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Author: Subject: Suspension Tuning Guide
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[*] posted on 28-5-2009 at 08:59 PM
Suspension Tuning Guide


Meant to replace the now derelict:

http://board.tercelonline.com/viewthread.php?tid=27467

will edit later....theres alot of HTML in my original article that i do not have time to change to fit TO's format


Note: Author WILL NOT take credit for the majority of this article, originally written by Sport Compact Car Magazine, and tailored to meet the Tercel/Paseo community's needs.

Part 1: Four steps to better handling



Suspension tuning is the black art of compact performance. With the majority of the world concerned about making horsepower, handling has traditionally taken a back seat. However, as all serious geeks know, every fast, well-rounded street car has as much suspension tuning as it does power tuning. With the popularity of drifting, time attack contests and racetrack hot lapping on the rise, suspension tuning and handling are becoming popular with enthusiasts who previously made all their efforts making power. Finding straight-line horsepower gurus to help you is relatively easy, but it's much harder to find an expert who can make your car corner well. The solution? Make yourself a guru. If your automotive interests are greater than a one-dimensional urge to blast straight down the 1320, then it's time to get to work.

Step One: Sticky Tires



Tires are by far the biggest contributor to finding more cornering force. By bolting on a set of stickies, you, in minutes make the biggest possible gain in cornering power. Generally, putting the biggest tires that will fit inside your wheel wells without rubbing is the way to go. Choosing an ultrahigh-performance tire is also important. The list of sticky street tires is miles long. Below are a few of our favorites organized by cost. The cost-no-object tires will ultimately yield more grip than the value-priced tires, but in most cases, the value-priced choices offer 90 percent of the performance of the more expensive tires at 60 or 70 percent of their cost. Some of our favorite ultra high performance street tires:
Cost No Object:

Michelin Pilot Sport and Pilot Sport2



BFGoodrich g-Force KD


Bridgestone Potenza S-03.



Value Priced:

Falken Azenis



Kumho ECSTA MX



If you're attending track events, autocrossing or simply wanting the most grip possible, try a set of DOT-approved racing tires. Some can be used for everyday driving, while others grip almost like racing slicks and last only slightly longer. These tires produce more road-sucking grip than any suspension mod you can make. The drawbacks are many. First, these tires can be expensive; second, they wear quickly; and third, the number of heat cycles( how many times you get them hot, as in burnouts at the strip) their rubber formulations can withstand before losing significant grip is limited. Many of them don't do well in the wet, and none work in the snow or ice. It's possible to end up with expensive, fast-wearing and not-so-grippy tires by using them on the street and by subjecting them to too many heat cycles. Most users of these race tires use them only on the track.

DOT-approved street-legal race tires:
Tires Durable Enough For Street Use:




Yokohama A032R



Yokohama A048



Toyo RA-1


Nitto NT 555 RII



Hankook Z211



Pirelli P Zero Corsa



Michelin Pilot Sport Cup



Avon Tech R



Kumho Victoracer V700


Track Only



Hoosier A3S04



Hoosier R3S04


Hoosier Radial Wet


Kumho ECSTA 710



Kumho ECSTA V700



Generally, it's possible to stuff a tire two sizes larger than stock into most cars' wheel wells. For instance, our cars came with a 155/70-13 tire on a 4.5-inch-wide wheel can usually accommodate a 205/50-15 on a 7-inch-wide wheel. Putting the tires on a wheel of the recommended width is important as well. Going up an inch in wheel diameter and running a lower profile tire is a good thing. A lower profile tire has shorter, stiffer sidewalls, which improve response to steering imputs and hold the tread flatter to the road surface during cornering load. However, it can be overdone.

Ultralow-profile tires are more sensitive to suspension tuning and camber changes. And stiff sidewalls don't conform to road surfaces easily. This makes ultralow-profile tires sensitive to shock, as the short, stiff sidewalls have very little compliance. Harsh surface inputs can make these tires skip and hop across the surface instead of digging in and finding grip. Large wheel and tire combinations also increase rotating and unsprung weight. For example, most of us run 17x7-inch wheels with a 205/40-17 tire. The big wheels and low-profile tire look cool, but this combo is too large and too heavy for optimal performance. Hard-core track geeks driving these same cars almost always fit a lighweight 15x7-inch wheel with a 205 to 215/50-15 tire, 225/45 may be possible.

Huge wheels also increase your car's final drive (slower acceleration, but higher top speed). Their added weight increases the flywheel effect, slowing the car's acceleration and increasing load on the brakes. this means wheels larger than 18 inches are rarely used for performance. 18 inches is the maximum practical wheel diameter. There are few choices for DOT-legal race tires larger than 18 inches anyway.

Huge wheels can greatly increase unsprung weight - the weight of the components that aren't suspended. This includes the suspension arms, brakes, the shock absorber and the wheel and tire. For the suspension to work well, the ratio of sprung weight to unsprung weight must be kept low. The best example of unsprung weight hurting handling is found on monster trucks. Even though they have wheel travel measured in feet instead of inches, it doesn't provide a significant measure of driver control.

This is why monster trucks have nearly as much unsprung as they have sprung weight, thanks to their huge tires and axles. This makes the shocks work much harder to damp wheel movement. Reduce unsprung weight and the suspension doesn't work as hard, the ride improves and the tires stay in contact with the ground. The obvious way to make up for the disadvantages of increased wheel diameter and width is to use a light wheel. Light wheels are easier to accelerate and brake. They also reduce unsprung weight.

