How to: Suspension Tuning
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This is a discussion on How to: Suspension Tuning within the Suspension & Wheels forums, part of the Tech & Modifying & General Repairs category; Hello ClubWRXers, this thread is something I wanted to create for quite sometime, yet haven't quite been able to put ...

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    Registered User wrx wagone's Avatar
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    How to: Suspension Tuning

    Hello ClubWRXers, this thread is something I wanted to create for quite sometime, yet haven't quite been able to put together in a well thought out structure. Recently however, I ended up in a discussion that really cemented much the information I intend to discuss in a coherent format. So here goes...

    Purpose of this thread: The information presented is designed to give members of our forum a general understanding of the basics of car setup. This is nowhere even remotely close to anything resembling a comprehensive guide. If you want that, people have written books. I would suggest looking up Fred Puhn. I will, however, discuss some fundamental concepts and ideas that I think are very useful to developing an understanding what you car is doing in a corner. My goal with this information is to give the reader the ability to develop a clear path to positively affecting the handling of their car on an asphalt or concrete surface.


    Disclaimer: With the purpose in mind, please read, enjoy, and ask questions; but also understand that whatever changes you make to your suspension will effect the handling dynamics of your vehicle. If this is not done properly or with care, you can very well influence the handling and stability very negatively. Enjoy at your own risk.



    Part 1: Tires

    How does one go about making their car handle better? What needs to be accomplished when your car sticks to the pavement like glue?

    The answer to this is pretty simple, better adhesion between tire and road. So the goal with suspension tuning becomes trying to make the best use of the available grip of your tires. So what's my best recommendation to make your car handle better? GET GOOD TIRES! Seriously. Once you have a good set of tires, please proceed with the following.

    Making the best use of your tires
    :

    I'm sure if you talked to Bridgestone, Yokohama, or Hoosier they would have a much longer, dorky, graph riddled answer for what makes a tire grip, as far as I'm concerned you should concern yourself with these three areas and little else:

    1. Contact Patch. To use cliche, this is literally where the rubber meets the road. You must maximize your tire's contact patch to have any hope of getting the most grip from that tire.

    2. Normal Force. This is force that will be used to press the tire into the pavement. It is this force, times the coefficient of friction between the tire and the pavement that produces the actual grip of the tire.

    3. Slip Angle. This is a term used to describe the amount or angle that the tread of the tire has moved in relation to the wheel. This website goes into a real good discussion on slip angles and how they influence handling: Slip Angles and Handling

    So from these items, you can quickly see that handling is, at its core, about maximizing your available contact area and increasing its effectiveness by controlling weight transfer.



    Part 2: Vehicle Dynamics and Suspension Equipment

    To look at the vehicle dynamics we will discuss the motion of the vehicle around three axes of rotation about it's CG (center of gravity). These are defined as:








    Pitch and roll are both created by the transfer of weight in the vehicle. Our end goal with controlling pitch and roll is to control the weight transfer of the vehicle and positively affect the vehicle dynamics is to produce the desired yaw (steering).

    So with all this discussion of the tires and the factors that influence their performance and the suspension components that allow us to control the vehicle dynamics to maximize the grip of our tires, what does that all mean for us?

    By tuning our suspension we are attempting to make changes to the pitch and roll of the vehicle to accomplish the following:

    Pitch:

    Control the amount of weight on the front tires during braking. Too much weight on the front end during braking means that the rear end isn't doing much at all to assist with braking and the rear end has become unweighted and lost grip. This reduced rear grip can create handling issues for the car on corner entry.

    Roll:
    1. Mitigate the loss of camber in the corner and maximize the contact patch of the tire.

    2. Control the transfer of weight to each tire to maximize the relationship between the total grip and the slip angles produced.

    It is this second concept which is a bit complicated and I will attempt to discuss in detail.

    From the "Slip Angles and Handling" webpage:
    Often times, in a corner, you will turn the steering wheel a certain number of degrees initially and then need to reduce the steering angle before you reach the apex of the turn. This is because the rear tires are generating a slip angle and some of the steering is taking place at the rear tires. It is even possible to travel through the turn with the steering wheel in the straight forward position because the rear tires are doing all of the steering.
    MAJOR POINT: You can set your suspension to actually steer the car with the rear tires as well as the front tires. The more tires you have working to get you around a corner the better. In reference to vehicle dynamics, this is yaw, or more commonly referred to as "rotation" or how well the car "rotates" through the corner.

