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  1. #1
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    Default Front Suspension & Steering Geometry for Track Performance

    This Forum Thread is for discussing & learning about “Front Suspension & Steering Geometry for Track Performance.”

    This thread has a narrow focus, just as the title says.
    For a thread focused on: Overall Handling & Tuning for Track Performance ... click HERE.
    For a thread focused on: Rear Suspension & Geometry for Track Performance ... click HERE.
    For a thread focused on: Measuring & Modifying Your Front Suspension Geometry ... click HERE.
    For a thread focused on: Designing Aerodynamics for Track Performance ... click HERE.
    For a thread focused on: Safety for Pro-Touring Track Cars ... click HERE.
    For a thread focused on: Brake Selection ... click HERE.

    I promise to post advice only when I have significant knowledge & experience on the topic. Please don’t be offended if you ask me to speculate & I decline. I don’t like to guess, wing it or BS on things I don’t know. I figure you can wing it without my input, so no reason for me to wing it for you.



    A few guidelines I’m asking for this thread:
    1. I don't enjoy debating the merits of tuning strategies with anyone that thinks it should be set-up or tuned another way. It's not fun or valuable for me, so I simply don’t do it. Please don’t get mad if I won’t debate with you.

    2. If we see it different … let’s just agree to disagree & go run ’em on the track. Arguing on an internet forum just makes us all look stupid. Besides, that’s why they make race tracks, have competitions & then declare winners & losers.

    3. To my engineering friends … I promise to use the wrong terms … or the right terms the wrong way. Please don’t have a cow.

    4. To my car guy friends … I promise to communicate as clear as I can in “car guy” terms. Some stuff is just complex or very involved. If I’m not clear … call me on it. I’m writing some books and want car guys to understand them. When you’re really not clear on something I said … please bring it up & help me improve.

    5. I type so much, so fast, I often misspell or leave out words. Ignore the mistakes if it makes sense. But please bring it up if it doesn’t.

    6. I want people to ask questions. That’s why I’m starting this thread ... so we can discuss & learn. There are no stupid questions, so please don’t be embarrassed to ask about anything within the scope of the thread.

    7. If I think your questions … and the answers to them will be valuable to others … I want to leave it on this thread for all of us to learn from. If your questions get too specific to your car & I think it won’t be of value to others … I may ask you to start a separate thread where you & I can discuss your car more in-depth.

    8. Some people ask me things like “what should I do?” … and I can’t answer that. It’s your hot rod. I can tell you what doing “X’ or “Y” will do and you can decide what makes sense for you.

    9. It’s fun for me to share my knowledge & help people improve their cars. It’s fun for me to learn stuff. Let’s keep this thread fun.

    10. As we go along, I may re-read what I wrote ... fix typos ... and occasionally, fix or improve how I stated something. When I do this, I will color that statement red, so it stands out if you re-skim this thread at some time too.

    ------------------------------------------------------------------------------------------------------------

    Welcome,
    I’ll outline some key strategies & concepts on front suspensions, optimum geometry, steering & more. We’re going to keep the conversation to typical Pro-Touring type cars … front engine, rear wheel drive … but for improving track performance.

    If the conversation bleeds over into general handling … I may suggest we move the conversation to that forum thread. Same with rear suspensions.

    Let’s talk front ends …

    Other than singular purpose built race cars … like Formula, Indy, GTP, Midgets & Sprint Cars … production type bodied cars’ typical limiting factor … is front tire traction during cornering. In all my Stock Car, GT & Sedan Road Racing ventures, we know we can’t go any faster through the corners than the front end has grip. If you haven’t gone off the range with track width split or tire size split … it is relatively easy to balance the rear grip to the front. Front grip is top priority & typically where I’ve have an edge over my racing competitors because I understand it well.

    ------------------------------------------------------------------------------------------------------------

    Before we get started, let’s get on the same page with terms & critical tuning concepts.

    Shorthand Acronyms
    IF = Inside Front Tire
    IR = Inside Rear Tire
    OF = Outside Front Tire
    OR = Outside Rear Tire
    *Inside means the tire on the inside of the corner, regardless of corner direction.
    Outside is the tire on the outside of the corner.

    ARB = Anti-Roll Bar
    FLLD = Front Lateral Load Distribution
    RLLD = Rear Lateral Load Distribution
    TRS = Total Roll Stiffness
    WT = Weight Transfer

    RA = Roll Angle
    RC = Roll Center
    CG = Center of Gravity
    CL = Centerline

    UCA = Upper Control Arm
    LCA = Lower Control Arm
    BJC = Ball Joint Center
    IC = Instant Center is the pivot point of a suspension assembly or “Swing Arm”

    .
    Last edited by Ron Sutton; 12-07-2014 at 01:57 PM.
    Feel free to chime in or ask technical questions. I am here to help where I can.

    Ron Sutton

    Ron Sutton Race Technology
    Your One Stop, Turn & Go Fast, Car Building Resource Center for Autocross, Track, Road Racing & Triple Duty Pro-Touring Cars

    Check out our 400 Page Car Building Catalog HERE

    Features: Suspension, Chassis, Cages, Brakes, Rear Ends, Engines, Transmisssions, Aero & Much, Much More!


  2. #2
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    TERMS:

    Roll Centers = Cars have two roll centers … one as part of the front suspension & one as part of the rear suspension, that act as pivot points. When the car experiences body roll during cornering … everything above that pivot point rotates towards the outside of the corner … and everything below the pivot point rotates the opposite direction, towards the inside of the corner.

    Center of Gravity = Calculation of the car’s mass to determine where the center is in all 3 planes. When a car is cornering … the forces that act on the car to make it roll … act upon the car’s Center of Gravity (CG). With typical production cars & “most” race cars, the CG is above the roll center … acting like a lever. The distance between the height of the CG & the height of each Roll Center is called the “Moment Arm.” Think of it a lever. The farther apart the CG & roll center are … the more leverage the CG has over the roll center to make the car roll.

    Instant Center is the point where a real pivot point is, or two theoretical suspension lines come together, creating a pivot arc.
    Swing Arm is the length of the theoretical arc of a suspension assembly, created by the Instant Center.

    Static Camber is the tire angle (as viewed from the front) as the car sits at ride height. Straight up, 90 degrees to the road would be zero camber. Positive camber would have the top of tire leaned outward, away from the car. Negative camber would have the top of tire leaned inward, towards the center of the car.

    Camber Gain specifically refers to increasing negative camber (top of tire leaning inward, towards the center of the car) as the suspension compresses under braking & cornering.

    Dynamic Camber refers to actual angle of the tire … compared to the track surface … when the suspension is compressed & the steering turned from braking & turning in a corner. For our purposes, we are also assuming the car is being driven hard, at its limits, so the suspension compression & chassis/body roll are at their maximum.

    Static Caster is the spindle angle (viewed from the side with the wheel off). Straight up, 90 degrees to the road would be zero caster. Positive caster would have the top of spindle leaned back toward to cockpit. Negative caster would have the top of spindle leaned forward towards the front bumper.

    Caster gain is when the caster angle of the spindle increases (to the positive) as the suspension is compressed, by the upper ball joint migrating backwards and/or the lower ball joint migrating forward … as the control arms pivot up. This happens when the upper and/or lower control arms are mounted to create Anti-dive. If there is no Anti-dive, there is no caster gain. If there is Pro-Dive, there is actually caster loss.

    Anti-Dive is the mechanical leverage to resist or slow compression of the front suspension (to a degree) under braking forces. Anti-dive is achieved by mounting the upper control arms higher in the front & lower in the rear, creating an angled travel. Anti-dive can also be achieved by mounting the lower control arms lower in the front & higher in the rear, creating an angled travel. If both upper & lower control arms were level & parallel, the car would have zero Anti-dive.

    Pro-Dive is the opposite of Anti-dive. It is the mechanical leverage to assist or speed up compression of the front suspension (to a degree) under braking forces. Pro-dive is achieved by mounting the upper control arms lower in the front & higher in the rear, creating an angled travel. Pro-dive can also be achieved by mounting the lower control arms higher in the front & lower in the rear, creating an angled travel.

    Total Roll Stiffness is the mathematical calculation of the “roll resistance” built into the car with springs, ARB’s, track width & roll centers. Stiffer springs, bigger ARBs, higher roll centers & wider track widths make this number go UP & the roll angle of the car to be less. “Total Roll Stiffness” is expressed in foot-pounds per degree of roll angle … and it does guide us on how much the car will roll.

