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Old 07-25-2011, 09:33 AM   #1
grambles423
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OFFICIAL Suspension Theory and Modification Thread

We're working to use this thread as a technical discussion thread for noobs.

USEFUL FORUM RESOURCES FOR SUSPENSION

SUSPENSION OPTIONS: http://www.golfmk6.com/forums/showthread.php?t=50438
DRIVER GEAR SPRING THREAD: http://www.golfmk6.com/forums/showthread.php?t=12098
OFFICIAL SWAY BAR THREAD: http://www.golfmk6.com/forums/showthread.php?t=45368

Coilover Discussions
http://www.golfmk6.com/forums/showthread.php?t=45518
http://www.golfmk6.com/forums/showthread.php?t=43113

....many more


OLD ORIGINAL POST

UPDATE ON Page 3 POST #44

Alright guys. I apologize for my absence lately, I've had a lot going on. As some of you may be aware of, I've tried to develop a thread that'll provide a good suspension understanding and a background of how we (As Automotive Design engineers) vaguely develop suspension characteristics and get an idea of what we shoot for. Now a lot of the main stuff is handed down from the big guys such as wheelbase target, Lateral and Logitudanol Force Targets, Ground height, Moment Roll, roll pitch, roll center, center of Gravity, etc. etc.

With a lot of known variables we can start solving, or at least get an idea of where we are at and deteremine suspension feasibility and performance characteristics before the vehicle is even made. Once prototyping begins, we can always tweak things to get exactly what we want.

So Lets start:

What is the one talked about topic on car enthusiasts forums? Lowering Right? Or from what I gather at least. Most people say "Stock GTI handles so good, there is no need to modify it." Right and Wrong, for everyday use right. For tracking wrong. There is literally an infinite route you can go to make your car perform well on the tracck, but at what cost to the daily driveability do you want to spend?

Theory:
I dont have any measurements for my suspension so I cant get any final numbers, but if anyone is willing, go for it.

From my suspension notes:
"A vehicle with a suspension is not a rigid body. The unsprung mass stays (mainly) on the ground. The sprung mass does not roll about the center of gravity. The sprung mass rolls about a kinematic point known as the ‘roll center’ which is defined by suspension geometry. (For preliminary design, without other domain knowledge, take roll center to be at height of axles.) If the roll center is above ground, then the lateral force on the tires in a turn must be directed towards the turn’s center of rotation in order to maintain the vehicle’s equilibrium. The tires scrub to the outside to generate this force. If the roll center is at the ground, there is no tire scrub. If the roll center is below ground, the lateral force is oppositely directed, and the tires scrub into the turn. This causes a break in traction, and the vehicle motion becomes unstable. Roll centers are placed enough above ground that no combination of driving or failure will cause them to move below ground. Roll centers in rear suspension are usually placed higher than those in front suspension so that the rear will be more stable than the front, and the rear will not want to over-rotate (oversteer)."

How do you calculate the roll center?
Moment of Roll (How hard you're vehicle rolls) = Roll Stiffness * Roll angle

Roll Stiffness = Front Roll Stiffness + Rear Roll Stiffness = FRS + RRS

FRS = 0.5 * Wheel Rate * Front Trackwidth^2 + Stiffness of Front Sway

RRS = 0.5 * Wheel Rate * Rear Trackwidth^2 + Stiffness of Rear Sway

*Sway thickness can be forund on the internet if you google Sway stiffness formula

*Assume your wheel rates are the same on each side

Moment of Roll = Sprung Mass * Gravity * (D) * Roll Angle + Sprung Mass * Laterall Acceleration * (D)

Lateral Acceleration = Speed into the turn / Radius of the Turn

(D) = Height of center of gravity above roll center (if roll center is above CG, d < 0 )

*This part is a little hard to determine WHERE exaclty the roll center is, but I will help you. From Happian-Smith (An Introduction to Modern Vehicle Design, 2002):

Front:
"The Roll center for a MacPherson Strut suspension lies on the upper defining line of the moment of inertia of the upper mount of the strut perpendicular to the strut axis." I'll discuss further:

The front roll center of a car with the Mustang's MacPherson strut suspension can be found as follows:

- Draw a line at an angle of 90 degrees from the top of the front strut
- Draw a second line through the lower control arm. The point where these lines intersect is the instantaneous center
- Draw a third line from the instantaneous center to the center of the tire contact patch. The point where this third line crosses the car's centerline at the roll center.


http://www.miracerros.com/mustang/t_rollcenter.jpg

This will be your (D) for the front.

