Spring Rate Changes (def. important for those who dont pay att. to this)
Modification - Effect on Suspension

Increase front and rear rate - Ride harshness increases; tires may not follow bumps causing reduced traction. Roll resistance increases.

Increase front rate only - Front ride rate increases. Front roll resistance increases, increasing understeer or reducing oversteer.

Increase rear rate only - Rear ride rate increases. Rear roll resistance increases, increasing oversteer or reducing understeer.

Decrease front and rear rate - Ride harshness decreases; tires follow bumps more effectively, possibly improving traction. Roll resistance decreases.

Decrease front rate only - Front ride rate decreases. Front roll resistance decreases, decreasing understeer or increasing oversteer.

Decrease rear rate only - Rear ride rate decreases. Rear roll resistance decreases, decreasing oversteer or increasing understeer.


Antiroll Bar Changes (aka sway bar)
Modification - Effect on Suspension

Increase front rate - Front roll resistance increases, increasing understeer or decreasing oversteer. May also reduce camber change, allowing better tire contact patch compliance with the road surface, reducing understeer.

Increase rear rate - Rear roll resistance increases, increasing oversteer or decreasing understeer. On independent rear suspensions, may also reduce camber change, allowing better contact patch compliance with road surface, reducing oversteer.

Decrease front rate - Front roll resistance decreases, decreasing understeer or increasing oversteer. More body roll could reduce tire contact patch area, causing understeer.

Decrease rear rate - Rear roll resistance decreases, decreasing oversteer or increasing understeer. On independent rear suspensions, more body roll could reduce tire contact patch area, causing oversteer.



Shock Absorber Changes (aka your struts)
Modification - Effect on Suspension

Increase rebound and bump rates - Ride harshness increases.

Increase rebound rates only - On bumps, tires may leave track surface.

Increase bump rates only - Body roll resisted; outside tire loaded too quickly; car won't stabilize into a turn.

Decrease rebound and bump rates - Ride harshness decreases; car may float over bumps.

Decrease rebound rates only - On bumps, tires follow track surface more effectively; car may continue to oscillate after bumps.

Decrease bump rates only - Body rolls quickly; car is slower to respond to turn-in.



Troubleshooting Tire Temperatures
Reading - Handling problem - Reason

All tires too hot - * - Compound too soft for track and ambient temperature conditions.

Front tires too hot - Understeer - Front tire pressures too low.

Rear tires too hot - Oversteer - Rear tire pressures too low.

Inside edges too hot - Too much body roll - Too much negative camber or too much toe-out.

Outside edges too hot Too - much body roll - Too little negative camber, too little toe-out or too much toe-in or wheel width too narrow for tire width.

Center of tread too hot - * -Tire pressure too high.

Edges on too hot - * -Tire pressure too low.

All tires too cold - * - Compound too hard for track and ambient temperature conditions or car not being driven to limit.

Front tires too cold - * - Inadequate load on front tires.

Rear tires too cold - * - Inadequate load on rear tires



Solving Handling Problems
Problem - Manifestation *Solutions

Steady state understeer - All turns or low-speed turns only
*If front tire temps are optimum and rears are low, stiffen rear antiroll bar; *if front temps are too hot, soften front (most likely).
*If front tire pressures are optimum, decrease rear tire pressure.
*Increase if chunking occurs.
*Improper front camber.
*Too much body roll at front, causing excessive camber change.

Steady state understeer - High-speed turns only
*If front tire temps are OK, increase front downforce.
*If front tire temps are too hot, reduce rear downforce.

Steady state oversteer - All turns or low-speed turns only
*If rear tire temps are optimum, with fronts too low, stiffen front antiroll bar;
*if rear temps are too hot, soften rear antiroll bar (most likely).
*If rear tire pressures are optimum, decrease front tire pressure. *Increase if chunking occurs.
*Improper rear camber.

Steady state oversteer - High-speed turns only
*If rear tire temps are OK, increase rear downforce.
*If rear tire temps are too hot, reduce front downforce.

Corner entry understeer
*Front shocks are too soft in bump resistance.
*Too much front toe-in; use a small amount of front toe-out.

Corner exit understeer
*Rear shocks are too soft in bump.
*Front shocks are too stiff in rebound.

Corner entry oversteer
*Rear shocks are too soft in rebound.
*Rear ride height is too high (too much rake) compared to front.

Corner exit oversteer
*Rear shocks are too soft in rebound.
*Too much rear toe-in or any rear toe-out.

