shmr=1/shmr;
end
wheelratestandard=srppi*spmr^2
wheelratemetric=wheelratestandard*lbf2n*m2i
cd=2*sqrt(wheelratemetric*cwlbs*p2kg)/lbf2n/m2i/shmr^2
vel=(0:0.1:20);

damp=lsd*cd*(0:0.1:knee);
hispeed=damp(end)+(0:0.1:20-knee)*cd*hsd;
damp=[damp hispeed(2:end)];
plot(vel,damp,'r','linewidth',2,'displayname',['LS:' num2str(lsd*100) '% Knee:' num2str(knee) ' ips HS:' num2str(hsd*100) '%'])
legend('off');legend('show','location','east')

Still looking for accurate suspension geometry coordinates (X,Y,Z). The current "best guess" is housed in a Google Docs spreadsheet: LINK

Special Thanks to MLA163, Wepeel, and Boredom.is.me for contributing measurements and discussion.

Saved for pretty pictures!

215/45/17 my friend. 8.4" Section width...

Sorry aboot that. I grabbed my info from the Car & Driver tire swap article:
LINK


Fixed and Thanks!

The stock front swaybar is 18mm, rear is 14mm.

The info I found from a Subaru forum suggested 17 and 13. Probably just difference in calipers and paint. Will change anyways.

How about a teaser, you say? Got the geometry style in... but no points or parameters have been entered.

http://www.ft86club.com/forums/pictu...pictureid=3277

Updates made to suspension parameters, but not specific coordinates. Can I get a second set of eyes on this?

Requires peer review:
Bump Travel = 4 in or 0.1 m
Rebound Travel = 6 in or 0.15 m
Max Roll Angle = 6 deg
Braking on Front = 60%
Tire Rolling Radius = 9 in or 0.225 m

Still looking for suspension geometry coordinates (X,Y,Z), if anyone has logged them

Right on brother.

I got 18mm / 14mm sways from measuring. I don't want to scuff up the coating to find pure steel, but would 1mm of powder coat be normal?

I misspoke. Powdercoating is generally a couple mils thick, which would only give you 0.1 mm or so, even considering both sides and caliper inaccuracy.

Turns out that 3" bump, 4" rebound, 5 deg roll is more appropriate (for now).

Front View
http://www.ft86club.com/forums/pictu...pictureid=3282

Driver's Side View
http://www.ft86club.com/forums/pictu...pictureid=3283

Top View (Passenger Side)
http://www.ft86club.com/forums/pictu...pictureid=3281


Values are all still "eyeballed" from the pictures and general information.

Next, the rear suspension~!

ooohhhhh...

Mac-Strut motion ratio STILL isn't 1.00... if you are going to bother being accurate, be accurate. ;)

I've got an excel sheet I made as part of my senior project for my undergrad... I'll dig it up and run some of these numbers through and see what I get... We need an actual front motion ration though hehe

According to racecomp, the front motion ratio is pretty darn close to 1. I'm not sure which side of one it sides on though.

-Acree

:scared0012: Airline food, I mean come on...

You're correct SCCABRZ192, it will be slightly less than one. What value that is... I couldn't tell you without some coordinates or parts in front of me.

This is not a traditional calculation for a wishbone, but the closest that I can imagine is to consider the knuckle and it's connections to be rigid. So, something like this:

distance from lower wishbone pivot axis to strut mount

Motion Ratio = --------------------------------------------------

distance from lower wishbone pivot axis to spindle center

Maybe 0.9? The good news is that a kinematic solver doesn't care about the parameter of motion ratio, it just solves a bunch of free body diagrams and spits out positions.

If you have more precise coordinates, I'd gladly put them in. :D

@Dave-ROR

Is it possible to sticky this? I think it's a great project that could end up helping to educate a lot of people here and would hate to see it lost under a bunch of 'Need help picking coilovers!' threads.

I'll be bumping it soon enough.

The rear suspension is not a normal type... so I've been testing alot of variants. It looks like "Double Wishbone + upper toe link + S link" should get the job done (although it's not really true here). The last few setups I've modeled have been "trailing arm + 2 links" or "H arm + upper link". I'm really a fan of H arms, since they kill toe movement, but otherwise allow for a smooth camber curve. Alas, Subaru didn't ask for my help.