    Increasing the slip angles at the rear of the car will make the race car looser (oversteer). Likewise, reducing the slip angles at the rear will often be interpreted by the driver as making the race car tighter (understeer).
    More to this point it is actually the ratio of the slip angle of the front tire to the slip angle of the rear tires that will determine if a car will understeer or oversteer. Ideally we would like to have a car which handles perfectly neutral, with maybe a slight tendency for oversteer. So with the Impreza platform in mind, we have our work cut out for us in trying to dial out the understeer that is inherent in the vehicle for many different reasons.

    To do this we should work to decrease the slip angle of the front tires and increase the slip angle of the rear tires.

    Weight transfer from the inside tires to the outside tires affects the slip angles. When your race car is in the turns, equally loaded tires will run at smaller slip angles. Conversely, a large difference between the inside tire's load and outside tire's load will cause the slip angle to increase.
    Effects on the Front End:
    From this we can see that adding more roll stiffness to the front end of the car will reduce the weight transfer and create smaller slip angles on the front tire. As this happens we also reduce the amount of weight which is applied to the outside tire in the corner. This is also the tire which should have the largest possible contact patch in the corner. By limiting weight transfer to this outside tire, we will be increasing understeer. With this interaction between camber loss, slip angle, and normal force; there becomes a maximum point in roll stiffness which will produce the most grip from the front tires.

    Effects on the Rear End:

    Increasing the weight transfer will produce more slip angle on the rear tires along with producing more camber loss. Both of these two will promote the car to oversteer. The slip angle will be very progressive and predictable in its loss of grip where the camber loss can produce very unpredictable results. Also a decrease in weight transfer will also produce oversteer as the normal force generated on the outside tire is no longer large enough to produce grip. Again you can see that there becomes a maximum point for roll stiffness where the maximum rear grip is produced.
    Last edited by wrx wagone; 11-07-2008 at 07:24 AM.
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    Registered User wrx wagone's Avatar
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    Part 3: Suspension Components

    So how are all of the above changes to the vehicle dynamics accomplished? Luckily for us suspension designers have created suspension components that will allow us to control pitch, roll, and their rate of change.

    Springs - Allow us to control both pitch and roll.
    Swaybars - Allow us to control roll.
    Dampers - Allow us to control the rate of change in pitch and roll.

    With these suspension components and how they are able to influence the vehicle dynamics, this leads me to develop the following sequence for proper suspension setup:

    Step 1: Since your springs are the only suspension component that will control pitch, (while also affecting roll) you must first set your spring rates based on your pitch control.

    Note: This is the source of much compromise for a street setup. You may find that the spring stiffness that yields the best results for handling are not the most ideally suited for comfort and daily driving. Comfort may very well become the driving factor in your selection of springs.

    Step 2: Now that you have selected springs and those springs have not only changed the pitch but have also changed the amount of roll in the vehicle, we must set the swaybars to produce the desired roll stiffness.

    Step 3: With springs and swaybars in place and dampers which are properly matched to the springs, we will now adjust the dampers to create the proper rate of change of both pitch and roll.

    Damper tuning articles:
    Koni Road Racing Setup Guide
    Neil Roberts Article on Shock Tuning



    Part 4: Real World Tuning Procedure

    At least in my mind with the basis of this procedure routed in the procedure as outlined above, the process for proper suspension tuning as based on the performance of the vehicle through a corner becomes as described below:

    Step 1: Alignment
    Start with a basic alignment. At least -2.0 front camber and about -1.5 rear camber with zero toe.


    Step 2: Testing and Data Acquisition


    This step is where you must break a corner into three separate parts and focus on what the car is doing or not doing during each of these three parts.

    1. Corner entry:
    Corner entry is completely front suspension related. How the car behaves at corner entry will tell you all of the following things about your front suspension:

    a) Pitch. How hard have you stood the car on it's nose during braking into this corner? Do you need stiffer springs - too much dive and unloading the rear end of the car? Do you need softer springs - too little weight transfer to load the front tires sufficiently to maximize braking?

    b) Dampers. How was the rate of transition in the corner? Did the car react too quickly or too slowly? Did you shock the tire with to quick of a weight transfer, or do you not transfer weight quickly enough to allow the tire to find grip?

    c) Roll. How much weight transfer did you get going into the corner? Was it too much or too little? Too much and you've taken the tire over it's contact patch and lost grip through camber loss. Too little and you've never transferred the weight the tire needs to stick.