    Front Lateral Load Distribution & Rear Lateral Load Distribution (aka FLLD & RLLD)
    FLLD/RLLD are stated in percentages, not pounds. The two always add up to 100% as they are comparing front to rear roll resistance split. Knowing the percentages alone, will not provide clarity as to how much the car will roll … just how the front & rear roll in comparison to each other. If the FLLD % is higher than the RLLD % … that means the front suspension has a higher resistance to roll than the rear suspension ... and therefore the front of the car runs flatter than the rear of the suspension … which is the goal.

    Roll Angle: is the amount the car “rolls” on its roll axis (side-to-side) in cornering, usually expressed in degrees.
    Pitch Angle: is the amount the car “rotates” fore & aft under braking or acceleration, usually expressed by engineers in degrees & in inches of rise or dive by racers.

    Dive = is the front suspension compressing under braking & cornering forces.
    Rise = can refer to either end of the car rising up.
    Squat = refers to the car planting the rear end on launch or under acceleration
    Roll = Side to side body rotation … aka body roll.
    Pitch = Fore & aft body rotation. As when the front end dives & back end rises under braking or when the front end rises & the back end squats under acceleration.

    Track width = is center to center of the tread.
    Tread width = is outside to outside of the tread. (Not sidewall to sidewall)
    Tire width = is outside to outside of the sidewalls.
    A lot of people get these confused & our conversations get sidelined.

    Spring rate = pounds of linear force to compress the spring 1”. If a spring is rated at 500# … it takes 500# to compress it 1”
    Spring force = total amount of force (weight and/or weight transfer) on the spring. If that same spring was compressed 1.5” it would have 750# of force on it.

    Anti-Roll Bar, ARB, Sway Bar & Anti-Sway Bar … all mean the same thing. Kind of like “slim chance” & “fat chance” ...
    ARB Rate = Pounds of torsional force to twist the ARB 1 inch at the link mount.

    Rate = The rating of a device often expressed in pounds vs distance. A 450# spring takes 900# to compress 2”.
    Rate = The speed at which something happens, often expressed in time vs distance. 3” per second. 85 mph.
    * Yup, dual meanings.

    Grip & Bite = are my slang terms for tire traction.

    Push = Oval track slang for understeer, meaning the front tires have lost grip and the car is going towards the outside of the corner nose first.
    Loose = Oval track slang for oversteer meaning the rear tires have lost grip and the car is going towards the outside of the corner tail first.

    Tight = Is the condition before push, when the steering wheel feels “heavy” … is harder to turn … but the front tires have not lost grip yet.
    Free = Is the condition before loose, when the steering in the corner is easier because the car has “help” turning with the rear tires in a slight "glide" condition.

    Good Grip = is another term for "balanced" or "neutral" handling condition ... meaning both the front & rear tires have good traction, neither end is over powering the other & the car is turning well.

    .
    Feel free to chime in or ask technical questions. I am here to help where I can.

    Ron Sutton

    Ron Sutton Race Technology
    Your One Stop, Turn & Go Fast, Car Building Resource Center for Autocross, Track, Road Racing & Triple Duty Pro-Touring Cars

    Check out our 400 Page Car Building Catalog HERE

    Features: Suspension, Chassis, Cages, Brakes, Rear Ends, Engines, Transmisssions, Aero & Much, Much More!

  3. #3
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    9 of the 20 CRITICAL TUNING CONCEPTS:
    These 9 are very pertinent to our front suspension & geometry discussions on this thread. The full list is in the thread: Overall Handling & Tuning for Track Performance

    1. What you do WITH & TO the TIRES ... are the key to performance. Contact patch is the highest priority.

    2. Geometry design, settings & changes should be to improve how the tires contact the road dynamically.

    9. A production based car, can go no faster through a corner than the front tires can grip. Balancing the rear tire grip to the front … for balanced neutral handling … is relatively easy … compared to the complexities of optimizing front tire grip.

    10. The front tires need force, from weight transfer on corner entry, to provide front tire GRIP. Too little & the car pushes … too much & the car is loose on entry. The rear tires need force, from weight transfer on corner exit, to provide rear tire GRIP. Too little & the car is loose … too much & the car pushes on exit.

    11. Tuning to allow a suspension corner … to compress quicker or farther … provides more force & therefore more grip to that tire … up to the limits of the tire. Tuning to allow a suspension corner … to extend/rebound quicker or farther … provides more force & therefore more grip to the opposite corner’s tire … up to the limits of the tire.

    12. Softer springs allow more compression travel & therefore more force onto the tire … for MORE GRIP. Stiffer springs reduce compression travel & therefore lessen force onto the tire … for LESS GRIP.

    13. Optimum roll angle works both sides of the car’s tires “closer to even” ... within the optimum tire heat range … providing a consistent long run set-up & optimum cornering traction.

    18. The car’s Center of Gravity acts as a lever on the Roll Center … separately front & rear. Higher CG’s and/or lower RC’s increases roll angle. Lower CG’s and/or higher RC’s decrease roll angle. Getting the front & rear of the car to roll similar is desired. Getting them to roll the same is not, because …

    19. Goal: To have optimum grip on all tires and disengage the inside rear tire (to a degree) to turn well … then re-engage the inside rear tire (to a higher degree) for maximum forward bite on exit. So, on entry & mid-corner, the car needs to roll slightly less in the front to keep both front tires engaged for optimum front end grip, while allowing the car to roll slightly more in the rear to disengage the inside rear tire, to a small degree, to turn better. For optimal exit, the car will have more roll in the front & less in the rear to re-engage the inside rear tire to a higher degree than it was on entry & exit, for maximum forward bite (traction) on exit.

    ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

    Front End Geometry

    Most everyone knows camber, caster & KPI/SAI work together, but most don’t really understand HOW they work together & how they affect each other. I’ll do my best to explain it, but we’ll need to peel the onion one layer at a time, so bear with me.

    For those that don’t know what this is, KPI stands for King Pin Inclination & SAI stands for Steering Axis Inclination. They mean the same thing.

    KPI was a term coined back in the day of solid front axles when spindles actually used king pins. Steering Angle Inclination is a more correct modern term & is calculated simply by running a theoretical line through the upper & lower ball joints & comparing that angle to the actual spindle pin the hub spins on (rolling axis in the photo). (I use both terms because many race car guys are used to the older term of KPI.) See illustration.

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    Last edited by Ron Sutton; 12-08-2014 at 11:38 AM.
    Feel free to chime in or ask technical questions. I am here to help where I can.

    Ron Sutton

    Ron Sutton Race Technology
    Your One Stop, Turn & Go Fast, Car Building Resource Center for Autocross, Track, Road Racing & Triple Duty Pro-Touring Cars

    Check out our 400 Page Car Building Catalog HERE

    Features: Suspension, Chassis, Cages, Brakes, Rear Ends, Engines, Transmisssions, Aero & Much, Much More!

  4. #4
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    Think of caster as “dynamic camber” … since caster has no affect on angle of the tires & wheels … until you turn the steering. Then caster is tipping the top of BOTH tires towards the inside of the corner you’re turning into (Good).

    One purpose of KPI angle is help make the scrub radius lower. You can look at the illustration above & imagine how big the scrub radius would be if the KPI was straight up & down. If the front tire stays in the same location more KPI angle makes the scrub radius smaller. Less KPI angle would increase the scrub radius.

    A bigger scrub radius means the tire is farther from the steering pivot … making the arc bigger that tire has to make in order to pivot. Not a big deal at low speeds, but in track conditions, at higher speeds, with the tire at its limit of grip … when you turn the wheel more, you are “torquing” the tire tread around a big axis to turn. This “rips” the tread across the pavement, causing the tire to lose a degree of grip. The bigger the scrub radius ... the higher degree of grip is being lost when you turn the steering. A zero scrub radius means the center of the steering pivot & the center of the tire tread are the same. This pivots the tire right in the center of the tread reducing lost traction to the minumum. KPI has pros & cons.

    Think of KPI as a different kind of caster that affects the “dynamic camber” … since it also has no affect on the angle of the tires & wheels … until you turn the steering. But unlike caster, it is not tipping both tires towards the inside of the corner you’re turning into. KPI is tipping the top of the outside tire out towards the outside of the corner you’re turning into (BAD) and tipping the top of the inside tire in towards the inside of the corner (Good).

    When the KPI/Caster Split favors the KPI … the tire & wheel, on the outside of corners, goes into a state of positive camber (BAD) … rolling over on the outside part of the tread and sidewall of the tire … with the inside part of the tread becoming unloaded. Basically, at this point, the actual tread making contact with the pavement (contact patch) gets narrower, making it incapable of maintaining the speed it was capable of an instant earlier, when it had a full contact patch.