In other news, my Hot Lava Auto FR-S has been ordered and will arrive somewhere between 2 weeks to 2 months from now. Getting plenty of access to a car for measuring the geometry coordinates will be much easier. Looks like I'll be spending alot of time with the calipers in the next few months (between this and the aero thread).

I'd say the most accurate suspension model for the rear would be the upper a-arm with three links. For those who have RCVD it's in section 17.6.

I've only used WinGeo (which I loathe) of the programs mentioned, but the suspension type is not an "option" when building the model.

WinGeo isn't that bad.... :bellyroll:

I agree. The lower links are just that... unaffiliated links.

Lotus is significantly easier to use than WinGeo (IMHO), but this is one of it's shortcomings. It prefers to shoehorn things into one of their included suspension styles (like 20 types), which greatly simplifies the process... you just have to get creative with your interpretation of the style.

Ex: double wishbone really means 4 body points, 2 suspension points, 2 shock/spring attachment points (all with 3 rotational degrees of freedom). Then, 2 body points + 1 suspension point = 1 rigid member (LOL). Lower shock/spring mount moves with the wishbone, upper moves with the body. All body points are rigidly connected. All suspension points are rigidly connected to the knuckle which is in series with a spring (tire) and ground.

Once the "fudged" version is up and running, you can build a "proper" version from scratch. If the results are close, you'll know that your assumptions are probably OK. Typically, the results are so close it's not worth your time to make the custom setup at all though. Take your time and get the fudged version perfect.

BTW that mention never got flagged in my notifications so I never saw it.

Stuck now :thumbup:

The measuring bit looks like a headache. This link was from a site Dave linked to for shocks.

http://farnorthracing.com/modeling.html

Do you have any of your own tips you can share?

Where he talks about telescoping rods attached to height gauges, I was thinking of a laser pointer instead (make sure it's parallel to the base and record the height above the surface, the set and zero your gauge accordingly).

Thoughts?

That guy is rather smart. Getting coordinates in Cartesian coordinates is difficult, but using plumbs and telescoping magnets is a brilliantly ghetto solution. It sounds perfect for items that can be +/- a couple millimeters, then bust out the calipers for small measurements.

I usually work in spherical, which sounds nuts, but it's easy to implement. Distances + angles. Rulers and string will give you point-to-point distances quite accurately. Up your precision by knotting 1 end of the string around a nail, and having a friend hold it. Redneck engineering! Magnetic angle finders will turn distances into positions. Of course you'd need a few quality reference parameters and some common sense "eyeballin' it" to ensure that the results mean anything.

What have you guys done to better understand your previous vehicles? For SAE Baja and Formula cars, we built from a model... this is reverse engineering an existing product, which I've only done a couple times. I stand to learn alot from you guys (and have high expectations!)

my cars on the rack...looks like we do some measuring tomorrow..

Robi

While we look forward to RobiSpec's analysis, I'll provide some brain food.

There are multiple systems interacting in a car suspension, with different natural frequencies and damping characteristics. The lowest decade (1-10Hz) is responsible for almost all system dynamics, but all of them should be mentioned. The system looka like dis:
http://ctms.engin.umich.edu/CTMS/Con...ures/susp1.png

1) Upper system (fix x_2 and w):
Sprung weight + suspension spring. This motion is damped by the suspension damper. These are very low frequencies (0.5-5 Hz).

f_nat1 = constant * sqrt(spring rate / sprung mass)
b_1 = damper constant / sqrt(spring rate / sprung mass)

2) Middle system (fix x_1 and w):
Unsprung weight + suspension + tires. This motion is damped by both the suspension damper and tire damping. These are generally much higher frequencies.

f_nat2 = constant * sqrt([spring rate + tire rate] / unsprung mass)
b_2 = damper constant / sqrt([spring rate + tire rate] / unsprung mass)

3) Lower system (fix x_1 and x_2)
If we assume that all of the car's weight is sent to the ground, the parts involved are the sprung weight + unsprung weight + tires. This motion is damped by the tire, which is generally a very low damping factor (tires bounce).

f_nat3 = constant * sqrt(tire rate / [sprung mass + unsprung mass])
b_3 = tire damping constant / sqrt(tire rate / [sprung mass + unsprung mass])

Due to the low damping capabilities of tires, it's best to let the dampers handle vibration control. For tire dynamics to minimally affect suspension dynamics, a decade of frequency separation should be sufficient.