Some examples of our favorite light wheels can be found in the table below. The mostly expensive wheels listed on this page are forged or semi-solid forged. These processes improve the grain structure and provide superior strengh for the weight.

Light wheels:

Cost No Object




Volk TE37



Volk CE28N



Rays Gram Light 57F



SSR Competition X



SSr GT2



SSR GT1



SSR GT7



5Zigen FN01R-F



Motegi Track Lite



Centerline Impulse


BBS RC



Value Priced



Axis Mag Lite



Axis Reverb


Kosei K1 Racing



Almost all Rotas

Enkei RPF1



Enkei NT03-M



Enkei RP03



Enkei RS+M


Rays Gram Light 57C



Team Dynamics Procomp



Team Dynamics Pro Race


Beware of some of the other lightweight, low-cost wheels offered on the market; we've found many of these bend like butter, sometimes just from normal track driving antics, like hitting FIA curbs or dropping a wheel off the track. The value wheels we have listed either have a history of surviving race conditons or are wheels we've had personal success with on the track.

Notes

* 17s are the largest wheel you can safely fit under the wheel well of our cars, using a 205/40/17.


* 15's are the optimal size, providing cheaper, more available options. They also provide the best weight/size ratio. Holeshot is the name of the smaller diameter wheel/tire size game.


* 205/40 is the widest wheel to be safely fit without rubbing, but 215 is possible in the rear with fender edge rolling and/or a Starlet rear axle, which is narrower, allowing a wider tire.


* The correct offset should be between +40-45. +38 is acceptable, however.



Step Two: Reduce Body Roll, Dive and Squat



The most important basic suspension trick is to reduce excess body motion. Toll under hard cornering, dive under braking and squat under acceleration all create problems for the driver. Contrary to popular belief, roll doesn't cause weight to transfer to the outside wheels. Rather, it hurts handling by slowing chassis response to steering, braking and accelerating - all critical inputs for controlling the car. Body motion also gives the impression the car isn't handling well. Roll, dive and squat all contribute to a lack of confidence behind the wheel. Watch a F1 car in a turn; it nimbly darts around the corner with no excess body motion. Now watch an SCCA showroom stock racer; it leans, squirms and squeals its way around the track. Extreme example, sure, but exactly the heart of the problem. More insidious are the other side effects excessive motion produces. Our softly sprung vehicles will roll and bottom the suspension on one or both ends when cornering hard. This shocks the tires and will cause an instant loss of traction on the end that bottoms first. The result usually involves a track tow truck.








Moving the suspension through a wide range of travel can also result in another problem. Most factory vehicles have compromised suspension geometry and several problems can occur when a car heels way over in a turn. First, the suspension can gain positive camber (top of tire leans to the outside). With our strut-type suspensions, it's worse, the car rolls, but the tires don't. This forces the tire to roll onto its outside edge and reduces its contact patch - clearly not the best way to use a tire. The other evil effect of roll is bump steer.

Bump steer is caused when the steering linkage and the rest of the suspension travel in different arcs throughout the range of motion. As a result, the tires can give steering input even if the steering isn't moved when the car heels over. This translates to the driver as a twitchy and unstable chassis. Combine dive and squat and all of these problems add up to a serious lack of control. Now that you know body motion is bad, what can you do to control it?

The first thing to do is run stiffer springs. Stiffer springs will resist roll and bottoming out under roll and combinations or roll, dive and squat. Of course, stiffer springs have more rebound energy. To prevent your car from bouncing like a pogo stick, you need shocks with more damping. Shocks don't affect how much a car rolls, but they do affect how the suspension responds to bumps and steering input. More rebound damping keeps the car from bouncing and floating over bumps and undulations. More damping also makes the car more responsive to steering imput. Too much rebound damping can prevent the suspension from returning once compressed, causing it to pack down and gradually bottom out.

Another way to reduce body roll is to install larger anti-roll bars. Anti-roll bars are torsion bars that connect the wheels. They don't come into play until the car starts to roll in a turn. During roll, they must be twisted for the car to lean over. Anti-roll bars don't affect the ride as much as stiffer springs and have no effect on dive or squat. Generally the shock damping doesn't need to be altered when the anti-roll bar diameters are changed.

Stiffening the suspension will degrade the ride, and it's easy to make your car too stiff. If this happens, the suspension will not be able to deal with bumps and will hop it's way around turns instead of compliantly absorbing the bumps and finding traction.

Notes

* Stiffer Springs are available as coilover sleeves, which are height adjustable, but will kill most shock absorbers, such as stock, and even KYB GR2's. Such available are made by Aerospeed, Eibach, and various other brands, but do stay away from the E-Bay specials, they do not have the right spring rates, and are not made of coil spring material, and will either break or cause an accident. Publisher's advice: Stay Away From Coilover Sleeves, Looks Only.


* Springs only are available through Tein, Eibach, and Goldline. The preferred ideal setup as recognized by the Tercel/Paseo community is the Eibach ProKit with KYB GR2's. The Eibach Sportline is recognized as more of a drop spring than a performance spring. If you are dropping, do not go any lower than 1.5 inches to prevent suspension stress and provide the best center of gravity/handling.


* Real Coilover setups are only available for the EP82 and EP91 Starlet. Either will work for 91+ Tercels and 92+ Paseos. Available are Tein, TRD Japan, and possibly Cusco and JIC. These are the ideal spring/damper combination, but are very expensive, and are more for the racing oriented.