    Now let’s talk about the tire on the inside of the corner. Some cars roll so much the inside suspension goes into a “droop” or state of extension … and if that car has negative camber gain built in … the droop actually helps the inside tire stand straighter. For cars don’t roll as much … and that compress the suspension on the inside tire & wheel when cornering, the negative camber gain on the tire on the inside of the corner is tilting that inside tire the wrong way. It is rolling over on the inside part of the tread and sidewall of the tire … with the outside part of the tread becoming unloaded. Also making the contact patch narrower, making it incapable of maintaining the speed it was capable of an instant earlier, when it had more contact patch.

    So your front tires that were already at their limit of grip … just lost a significant amount of contact patch & essentially got narrower … and lost even more front traction … creating an instantaneous push or understeer condition.

    The amount of dynamic camber loss is minimal with slight amounts of steering input on large sweeping corners, but grows exponentially worse with higher rates of steering input (front wheel steering angle) on tighter corners. More caster would help both situations … creating more dynamic camber the correct way for both tires … keeping the tire contact patches flatter on the track surface. But how much is enough? Read on.

    Now let’s discuss how Caster & KPI can work together for an optimum combination …

    Racing spindles run the gamut of KPI angles. In oval track stock cars where the control arms are pretty average in length & the wheels are out wide … making the scrub radius quite large … racers typically run spindles with 8-12 degrees of KPI. In Trans Am & other road race cars where the control arms are longer & the wheels have a lot of back spacing … making the scrub radius smaller if not zero … racers typically run spindles with 3-5 degrees of KPI.

    For the street & Pro Touring market, there are a lot of spindle options … usually with KPI somewhat in the middle of the extremes outlined in the racing paragraph above. OEM Ford, Mustang II & the ATS spindle from Speedtech all have 8.0 degrees of KPI. Correction: Wilwood Pro Spindles & some aftermarket dropped Pinto/Nustang II spindles have 11° KPI. A lot of GM spindles have 8.75. The C5/C6 spindles are 8.66° KPI. For our conversation … and math purposes … let’s use a spindle with 8.0 degrees of KPI/SAI as our sample.

    If you were to set both the caster & camber at zero … and rotated the spindle 90 degrees each direction … the difference would be 2x the KPI/SAI angle of 8.0 degrees … so in this case 16.0 degrees.

    We know the wheels don’t turn anywhere near 90 degrees, but this example makes everything more clear. Please humor me & follow along closely, because I’m about to share something that is one of the most overlooked keys to proper cornering set-up. We will account for the ACTUAL steering turning radius later.

    If you rotate the spindle 90 degrees toward the front (like the wheel is turning on an outside corner) the tire & wheel experience 8.0 degrees of camber loss (goes into positive camber). Bad … very bad for the outside tire of a corner. :(

    If you rotate the spindle 90 degrees toward the rear (like the wheel is turning on an inside corner) the tire & wheel also experience 8.0 degrees of camber loss (goes into positive camber). But this good for the inside tire of a corner.

    Reminder, obviously we are not turning the wheel 90 degrees in the real world, so don’t lock in on the numbers “too much” … just the concept.

    Caster is different. If we set caster at 8.0 degrees positive (top to the rear) & leave KPI/SAI out of the equation, as if we had a spindle with zero KPI/SAI … and you rotate the spindle 90 degrees toward the front (like the wheel is turning on an outside corner) the tire & wheel experience 8.0 degrees of camber gain (goes into negative camber). The right direction for the outside tire in a corner. If you rotate the spindle 90 degrees toward the rear (like the wheel is turning on an inside corner) the tire & wheel experience 8.0 degrees of camber loss (goes into positive camber). And this is the right direction for the inside tire of a corner.

    So … caster helps both the inside & outside wheel & tire.

    Here’s the most important piece of info to know at this point. It is the first & most important key to getting the front tires to use their full contract patch when cornering … increasing front end grip & turning speed. Drum roll please …

    Caster helps offset (reduces) the KPI effect on the wheel & tire on the outside corner … and compounds (adds to) KPI/SAI on the wheel & tire on the inside corner.
    Read that again. It’s very important.


    This is called KPI/Caster Split. Caster helps offset the negative effects of the spindle KPI angle on the outside front wheel ... and caster compounds & adds to the advantages of the KPI angle on the inside front wheel. When the KPI is significantly greater than the caster (unless the car has a LOT of Camber) the outside wheel is going to lose camber as the steering is turned & roll over on the outside front tire. Ugly.

    The greater the KPI/Caster Split is favoring the KPI, the worse the problem. On the other hand if the KPI/Caster split favors the caster … meaning the caster is greater than the KPI, the outside wheel is going to gain camber as the steering is turned, creating a flatter, better tire contact patch. The inside wheel also gets cambered the correct direction (for the inside wheel) and both front tires stay flatter to the road, have more grip, better turning & higher corner speeds.

    Sooo … if we set the car up using spindles with 8.0 degrees of KPI/SAI and 8.0 degrees of caster … and you rotate the spindle 90 degrees toward the front (like the wheel is turning on an outside corner) the tire & wheel experience 0 degrees of camber gain or loss. Because KPI & Caster are not linear in their rotation, it is only true zero at 0° & 90° rotation, but that doesn't change the concept, it just makes the math harder. Frankly I don't use the trig formula. I use Performance Trends Suspension Analyzer to see the Dynamic Camber in both numbers & 3D drawing. (See it HERE.)

    I'm going to explain it like they are linear ... even though the math below is not spot on ... so it is easier to understand. The actual result will be only slightly different. If you rotate the spindle 90 degrees toward the rear (like the wheel is turning on an inside corner) the tire & wheel experiences 16.0 degrees of camber loss (goes into positive camber). This is the right direction for the inside tire of a corner … way too much ... but we’re not turning 90 degrees. We’re turning somewhere from 0 to 25 degrees. What if the wheels were turning 15 degrees? … that’s 1/6 of 90 degrees … times 16.0 … equals 2.67 degrees … the right direction.

    At this early point in peeling the layers of the onion ... we have:
    Outside Front Tire at 0.0 degrees (OK)
    Inside Front Tire +2.67 degrees (Good)

    You’re probably going “Hmmmm” … but we don’t have the whole picture yet.
    We have a lot of other geometry to factor in. Remember, we’re peeling this onion a layer at a time, so we’ll get to camber gain, chassis/body roll angle & static camber in steps.

    Camber gain & chassis roll angle are next. Chassis roll angle hurts the contact patch of both tires. Camber gain (towards negative) helps the contact patch for your outside tire & hurts the contact patch for the inside tire.

    Chassis/Body Roll Angle
    Stock production cars have a HIGH roll angle when pushed to their limits. Pro level race cars running high travel/low roll angle suspensions … obviously have pretty low roll angle. But’re we’re talking modified PT cars for track purposes. Way better than stock production cars, but not quite race cars.

    Because higher roll angles are an enemy of proper geometry & optimum contact patch … for this conversation let’s say we’re running a milder version of the high travel/low roll suspension … and have 1.5 degrees chassis/body roll in the corners. Obviously any chassis/body roll is ot the outside of the corner … and therefore hurtful to the contact patch angle of both tires.

    If we add chassis/body roll angle into our numbers above, now we have
    Outside Front Tire +1.50 degrees (Bad)
    Inside Front Tire +1.17 degrees (Good)

    Camber Gain
    On the outside tire, if you worked out your camber gain to be 1.5 degrees negative “in dive” … and assuming we have a modern low roll angle suspension with chassis/body roll angle of 1.5 degrees … those two just neutralized each other. The inside tire, of this car in the same corner, is compressed, but not as far, so it doesn’t have as much camber gain towards negative (reminder: camber gain towards negative is bad on the inside tire).

    Let’s say we end up with 2/3 the compression travel on the inside tire & end up with 1.0 degrees negative camber gain (the bad direction for the inside tire) … so what does that do for us at this level of “onion peeling”?

    Dynamically we have:
    Outside Front Tire 0.0 degrees (OK)
    Inside Front Tire +0.17 degrees (OK)
    Not optimum yet, but we’re going the right direction & we’re not done yet. :thumbsup:

    The next layer of the onion is static camber.
    You need SOME static camber … to help with initial steering turn-in responsiveness. Just don’t get greedy. In road racing or AutoX where you’re turning left & right, static camber is like camber gain. It helps the contact patch on the outside tire & hurts on the inside tire. For this example, let’s add 1.0 degrees of static camber.