F_tire > 10 * F_susp
sqrt(tire rate / unsprung mass) > 10 * sqrt(spring rate / sprung mass)
If both values are more than one,
tire rate / unsprung mass > 100 * spring rate / sprung mass
tire rate / spring rate > 100 * unsprung mass / sprung mass

for our example:
6500 / 131 > 100 * 83 / 618
49.6 > 13.43 --> sufficiently separated

I guess I should amend the above statement. Thanks for pointing it out!

Continuing this thought:
If we calculate the max spring rate that can be used without being affected by tire dynamics (at stock pressures):
max front wheel rate = 484 lbs/in
max front spring rate = 526 lbs/in
max rear wheel rate = 422 lbs/in
max rear spring rate is = 548 lbs/in

At autox pressures, max spring rates would be 809 (front) and 843 (rear). In metric, that's 14.2k and 14.8k. Interesting, not that anyone would want to run them that stiff anyways.

After updating the original post to include Newtons, I re-verified the natural frequencies and added the new ones using the racing aspirations calculator (LINK

f_nat1 = 1.3 Hz front, 1.5 Hz rear
f_nat2 = 31.3 Hz front, 22.1 Hz rear (stock pressures)
f_nat3 = 9.03 Hz front, 8.08 Hz rear (stock pressures)

From this, we can infer that there is some small interaction caused by the tires at stock pressures. Bumping up the pressures to autocross level (10000 lbs/in rate):
f_nat2 = 30+ Hz (autox pressures)
f_nat3 = 11.1 Hz front, 9.9 Hz rear (autox pressures)

Now we see that the tire's natural frequency jumps up ~23%, which decreases it's involvement in the bounce dynamics of the car. The added pressure also decreases sidewall flex, which maintains a more even contact patch.

New discussion point: Sway bars!

Roll center (according to SAE) - The point in the transverse vertical plane through any pair of wheel centers at which lateral forces may be applied to the sprung mass without producing suspension roll.

Layman's definition - This is a neutral point for your suspension. Applying lateral force at this height will generate no roll (vertical movement at either corner).

Let's consider a 1.0 g turn in a 2645 lbs car. That means ~2645 lbs force will be applied to the center of mass/gravity. Here's an illustration:
http://www.mitchellsoftware.com/ForceB1.jpg

4 possible options:

1) Roll center height = center of gravity
There is no roll. If there is sufficient grip in the tires, your car will turn like a go-kart or a door-hinge (flat). This does sent alot of force through the control arms, and the spring/damper are not used at all.

2) 0 < roll center height < center of gravity
There will be a moment (torque) generated, since the lateral force is applied at a different height than the reaction force.

Torque = Force * distance
Roll Torque = 2645 * abs(center of gravity height - roll center height)

Since there is a torque, there will be reaction forces. Typically this duty falls on the springs and sway bars. Most race cars try to keep the roll center height at 15-30% of the center of gravity height.

3) roll center height = ground height
The control arms won't be loaded, and all forces will be sent through the spring/damper. Not horrible, just sub-optimal.

4) roll center height < ground height
The spring/damper will see an amplified force, and can cause the control arms to see the wrong type of force (compression vs tension). This isn't necessarily bad, but I can't recommend ever having an underground roll center, unless you overbuild the spring/damper to compensate for it.

TL;DR - Try to keep the roll center between the center of gravity and the ground. Lower is better, but don't go underground.