Step Three: Balance the Chassis

Now that you've reduced body motion and improved steering response, we can work on the next major area of improvement. The goal for most of us is to have neutral balance, where all four tires slide the same amount, is the fastest way around a corner most of the time. This way you use each tire's maximum grip. It might seem odd, but many experienced drifters perfer a neutral car because it allows them to have many control options for getting sideways.

Unfortunately for the enthusiast, most cars are factory tuned to understeer. Understeer occurs when the front tires slide first when at the limit. Manufacuturers do this because it's the easiest handling mode for the average driver to control. Understeer isn't efficient for extracting maximum literal acceleration because the car will use the front tires excessively, while the traction contribution of the rear tires is wasted. It's also the slowest and most boring way around a corner.

Bottom line? Understeer sucks. If we go too far in the quest to eliminate understeer, we'll inevitably create oversteer. Oversteer occurs when, at the limit, the rear tires slide before the front tires do. How do we tune a car's handling balance? By manipulating the tire's slip angle. Slip angle is defined as the difference between the direction the tire is moving and the direction the contact patch of the tire is pointing. At extreme slip angles, the contatt patch actually slides across the pavement. The primary dynamic contribution to slip angle is the load placed on each individual wheel while cornering. A greater load on a given wheel/tire results in a greater slip angle of that wheel/tire when subjected to a sideways cornering force. A nose-heavy FWD car has more weight and thus cornering load on the front tires. The front tires start to slide first, causing understeer. Properly manipulating tire load and slip angle by controlling weight transfer is key to balancing the chassis. By altering weight transfer and tire loading during cornering, much can be done to change the car's natural handling tendencies. Can you make a nose-heavy fWD car oversteer? Sure. Look at the most successful front-drive racecars; they oversteer like crazy. How does a tuner manipulate tire loading and slip angle? By tweaking the spring rates, anti-roll bar rates, tire sizing and pressure, and to a lesser degree, the shock damping.







The first option a tuner has is to increase the tire pressure. The harder a tire is inflated, within reason, the smaller slip angle it develps. In the case of a nose-heavy FWD car, if you add several psi to the front tires and take some pressure out of the rear, the front tires' slip angles will increase. This alone can do quite a bit to reduce understeer. Changing the spring rate and anti-roll bar rates has a large impact on slip angle. Running a stiffer spring or anti-roll bar on one end will cause more weight to be transferred onto the outside tire as the car tries to roll in a corner. The softer end will compress and the more stiffly sprund end will resist compression, putting more weight into the tire and causing it to run at a bigger slip angle. The best thing to do for your understeering, front-wheel-drive car is run a bigger anti-roll bar to tune out understeer. Conversely, stiffening the front suspension and increasing the rear tire pressures can tame oversteer.




Shocks can improve response and help balance the car right after the initiation of a turn; soft shocks get the car to a steady point of weight transfer faster. When stiff, they can delay weight transfer. Thus, shocks affect how the car feels at turn-in and also how it feels past mid-turn. A car with the shocks set fairly hard will turn in sharply. If the shocks are set too hard, the balance might change later in the turn in an unpredictable way as the heavy damping slows the body roll and weight transfer. Tire sizing can also affect chassis balance. Installing a wider tire on the end that needs traction most is obvious. Many FWD autocrossers and road racers install a wider front tire to get more front grip. At the limit of adhesion, a car that slides all four wheels without brake or throttle input is considered ideal; it also doesn't exist. Being able to provoke slight oversteer by lifting the throttle and more aggressive oversteer with slight braking while cornering at the limit is useful as well. Being able to slow rotation with slight throttle application makes FWD cars easier to control.


Step Four: Weight Transfer

Weight transfer is the movement of weight from the inside to the outside wheels during cornering. Excessive lateral weight transfer hurts handling. It's caused by centrifugal force working on the chassis' center of gravity, which loads the outside wheels and unloads the inside wheels. Contrary to popular belief, very little weight transfer can be attributed to lean in a corner. Even at large roll angles, weight transfer due to roll is quite small. So lowering a car's center of gravity and widening its track width will reduce weight transfer more effectively than reducing roll angle.

Lowering is best accomplished with shorter springs. The smartest approach is to use shorter springs and shorter-bodied shock absorbers or struts that maintain stock compression travel at a lower ride height. Excessive lowering can change suspension geometry, causing positive camber during roll and contributing to increased bump steer. The easiest way to increase track width is to use wider wheels and tires that fill out the wheel wells. This also increases the amount of rubber on the road. Using wheel spacers and wheels with a more positive offset can also increase track width. Any positive change in track width, and therefore offset, increases the scrub radius.







Scrub radius is the distance from the centerline of the tire's contact patch to the point where the steeing axis intersects the ground, also known to enthusiasts as "The Dave Point." Increasing the scrub radius allows forces generated by the tire more leverage to act on the steering. To the driver, this translates as torque steer under acceleration and braking. To minimize the change in scrub radius, it's important to try to increase wheel width to the inside as well as the ouside by paying close attention to the wheel offset. This puts more rubber on the road and increases the track width while maintaining the same scrub radius. Increasing track width also changes the motion ratio of the suspension, which effectively reduces spring and anti-roll bar rates.