    2. Mid-corner:
    This tells you about the steady-state balance of the car. Does the car have too much understeer or too much oversteer? This is the location of the corner where it becomes critical that the front and rear grip is very well matched.

    3. Corner exit:

    Just like corner entry is to the front end, corner exit is to the rear end.

    a) Pitch: Are the springs too soft - the car squats under acceleration and weight transfers from the front causing corner exit understeer? Are the springs too stiff - corner exit oversteer?

    b) Dampers: These are used to dial in oversteer on corner exit. Tight transition heavy course - dial the dampers back. Open sweeper corners - dial them up.

    c) Roll: Once you have your rear springs set, use your rear swaybar to dial mid-corner oversteer.


    Step 3: Tuning

    From the information gathered by understanding what a car is going throughout the corner, the proper procedure to car setup involves adjusting these components in this order:

    Start at corner entry.
    1. Front springs - Set amount of corner entry brake dive.
    2. Front dampers - Adjust to course layout and surface to control rate of weight transfer.
    Move to corner exit.
    3. Rear springs - Set amount of squat under acceleration.
    4. Rear dampers - Adjust to course layout and surface to control rate of weight transfer.
    Dial balance mid-corner.
    5. Front swaybar - Set amount of roll stiffness in mid-corner (understeer).
    6. Rear swaybar - Set amount of roll stiffness in mid-corner (oversteer).


    Step 4: Alignment and Tire Pressure


    With your suspension setup largely finished, you can now adjust your final alignment and tire pressure to tweak this setup to its full potential. These adjustments in alignment and tire pressures can be fairly effective adjusted based on tire temperatures. This is a very good article on this: Reading Tire Temperature

    Tire pressure:

    Increasing tire pressures will yield an increase in grip upto the point where the tire's carcass can no longer hold the shape of the tire tread area and the tread will balloon out decreasing the tire's contact patch.

    Decreasing tire pressure will yield and increase in the slip angle produced by the tire. Good for rear tires, bad for front tires.

    Front tires - Increase the pressure in the front tires to yield the maximum amount of front grip.

    Rear tires - To create oversteer, you can either increase the tire pressures to reduce the contact patch, or you can reduce the tire pressure to increase the slip angle produced by the tire. I would recommend lowering the pressure. This method of inducing oversteer is much more consistent.

    This is a great section from Dennis Grant on Tire Pressures: Autocross to Win: Tire Pressures

    Alignment:

    1. Adjust your static camber to produce the largest possible contact patch.

    2. Add toe out to the front to promote turn in and to the rear to promote rotation.


    Part 5: Conclusion

    I would like to leave you with these thoughts in regard to suspension setup:

    1. Maximize front grip first and then dial the rear suspension to match.

    2. You can steer the car with the rear tires. Loose is fast.

    3. Everything with suspension tuning must be done in balance. It is the ability of the suspension tuner through the understanding and experience of all the different variables that allows them to find this balance. This is the art of suspension tuning, mixing all the correct parts to make the end result the best it can be.


    Thank you very much for reading this setup guide. It is by no means complete and is very much a work in progress. Please feel free to discuss items, make suggestions for improvements, correct any errors, or clarify any over simplifications I have made. I'm still very much growing my understanding of car setup, and will continue to learn over many years to come.


    Interesting reading:
    Dennis Grant - Autocross to Win
    Last edited by wrx wagone; 02-27-2009 at 09:48 PM.
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    Moderating on the run! Big Sky WRX's Avatar
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    JR- nice informative thread- stuck

    I'll add a couple of tidbits from my experience. Sway bars are a great fine tuning tool, obviously adj ones are advantageous in that regard.

    As the Impreza uses a MacPherson strut front and rear (well up to 08 anyways) the camber changes cornering are significant over other suspension designs. This is the primary reason why large front sway bars work so well on this platform.

    ride heights- again due to the nature of MacPherson struts, lowering too far will degrade performance (even though you are lowering the center of gravity)- keep the lowering reasonable

    overall handling can be very driver subjective, the perfect setup for JR's driving style for example could be a nightmare for me or vice versa- that's where the fine tuning/experimenting really pays off
    "Simplify and add lightness." - Colin Chapman

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    Admiral Ackbar the 1st mycologist's Avatar
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    Awesome thread.
    "From a little spark may burst a mighty flame." - Dante
    "The stitch is lost unless the thread is knotted." - Italian proverb

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    Registered User Acrdys's Avatar
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    This... is a lot esier to understand than going through unabombers suspension faq's.