Rear:
"The roll center for a trailing arm suspension lies in the ground plane on the center line of the vehicle" MEANING...its basically on the ground. Your (D) would equal the distance from the COG to the ground.

Just connect to the two dots...and you get your roll axis:


http://www.miracerros.com/mustang/t_rollaxis.jpg

This is what your car rolls about when you turn. FYI that is just an example....but you get the idea.

What does this all mean?

You know your roll stiffness, now you need to find your Roll Moment, but in order to do that you must solve for your roll angle. AND in order to do that, you must solve an average roll center between the front and rear. Shouldnt be hard, Front + Rear divided by 2. Then just rearrange that roll angle formula:

Roll Angle = [Sprung mass * Laterall Acceleration * (Daverage) ] Divided by [Roll stiffness (Front+Rear) * Sprung Mass * gravity * (Daverage)]

And guess what? We're not done. You can now take this formula and find your wheel deflection:

Wheel Deflection = roll angle * trackwidth / 2, You can homogenize to either the front or rear, but then you'd have to break up the roll angle formula and just use the (D) for each specific case.

Wheel deflection of the front = roll angle of the front * front trackwidth / 2
and ditto for the rear.

What does this Ultimately entail?
Well...you can seen it can get pretty hairy, but as long as you pace through it, its not so bad.

I can safely say now, you can see theres an affect going on when lowering your vehicle. When lowering you drop the effective roll angle closer to the center of gravity and you stiffen the roll. Meaning, you can have hard springs and hard dampers and never see your car roll. Obviously the more roll, the more wheel deflection you get thus removing how much power you put to the ground when coming out of a turn. Thats it.....thats the whole idea of wanting a stiff suspension while tracking.

Now for daily driving, you can sacrifice some stiffness for some body roll. This helps for a smooth ride and happy passengers. I myself use Bilsteins on my new MKV but they're a little stiff. But DAMN do they handle nicely. They deflect the wheel barely an inch coming hard into turns. Do I track the car much? Not really, so in my honest opinion, I will be getting rid of them and sacrificing that performance for a better ride. But others will do differently.

Obviously all the equations are limited to suspension geometry. Meaning, you cant slap the wrong hardware and expect ultimate performance. Its all a matter of balance.

Lateral Load Transfer

This is a bit harder to explain. You have many many many many factors to consider. But its a good idea to touch on this for those you have a grasp on the concept.

Here are some references to read if you're curious:
http://en.wikipedia.org/wiki/Weight_transfer
http://www.neohio-scca.org/comp_clin...educed%202.pdf

Heres just a reference formula to blow your mind:

Front Turning Force = [mass of the vehicle * distance between CG and front trackwidth* Lateral Acceleration * height of CG above ground] / [wheelbase * front trackwidth]

Front Turning Force = All of that mess above + [Roll stiffness * Sprung mass * Lateral Acceleration * (Dfront)] / [front trackwidth * (roll stiffness - sprung mass * gravity * (Dfront))]

Ugh......then you can do the same for the rear. Now that you have both Front and Rear Turning Forces you can do the following:

If, Front Turning Force + Rear Turning Force > (vehicle mass * gravity) /2

Then the vehicle rolls over.

Eventually you'll get to a point where your suspension can handle the force its taking from the turn, but your tires cant. Keep that in mind when you're tracking the vehicle. Always have some nice sticky tires.

I KNOW I've missed some stuff and probably confused the hell out of most of you, but people have wanted to see if for quite some time. Any questions? Post them!

This should offer some insight as well:
http://forums.vwvortex.com/showthrea...rum-FAQ-Thread
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Last edited by grambles423; 07-27-2011 at 09:48 AM.
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Old 07-25-2011, 10:23 AM   #2
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Another great post as usual. I look forward to seeing this with some real world calculations. I have always been part of the "stock is best" crowd on the Mk6, mainly because of TechEd's influential posts while I was looking for a car. Specifically, this one: http://forums.vwvortex.com/showthread.php?4738232

That said, I would love to see this with actual numbers. I trust you walking us through it more than anyone saying what is right or wrong.


