Lol tell me how that goes. What school are you at? A basic solid works class teaches you nothing more than how to use the tools within solid works. It's still up to the user to understand wtf is going on.


Sent from my iPhone using Tapatalk

Not currently in my school yet, but solidworks was the only CAD program that came to mind lol

Developing a Proper Suspension Model
 

And so it begins....Attachment 95797

I'm still waiting on the actual book to arrive


Sent from my iPhone using Tapatalk

That is a good book. I have all three that are listed above. Another good one is Chassis Design

[ame="http://www.amazon.com/Chassis-Design-Principles-Analysis-R-206/dp/0768008263/ref=sr_1_1?ie=UTF8&qid=1416168491&sr=8-1&keywords=chassis+design"]Chassis Design: Principles and Analysis [R-206]: William F. Milliken, Douglas L. Milliken, Maurice Olley: 9780768008265: Amazon.com: Books[/ame]

I was going to go to sleep two hours ago, but I started reading this thread. Curse my love of knowledge.
But otherwise, awesome stuff! :thumbsup:

Ive searched but couldn't find it. Has anyone measured and graphed front and rear camber curves and toe change.

Not that I've seen anywhere.

If anyone has those curves, it would be a huge service to the community for choosing the "optimal ride height." I use quotes because you can always set the static values of camber and toe... but if it changes quickly under the normal weight transfers of racing, we might have to tweak the geometry.

Some guy from the WRX world posted this up, but it comes with no context or guarantee. I assume that 1 and 2 inch are the levels of drop. 10, 20, 40 might be wheel spacers (or offsets).

http://i48.photobucket.com/albums/f2...-1inchdrop.jpg

http://i48.photobucket.com/albums/f2...-2inchdrop.jpg

Yeah... I would toss in a couple dollars for an accurate camber curve.

Found a cool bit of code that might be useful for this crowd.

It's a MATLAB code that simulates shock dyno graphs.

Here's a cheat guide for our front suspension:
cwlbs = 618; srppi = 131; spmr = .92; shmr = .92; lsd = .65; knee = 3; hsd = .1;

And the rear suspension:
cwlbs = 539; srppi = 211; spmr = .83; shmr = .83; lsd = .65; knee = 3; hsd = .1;

TL;DR - calculate wheel rate, calculate the critical damping force, build a set of low speed damping forces, build a set of high speed damping forces, graph everything.

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')

If no one does it by the time I pull my car out of storage in spring I will likely do it. I did it on my Miata, it's not hard, just takes time.

Just as a quick exercise, I wanted to see what spring rates would yield different natural frequencies.

Ks = 4*pi^2*f_natural^2*m_sprung*motion_ratio^2
note: OptimumG uses motion ratio of wheel/spring (>1)

Here's what I found (fronts and rears in kg/mm).
1.5 Hz = 3.000 / 3.213
1.6 Hz = 3.413 / 3.656
1.7 Hz = 3.853 / 4.127
1.8 Hz = 4.319 / 4.627
1.9 Hz = 4.813 / 5.156
2.0 Hz = 5.332 / 5.713
2.1 Hz = 5.879 / 6.298
2.2 Hz = 6.452 / 6.912
2.3 Hz = 7.052 / 7.555
2.4 Hz = 7.679 / 8.226
2.5 Hz = 8.332 / 8.926
2.6 Hz = 9.012 / 9.655
2.7 Hz = 9.718 / 10.412
2.8 Hz = 10.452 / 11.197
2.9 Hz = 11.211 / 12.011
3.0 Hz = 11.998 / 12.854

That covers most of the coilover rates out there.

Far North Racing suggests:
Street car: 0.8 Hz
Occasional autocrosser: 1.0 - 1.5 Hz
Full-bore autocrosser: 2.2 - 2.5 Hz

Most people don't recommend a completely flat ride. For a street car, you might use 1.5 Hz front and 1.7 Hz rear. That puts us at 3K/4K. Most of the lowering spring kits are in that ballpark or slightly stiffer.

For a track car, 2.3 Hz in the front and 2.5 Hz. That puts us at 7K/9K. KW, Ground Control, and RSR coilovers have similar ratios, but slightly softer. Interesting to see how many coilovers are super-stiff in the front. That probably leads to a better feel and less actual grip.

Cool thread. I'll see what I can contribute after this weekend.

This is the inverse of what is usually used for motion ratio.
For the FR-S/BRZ, motion ratios are ~0.95 front and ~0.75 rear, from what I've gathered... So in the equation above you would use (1/0.95) and (1/0.75) for "motion ratio".