Straightline instability
*Tire pressure is too low in one or more tires.
*Too little positive front caster.
*Too much front toe-in or any toe-out in rear.

Straightline speed too slow
*Too much overall downforce.
*Too much toe-in or toe-out.
*Ride height is too hight.

Excessive steering effort - All turns
*Too much positive caster.
*Front tire pressures are too low.

Chassis or suspension bottoms
*Spring rates are too soft.
*Shock absorber bump rates are too soft.
*Inadequate suspension travel.
*Inadequate ride height.

With toe-in (left) a deflection of the suspension does not cause the wheels to initiate a turn as with toe-out (right).



Caster



(TOP LEFT) Positive camber: The bottoms of the wheels are closer together than the tops. (TOP RIGHT) Negative camber: The tops of the wheels are closer together than the bottoms. (CENTER) When a suspension does not gain camber during deflection, this causes a severe positive camber condition when the car leans during cornering. This can cause funky handling. (BOTTOM) Fight the funk: A suspension that gains camber during deflection will compensate for body roll. Tuning dynamic camber angles is one of the black arts of suspension tuning.

Front spring rate increase:
More under steer; increase in proportional weight transfer to the front when rear wheel rate is not increased; reduces front traction when rear rate is not changed.
Usable adjustment: 150-600 lbs/in
Symptoms of too much adjustment: terminal under steer; front of car hops in corners; excessive wheel spin on inside front tire on FF cars.

Front spring rate decrease:
Less under steer; decreases proportional weight transfer to the front when rear wheel rate is not increased; increases front traction when rear rate is not changed.
Usable adjustment: 150-600 lbs/in
Symptoms of to much adjustment: Too much over steer; over steer then under steer if spring is so soft that the car bottoms out on lean, car bottoms out excessively with a jolting ride.

Rear spring rate increase:
More over steer; increase in proportional weight transfer to the rear when front wheel rate is not increased; increases rear traction when front rate is not changed.
Usable range: 100-600 lbs/in
Symptoms of too much adjustment: too much over steer; sidestep hop in corners; twitchy; pretty scary.

Rear spring rate decrease:
Less over steer: decreases proportional weight transfer to the rear when front wheel rate is not changed; increases rear traction when front rate is not changed
Usable range: 100-600 lbs/in
Symptoms of too much adjustment: car under steers; if way to soft car under steers then over steers as car bottoms out on lean; car bottoms out excessively with a jolting ride.

Front anti-roll bar stiffer: more under steer
Usable range: none to 1.25 inches in diameter
Symptoms of to much adjustment: terminal under steer; lifts inside front tire off the ground witch can cause massive wheel spin on FF cars; also not good for most effective tire usage as inside tire is now doing nothing.

Front anti-roll bar softer: less under steer
Usable range: none to 1.25 inches in diameter
Symptoms of to much adjustment: overstate scary; more like fun

Rear anti-roll bar stiffer: more over steer
Usable range: none to 1 inch in diameter
Symptoms of too much adjustment: Big-time over steer. Can cause inside rear tire to lift off the ground.

Rear anti-roll bar softer: less over steer
Usable range: none to 1 inch in diameter
Symptoms of to much adjustment: under steer; slow and boring

Front tire pressure higher: less under steer by reducing slip angels on most tires
Usable adjustment: up to 55psi hot
Symptoms of too much adjustment: no traction- tire crowned so more under steer; adds wheel spin in FF cars; jarring ride; center of tire wears out

Front tire pressure lower: more under steer by increasing slip angles on most tires
Usable adjustment: not less then 20psi
Symptoms of too much adjustment: edges of tire wear quickly because tire is folding over; feels mushy; tires chunk because low pressure means heat build up.

Rear tire pressure higher: less over steer by reducing slip angles on most tires
Usable range: up to 45psi hot
Symptoms of too much adjustment: no traction—tire is crowned so more over steer; bad wheel spin on FR cars; jarring ride; center of tire wears out.

Rear tire pressure lower: more over steer by incresing slip angles on most tires.
Usable range: not less then 20psi
Symptoms of too much adjustment: edges of tire wear quickly because tire is folding over; feels mushy; tires chunk because low pressure means heat build up

More negative camber front: less under steer because of better lateral traction as tread is flatter on the ground under side load.
Usable range: up to 3.5 degrees negative
Symptoms of too much adjustment: poor braking; car is road crown sensitive; twitchy; front tires wear on inside edge

More negative camber rear: less over steer because of better lateral traction as tread is flatter on the ground under side load. More rear grip
Usable range: up to 2.5 degrees negative
Symptoms of too much adjustment: more over steer; car feels twitchy in back; tires wear out on inside edge; less breakaway warning when limit is exceeded.