Lastly, a very positive offset wheel puts a large strain on wheel bearings, ball joints and steering linkage, making them wear much faster. All of these are good reasons not to go overboard with this method of increasing track width. A good rule of thumb is it's safe to use the largest wheels and tires you can stuff in your stock wheel wells by rolling the inner fender flange. A good guideline is to increase the track width and lower the car more on the end that slides first in a corner.

An understanding, nose-heavy, FWD car can use more track width in the rear. This play on physics will help you reduce weight transfer in both cases.


Step Five: Add Negative Camber

For a tire to grip well, it must use all of its contact patch. Thanks to problems like tire distortion and compromised suspension geometry, this rarely happens. When a tire is subjected to side load, its sidewalls flex, digging the outside tread into the ground and lifting the inside. If you drive hard, you’ve probably noticed the outside-edge of your tires get chewed up much faster than the rest of the tread. That means the tire isn’t using all of its contact patch effectively. As a car rolls in a corner, the chassis rolls the tire onto its outside edge, making the problem worse. Keeping the tires flat on the road is the primary reason to reduce roll.







In part one we listed several ways to do this; the easiest ways are to increase spring rate or use larger anti-roll bars. The primary tool, however, used for combating tread lift is to dial in more negative camber. Camber is the inward or outward tilt of the tires when looking at them from the front. If the top of the tires leans outward, camber is positive. If the top of the tire leans inward, camber is negative.

Dialing in negative camber helps combat tread lift and wheel tilt. The trick is to add just enough negative camber so the tread stays flat and 100 percent engaged with the ground under side load and roll. But, adding too much negative camber will hurt more than it helps.

Too much negative camber will:

1. Reduce traction

2. Reduce acceleration traction if its applied on the drive wheels

3. Increase the tendency to follow cracks and grooves in the pavement

4. Increase wandering caused by road crown

5. Affect tire wear; the insides of the tire tread will wear faster with more negative camber if you don’t corner hard.

Conversely, if you constantly corner hard, your tires will wear more evenly and last longer. Your car and your driving style together determine how much negative camber you need. Aggressive drivers should use more. Those concerned about tire life should use less. Suspension design also matters. Our cars need more negative camber to work well under cornering load. The following table provides a rough guideline on how much camber to use based on your driving style and accounting for tire wear.

Camber Adjustment Guidelines

Driving Style/Front/Rear/

Aggressive Street Driver

-1.75, 1

Weekend Hot Lapper

- 1.75-2.5, 1.2-1.5

Racer Only

- 2.5-4.5, 1.5-2.5

Unfortunately, camber is not adjustable on most modern cars. Even if camber is adjustable, it’s rarely adjustable enough to align a lowered car correctly. The best way to adjust camber is to use a camber plate. Camber plates use an adjustable top mount that locates the upper shock mount in a retainer plate that slides laterally on a slotted track (Typically called Pillowball Mounts). Simply enlarging one of the two lower mounting bolt holes in the strut housing about 1/16-inch with a drill, leaning the upright into the strut and retightening the bolts can give you quite a bit of no cost camber adjustment. Avoid using undersized shaft or eccentric bolts sold as crash bolts. Crash bolts are sold as a cheap way to adjust camber on crash damaged cars. Because of the small shaft diameter, they usually stretch and allow the camber adjustment to slip under the load of hard driving with sticky tires.

Adjusting camber is well worth the effort. Optimizing the camber for your car and driving style can often make a bigger difference in the amount of grip the car can generate than any other mod except tires.


Step Six: Tune Your Toe


Toe refers to the direction a car’s tires are pointed relative to each other when viewed from above. Toe-in means the front of the tires are closer to each other than the rears. The opposite is toe-out. Toe is measured in inches relative to straight ahead, or zero toe. With zero toe, a car’s tires are exactly parallel to each other. Fine-tuning toe settings will allow a measure of control that’s often overlooked. It also has a significant effect on how a car behaves in a corner.



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Front toe settings make a big difference in how a car handles in the first third of the turn, the critical turn-in phase where cornering force is initiated. Like all chassis tuning, too much of a good thing will cause problems. Too much toe-in or toe-out will create tire wear on the inside and outside edges of the tire. Any toe setting past 1/8-inch will cause excessive tire wear.

Aggressive toe has probably ruined more tires on lowered cars than any other chassis adjustment. Below are the guidelines for setting toe and how it can affect feel and handling:

Front Toe-Out:

Just Right:

- Reduced understeer at turn-in

- Improved steering response

- Counteracts the natural tendency for FWD cars to toe-in under throttle


Too Much:

- Instability during braking

- Straight-line instability, especially over single-wheel bumps or split-traction surfaces

- Unrecoverable understeer


Front Toe In:

Just Right:

- Generally helps make the car feel more stable

Too Much:

- Wandering under braking

- Refusal to turn in or rapid turn-in followed by understeer

Rear Toe-Out

Just Right:

- Easy midturn rotation. Less front tire load

Too Much:

- Violent lift-throttle or trail-braking rotation



Below are some typical toe adjustments, tire wear expectations and styles of driving.

Aggressive Street Driver:

Front: 0

Rear:: 0

Weekend Hot Lapper:

Front: 0-1/8" out

Rear: 0-1/8" out

Racer Only

Front: 1/8-1/4" out

Rear: 0-1/4" out


Step Seven: Make It Stiffer



Chassis stiffness is a critical element in suspension tuning. A flexible chassis doesn’t allow the suspension to keep the tires in contact with the road and is less responsive to critical suspension changes like increased spring and anti-roll bar rates. The best way to combat chassis flex is by seam welding every spot-welded panel in the unibody and installing a welded-in roll cage. Unfortunately, these are also the least practical ways to solve the problem. Chassis braces are better.