Load Transfer
Let's assume that we stay between #2 and #3 (because that's how a properly designed suspension should be). In a cornering maneuver, the reaction forces are generated when the outside spring is compressed and the inside spring is extended. For simplicity, let the roll center height be 10 in. Let's initially assume that there is no sway bar and the reaction force at both ends is half of the total reaction force:

Reaction force = 1/2 * roll torque / half track
Reaction force = 1/2 * [2645 lbs * 0.53] * abs(18.1-10 in) / 29.9 in
Reaction force = 189.3 lbs

Deflection = reaction force / spring rate
Deflection = 189.3 lbs / 131 lbs/in
Deflection = 1.445 in

Roll angle = arctan(deflection / half track)
Roll angle = 2.767 deg

Sway Bars
Sway bars add coupling between the wheels. Any difference in height will create a torque that will "lift" the outside wheel in an attempt to equalize the wheel heights again.

Situations:
1) In a single wheel bump, the full length of the bar is used to control one end, so the bump stiffness is halved from the numbers calculated.

2) In a two wheel bump, the whole bar has no effect.

3) In a turning maneuver, it should follow the formula:

K = pi*G*d^4*(MR)² / (16*R²*L)
where
pi = 3.141592653
G = elastic modulus
d = bar diameter
MR = motion ratio of control arm swaybar link
R = radius arm of sway bar
L = length of sway bar

basic data:
G = 8.14 x 10^10 Pa for spring steel
d = 0.018 m (front) 0.014 m (rear)
MR = not sure... but it's 0.6 on a Miata
R = not sure... but it's 0.225 m (front) and 0.121 m (rear) on a Miata
L = not sure... but it's 0.830 m (front) and 0.850 m (rear) on a Miata

Therefore:
K (front) = 14375 N/m
K (rear) = 17761 N/m

From this, we see that the FR-S is a sports car built with soft springs and stiff roll bars. This means that it will feel silky smooth on the highway, but any difference in wheel height will be heavily resisted. Strictly speaking, this is not good race car dynamics... but it works great on street cars.

wow! This thread should be called Suspension 201. I am learning a lot of good and interesting stuff. Thanks Shank for sharing.

It's good outlet for creative thought. The 86 community has alot of smart people (including a few manufacturers). I get to throw down the theory in my own words, and it only helps the community.

Experienced people will peer review it (and hopefully correct me), and newer people will learn from it. Once my FR-S comes in, I'm sure things will get less theoretical and more hands-on.

Thanks for reading, and know that questions/comments are always welcome.

What is the purpose of the constant in the natural frequency calculations? I haven't seen that before.

Can you explain how you get to this conclusion from what you presented above? You can use mathematics/vibrations if you'd like.

As stated, we have a highly intelligent crowd for peer-review. Thanks for signing up to make a post, Josh.

I pulled most of the info from a guide found here: LINK to PDF

First point. Here's the traditional formula for a spring-mass oscillator:
F = sqrt(spring constant / mass)

It will output in rad/s. Radians are wondeful for Bode plots, but not good for normal people... so 1/2/pi = 0.159 is the conversion factor for Hz.

Second point may be very valid. It's a fine handwaving argument, but it may not be correct. I picked that up from a random forum post on the miata forums. Let's find out:

Tire dynamics should minimally affect suspension dynamics.
A decade of frequency separation is considered sufficient.
F_tire > 10 * F_susp
sqrt(tire rate / unsprung mass) > 10 * sqrt(spring rate / sprung mass)
If both values are more than one,
tire rate / unsprung mass > 100 * spring rate / sprung mass
tire rate / spring rate > 100 * unsprung mass / sprung mass

for our example:
6500 / 131 > 100 * 83 / 618
49.6 > 13.43 --> sufficiently separated

I guess I should amend the above statement. Thanks for pointing it out!

Continuing this thought:
If we calculate the max spring rate that can be used without being affected by tire dynamics (at stock pressures):
max front wheel rate = 484 lbs/in
max front spring rate = 526 lbs/in
max rear wheel rate = 422 lbs/in
max rear spring rate is = 548 lbs/in

At autox pressures, max spring rates would be 809 (front) and 843 (rear). In metric, that's 14.2k and 14.8k. Interesting, not that anyone would want to run them that stiff anyways.

in a non gay way I love you man

Didn't see this got stickied. I applaud your efforts. I want to develop a full system model, including a driver, using state space equations so you can change the initial conditions easily for simulation purposes. The problem is I hate matrices lol.