Important to use the right units of course, kg for mass and N/m for spring rate is easiest. N/m is kg/mm * 9.81 * 1000

Your spring rates look too low and too close together to get those natural frequencies. What did you use for mass and motion ratio?
Assuming 2950 lb. car (with driver), 54/46 weight distribution and subtracting 50 lb. from total weight at a corner to get sprung weight, I came up with 338kg sprung mass for a front corner and 286kg sprung mass for a rear corner.
Looking at 2Hz for an FR-S/BRZ, I get:

front Ks = 4pi^2 * 2Hz^2 * 338kg * (1/.95)^2 = 59,141 N/m = 6.0 kg/mm

rear Ks = 4pi^2 * 2Hz^2 * 286kg * (1/.75)^2 = 80,290 N/m = 8.2 kg/mm

Running your 5.332 and 5.713 kg/mm spring rate numbers, or 52,307 N/m and 56,045 N/m, I get
f = (1/2pi) * sqrt(k*motionratio^2/m)
f front = (1/2pi) * sqrt (52,307*0.95^2 / 338kg) = 1.88 Hz
f rear = (1/2pi) * sqrt(56,045*0.75^2 / 286kg) = 1.67 Hz

2 Hz is generally a pretty good street/track compromise. That is, too stiff for the street and too soft for the track!

The rule *used* to be to have a slightly higher rear natural frequency than front, but that has reversed over the past ~15 years or so.

For feel, my impression is that biasing stiffness to the rear improves feel and biasing to the front reduces it, but "feel" is often subjective...

So here is the front info I can gather. I admittedly have never analyzed a strut suspension in this software before so maybe I input something wrong but the data points are exact.

Front suspension @ stock ride height
Roll center height: 3.140" (Drops below the ground plane at only 1" bump)
Caster: -5.937 deg
Motion ratio: .95 and rising rate with bump. -1.5" bump (or lowered ride height) =.997MR. -2.5" bump = 1.050MR
Camber gain: -0.262 deg at 1" bump travel. Parabolic curve bottoms out at only 1.6" of bump travel, camber gain switches to positive

http://img.photobucket.com/albums/v3...p%20camber.jpg

Pure roll scenario from -3 deg (right turn) to +3 deg (left turn) @ stock ride height
The black vertical line indicates -1 deg of roll and the data points at -1 deg are in the colored boxes on the right.

http://img.photobucket.com/albums/v3...-MomentArm.jpg

This one is particularly nasty. This is a plot of -3 to 3 deg of roll at a ride height of -3 on the far left to +2 on the far right in 0.10" steps. The vertical black line is at -1" ride height which is right around where the roll center goes crazy shooting off to infinity and switching back and forth above and below the ground plane. I wouldn't personally ever set a BRZ to a 1" drop after looking at this without roll center correction.

http://img.photobucket.com/albums/v3...-MomentArm.jpg
Unfortunately, I think there is very little I could say about this layout that's positive. I've heard the OEMs have used the rubber bushing deflection to their advantage which I can't do, perhaps that was done with this suspension but installing sphericals, delrin, or polyurethane would eliminate that anyway.

If anyone thinks this was worth a little donation for my time, donate something to alz.org. On to the rear...

Could you please explain (in simple language :) ) what the consequence of dropping more than 1" inch is?

I can't believe I'm condoning this but if this is all correct, you're going to want to drop your car at least 1.5" and probably increase caster a degree. At that point you still have pretty bad but controllable camber gain (about +1deg per deg of roll), you keep your roll center from shooting off to infinity but it is still passing back and forth through the ground plane (not good) and the moment arm to the CG is seriously non-linear. So with that drop I'd get some kind of roll center correction but I haven't analyzed that yet.

-3 to +3 pure roll, no steering, at -1.5" drop
http://img.photobucket.com/albums/v3...-MomentArm.jpg

These graphs are REALLY helping me understand what is going on with our suspension and I'm starting to understand the problems you are outlining.

Thank you for your time, a donation will be made later today.

Awesome stuff, thanks Ryan!

What software is that?

- Andy

WinGeo

@RBbugBITme

What ride height, relative to stock, would you say is best? Or is it just a matter of also preforming the roll center correction when lowering?

do you have a higher rez version?

how much effort would it be to integrate steering and roll into the geometry calculation?


as a car would only roll during steering, so this is rather important.

I could be wrong, but here is my impression:

I don't think that a *virtual* point "shooting off to infinity" necessarily means bad things happen in the real world. It's just that the mathematical model is running into a discontinuity, probably due to a divisor getting close to zero somewhere.

Looking at the plots where everything goes to hell at -1 ride height, that might be a mathematical artifact and not representative of physical reality. The *actual* roll center height doesn't go from -5ish to +5ish by going to +infinity and flopping to -infinity first.

It's just a limitation of the mathematical model.

I *think*!

Sorry Calum, I would rather not and really can't recommend the best ride height. The intent here is to supply you with enough info to be able to make better educated decisions about your own setup. I have driven this car during a test drive once so I'm not the best guy to talk to about specific setups.

7thgear, if you go to the photobucket page with the image you can blow it up pretty big with good resolution. Yes I'll do a static roll angle and iterate steering angle at two different ride heights when I get home tonight. The point of these camber curves is to isolate your camber change due to chassis roll since you can be at the same roll angle for every degree of steering angle depending on vehicle speed.