    Now with static camber added … with your car hard in the corner … suspension in dive, wheel turned 15 degrees for a tight corner …

    Dynamically we have:
    Outside Front Tire -1.0 degrees (Good)
    Inside Front Tire -0.83 degrees (Bad)
    Not optimum yet, but we’re we’re not done yet.

    [B]Now, here is another part I love. You simply add caster until the contact patches of both tires are flat & happy. And from this point the math is easy.

    Add 1.0 degree of caster and …
    Outside Front Tire -2.0 degrees (Good)
    Inside Front Tire +0.17 degrees (Good)

    Add 1.25 degree of caster and …
    Outside Front Tire -2.25 degrees (Very Good)
    Inside Front Tire +0.42 degrees (Very Good)

    Add 1.5 degree of caster and …
    Outside Front Tire -2.5 degrees (Very Good)
    Inside Front Tire +0.67 degrees (Very Good)

    There are many factors that will define your optimum set-up, but this creates a baseline that is darn close.

    **P.S. I like to end up “around” 1.5-3 degrees more dynamic camber on the outside tire, since the outside tire is loaded so much more. This varies with tire grip (think TW200 versus Slicks) and with sidewall height, design and rim width.

    There is a little more involved in this, when you involve exact steering angles for specific corners. The good news is … when you run tighter corners requiring more steering angle than 15 degrees … the caster increases the dynamic camber to help the tires maintain flat contact patches. I use Performance Trends Suspension Analyzer (HERE) to plug in all the info & know exactly what dynamic camber I have at different steering angles & different camber gain & different suspension travels.

    The best way I have found to work out a front end setting is to start with KPI/SAI & caster … then bring in camber gain … and finally static camber … to achieve the optimum dynamic camber for BOTH tires. Regardless of how you get there ... all of these geometry pieces need to work together in harmony to achieve full, optimum contact patches for both front tires in hard cornering situations … for optimum cornering grip & speed.

    It probably is clearer now why getting advice on one setting that worked for a buddy’s car … without knowing the whole picture … can be misleading. As a tuner, I couldn’t imagine setting the caster without knowing the spindle KPI & the car’s camber gain … and then of course testing on track with tire crayon on the edges every run (plus taking tire temps).

    It’s been said a zillion times. It’s the whole package, not one part or one setting.

    -------------------------------------------------------------------------------

    Remember the KPI/Caster Split concept ... if the caster is greater than the KPI, the outside wheel is going to gain camber as the steering is turned, creating a flatter, better tire contact patch. The inside wheel also gets cambered the correct direction (for the inside wheel) and both front tires have more grip, better turning & higher corner speeds.

    When I'm designing a front suspension for a specific class with rules on what spindle we can run, I pick the best spindle available under the rules and design everything else to either fix or compliment that spindle. Factory spindles usually have KPI/SAI ranging from 7-10 degrees. When I have to run a factory spindle ... I know I'm going to end up with 1-2 degrees of caster more than the KPI.

    I designed & raced NASCAR Modifieds with Factory GM #2 spindles with 8.75° KPI. 10-10.25 degrees of caster produced awesome results. We had a crew chief go off the range with set-ups & try 6-7 degrees of caster, but the cars always pushed in mid corner ... and snapped loose on exit. He was used to running less caster, but didn't take into account the KPI of the spindles we had to run.

    -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

    The whole combination of KPI/SAI, caster, caster gain, camber, camber gain, Ackerman, toe, steering ratio, etc. ... ALL have to be designed together for optimum cornering performance. All of them are important, but the KPI/Caster Split is critical & often not fully understood.

    .
    Last edited by Ron Sutton; 02-25-2016 at 08:25 AM.
    Feel free to chime in or ask technical questions. I am here to help where I can.

    Ron Sutton

    Ron Sutton Race Technology
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  5. #5
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    When I'm designing a front suspension with no rules on what spindle we can run, I design the spindles & have them built. Then I'm not trying to fix anything ... and everything else in the front suspension can be designed to compliment that spindle. For a road racing car, I designed the spindle with 3 degrees of KPI/SAI ... and designed the rest of the the front suspension around what is called a "zero scrub" set-up ... & we ended up with 5 degrees of caster for optimum handling. This car did NOT require a high caster number, because the spindle KPI was lower. What is optimum for tight cornering is having the KPI/Caster Split slightly favoring the Caster.

    Another successful car I designed with 5 degrees of KPI/SAI ... ended up with optimum handling with 7 degrees of caster ... depending on the track. Again, the key was the KPI/Caster Split slightly favoring the Caster.

    This higher amount of caster seems odd to most veteran mechanics & street car guys used to running 1-3 degrees of caster. But when you look at the newer Corvettes, Vipers, BMW’s, Mercedes, Etc. … you'll see they run a lot more caster than what most think of as "typical”. The Factory GM specs for the C6 ZR1 is 7.7°-8.3° of caster. The spindle has 8.66° of KPI ... so owners that compete & win in the C6's typically increase the caster to 9.5°-10.5° of caster ... with the KPI/Caster Split favoring the Caster.

    The common denominator is we are almost always running more caster than KPI/SAI ... if we can. We run spindles with lower KPI where we can too ... but it requires running wheels with DEEP back spacing to get the scrub radius low (or sometimes zero). So simply ordering a spindle with smaller KPI is NOT a bolt-on solution.

    There are a lot of spindle options, including custom spindles, which cost less than most folks think. One BIG question in this equation though is Scrub Radius. For optimum AutoX performance. Scrub Radius is important. The smaller the better … down to zero dynamically … and KPI/SAI plays a big role in this.

    The first goal with Scrub Radius … if possible … is reducing it, so the tire is not having to torque itself around a big radius when you turn the wheel. If that is not possible … the 2nd goal would be … at least not making it bigger. Caster, KPI/SAI, wheel back spacing & Scrub Radius … all need to be worked out together. You should not arbitrarily pick a spec for any one of these things without considering how they affect each other.

    How do you work out a front end geometry combination?

    All of us know that each front suspension geometry setting affects the overall picture ... and to a degree ... each other. As a race car Designer & Crew Chief, I can't look at just one individual setting. I have to look at the spindle KPI/SAI, Caster, Caster Gain/Loss, Camber, Camber Gain/Loss as a team ... a team of geometry devices that I need to work together in harmony to improve how the tires contact the road dynamically.

    Having done this for years, designing, building, tuning & racing a lot of cars, in just about every type of racing, has given me some firsthand insight into how all these things work. So now it's a little easier & quicker for me to "get a set-up there" to the sweet spot. I have a process that gets me there quickest ... with the least back-n-forth.

    First … some guidelines:
    Zero scrub radius with long control arms & deep backspaced wheels is optimum, but many race series rules prevent us from achieving this, with rules on the LCA. If the rules allow, we're running long control arms & a low KPI spindle with deep backspaced wheels & achieving low to zero scrub radius. (Never zero statically, but close)

    But when the racing series rules restricting our LCA choices prevent us getting the ball joints "out there" … making us choose between track width & scrub radius … we are “usually” going with wheels with less backspacing to achieve the maximum track width allowed by rules … then running higher KPI spindles to reduce the scrub radius as much as we can.

    I've had people ask me why don't we give up track width to achieve a lower or zero scrub radius ... but in the big picture ... track width trumps scrub radius, up to a point. Everything has its limits & there are exceptions for everything. Tight AutoX courses sometimes favor narrower cars with narrower track widths. But most anything faster than that … like road courses … favor a wider track width. When you have no rules or limitations, you making everything optimum. When rules prevent that, you’re shooting for the best overall compromise.

    I have a step-by-step process I follow ...

    with the goal being optimizing the contact patch of both tires, while turning hard on tight corners of AutoX tracks or Road Courses .

    A. I have to work out the Spindle KPI/SAI with the length of the A-arms, tire width & wheel backspacing ... to end up with a desirable scrub radius.

    When I’m working out a car for class rules, those rules often limit what you can do for spindles & wheels. In more unlimited series there are less rules. On the street, there are no rules, except the ones you impose based on your desires, budget & priorities.

    If I have a scenario where we are running front wheels with a lot of backspacing … that gets the KPI/SAI closer to the center of the tire … reducing Scrub Radius. “Zero Scrub” is optimum, but challenging to achieve for packaging reasons. It requires very wide wheels & deep backspacing. You can go too far ... with REALLY DEEP back spaced wheels & HIGH KPI/SAI spindles … you can get into a situation of negative Scrub Radius, which isn’t desired. Reminder: Your goal is not zero scrub radius static ... because it will result in negative Scrub Radius. The goal is zero scrub radius (or close to it) dynamically with the car in dive, roll & the tire at slip angle.