Clarification edit: I say the above with the H&R Street Performance Kit in mind as my next major mod, once stage 2 is completed. So seeing how the math works out on this is critical to me.
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Old 07-25-2011, 10:35 AM   #3
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Quote:
Originally Posted by Bender View Post
Another great post as usual. I look forward to seeing this with some real world calculations. I have always been part of the "stock is best" crowd on the Mk6, mainly because of TechEd's influential posts while I was looking for a car. Specifically, this one: http://forums.vwvortex.com/showthread.php?4738232

That said, I would love to see this with actual numbers. I trust you walking us through it more than anyone saying what is right or wrong.
I agree. I love stock dynamics. Had I purchased my current car without modifications I'd have a hard time justifying whether to spend the money. But since its lowered 1.25" and has hard Bilsteins, I can definitely see the performance benefit. However, most engineers design for comfort vs. performance balance. Since my bar is set with the Bilsteins, I can go softer shocks but will only see decreases in performance. But, thats just the price I pay for a happier wife lol. I KNOW you understand that one.

As far as actual numbers. They're all pretty finite. Thats the best part of these equations. It might get a little vague around the sway bar stiffness or the roll centers, but realistically they all have a real value you can put with it, so really theres no debate in the numbers.

Also, keep in mind that there are standards that need to be met when designing a vehicle. AND that EURO spec is lower than US spec. I'd say go as low as euro spec, adjust your damping values...and enjoy the car how it was SUPPOSED to be enjoyed.
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Old 07-25-2011, 10:46 AM   #4
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EU spec also have different suspension tuning than US/Canada car though. Considering damper is a "energy management" mechanism and through altering ride height and travel you alter the way suspension energy is dissipated, without corresponding change in damping force will change how that movement is managed. In EU car's case they also are available with adaptive dampers that provides a much more proactive way in managing movement....Its not just a case of "EU car ride this low so I can too". If EU(non-adaptive) shocks are stiffer to control the reduced travel, you need to have that also to achieve the same effect).

BTW the pics didn't work for your first post, I am always curious on how to find RC for a strut....SLA is much easier in that respect...
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Old 07-25-2011, 10:54 AM   #5
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EU spec also have different suspension tuning than US/Canada car though. Considering damper is a "energy management" mechanism and through altering ride height and travel you alter the way suspension energy is dissipated, without corresponding change in damping force will change how that movement is managed. In EU car's case they also are available with adaptive dampers that provides a much more proactive way in managing movement....Its not just a case of "EU car ride this low so I can too". If EU(non-adaptive) shocks are stiffer to control the reduced travel, you need to have that also to achieve the same effect).

BTW the pics didn't work for your first post, I am always curious on how to find RC for a strut....SLA is much easier in that respect...
Understandable, which is why you MUST alter your damping values when you lower. Otherwise you're not meeting that "spec". Euro springs on US Dampeners calls for a non complient suspension. Your control variables will probably underdamp the system and in essence you'd bounce differently.

I LOVE adaptive damper systems. They solve the all around problem that most suspensions face and curb to the driving characteristics and change respectively. Too bad we dont get them here, which is why finding dampers to suit the needs.

SIDE NOTE: In my most professional opinion, you should NEVER just change the springs and call it a day. Always find shocks to will compliment your springs
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Old 07-25-2011, 10:59 AM   #6
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Adaptive damper will help solving a lot of problem associated with our big heavy stock wheels.....

That BTW is what I work on, albeit on different application than passenger car, we also use the same technology as VW does in EU GTI....
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Old 07-25-2011, 11:03 AM   #7
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Adaptive damper will help solving a lot of problem associated with our big heavy stock wheels.....

That BTW is what I work on, albeit on different application than passenger car, we also use the same technology as VW does in EU GTI....
Are you using magnetic or air? I've got a patent running for mass production assist for an air system used on a Luxury brand.
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Old 07-25-2011, 11:16 AM   #8
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Solenoid driven valve, not the MR fluid based Delphi/Lord based Magnaride system, though they are our benchmark. We started on passenger car but my parent company dropped light vehicle business after the 2008 sh*tstorm....We then started working on the technology for hi-mobility heavy off-road vehicle(instantly moving the load to a order of magnitude more, lol, which is shocking for someone came from Formula SAE and used to work on cars that weighs less than 600lb with driver). Right now my projects are primarily adaptive damping for seats in commercial trucks and agricultural vehicles, which have comparable damping force need to passenger cars due to their horrid motion ratio....