Ride height to low (typical beginner mistake): car is twitchy with unpredictable dynamics. Bump steer make you life miserable.
Usable range: usually 1.5-2.0 inches lower then stock unless car has been modified to go lower.
Symptoms of too much adjustment: everything that could possibly go wrong: sudden over/under steer; twitchy due to bump steer; very harsh ride; premature tire wear.

Toe in – front: car is stable going straight. Turn in is average
Usable range: 0-1/8th inch
Symptoms of too much adjustment: car has slow twitchiness under braking; feels odd; kills outside edge of tires

Toe out – front: Car turns in well; works pretty well on FF car as they tend to toe-in under load.
Usable range: 0-1/4 inch
Symptoms of too much adjustment: Car is really twitchy under braking; car wanders on straight road; kills inside edge of tire

Toe in – rear: car is less likely to over steer when the throttle is lifted
Usable range: 0-1/8th inch
Symptoms of too much adjustment: weird, slow, rocking movement in back; feels slow but still unstable; wears outside edge of tires.

Toe out – rear: Helps car rotate useful in low speed and slalom courses; very common on FF pro rally cars.
Usable range: 0-1/8th inch
Symptoms of too much adjustment: not to good for street driving; causes lift throttle over steer; makes violent side to side rocking motions in the rear; tie wears on inside more.

Positive front caster: helps stability; suspension will get more negative camber when turning; reducing positive caster reduces steering effort. (Negative caster is not usable)
Usable range: 4-9 degrees positive
Symptoms of too much adjustment: can increase under steer especially in cars with wide low-profile tires. Can increase steering effort.

Single adjustable shock stiffer: Better turn in; better transient response; causes slower onset of over/under steer by slowing weight transfer depending on what end of the car is adjusted.
Symptoms of too much adjustment: suspension becomes unresponsive; ride gets harsh; car skips over bumps, loosing traction; Causes a big delay in weight transfer resulting in strange handling like under steer then late corner stage over steer.

Single adjustable shock softer: slower transient response; quicker onset of over/under steer
Symptoms of too much adjustment: car oscillates due to under dampened spring motion, like a boat. Car gets twitchy in turns. Feels unstable.

Alignment Angles Terminology

Caster Angle

Caster is the angle between an imaginary line drawn through the upper and lower steering pivots and a line perpendicular to the road surface (viewed from side of vehicle). If the top of the line tilts rearward, the vehicle is said to have "POSITIVE" caster. If the top of the line tilts forward, the vehicle is said to have "NEGATIVE" caster.




Positive caster can also be defined as when the spindle is tipped so that the pivot support centerline intersects the road surface at a point in front of the initial tire contact. Negative caster would then be the center line intersection to the road surface behind the initial tire contact.

Most vehicles produced today do not have adjustable caster angle. Many early model vehicles have adjustable caster in which road crown is compensated for (along with camber). By setting the caster angle on the driver's side 1/2 degree less than the passenger side for positive caster specifications or 1/2 degree more for negative caster specifications, the road crown should not cause vehicle pull in either direction.

Vehicles equipped with manual steering use very little positive or negative caster. This helps reduce the steering effort at the steering wheel. The advantage of caster adjusted toward negative is greater maneuverability; however, direction stability on open road driving is reduced. The advantage of positive caster is the strong directional stability and the ease of returning the steering to a straight-ahead position.

Caster will not cause tire wear unless extreme misadjustment or worn parts are involved. Always set caster (if adjustable) to specifications and within 1/2 degree from side to side. Keep road crown in mind and adjust as necessary if a pull is present after a proper alignment has been completed.

Front Camber Angle

The camber angle will affect the wear on the inner or outer edge of the tire. Camber is the inclination of the centerline of the wheel from the vertical as viewed from the front of the vehicle. Camber angle is measured in positive or negative degrees. Positive camber is the outward tilt of the top of the tire. Negative camber is the inward tilt of the tire at the top. If a tire was absolutely vertical, the degree of camber would be zero.



Unlike the caster angle, camber will change with vehicle load and ride height. With the weight of the driver in the vehicle, front left camber will increase and front right camber will decrease. As rough road conditions are encountered, the downward thrust of the vehicle body will cause front camber to go negative. As the vehicle body movement returns upward, front camber will go positive. As camber oscillates, toe adjustment will also change with each movement of the control arm.