    I recently designed a front end with 12” wide wheels & 10” of backspacing. A 8-10 degree KPI/SAI spindle would have put it into negative Scrub Radius. The spindle design & hub design also play a part on the packaging with the distance from the steering axis to the wheel hub face. With the hub we used & a 5 degree spindle, it created a zero scrub radius package (dynamically).

    ------------------------------------------------------------------------------------------------------------

    B. Once I know the spindle KPI/SAI, that guides me on how much “static” caster I need to build in to achieve a KPI/Caster Split favoring the caster.

    Optimally, we need the 1.0-3.0 degrees more caster than KPI/SAI angle. A lot of Ford passenger cars run 8.0 KPI, so we’re shooting for 9.0+. A lot of GM spindles are 8.75 KPI, so we’re shooting for 9.75 +/-. If we can run a lower KPI/SAI spindle … like in the example above using a 5 degree KPI spindle … we’ll need 6.0+ degrees of caster.

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    C. I need to work out if we want caster gain, or for the caster to stay the same, as the front suspension compresses fully under braking & turning (Term = "Dive") Think of this as “dynamic” caster, because it only happens when the suspension is compressed.

    Quick Primer: When the front suspension compresses, if the angles of the lower control arm cause the lower Ball Joint to move forward and/or the angles of the upper control arm cause the upper Ball Joint to move back … that creates caster GAIN … in dive (compression). When the front suspension compresses, if the angles of the lower control arm cause the lower Ball Joint to move back and/or the angles of the upper control arm cause the upper Ball Joint to move forward … that creates caster LOSS … in dive (compression). We never want loss. When the front suspension compresses, if the angles of the lower control arm and/or upper control arm combine for no change in the ball joint locations … that is considered Caster Neutral. *Ask if you want the “how” explained more in-depth.

    If we can get all the “static” caster we need, we will typically run only a small amount of caster gain. If we can’t get enough static caster, we need to build in more of caster gain to help us get to the desired total number.

    ------------------------------------------------------------------------------------------------------------

    D. If I can get the caster I want ... statically and/or with gain ... so the KPI/Caster Split favors the caster at least 1.0+ ... I can run less Camber, which is the goal. We're always going to run SOME static camber (negative) … say –0.5 at a MINIMUM … and we always want SOME Camber gain … but if we don’t end up with a KPI/Caster Split favoring the Caster by 1.0+ degrees … then we need to make the difference with Static Camber & Camber Gain.

    This is NOT ideal, because Camber by itself helps the angle of the outer tire achieve optimum contact patch … and hurts the angle of the inner tire, preventing optimum contact patch. The more static camber & dynamic camber we have to run … to make the outer tire work best … the more it hurts the contact patch of the inner tire.

    If we can get to the optimum angle with a KPI/Caster split favoring the caster by 1.0-3.0 degrees … either statically or in dive … we will run smaller amounts of static camber & camber gain. This is optimum. But if we don’t … we’ll run all the camber we need to … to make that outer tire WORK.

    ------------------------------------------------------------------------------------------------------------

    E. Once I know how much camber I need for that combo, I work out how much of it is going to be static camber & how much is going to be camber gain.

    We always want some static camber. -0.5 is like the minimum. I like -1.0 to -2.0 … IF everything above falls into place. But even this depends on how much camber gain we end up with.

    ------------------------------------------------------------------------------------------------------------

    F. How we get the camber gain, & how much, affects the car's static & dynamic roll center ... so they have to work as a team too.

    Our desired roll center plays a role in this decision, because all the A-Arm angles creating Instant Centers determines where the static Roll Center is ... and where it goes dynamically in dive.

    It should be clear now why I don't start with Camber.

    ------------------------------------------------------------------------------------------------------------

    All of this ... is to optimize both front tire's contact patch with the asphalt ... In the car's dynamic states when it's driven HARD ... meaning turning, braking, rolling, unwinding & accelerating to the limits of the car, tires & driver ... sometimes beyond.

    I realize this is a lot to digest. Don’t be afraid to ask questions where ever I wasn’t clear.

    .
    Feel free to chime in or ask technical questions. I am here to help where I can.

    Ron Sutton

    Ron Sutton Race Technology
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  6. #6
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    *Caster gain is achieved when the front A-arm geometry is set for anti-dive. Zero caster loss or gain is achieved with zero anti-dive. Caster loss is achieved when the front A-arm geometry is set for the opposite of anti-dive … called “pro-dive.”

    ** The more static camber you have to run to optimize the outside tire, the more you’re hurting the inside tire. I like to get 50%-60% of the total camber desired … through camber gain. Here is why: The suspension on the inside of the corner is not compressed as far as the suspension on the outside corner. So effectively … the inside tire is not getting as much negative camber gain to fight & overcome.

    *** Don’t get greedy with static camber. Yes it improves initial turn-in steering response, which is good. But two things:
    1. Camber helps the outside tire & hurts the inside tire. If you run too much static camber, you can’t get the inside tire to work optimum.
    2. There are a LOT of other things that help turn-in response … so use some of those … & don’t get greedy with static camber.

    -----------------------------------------------------------------------------------------------------------

    You know how people say a little information can be dangerous? This is one of those times.

    If your PT car has a Moderate to High scrub radius … you have to be careful with how much caster you can put in the car … because caster combined with a moderate to high scrub radius creates a “jacking effect” when you turn the wheels. Dynamically, this jacking effect “de-wedges” the car … loading the inside front & outside rear tires more … while also unloading the inside rear tire & increasing the degree the car diagonally rocks & loads the outside front tire.

    All of this helps the car to turn better. But go too far … and the car will get loose on entry. This effect increases with speed, so is more pronounced on road courses than AutoX. This is where track tuning comes into play. If you have a moderate to high scrub radius … sneek up on the caster you put in the car … until you get the car “free” on entry … then reduce the caster a tick … or tune something else to allow you to keep that amount of caster, so the car turns well in the middle. But do NOT keep a set-up that makes the rear loose & stepping out on corner entry.

    Make sense?

    -----------------------------------------------------------------------------------------------------------

    In my experience & by my personal scale ...
    0-1/4" = Zero
    1/4"-1" = Very Low
    1-2" = Low
    2-3" = Moderate
    3-5" = High
    5-7" = Very High
    7" + = Extreme

    * But this relative to the situation. When we designed a new Modified chassis that dropped the Scrub Radius from over 7" to under 5" ... we thought of that as "low scrub radius” ... for the NASCAR Modified class we raced in.

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    Steering Feedback to the Driver

    Less scrub radius decreases the feel of the track in the steering wheel ... communicating to the driver what the front end is doing. More scrub radius increases this feel.

    More caster increases the feel of the track in the steering wheel ... communicating to the driver what the front end is doing. Less caster decrease this feel.

    Since reducing scrub radius is good for turning ability ... but takes away the feel ... and adding caster also helps the turning ability ... and returns the feel ... these are two good tuning changes to do together.

    Now “driver feel” is very subjective … no doubt. I have had talented race drivers we developed to the nth degree ... with amazing feel & feedback ... that can tell you if you changed the tire 1/4 pound of air pressure, took out .030" of bump stop shim, which spring coil bound first, which suspension corner stopped traveling 1/16" earlier than the other or if there was extra clearance in your ring & pinion gap ... and that's not even a tiny bit exaggerated. A Crew Chief/Tuner can have a lot of confidence in those situations. These particular drivers & I won a lot of races together.

    And on the other hand ... I've seen drivers that couldn't tell you if they hit a pothole or not. So I think "driver feel" is a relative term.

    .
    Feel free to chime in or ask technical questions. I am here to help where I can.

    Ron Sutton

    Ron Sutton Race Technology
    Your One Stop, Turn & Go Fast, Car Building Resource Center for Autocross, Track, Road Racing & Triple Duty Pro-Touring Cars

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  7. #7
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    Tuning the Track Car with Tire Temps

    Tire temperatures are an excellent guide for tuning race cars in general, and specifically the front end geometry. Tire pyrometers are a simple tool to get accurate results … if used correctly. On my race teams, we use either the Longacre or Intercomp digital pyrometer with memory. You stick the probe into the tire tread (inside first, center next, outside last) … listen for the “beep” telling you it has the temp … push the button to “capture” the # … and move onto the next spot … all the way around the car. It stores & shows all 12 numbers (4 tires x 3 spots) in one display. We then write the numbers down in what we call a “run sheet” of the car’s tuning notebook.

    What? You don’t have run sheets? No notebook? If you’re going to get fast & win events … better get a notebook & run sheets. Because if you can “remember” all the info from your testing … you are NOT testing enough and/or not measuring enough key details. I’ll post a version of a run sheet on here down the road, so you can download them, customize & print them out.