The most common brace is the strut tower brace, which connects the strut towers in the engine compartment. Triangulated strut tower braces are the most effective and tie both shock towers to the firewall. There are also lower crossmember braces and subframe braces available for most cars. Harness bars, which are stout bars that connect to the upper shoulder harness bolts and the floor, also significantly stiffen the chassis. Hinge braces tie the shock towers to the sturdy base of the A-pillar via the door hinges. These make a huge difference.

Any chassis bracing has the potential to bump your car up several classes in virtually any competition series (especially autocross), so be sure you read the rules if you plan to add braces to a car you race. Another way to stiffen is to inject Foamseal-brand two-part catalyzed polyurethane structural foam into the hollow structural members of the unibody. Although it’s time consuming and messy, it can produce significant gains in chassis stiffness without resorting to a roll cage. Some manufacturers use this treatment to increase chassis stiffness from the factory.

Beware of the all-too-common inferior chassis brace. These are usually spindly-looking devices with small tubes and no gussets. Contrary to popular internet wisdom, it’s impossible to make a chassis too stiff. Fortunately, chassis braces are rarely expensive and have few negative side effects. Certain chassis braces in combination with aggressive suspension tuning will cause handling problems, which should be tuned out through suspension adjustment. You paid good money for your adjustable suspension, so be sure you adjust it correctly.


Step Eight: Adjust Caster



Every car’s front wheels turn on pivots attached to the suspension. Caster is the angle of the imaginary line drawn through the pivots. It’s measured in degrees relative to vertical. If the top pivot point is behind the lower pivot point so the caster angle slopes backward like on a bicycle (as viewed from the side), the caster is positive. If the angle slopes forward (which it never does), the caster is negative.






Kingpin Inclination Angle (KIA) is the angle of the line drawn through the same pivots as caster but viewed from the front of the car. KIA always slopes toward the center of the car and is expressed in degrees from the vertical plane. KIA is a design constraint and is not adjustable. Caster and KIA together affect straight-line stability and camber while the wheel is turned.

Increasing positive caster projects the Dave Point (the point where the steering axis meets the ground) further in front of the tire’s contact patch. This distance is called caster trail. When the tire’s contact patch is behind the Dave Point, the tires want to stay centered behind the Dave Point the same way a shopping cart’s casters naturally align its wheels in the direction of travel. Like shopping carts, caster, the distance between the Dave Point and the tire’s contact patch creates a torque reaction, which causes the steering to self-align. The driver perceives this reaction as greater stability and on-center steering feel. More positive caster means a bigger torque reaction as well as increased stability and feel.

Unfortunately, this virtual lever arm also increases torque steer on FWD cars because the force is reversed when the wheel is driven. This is why most FWD cars don’t have as much caster as rear wheel drive cars. Lots of positive caster causes the outside wheel to gain camber in a turn when you need it most. Think of a parked chopper with the wheel flopped to the side. That’s an extreme example of negative camber gain with positive caster. Too much positive caster can increase tire loading and understeer.

KIA increases stability by making the axle path travel in an upside down, U-shaped arc (when viewed from the side) as the steering wheel is turned. The axle is at the apex of the arc when the steering wheel is centered. As the wheels are turned, they actually lift the front of the car. This lifting effect increases effort the more the wheel is turned, which contributes to steering feel and straight-line stability. KIA also tilts the wheels outward in a turn, which reduces camber.

Positive caster and KIA are both huge considerations for design engineers. Balancing positive caster’s ability to increase camber in a turn with KIA’s ability to decrease it is critical to achieving the right combination of stability and steering feel. Caster can be adjusted with tension rods, adjustable arms and universal (racing) camber plates. These parts are available for almost any car that’s been raced.

Here are some basics guidelines for adjusting caster:

Positive Caster:

Just Right:

- Improves straight-line stability

- Sharpens turn-in

- Improves traction everywhere in the turn

Too Much:

- Very high steering effort

- Provides sharp turn-in but increases understeer from midturn onward

- Increases torque steer in FWD cars

Caster Adjustment Guidelines:

3-4 degrees
PART 3: It's All In The Geometry



In the first two parts, we covered relatively basic suspension tuning techniques. Now it's time to bury ourselves in suspension geometry. Making changes on this fundamental level is what race car suspension engineers do for a living. But we've found that with the more popular performance cars in this market, there are parts available that will allow you to make these changes.

Roll Center:

Roll center is the virtual pivot point in space that a car rotates around when subjected to cornering forces. The roll center is significant because its location determines how a car will handle and what factors must be considered when tuning its suspension. To find a car's roll center, first locate the "instant centers" of it's front and rear suspension. The instant center is the point in space around which the suspension's links rotate. Locating your car's instant centers can be done by measuring its suspension and creating a scale drawing. Measure how high the pivot points are above the ground and know the exact dimensions of the control arms.

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To find the instant centers on a car with upper and lower control arms, draw lines from the center of the ball joint through the inner pivots of the upper and lower control arms and extend them inward toward the center of the car until they meet. Now draw a line from the center of the tire's contact patch to the instant center on both sides of the car. The point where these two lines intersect is the roll center.