Part of the problem it represents is that your jacking forces will switch from positive to negative (anti-jacking) mid-corner. No one ever wants to design that in on purpose. You either maintain a RC above ground or below and keep it there. The other issue is the wild swing in the moment arm length.

I don't think it's a problem. You'd never notice the difference between the roll center being 0.1" above the ground vs. 0.1" below the ground. Yeah, there will be an optimal range for RC height (highly dependent on the application), but nothing magical or bad happens when it passes through zero.

There *is* some wild movement of virtual mathematical points, but if you look at the actual loads going into the suspension mechanisms, you aren't seeing radical changes in actual physical moment arms.

I really think that the crazy curves going to +/-infinity are more indicative of localized regions where the *math* goes unstable and not an indication of actual problem areas.

I would have no qualms about testing an FR-S/BRZ at 0", -.5", -1", -1.5", -2" to see what happens. My guess is that nothing particularly weird happens at -1".

You've measured this? You can certainly calculate it but I'm not going to since I have no vested interest in this car.

The RC width going to infinity isn't an unstable equation. That is physically when the angle between the top of the strut and the lower control arm change from acute to obtuse, meaning the instant center jumps from one side of the vehicle to the other. You may not notice this in a car full of rubber bushings but no race car ever behaves like this.

The only benefit to the digressive curve of the moment arm is that it is digressive. If it were progressive the car would want to continually roll further and further without a progressive spring rate to match it.

Ryan, can you post the front view of the car at a 1 inch drop? It might clarify what you mean about roll center location at that height...

- Andy

Yeah, when I get home.

I think another thing to keep in mind here is that this is only taking into account the geometrical roll center http://balancemotorsport.co.uk/suspension-geometry. I haven't personally felt any of the unpredictable behaviors that all of this jacking --> anti-jacking would induce.

What you can absolutely get from the model is the camber gain in roll, heave, (steering if you had it).

I would be much more inclined to see simulations/data with the force based roll center.

Also, did you measure the suspension points on the car? Are you sure they are accurate? I just want to make sure you didn't use the points from the spreadsheet on this thread. :)

well, lateral overload or unloading would, leading to the tire giving out sooner rather than later is your immediate result of improper geometry


At the end of the day, we want a chassis that remains relatively stable while the wheels hug the road for dear life, with any change in load intuitive and proportional to a drivers inputs.


[ame="https://www.youtube.com/watch?v=zZQKepbGKiI"]- Robert Kubica Test - Flat out & Max Attack Moments - Test Mc 2015 - - YouTube[/ame]

I love the way rally cars sound!

I wasn't saying that I don't know what it feels like (though I probably don't).. I was saying that after tracking the car it feels predictable and stable to me at stock ride height.. But I'm not a racecar driver either and may or may not feel it unless I drove it after it was "fixed" and could compare.

These are based on Toyota/Scion 3D models that have been paid for so I can't share the data points.

http://img.photobucket.com/albums/v3...rz%20shell.jpg

I have all the FAP data as well and will try to apply some FAP theory but I'm limited on time, I have a side business developing suspension parts for other cars and I was at this until I think 3am last night.

Sweeeeet.. Subaru wouldn't give me any data when I asked them.. for obvious reasons, but it was worth a shot!

For anyone that wants to play with a model and move the car around to see what the effects are, this is fun http://www.racingaspirations.com/app...try-calculator.

I also wonder what sort of adjustments (for better or worse) the roll center correction kits for this car do.

Thanks for the info Ryan and keep it up!

how much does something like this cost?

I think only a few hundred a year plus the computer/RAM/GPU to handle some of the file sizes, and CAD software.

for all mass production macstrut vehicles roll adjustment kits are, imo, a universal good to a balanced degree


and more so for lowered cars (usually returning the geometry to OEM levels)




ideally you want a custom made tubular crossmember, front and rear, for ultimate customization (what those rally cars have).

Funny thing is, making a production car like this is easy peazy, but they will be so capable that they'll offer no fear for average drivers and they'd just crash in the canyons.

Does anyone have any information in regards to the actual amount of roll we see? (yes, I know it is car specific dependent on modifications and conditions) I have a fender video from last year's autocross season that shows what looks like 3-4" of roll total if turning left to right. This was with 245-40 200TW tires and the strano front bar.

Now that I have my coilovers and I'm lowered it would presumably be much less, I'm guessing in the range of 2.5" based on watching others run my car around the track. This may be a HUGE oversimplification, but are we aiming to have the outside tire flat on the ground while cornering?

Edit: after doing some reading it seems the increased spring rate is probably offset by the by the leverage of the center of gravity over the roll center. I'll shut up until i have a deeper understanding.

I have a question about suspension design and it probably requires too large of an answer but here goes. What values of caster, camber, etc are optimal throughout the range of motion of the suspension? I'm guessing this is determined by tires, body roll and forces expected at given angles.