    The digital pyrometers we use with memory, save a lot of valuable time … when the tire is cooling off fast … because you don’t have to write as you go. But they are expensive. The versions we use, also have a 4-car lap timer built in them, so the crew can time our car & up to 3 competitors at the same time. It stores all the info for putting into the run sheets after the session.

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    It is expensive, but will make you go faster than any other $400 you’ll spend. Go HERE.
    For a little less expensive version, with no stop watch, go HERE.
    For a lot less expensive version with no memory, but just as accurate, go HERE.
    For a combo unit ... infrared for brakes & probe for tires go HERE.

    Do not use an infrared temp gun. It’s not ok. The misinformation can is worse than no information. The reason is simple. The surface of the tire cools fast … and equalizes. Surface temps will not show you accurate differences across the tread or from tire to tire. Use a probe type.

    Being consistent is critical, or numbers varying from run to run will make you crazier than you already are. Slicks are easy … we measure 1” in the inside … dead center & 1” in from the outside. Note: most slicks have relatively thin surfaces, so stick the sharp probe into the rubber deep at a 45 degree angle, so you don’t pop the tire.

    On treaded or grooved tires, the 1” number may put the probe in a funky spot on the tire blocks or close to a groove. My rule of thumb is to put the probe into the center of the outside tread blocks or runners, hopefully in the ½” to 1” range for the edges of the tire. With thicker street tires you can go straight in with the probe. Go deep.

    Be consistent with your depth & placement of the probe into the tire tread … and always go in the same order … as quickly as you can before the tires cool too much … and you’ll have the most valuable data available for tuning.

    Let’s run through some examples … assuming we have a PT track car on a road course running equal size Hoosier R6 tires front & rear … with an optimum tire temp around 190 degrees. I’ll start with the basics & progress. Here is what the tire temps tell me

    Across the face of the tread
    LF 190 180 190 RF 190 180 190
    LR 180 190 180 RR 180 190 180
    Conclusion: Front tires are under inflated & rear tires are over inflated.

    Difference left to right
    LF 180 180 180 RF 190 190 190
    LR 180 180 180 RR 190 190 190
    Conclusion: Either the track has more LH turns than RH, the car has more right side weight bias, the car’s suspension is set to for a higher roll angle on LH turns & lower roll angle on RH turns or some combination.

    Difference front to back – Part 1
    LF 210 210 210 RF 210 210 210
    LR 190 190 190 RR 190 190 190
    Conclusion: Car is tight and/or pushing, the driver is over driving the car on corner entry or both.

    Difference front to back – Part 2
    LF 170 170 170 RF 170 170 170
    LR 220 220 220 RR 220 220 220
    Conclusion: Car is loose, the driver is over powering the tires on corner exit or both.

    Difference opposing corners
    LF 180 180 180 RF 190 190 190
    LR 190 190 190 RR 180 180 180
    Conclusion: Car has cross weight in the suspension set-up causing it to be tighter on LH corners & more free on RH corners.

    Difference on edges – Part 1
    LF 200 190 180 RF 180 190 210
    LR 190 190 190 RR 190 190 190
    Conclusion: Front wheels & tires do not have enough dynamic camber. (Do not confuse this with static camber or camber gain.)

    Difference on edges – Part 2
    LF 185 190 195 RF 195 190 185
    LR 190 190 190 RR 190 190 190
    Conclusion: Wheels & tires have too much static camber.

    What if one tire is too cool
    LF 190 190 190 RF 190 190 190
    LR 180 180 180 RR 190 190 190
    Conclusion: [/B]Either due to track or driver, the car is exiting some RH corners easier or gentler.

    What if one tire is too hot
    LF 205 205 205 RF 190 190 190
    LR 190 190 190 RR 190 190 190
    Conclusion: Either due to track or driver, the car is entering some RH corners deeper & more aggressively. Or you have a brake dragging on that corner.

    What if all the tires are too cool
    LF 160 160 160 RF 160 160 160
    LR 160 160 160 RR 160 160 160
    Conclusion: Either the driver is under driving the car, the car has too small of a roll angle, the track surface is cold … or a combination.

    What if all the tires are too hot
    LF 210 210 210 RF 210 210 210
    LR 210 210 210 RR 210 210 210
    Conclusion: Either the driver is over driving the car, the car has too large of a roll angle, the track surface is super hot … or a combination.

    There is a lot more to this. These were just some samples. With experience, you can learn how to read tire temps, differences & even the face of the tire & know what the car is doing. Remember harder compound tires have lower operating temperatures than my samples. And as tires age, the rubber hardens, so they will run cooler too … because they’re “dead.”

    .
    Feel free to chime in or ask technical questions. I am here to help where I can.

    Ron Sutton

    Ron Sutton Race Technology
    Your One Stop, Turn & Go Fast, Car Building Resource Center for Autocross, Track, Road Racing & Triple Duty Pro-Touring Cars

    Check out our 400 Page Car Building Catalog HERE

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  8. #8
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    Having & using a tire durometer is smart too. (like THIS) It will let you know the condition of tires & if you have a “bad” tire. We ran 6 USAC Midgets and set-up them up as identically as is possible with humans. Spring rates within 3#, shock valving within 5#, ride heights within .010” … you get the point. When we roll all of them out at the track on new rubber & one of them handles differently than the rest … we check the tires with a durometer before anything else. Sure enough … if a front tire is harder than is normally should be … or a rear tire is softer than it should be … that car pushes. Vice versa too. You can tune & tune & tune, but all you’re doing to putting a band-aid on the problem and you’re going to have bad days as long as that tire is on the car. We simply change the tire.

    A lot of people ask me what are acceptable split numbers … either across the tread … or from side to side. “Acceptable” varies with the person … like saying it’s cold. My wife says it cold when its 55 degrees. Having lived in parts of the country where it gets in the teens or occasionally single digits … 55 is a pleasant spring day … to me.

    An acceptable tire temp variance is different with each tuner. I know a lot of racers have a larger "acceptable" window than me & are ok with ... 5-10-15 degrees differences. I’m not looking for “acceptable” … unless that is the new term for "optimum." Optimum is 2 degrees or less across the face of the tire. Any more than that, and my team has tools out.

    As with all guidelines, there are exceptions. I’ve seen series with tires so hard the only way to get heat in them was to run them on the edges or over inflate them & run them on the center. But for racing tires & most low tread wear performance street tires … my number is 2 degrees. If I’m road racing, I allow no more the same 2 degrees from side to side. If it’s more, we’re tuning on it to get more grip & speed.

    When I tell you I have often achieved tire temps within that same 2 degree window … comparing side to side … when I was oval track racing stock cars … after you stop calling me a liar … realize we ran very low roll angle ... and we can tune some things "special" because we're only turning left.

    Heck, we have had the left front & left rear tires HOTTER than the right side tires on an oval track. That’s not optimum. I just don’t want you to think the outside tires always have to be hotter & “accept” that.

    Maximizing front tire grip is the key factor to corner speed & lap times. Front tire grip is my highest priority. Because adjusting the rear grip to match the front grip ... for a balanced "neutral" handling car ... is relatively easy.

    Said another way, If the front tires can only maintain 52.3 mph in corner X ... then the car is only going to go 52.3 mph through corner X. Anything you do to adjust rear grip isn't going to make the car going faster than 52.3 mph through corner X. You could make the car loose ... or push ... and go slower than 52.3 mph ... but not faster.

    If we tune on the front geometry & raise the front tires grip so it would carry 52.7 mph through corner X ... and then we tune the rear grip to balance that ... we raised our corner speed .4 mph in corner X ... and probably most of the other corners too.

    So if a car I'm working on has more than 2 degrees temp split across the tire ... or difference side to side … we're getting tools out.

    .
    Feel free to chime in or ask technical questions. I am here to help where I can.

    Ron Sutton

    Ron Sutton Race Technology
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    Front Roll Centers

    I’ll be very basic for any readers following along that are completely new to this & apologize in advance for boring the veterans with more knowledge of this. Cars have two roll centers … one as part of the front suspension & one as part of the rear suspension. I’ll first explain what role they play in the handling of a car … then how to calculate them … and finally how to tune with them.

    Think of the front & rear roll centers as pivot points. When the car experiences body roll during cornering … everything above that pivot point rotates towards the outside of the corner … and everything below the pivot point rotates the opposite direction, towards the inside of the corner. Because the front & rear roll centers are often at different heights, the car rolls on different pivot points front & rear … “typically” higher in the rear & lower in the front.