A tire with positive camber can influence the vehicle with a directional pull. The vehicle will go towards the side that has the tire with the most positive camber. It is the normal tendency of the tire to roll around the center of a circle when the top of the tire is inclined towards the center of that circle.

Positive camber tends to place the tire-to-road contact area nearer the point of load. This assists in easier steering and forces the thicker inner portion of the spindle to carry most of the load. Modern suspension design has reduced the need for considerable positive camber. Many manufacturers specify a slight amount of negative camber.

Some manufacturers recommend an additional 1/4 to 1/2 degree positive camber on the left wheel to compensate for road crown. The car will then pull toward the side with greater positive camber. This will offset the pull effect of the road crown. Always set camber within specifications.

Rear Camber Angle - Front Wheel Drive

Rear wheel camber angle is being relied on for improved steering and general handling performance. In the past FWD vehicles and independent rear suspension vehicles were most likely to have adjustable rear camber. On vehicles currently being produced, rear camber adjustment capabilities are being found on all types of models.

Note: Always use full-floating tables under wheels whenever alignment is being done.

When alignment problems are reported on vehicles with fixed rear axles and no rear wheel camber adjustment capabilities, a thorough inspection of the rear suspension should be made. Damaged or worn components can cause alignment and/or steering problems. Replacing or repairing the defective components should bring the rear wheel assemblies into specification.

On vehicles where rear wheel camber is adjustable, all previous precautions apply. If camber adjustment requirements are excessive, a thorough inspection must be performed. Replacing any defective components could bring the camber into specification and adjustment may become unnecessary. As with the front suspension, DO NOT perform alignment on vehicles with damaged or worn components.

Whether the rear suspension is adjustable or not, if all components are in good condition and the proper specifications cannot be obtained, aftermarket correction kits may be needed.

Rear Camber Angle - Rear Wheel Drive

On RWD vehicles, where rear camber is usually not adjustable, camber will normally be fixed at zero. Even though this angle cannot be changed through adjustment, if rear suspension abnormalities exist, a thorough inspection must be made.

Not to be overlooked are the rear springs. Worn or weak rear springs will alter riding height and because of a reduction in tension, will bring the shock absorbers out of the optimum range of their dampening ability. The result will be excessive tire movement. This condition reduces operator control and contributes to abnormal tire wear. As in FWD vehicles, replacing worn or defective components may bring rear wheels within specification.

Toe-In and Toe-Out

Unlike caster and camber, which are measured in degrees, toe is most frequently measured in fractional inches, millimeters or decimal degrees. The system of measurement selected will depend on the type of equipment available. An incorrect toe setting is one of the main alignment factors that cause excessive tire wear. Front and rear toe are the same in definition, with the adjustment capabilities and procedures being the only actual difference.




Toe is the difference between the leading edge (or front) and trailing edge (or rear) of the tires. Toe-in is the measurement in fractions of an inch, millimeters or decimal of degrees that the tires are closer together in the front than they are in the back. Toe-out is the same measurement, except the tires are further apart in the front than in the rear.

Some manufacturers measure the angular change from straight-ahead in degrees. Slight toe-in is preferred to toe-out on most vehicles because steering is aligned while the vehicle is stationary. When the vehicle is moving, linkage components flex causing a change in alignment angles. This is classified as "Running Toe." Running toe should be zero to maximize tire life and achieve the least rolling resistance.

The usual tendency is for the tires to turn outward while the vehicle is in motion, so most vehicles are designed with a static toe-in setting. The static toe-in setting will become zero as the linkage flexes when the vehicle is in motion. Always set toe to the manufacturer's specifications.

On vehicles with toe adjustment capability on the rear, an alignment specialist can go beyond manufacturer's specifications according to vehicle usage and customer requirements. With the proper equipment, the rear axle can be adjusted to perform aggressively toward demanding load and road conditions. Vehicles with FWD and independent rear suspensions are more likely to have adjustable rear toe. As with rear camber, properly adjusted rear toe will contribute to improved steering and handling characteristics. Full floating tables must be used under the rear tires whenever toe is to be adjusted.

If rear toe is out of specification a thorough inspection must be done, whether or not rear toe is adjustable. Components found to be defective must be replaced. On vehicles that do not have rear toe adjustment capability and toe is not within specifications, replacing defective components may bring toe within specifications.

Toe-Out on Turns

When a vehicle enters a turn, the outer tire must travel a greater distance than the inner tire. The tire center is tangent to the turn circle. If the tires were to remain parallel in a turn, one tire would drag across the road surface. This would create tire squeal, excessive tire wear and reduce handling performance.