Be careful with roll centers. Over the last few years, it has become one of the most misunderstood terms on the internet. You are using the proper, SAE definition, but you are giving 2D, kinematic scenarios of roll center height. What we need to determine are the force application points (FAP) and use the force-based roll center, not the kinematic roll center.

The KRC tells you an arbitrary point in space. It only works in a symmetrical 2D case. As soon as you turn the wheel and the suspension begins to compress/decompress as the car begins to roll, the model is no longer valid. So the KRC only works when the car is static and what good does that do?

Worrying about roll center migration is also a load of crap. If the roll center can migrate up-and-down and side-to-side, what happens when the roll center is outside the wheelbase? All four corners would simultaneously be in tension or compression, a physical impossibility.

Mitchell himself discusses the KRC vs FBRC here: http://www.neohio-scca.org/comp_clin...namics2007.pdf

There is nothing wrong with having a KRC below ground per se. Sometimes, such as in the case of F1, there's a below ground roll center, but that's because suspension as a whole is compromised around the aero. But a below ground roll center means the suspension has anti-jacking built into it, creating a more stable aero platform in return. If you are going to discuss roll center height, you must include jacking/anti-jacking forces as a result.

Preface: I no engineering background.

I get that you're basically trying to develop a model of the car's suspension. Once you have this created, what kind of information can we learn from this? How does this benefit the owners of these cars?

If I'm understanding this correctly, you're using a simplified version of this car (model) in order to get a better understanding of this car's vehicle dynamics, correct?

hehehehe

I suspect that many people will find many different uses for a suspension model. I know that this information is really beneficial to me as I try to input the correct variables into this suspension dynamics calculator. It should be useful in finding theoretically ideal spring rates and damper settings (either through re-valving or adjustments), among other things that will maximize the car's potential grip and performance.

Unfortunately, the accuracy of that data will be limited by the accuracy of the inputs, and most of the input data can only be determined from having a very precise model of the car's suspension. (And fancy software :D)

I wonder if damper companies (Bilstein, Koni, Eibach, etch) go into this much detail when developing their dampers.

@ EarlQ, :word:

All of the suspension nerds on here should read that paper. Not just because of the FAPs and jacking forces, either. Mitchell is the man, and he has a way of elegantly poo-pooing on everything you've ever learned.

My interpretation of roll center isn't horrifically wrong, but it isn't terribly useful either. Hopefully it will atleast serve as an elementary-level understanding for the next-level theory and kinematic simulations.

@ Ninjin,
A model is just that, a simplified and solvable interpretation of reality. A good model is reproducible and transferable. My sincere hope is that we can make something that anybody can validate for themselves and apply to their specific modification.

Ex: Somebody has heard that RCE Yellow lowering springs are the bee's thorax. They want to buy some, but would feel more comfortable knowing what the rear camber curves look like (since that's not factory adjustable). If we validate the base model, he can drop in the new spring rate and height... then be reasonably confident in the modest tire wear rates. Similarly, they could test the eccentric bushings, and see if that would correct the problem (before buying and installing them).

@ Ayau,
They have modelling, but when you're ballin' enough... you get big boy tools. 7 post rigs like this:
[ame="http://youtu.be/aYgR38lC8JI"]LINK to KW's rig[/ame]

and shock dynos like this:
[ame="http://youtu.be/32XLPjdDlRA"]LINK to Ohlins' shock dyno[/ame]

Even if it's a technician running the test and one experienced engineer calling the shots ... you can get a great setup. KONI and the rest have team of people on this, since it's their bread and butter. Just so it's said, 4 of the "posts" are the wheels, and the remainder are for the chassis.

I've started a spreadsheet of the suspension coordinates.

LINK to Google Docs Spreadsheet

Very few points are correct yet. The front has been approximated based on pictures and specifications. The rear has not been configured, and is simply the program defaults. I'll fill these in shortly.

I'll leave this publicly viewable. If anybody wants "edit" capabilities, just PM me.

That is awesome.

For people who track their cars: if you have data logs or track maps, can you please PM me? I want to build, verify, and correlate a few different simulations.