    If you were to draw a line parallel down the middle of the car connecting the two roll centers … this is called the roll axis … that line would represent the pivot angle the car rolls on … again “typically” higher in the rear & lower in the front.

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    The forces that act on the car to make it roll … when a car is cornering … … act upon the car’s Center of Gravity (CG). With typical production cars & “most” race cars, the CG is above the roll center … acting like a lever. The distance between the height of the CG & the height of each Roll Center is called the “Moment Arm.” Think of it a lever. The farther apart the CG & roll center are … the more leverage the CG has over the roll center to make the car roll. Excessive chassis roll angle is your enemy, because it is over working the outside tires & under utilizing the inside tires.

    Some people like to look at the car as a unit. I look at it as two halves. Here are some examples … using a typical 3500# Pro Touring Car with 53% front weight… to provide more clarity:

    If the CG is 20” high … and the front roll center is 2” below ground … the car has 53% of the 3500# weight with 22” of leverage to roll the front of the car.
    If the CG is 20” high … and the rear roll center is 11” above ground … the car has 47% of the 3500# weight with 9” of leverage to roll the rear of the car.
    * Rolling the car that much more in the front overloads the outside front tire & under utilizes the inside front tire when cornering.

    If you lowered the car 2” … the CG drops 2”. The front roll center probably moved too … but it’s not linear … it is based on A-arm angles. Let’s say it dropped 1” in the front to 3” below ground and the rear stayed the same at 11”.

    Now …
    If the CG is 18” high … and the front roll center is 3” below ground … the car has 53% of the 3500# weight with 21” of leverage to roll the front of the car.
    If the CG is 18” high … and the rear roll center is 11” above ground … the car has 47% of the 3500# weight with 7” of leverage to roll the rear of the car.
    * The front now rolls over less & the rear too, making the car run “flatter” … not flat, just less roll angle … working the inside tires better.

    Any weight you can remove from high up … or relocate to lower in the car … moves the CG down … reducing the leverage it has over the roll center … allowing the car to have less roll angle during cornering … working all four tires more evenly … and the grip of four tires is faster than two. 

    We’ll discuss moving the roll center in the final section. Next, let’s cover how to figure out where your front roll centers is at.

    ---------------------------------------------------------------------------------------------------------------------------------

    Finding the Front Roll Center:
    Measuring all the pivot points in the front suspension to calculate the roll center in the front suspension of a double A-arm suspension car can be tedious … but the concept is quite simple.

    Your UCA & LCA have pivot points on the chassis … and they pivot on the spindle at the BJC’s. Forget the shape of the control arms … the pivots are all that matter.

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    If you draw a line through the CL of the UCA pivots & another line though the CL of the LCA pivots … they will intersect at some point (as long as they are not parallel). That point is called the instant Center (IC) … and the UCA/Spindle/LCA assembly travels in an arc from that IC point. However far out that IC is … measured in inches … is called the Swing Arm length. More on this later.

    Next you draw a line from the CL of the tire contact patch at ground level … to the IC. Do this on both sides … and where the two “Tire CL-to-IC” lines intersect … is the front roll center. Look at the drawing below. The colored dots represent the IC for the same color LCA/UCA. The black dot represents the static RC at ride height.

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    Make sense?

    .
    Feel free to chime in or ask technical questions. I am here to help where I can.

    Ron Sutton

    Ron Sutton Race Technology
    Your One Stop, Turn & Go Fast, Car Building Resource Center for Autocross, Track, Road Racing & Triple Duty Pro-Touring Cars

    Check out our 400 Page Car Building Catalog HERE

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    Now I’ll throw you a curve ball. The static RC at ride height doesn’t mean much. It is the dynamic roll center “in dive” that really matters. “In dive” means when the front suspension is compressed & the car is in roll. So in the corner … when you have the front suspension compressed & the car is rolled over … all those angles change … and therefore the roll center moves. It typically goes down … and may migrate to the left or right of center. In the drawing below, the car is making a right hand corner … compressing the suspension 2” in the center and rolling over at a 3 degree angle.

    Again, the colored dots represent the IC’s for the same color LCA/UCA. See how the IC’s move the swing arm lengths change? The black dot represents the dynamic RC in dive. Notice the RC is lower but also “migrated” to the left.

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    Optimizing the Front Roll Centers:

    First off, most people find working with the front roll center difficult, tedious, confusing & laborious … and therefore they don’t do it much. I love those people as competitors because they’re easy to beat. Getting fast … faster than everyone else … takes work, testing, work, smarts, more work & more testing. And the front suspension … which is the most complicated … is the most important key to cornering performance. Because I understand it so well, that has been to my advantage over the years.

    You don’t have to become a tuner to have fun with your Pro Touring car. You can buy & install many good suspension packages available on the market that have a “much better than factory” set-up for your car … because the aftermarket manufacturer worked out a good basic geometry package. The car will handle well, outperform most factory cars and be a lot of fun. Just don’t disillusion yourself into thinking you’re going to show up at serious competitions & beat the “thinkers & tuners” with a bolt on package.

    If running “good” at autocross & track days isn’t good enough, and you want to compete at a higher level and win events … you need to learn about suspension geometry & tuning … and do lots of testing & tuning. I figure I have over 2500 test days under my belt in my 35 years of racing. I’m not the smartest guy at the track … but when he goes home … I’m still there testing, tuning, learning & getting faster. To win … you gotta be willing to do the work. If you’re not, be clear on that and set your goals accordingly. We’re all here for fun. Some of us find the fun of winning worth the effort & sacrifices it takes to do so.

    ---------------------------------------------------------------------------------------------------------------------------------

    Moving the Front Roll Centers:

    Let’s start with the understanding that to move the front RC … you are changing the angles of the UCA and/or LCA … to achieve a different IC. Some changes affect RC only dynamically in dive … while most changes affect the RC both statically at ride height & in dive. I’ll put an * next to the item that only changes RC dynamically & doesn’t show up statically.

    What are (or can be) your tuning tools to change angles:
    1. Spindle heights and/or distances from spindle pin to ball joint surfaces
    2. Ball joint pin heights
    3. Control arm length*
    4. Adjustable control arm mounts on the chassis.
    5. Also, obviously, any changes in ride height.

    Direction:
    a. Raising the RC, places it closer to the CG, reducing the CG leverage, reducing roll angle … and working the front tires less.
    b. Lowering the RC, places it farther from the CG, increasing the CG leverage, increasing roll angle … and working the front tires more.
    c. For faster corners found at big road courses I’ve found the happy window to be 1” to 2.5” … and 0 to 1.0” for tight AutoX events ... depending on the suspension strategy.
    d. If the RC migrates to the inside of the corner under dive … it will work the front tires more … but roll more if not controlled by the suspension.
    e. If the RC migrates to the outside of the corner under dive … it will roll less, but work the front tires less.

    *KEY NOTE: For optimum cornering ability, you need to WORK the tires … and low RC’s combined with big front sway bars & higher rate rear spring /sway bar combos work the front tires while keeping the roll angle low. In other words, don’t use the front RC as your primary tool to control the car’s roll angle.

    For the Warrior Cars (see HERE) I designed & offer as turn-key complete cars, I have four front RC locations that range from 0" in dive for AutoX to 1.2” in dive for high speed road courses. All we have to do is change the slugs in two control arms. Yes, these have BIG front sway bars.

    ---------------------------------------------------------------------------------------------------------------------------------

    Also know, when you are changing these control arm angles, you are changing the camber gain. Use this to your advantage.

    A lot of stock production cars have the swing arms so far out … there is little to no camber gain … often camber loss. Plus, in many stock production cars the A-arm angles put the roll center so low it is below ground ... and the CG is high … giving it a ton of leverage to roll the car … which is part of why many stock production cars roll so much.

    Typically, when you dial in your front geometry … you’re goal is to place your RC for optimum handling for the type of driving you do (or find the best compromise) … and end up with the desired camber gain.

    Some quick tips:
    • Anytime you’re shortening the “swing arm” … you’re increasing camber gain … regardless of how you did it.
    • Anytime you’re shortening the swing arm length … & keep the IC at the same height … you’re raising the RC.
    • Conversely, lengthening the swing arm length … & keeping the IC at the same height … lowers the RC.
    • Anytime you’re raising the IC of the swing arms … and keeping the same swing arm length … you’re raising the RC.
    • Conversely, lowering the IC of the swing arms… and keeping the same swing arm length … lowers the RC.

    There are several software programs out there to calculate roll centers. I own & use several. I suggest Performance Trends to car guys & gals often because it is the easiest to use.
    A great value program is the Roll Center Plus version HERE.
    There full blown Suspension Anlyzer like I use is more expensive, but a small cost compared to what we spend making these cars handle well. See it HERE.