Roll center affects many critical elements of a car's handling. The most critical are steering input, body roll, balance and mechanical grip. The center of gravity location (CG) for each end of the car can be found by jacking the car up a known distance on each side while it's on corner scales, and observing the change in corner weights. This data can then be fed into an equation to give you the coodinates of the CG. Since most people don't have a perfectly flat surface and expensive corner scales, it's usually safe to estimate the CG for the front suspension around cranshaft height in a front-engine car. In the rear, it's usually at the floor of the trunk.

The distance between the roll center and the center of gravity is called the roll couple. The roll couple is the lever arm that centrifugal force working on the CF uses to make a car lean over in a turn around the roll center. The longer the roll couple, the more weight is transferred to the outside wheels during cornering and the more the car will want to roll in a turn. A longer roll couple makes cars slower to respond to steering input. The resulting weight transfer from a long roll couple also uses the inside tires less effectively during cornering, thereby reducing the available grip.





The often-overlooked disadvantage to lowering is that the roll center drops more radically than the center of gravity on most cars. This increases the roll couple and can cancel any weight transfer advantage. The huge roll couple created by overlowering will require an overly stiff suspension to control body movement. And when your suspension is too stiff, it won't absorb road irregularities effectively, which will make it harder to keep the tires in contact with the ground. You can't drive fast if your tires aren't on the ground.

On most cars, the ideal location for the roll center is 2 to 5 inches above the ground for the front suspension and 4 to 10 inches above the ground for the rear suspension. With the rear roll center higher than the front, the car will transfer more weight to the front, making it more likely to understeer. Most purpose-built racecars utilize this design because it allows them to be tuned for slight understeer at high speed and more oversteer at lower speeds. The mass and roll center locations can be used to predict a car's natural handling characteristics.

If the front and rear roll centers are plotted and a line is drawn between them, the line indicates the roll axis of the car. The roll axis is the axis that the car rolls around in a turn. The mass axis is a line drawn between a car's front and rear centers of gravity, which can be determined using the method discussed above. Mass axis can be roughly plotted by drawing a line through the center of gravity points in the front and rear of the car. Since there isn't already a preexisting engineering term for this axis, we'll call it the Mike Axis. When the roll axis and the Mike axis are plotted next to each other, the distance and slope between the two are useful in determining a car's natural handling tendency. If the space between the two lines is greater in the front of the car, within an upward sloping Mike axis, the car will tend to understeer due to greater weight transfer to the outside wheels at that end of the car. Front-engine, front-wheel drive cars strongly exhibit this trait.

Conversely, if space is greater in the rear of the car, with a downward sloping Mike axis the car will tend to oversteer. FWD cars will usually have a Mike axis that slopes upward at a steeper angle. Since the roll axis on a well-designed car tends to slope downward toward the front of the car, it's easy to see why front-heavy cars tend to understeer. Roll center can be adjusted by using aftermarket control arms with adjustable pivot points. Or, if you're ambitious, it's not impossible to find a fabricator capable of modifying control arms to suit your needs. Remember, if you can adjust roll center, you can reduce the roll couple and lower the center of gravity effectively. This is an effective way to change your car's dynamic balance by reducing roll couple and weight transfer. But most importantly, it's critical to remember that overlowering a car will create more problems than it solves.


Overlowering: Don't Do It

Almost everybody does it. Lowering your car is paramount to improving it's handling. The key, however, is to lower it just enough to gain the benefits it creates without suffering the potential drawback. The aftermarket does little to help us in this regard. Nearly every company that makes suspension components, even very reputable ones, spews out thousands of sets of lowering springs that are both too low and too soft for optimal handling. Why do they do this? Are the engineers at these companies incompetent? Is it a conspiracy to make our cars suck? No, the enthusiast is to blame. The majority of enthusiasts want a low ride height to fill the ugly gap in their stock wheel wells. They also won't accept a ride that, for the most part, is a lot harsher than stock. Macho or not, most enthusiasts don't drive hard enough or well enough to realize that their cars actually handle worse than stock, mistaking reduced roll for better handling.

The problem is that we only have about 2 inches of compression travel at the stock ride height in the front suspension. Let's say you lower the car the typical 1.5 inches. That leaves a half inch of travel before you hit the bump stops. Your typical aftermarket lowering spring might only up the spring rate a paltry 20 percent or so, which isn't nearly enough to keep the car off the bump stops with only a half-inch of travel. The result is poor ride quality and sub-standard handling.

As the car leans in a corner, the suspension will settle onto the bump stop. As the bump stop compresses, the spring rate ramps up infinitely, which causes massive weight transfer and relentless understeer. Believe it or not, it gets worse. With the lower control arms pointing upward, the instant center of gravity starts to drop rapidly and the roll couple greatly increases. The bigger roll couple causes more weight transfer to the outside wheels and more body roll. Finally, the steering tie rods start to point upward more radically, because they are shorter than the lower control arm and positioned out of place in the lowered chassis. This causes toe-out when the wheels deflect, making the steering twitchy and the car feel unstable. There are lowering springs available that are capable of causing these or similar problems on just about any car. What can you do to work with the drawbacks of overlowering or avoid it completely?

Make sure your car doesn't use the bump stops under maximum cornering load. The easy way to detect this problem is with a zip-tie telltale on the shock shaft. If the zip-tie is pushed up flush or into the bump stop after a hard turn, then your car is using the bump stops every time you corner hard. If you must run low, do it race car style. Get short-bodied high-end coilovers shocks or struts with higher rate springs. Independently adjustable ride height and spring preload are also critical. Suspension components with these features are designed to work at low ride heights.