    Well, it leads me to the right direction...

    Thanx a ton for the info, I'll keep this in mind when I decide to order suspension parts.
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    Registered User Aphrodisiac's Avatar
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    When adjusting sway bars....

    a) are they hard to adjust in the driveway?
    b) how much do they adjust?
    c) where do you go to adjusting them tighter or looser?
    d) what set up (size wise) gives less understeer?
    e) what are the best sway bars and why?

    When scouting springs, struts, coilovers and such...

    a) what are the set up differences?
    b) what types are out there, and which work better?
    c) why springs VS why coilovers?

    Thanks for the attention, I take suspension much more seriously then anything else on a car and I feel my knowledge is good but lacking (like my experience).
    Fey
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    Registered User wrx wagone's Avatar
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    Answers in Bold.

    Quote Originally Posted by Aphrodisiac View Post
    When adjusting sway bars....

    a) are they hard to adjust in the driveway?
    No. Use ramps.
    b) how much do they adjust?
    The adjustment holes do make a very noticeable difference.
    c) where do you go to adjusting them tighter or looser?
    Shorter "arm" = stiffer bar.
    d) what set up (size wise) gives less understeer?
    Check out my swaybar FAQ for suggestions.
    e) what are the best sway bars and why?
    The ones that accomplish your goals.

    When scouting springs, struts, coilovers and such...

    a) what are the set up differences?
    b) what types are out there, and which work better?
    c) why springs VS why coilovers?
    The above questions end up being based on the goals that you have for your suspension and what compromises you are willing to make. A good street setup /= good track setup /= good autox setup.

    Unless you need stiff spring rates and the ability to corner balance the car there isn't really an overwhelming reason that coilovers are a must. A quality set of springs and struts will work just fine.


    Thanks for the attention, I take suspension much more seriously then anything else on a car and I feel my knowledge is good but lacking (like my experience).
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    Registered User Hold_Fast's Avatar
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    Just ordered Fred Puhn's book How to make your car handle.


    Awesome post though. I'll more than likely be back with questions after reading Puhn's book.
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    Registered User yo vanilla's Avatar
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    This is great theory, but how about creating one or two examples. For instance, say a guy has a stock 2007 WRX He's already got a good set of street tires and wants to make some minimal suspension upgrades. Meaning non-adjustable springs and dampers, so you can't test n tune them after the fact. So first he's got to pick a spring rate, but then how do you calculate what damper works with that spring? What is the stock specs?
    07 WRX TR | 03 mazda6
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    Registered User wrx wagone's Avatar
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    Quote Originally Posted by yo vanilla View Post
    This is great theory, but how about creating one or two examples. For instance, say a guy has a stock 2007 WRX
    I think I see where this is going...

    He's already got a good set of street tires and wants to make some minimal suspension upgrades. Meaning non-adjustable springs and dampers, so you can't test n tune them after the fact. So first he's got to pick a spring rate, but then how do you calculate what damper works with that spring? What is the stock specs?
    Your stock struts will not handle much, if anything, more than ~230 lb/in Front & ~200 lb/in Rear. Most WRX lowering springs are in this range. e.g.
    - STi Take offs & Pinks
    - Tein H-Techs
    - Prodrive
    - Eibach Prokit
    etc. etc.

    Also, since your dampers are limiting your spring rate selections, your suspension "tuning" will be limited to using swaybars to tune your roll stiffness. This is the situation that a majority of WRX owners end up with.
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    Registered User yo vanilla's Avatar
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    Sure, I've already got that. I'm not looking for lowering springs or anything. What I meant was, how to furmulate which dampers will match your selected springs.

    For instance say our guy chose 400/380lbl F/R springs. Random numbers for examples sake. How to determine what damper will match that? I want to know how to calculate it.
    07 WRX TR | 03 mazda6
    man a whole mess of nachos sounds good right now
    ***Hawk-Eye Alliance #95*** Kachow!!

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    Moderating on the run! Big Sky WRX's Avatar
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    it can be done, but not easily

    there are several things one needs before doing that

    you need Motion Ratios- I've provided how to calculate MR in my wagon thread

    then you will need a dynograph of the damper- not always easy to get ahold of

    lastly you need to figure out what % of critically damped you want your dampers to be- lots of folks feel in the 65-80% range is optimum

    good discussion of dampers here:

    DGs Autocross Secrets aka Autocross to Win
    "Simplify and add lightness." - Colin Chapman

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