We buy the valves from third party company and we make the damper hardware for it sized the flow characteristic based on the damping force requirement. And our control group develop controls for it. There are some advantage/disadvantage vs the MR based system....The core technology is on passenger cars in other brand for a while now, employed by Tenneco and Magneti-Marelli in various cars, we buys the same valve as them from the same company just using them in different application. We also are working on developing another proprietary valve in different packaging space with a different US company.
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Old 07-25-2011, 11:45 AM   #9
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What adaptive dampers are out there on the consumer market for reasonable prices besides the FSDs?

From a technical standpoint, what are your thoughts on an FSD (for example) paired with a VW DG spring?
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Old 07-25-2011, 12:52 PM   #10
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Originally Posted by Bender View Post
What adaptive dampers are out there on the consumer market for reasonable prices besides the FSDs?

From a technical standpoint, what are your thoughts on an FSD (for example) paired with a VW DG spring?

They don't sell aftermarket adaptive shocks....as damper is only part of the equation in how they work. The damper varies damping force, but to know how much damping force is needed when, that is up to the vehicle dynamic controller, the same thing that decide how much brake XDS needs to use or how much TCS or ESC needs to intervene to control the car. All those things needs to be linked, and powered....

FSD is still a passively valved damper, they work due to the different domain of vibration for something like low amplitude, high frequency event like road texture and surface imperfection, vs handling event like body roll and pitch/dive motion. And they built 2 hydraulic circuits tuned to accommodate both. They are still passive and when you operate outside of their tuning zone you are still compromised. A true adaptive(actually, semi-active may be more accurate) can almost infinitely vary the force level(in a given window) that gives you the right level of damping when you need it.

FSD is pretty novel though and I always thought its kinda neat in how it works....
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Old 07-25-2011, 12:54 PM   #11
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Quote:
Originally Posted by Bender View Post
What adaptive dampers are out there on the consumer market for reasonable prices besides the FSDs?

From a technical standpoint, what are your thoughts on an FSD (for example) paired with a VW DG spring?
I would also like to know what is thought of the FSD's. I am about to get the Neuspeed springs and thought the FSD's would compliment them very well.
Please tell!
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Old 07-25-2011, 01:03 PM   #12
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I think its hard to say. All the aftermarket product(non-adjustable ones especially) should be tuned to their application. But as damping and springing works hand in hand, what spring the dampers are tuned with will determine the effectiveness, as your natural frequency(just one parameter for sure), is dependent to both spring rate and mass. Which is always why its nicer to buy matched set of coil-overs as you know they are tuned together. Having never bought stuff like these for my car though I don't really know how they are usually marketed, do they suggest a spring to use with, suggest a rate or a range of rate...etc.
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Old 07-25-2011, 01:53 PM   #13
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I think its hard to say. All the aftermarket product(non-adjustable ones especially) should be tuned to their application. But as damping and springing works hand in hand, what spring the dampers are tuned with will determine the effectiveness, as your natural frequency(just one parameter for sure), is dependent to both spring rate and mass. Which is always why its nicer to buy matched set of coil-overs as you know they are tuned together. Having never bought stuff like these for my car though I don't really know how they are usually marketed, do they suggest a spring to use with, suggest a rate or a range of rate...etc.

Agreed. Definitely has a lot to deal with the damping coefficient too. Having high peaked osscillations from an underdamped system is what causes that "bouncy" feel. Something that takes fine tuning.


It would realistically be better if every manufacturer could display their damping coeffcients or spring stiffnesses on the spec sheet. But its all about the money and to hand out such information can create copies.

I would like to try out some FSDs because the engineering is quite cool. The fact that you have a multi variable damping coefficient isnt somehting particularly hard to come up with, but its the fact they pulled it off and packaged into a performance part to enthusiasts. I'm just worried about a squishy feel. Thats why I'm considering Adjustable yellows or FSDs.

Again, this brings me back to the fact it woould be easy to figure out what you're looking for if you had the values to plug into these formulas. I could easily detect sharp bumps versus low frequency force inputs and even relate them to the bode plots to find natural frequencies.....etc...etc.. I'm sure RacingManiac, you know most of this, but for others....
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Old 07-25-2011, 01:18 PM   #14
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TechEd's posts about suspension are what made me feel like I was right and it wasn't a waste to return my suspension to stock recently. In hindsight, I likely wouldn't have gotten coilovers at all had I stumbled across his posts prior to buying them.
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