Many popular performance cars have kits to adjust and correct roll centers, camber curves and bump steer. If you can't get a decent rate drop-in spring for your car, Ground Control makes kits for many cars allowing the use of Eibach 2.5-inch ERS racing springs, which come in nearly an infinite selection of rates and lengths. With Ground Control's threaded spring perches, you can also adjust the ride height. If you can't do this, run short, soft progressive microcellular urethane bump stops so the wheel rate will ramp up gradually if the bump stops are used. Koni makes excellent bump stops.


Bump Steer:

Steering precision and stability - both of which are affected by bump steer - are the next victim of overlowering. Bump steer is steering input created by the suspension moving through its stroke in response to bumps and roll. It's caused by the suspension's control arms moving in different arcs than the steering linkage as the suspension follows its stroke. It's fairly easy to design a suspension system that doesn't have bump steer. Our cars have the steering rack placement compromised by packaging constraints so steering tie rod location is often less than optimal. Lower the car and the problem gets worse.

What can you do to reduce bump steer? Many cars have aftermarket parts available to relocate the tie rod ends of the steering linkage. Tie rod ends with spherical bearings and spacers can be tuned to reduce bump steer by placing the tie rods at a more favorable angle. If these parts are not available for your favorite car, they can be easily fabricated. By learning what effects changes in suspension geometry have on a car's behavior, you can tune and adjust your suspension to work like you want. Understanding these geometry traits and making them adjustable is a powerful tool when trying to squeeze out the last bit of cornering performance.

If you're a racer, autocrosser, drifter or just a hardcore canyon carver, these tools will give you a significant edge.

Toe Steer:

Like bump steer, which we discussed in the previous installment, toe steer can adversely affect your car’s handling. Toe steer is a product of suspension components of different lengths moving through different arcs at the same time. The resulting changes in toe in the rear suspension can cause unpredictable handling. This usually happens when lowering the car. If kits are unavailable for your application, a racecar fabricator can reposition the trailing arms and other links to a corrected position on severely lowered cars. It’s also common to slot control arm mounting points to alter their path of travel and correct toe on lowered cars.

Excessive lowering can make toe steer worse by placing the suspension links in a static position and range of travel they were never designed for. A beam axle suspension with trailing arms, as found in the rear of many small front-wheel-drive cars is a good example of where this might occur. At stock height, the trailing arms are usually parallel to the ground. When the car rolls, the outboard and inboard arms swing in different directions. The resulting arc-shaped axle path shortens the car’s wheelbase during compression and droop. Since each arm swings equally in different directions, the axle’s toe doesn’t change because each end of the beam axle is pulled the same distance forward.

If the car is lowered too much, both trailing arms point downward toward the front of the car. At this angle, the arms don’t move equally in opposing directions. Under roll, the inside arm will push its side of the axle rearward while the outer arm will pull its side of the axle forward, causing understeer.

Anti-Dive and Anti-Lift

Anti-dive and Anti-lift are tricks that can be applied to a car’s front suspension geometry to control brake dive and acceleration lift. Lift and dive can be mitigated by carefully locating suspension pivot points to take advantage of the “force reaction” on the chassis created by acceleration or deceleration. This avoids the need to increase spring rates to reduce pitching – an important issue for ride quality in all street cars. Anti-dive helps prevent the nose of the car from diving during hard braking. Most cars have some degree of anti-dive designed into the stock suspension geometry. Anti-dive utilizes deceleration forces to increase the front wheel compression rate and reduce brake dive. By changing the angle of the suspension links, the amount of anti-dive can be manipulated.

However, excessive anti-dive can hinder performance causing the front suspension to stiffen while braking for a corner, which can cause understeer. In extreme amounts, anti-dive geometry will cause wheel hop and caster changes under braking. Most racecar suspensions have much less anti-dive and anti-lift than street car suspensions. On racecars, the stiff suspension is used to control body motion instead of redirecting braking or acceleration forces. Most kits available for serious wrenchers to alter anti-dive and anti-lift work to reduce these factors. On FWD cars, this same geometry also resists front-end lift under acceleration.

Anti-lift geometry greatly affects launch traction for FWD drag cars, yet only recently have tube-frame pro class front-wheel-drive drag racers begun to consider using front-end anti-lift geometry. Whiteline has suspension mounts and bushings to tune the anti-lift and anti-dive out of our cars.


Calculate Anti-Dive, Anti-Lift and Anti-Squat

1) Find the Center of Gravity The calculated center of gravity (C.G.) usually ends up 15 to 20 inches above ground – a few inches higher than the plane of the crankshaft – in a typical sedan. On a front-wheel-drive car, it is about even with the driver’s seat. Draw a vertical line from the C.G. to the ground.

2) Find the Instant Center Calculating anti-lift and anti-dive requires finding the side-view instant center for the front suspension. Anti-squat calculations require finding the side-view instant center for the rear suspension. To find either instant center, draw lines through the suspension pivots of the upper and lower control arms at their attachment points to the chassis. These lines should converge somewhere in the middle of the car between the wheels. This intersection is the side instant center.

3) Find the Percentage Anti-dive, anti-lift and anti-squat are expressed as percentages of the C.G. height. Draw a line from the center of the tire’s contact patch up to the instant center intersecting the line from the C.G. to the ground. This line represents the force vector where the acceleration or deceleration force acts on the mass of the car.