    *KEY NOTE: When you change the swing arm IC … length or height … you are changing the bump steer … because you are changing the arc the UCA/Spindle/LCA assembly travels in. Another reason why so many people don’t like tuning on it.

    For the Warrior cars & my Front Frame & Suspension Packages (HERE) , I have worked out the bump steer shim packets for each of the four RC setups … which includes a tie rod slug & shims. I simply keep a “set” for each of the 4 RC locations, making the change over quicker & easy … after the initial work is done. Takes about 4 minutes.

    Lastly,
    once I have tested & worked out an optimum front end set-up for a type of track … we “lock it in” and don’t change it at the track. So this is NOT something you’re constantly tuning on … just initially. At the track, the optimum front end geometry is the optimum front end geometry … so as the track changes throughout the day … we’re tuning on other stuff to keep the car “balanced”.

    ---------------------------------------------------------------------------------------------------------------------------------

    These posts delved into the details of front roll centers (RC) ... but I think it's helpful for car guys & tuners to take several steps back & look at the big picture of handling ... to better understand the role of the front RC with the rest of the car.

    Total weight ... weight distribution front to rear ... and height of this weight (CG) act like a lever over the roll centers. As discussed earlier, lowering the CG shortens that lever, as does raising the RC ... but works the tires less. Raising the CG lengthens that lever, as does lowering the RC ... and works the tires more.

    Your goal is to move them both ... to the degree possible ... where you find the optimum balance of working the tires & roll angle. BUT ... and this is KEY ... modern day tuners do not use the RC height as the primary means of controlling roll angle. They use the suspension tuning items as their primary tools to control roll angle & the RC height secondarily. They use the RC primarily to load the tires more.

    So to achieve the optimum balance of roll angle & working the 4 tires optimally ... this all has to work with your suspension ... springs, anti-roll bars & shocks ... and track width ... to end up at the optimum roll angle for your car & track application.

    Hopefully, this brief overview, helps clarify roll centers place in the tuning picture.

    .
    Feel free to chime in or ask technical questions. I am here to help where I can.

    Ron Sutton

    Ron Sutton Race Technology
    Your One Stop, Turn & Go Fast, Car Building Resource Center for Autocross, Track, Road Racing & Triple Duty Pro-Touring Cars

    Check out our 400 Page Car Building Catalog HERE

    Features: Suspension, Chassis, Cages, Brakes, Rear Ends, Engines, Transmisssions, Aero & Much, Much More!

  11. #11
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    I prefer to recommend Performance Trends software to calculate suspensions for a few key reasons. Primarily, it is the easiest to use, comes with good instructions if you‘ll read them, tech support is a phone call away & they’re a good value for what you get.

    Options are:

    A. Get the full 3D version, which is more complex to use, mainly because you have to sort through a gazillion input options most car guys won't use. This is what I use when mapping out everything on a new chassis design. Performance Trends software is the easiest to use of the complete, 3D software brand options, but that's like saying brain surgery is easier than rocket science. It's about $400 & you can see it HERE.

    B. A better value for most is to get their basic front suspension software, called Roll Center Calculator, which I highly recommend for car guys working out their roll center, camber gain, dive evaluations, etc. It is under $100. Go HERE.

    C. The best value would be getting their "Roll Center Calculator PLUS" for under $150 that includes rear suspension & overall handling calculations, which I think is important. But it doesn't have options for Torque arm & Watt's link. Frankly, you could fake* the information into the program quite easily & have a full suspension program for under $150. See it HERE.

    D. For Oval track racer, their Circle Track Analyzer program is a good total car package.

    * What I mean by fake the input info for a Torque Arm and/or Watt’s link, is:
    1. Choose a 3-link & input your lower trailing arms as normal (4 points) & make the top link achieve an instant center at the same dimension as your torque arm instant center.

    2. Find the height of your Watt's link pivot and enter that number into the software for both sides of the panhard bar measurements. These simple softwares do not account for the PH bar arc anyway, so you'll end up with the correct geometry in the software ... and the ability to do full suspension calcs.

    The FLLD calculation in the "Plus Version" is worth the price of the software all by itself, because you can work out combinations of roll center, spring rates & sway bar rates and KNOW how balanced the car will be. I have found I can rely on the FLLD calcs when coming up with combinations. I typically start with 4.5-5.5% more FLLD % than the car’s front weight bias. In other words, if the car has 52% front weight bias … I am targeting at suspension set-ups with a 56.5-57.5% FLLD as baseline.

    .
    Feel free to chime in or ask technical questions. I am here to help where I can.

    Ron Sutton

    Ron Sutton Race Technology
    Your One Stop, Turn & Go Fast, Car Building Resource Center for Autocross, Track, Road Racing & Triple Duty Pro-Touring Cars

    Check out our 400 Page Car Building Catalog HERE

    Features: Suspension, Chassis, Cages, Brakes, Rear Ends, Engines, Transmisssions, Aero & Much, Much More!

  12. #12
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    To figure out my sai on my Dart spindles Ron, would I draw a line through my ball joint tapers then the difference between that angle and the face of the spindle is my sai?

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    Quote Originally Posted by 72BBSwinger View Post
    To figure out my sai on my Dart spindles Ron, would I draw a line through my ball joint tapers then the difference between that angle and the face of the spindle is my sai?

    That's correct. I word it slightly different. Draw a line through your ball joint centers & compare that to the wheel mounting surface of the hub.

    I have special mock up tools that allow me to run a rod through ball joint simulators. Then I make the hub surface perfectly 0.0° level & using an accurate inclimeter (angle finder) lay it on the rod to read the KPI.

    Best wishes.


    Feel free to chime in or ask technical questions. I am here to help where I can.

    Ron Sutton

    Ron Sutton Race Technology
    Your One Stop, Turn & Go Fast, Car Building Resource Center for Autocross, Track, Road Racing & Triple Duty Pro-Touring Cars

    Check out our 400 Page Car Building Catalog HERE

    Features: Suspension, Chassis, Cages, Brakes, Rear Ends, Engines, Transmisssions, Aero & Much, Much More!

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    Thanks Ron, that is how I pictured it and it is reassuring that the squirrels in the cage are running in the right direction lol.

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    Ron:
    Thanks for the very concise tutorial on suspension geometry; although it was little like drinking from a fire hose - lol. I own 2 different books on handling and suspension tuning, but never fully understood the effect of KPI on dynamic camber until I read your posts. I have a question about scrub radius. Is the scrub radius measured from the center line of the wheel or from the mounting flange of the hub?
    Rodney
    Rodney Meyers
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    I was taught SAI= Steering AXIS Inclination btw! Just pulling your chain man....

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    One of the books I mentioned uses KPI the other book (which is newer) uses SAI. I just like KPI. LOL
    Rodney Meyers
    72 Olds 442 Rest-mod clone

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    Subscribed!
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    Quote Originally Posted by cdrod View Post
    Ron:
    Thanks for the very concise tutorial on suspension geometry; although it was little like drinking from a fire hose - lol. I own 2 different books on handling and suspension tuning, but never fully understood the effect of KPI on dynamic camber until I read your posts. I have a question about scrub radius. Is the scrub radius measured from the center line of the wheel or from the mounting flange of the hub?
    Rodney

    Hi Rodney,

    It is measured from the centerline of the tire tread ... so same as the centerline of the wheel when static.



    Feel free to chime in or ask technical questions. I am here to help where I can.

    Ron Sutton

    Ron Sutton Race Technology
    Your One Stop, Turn & Go Fast, Car Building Resource Center for Autocross, Track, Road Racing & Triple Duty Pro-Touring Cars

    Check out our 400 Page Car Building Catalog HERE

    Features: Suspension, Chassis, Cages, Brakes, Rear Ends, Engines, Transmisssions, Aero & Much, Much More!

  20. #20
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    Quote Originally Posted by 72BBSwinger View Post
    I was taught SAI= Steering AXIS Inclination btw! Just pulling your chain man....

    I read this and went "whaaaa" ... and went back & read my post. Oops. I corrected it. Thanks for the catch!




    Feel free to chime in or ask technical questions. I am here to help where I can.

    Ron Sutton

    Ron Sutton Race Technology
    Your One Stop, Turn & Go Fast, Car Building Resource Center for Autocross, Track, Road Racing & Triple Duty Pro-Touring Cars

    Check out our 400 Page Car Building Catalog HERE

    Features: Suspension, Chassis, Cages, Brakes, Rear Ends, Engines, Transmisssions, Aero & Much, Much More!

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