To calculate the percentage of anti-lift, anti-dive, or anti-squat, compare the overall height of the C.G. to the distance between this intersection point and the ground. For example, if the force vector intersects the line between the ground and C.G. at one-quarter of its height, the suspension has 25 percent anti-dive.



Ackerman Angle Ackerman steering is a dynamic toe out that compensates for the difference in turning radius of the inside and outside wheels. Ackerman angle is the difference in steering angle between the inside and outside wheel in a corner. Ackerman Angle: When cornering, the inside and outside wheels have different distances and different arcs. If both wheels are at the same steering angle, then one or both tires would be scrubbing.

Think of Ackerman as dynamic toe-out, which increases toe-out as the steering wheel is turned. This gives the quick turn-in advantage of having toed-out alignment while turning without the handling and tire-wear drawbacks of static toe-out in a straight line. Just about all cars have Ackerman built into their steering geometry. It is, for all practical purposes, non-adjustable. The Ackerman angle can only be changed by moving the steering rack. A simpler solution is to just dial in some static toe-out, which will multiply the Ackerman effect that’s already engineered in the car’s steering geometry.


Camber Curve:

Multilink and A-Arm suspensions are designed with shorter upper links or different rotation points so the upper components sweep in a tighter arc than the lower links. The different arcs make the wheel gain negative camber as the suspension compresses. However, there can be drawbacks.

Too much camber gain can cause side scrub. Side scrub occurs as a control arm sweeps through its range of motion and pulls the tire laterally, which adds further traction load on the tire. This can cause instability over bumps, especially if the bumps only affect one side of the car. Placing the suspension links for negative camber gain also affects roll center location. Fortunately, it’s easy to find a good compromise between roll center location, negative camber gain and minimal side scrub.


Corner Balancing:

Corner Balancing is the adjustment of weight distribution at each wheel. Ideally, the cross-weight percentage is the same diagonally between the car’s corners. This is done so a car’s understeer/oversteer balance is the same in a right- or left-hand corner. Corner weight can be adjusted on any car with a height-adjustable suspension. There are applications for nearly all popular performance cars nowadays. For older or non-mainstream cars, companies like Ground Control sell parts to make any car’s coil-over suspension height-adjustable.

Corner weights are set by adjusting the suspension ride height at each corner while the car is on four linked electronic scales. The scales display the weight supported by each wheel. With the driver in the car, the spring perches are adjusted to achieve the desired cross-balance. Raising the perch increases the weight at that corner; lowering decreases it. Weight also tends to be transferred diagonally across the car when changing perch height. Through trial and error, the weights should be adjusted to be as close to equal as possible from side to side and diagonally.

The complexities of modern suspension design, from anti-lift geometry to camber curves, are fundamentals that must be understood before making effective changes to a vehicle’s suspension.




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geocool69
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[*] posted on 29-5-2009 at 02:15 AM


amazin!
this is the mother of all suspension guides and covers everything

would you be able to submit this to my wiki guide?


(clicky)





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Tercel5efe93
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[*] posted on 29-5-2009 at 02:28 AM


We needed somthing like this for a long time your greatness thank you!!!!

:thankyou:
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[*] posted on 29-5-2009 at 10:47 AM


Just to add an observation I have 17 x 7's and a built suspension , I have a pretty much stock 3ee and I know the bigger wheels have to be eatting up some of my 79 rwhp. But honestly I don't see a problem with acceleration in day to day driving. And my little Tercel is a hard core corner carver.

It changes lanes lighting fast, brakes better and corners, harder my other car is a 944 and the Tercel easily handles as well as the Porsche.

If I had it to do over again I might try 16's if the steering would be as quick with a 40 profile tire. The Porsche has 225 50's on it and they have twice as much side wall as the 40' s and it does show.

Maybe if the Porsche had 17's it would be different but the 17's aren't just for show.:bananarock:

[Edited on 29-5-2009 by chip18sw]




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[*] posted on 29-5-2009 at 10:48 AM


sticky this thread ASAP
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[*] posted on 29-5-2009 at 09:18 PM


I've been referring to this piece since I've owened this Tercel. iced, iced, baby has done a great by putting this together for us. Thanks yous Daniel Son.:2thumbsup:

As congested as this suspension section is with stickies, this is no doubt worthy.




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[*] posted on 28-7-2009 at 02:25 AM


wow.... great job! :2thumbsup:



Traaaaankii

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[*] posted on 15-1-2010 at 11:37 AM


I didn't understand half of it... but I'll show my friend that is studying auto mech.. I think this will help him to get an idea !!! The info seems to have it all... Let's see if I can manage to make it work into my car!!!

:bananarock:

smell ya later.... keep posting this kinda stuff...

reply with links to other guides if posible!:rotf:




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[*] posted on 15-1-2010 at 12:17 PM


this is a very thorough thead daniel and i never realized afteryou left exactly how much you have to offer this forum and what we were going without after you left.

your the shyt bro

hows that megasquirted silvertop coming?
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[*] posted on 28-4-2010 at 04:41 PM


:yikes: :yikes: :yikes:

Interesting, love the suggestions of wheels and rims as well as the suspension setup.
Thanks for taking the time to write this :thumbup:




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[*] posted on 23-7-2010 at 07:30 AM


Sport compact car definitely writes a good article, no doubt about that.. I have all of the installments of this series, "making it stick"




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[*] posted on 23-7-2010 at 07:35 AM


Sport compact car definitely writes a good article, no doubt about that.. I have all of the installments of this series, "making it stick"




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