The outside front wheel must therefore be turned at less of an angle than that of the inside front wheel. This will keep both wheels tangent to their respective turning circles and prevent tire squeal and/or damage. As the vehicle enters a turn, the tie rod ends will travel an equal distance, but due to the angle of the steering arms the tires will progressively toe-out.

Although this angle is never adjustable, it is easily checked on the alignment rack by turning the tires 20 degrees on full floating tables. First turn the front right tire 20 degrees and read the indicator on the left wheel. This is the angle of toe-out for the left tire. Repeat the procedure for the remaining side. Compare reading with specifications. Readings not within specifications are an indication that the steering arms are bent and should be replaced. Never bend or heat components to repair them.

Steering Axis Inclination (SAI)

Steering Axis Inclination (SAI) can be a difficult angle to understand. SAI is also referred to as the ball joint angle or kingpin inclination (on I-Beam suspension). The easiest way to understand SAI is to first define steering axis. The steering axis is an imaginary line intersecting the spindle support. In a conventional steering system, the spindle supports are the upper and lower ball joints or the kingpins. With MacPherson strut systems, steering axis is the angle beginning at the ball joint and extended through the strut assembly.



Viewed from the front of the vehicle, SAI is the angle between the steering axis and a true vertical line established through the tire. The SAI is a stability angle and is measured in degrees. If these imaginary lines were extended to the road surface, the area covered between them would be identified as the point of load or scrub radius.

The vehicle body will be closest to the road surface when the wheels are pointed straight-ahead as a result of SAI. A spindle with SAI will have the outer end of that spindle at the highest point when the wheels are pointed straight-ahead. Therefore, as the weight of the vehicle pushes downward, the spindle will always attempt to move upward to return the wheels to a straight-ahead position.

After a turn, the SAI helps to return the tires to straight-ahead position. SAI also aids in vehicle directional stability by resisting road irregularities that attempt to turn the wheels away from the straight-ahead position. SAI produces many of the same benefits that improve steering stability as positive caster. Correct engineering of SAI can reduce the need for high positive camber.

The effect of SAI on directional stability is usually greater then that of caster. Some vehicles with power steering require a greater amount of steering wheel returning force than those with manual steering. SAI is often used with positive caster on power steering equipped vehicles to assist in steering wheel returnability.

Scrub Radius

Scrub radius is the term used to describe the distance between the projected steering axis and the tread centerline at the road surface. Scrub radius is positive when the centerline of the tire lies outside the projected steering axis. It is negative when the centerline of the tire is inside the projected steering axis. The scrub radius is a distance measurement and it is therefore measured in inches or millimeters.

The size of the scrub radius depends on the steering axis inclination, wheel offset and the distance the spindle centerline is above the road surface. By carefully considering the correct SAI and the proper wheel offset for the designed spindle height, the required amount of scrub radius is designed into the suspension.

Although the spindle height has an effect on the scrub radius, little can be done to change this height because tire height is limited by the clearance space under the fender and body. Since all handling sensations pass between the tire and the road, the scrub radius provides the necessary feedback to give the driver road feel.

Setback

Setback or front end squareness is a condition in which one wheel is rearward of the other. If setback is present the turning radius will not be correct when the vehicle turns. With this condition, the tires will wear very much in the same manner as if they were under inflated. Generally, setback is the result of collision damage. It is preferable to have the front tires square with each other before alignment is done.



Considering the many different types of alignment equipment available, it is not possible to cover each checking procedure. Use the alignment machine manufacturer's instructions for checking setback. The most accurate way of checking is with four wheel alignment equipment.

Depending on the severity of setback and the type of alignment equipment being used, false readings can mislead a technician into thinking that an incorrect adjustment is within specification. These false readings are experienced more frequently with two wheel alignment methods.

Thrust Angle

Thrust angle is the line that divides the total angle of the rear wheels. The rear tires are not just following the front tires, they are actually establishing direction of the vehicle. In doing so, a direction of thrust is developed. The thrust angle created by the rear wheels is used as a reference for aligning the front wheels. Ideally, the thrust angle should be identical to the geometric centerline of the vehicle.




If thrust angle and geometric centerline are identical, the position of the tires would then form an absolute rectangle and the front tires could be aligned to the rear tires, resulting in a perfectly centered steering wheel.

Because of unitized construction, factory tolerances and a varying degree of damage and/or wear, it is increasingly unlikely that the axles will be parallel. When the rear axle projects a different angle than the front axle, the driver will need to turn the steering wheel to compensate in order to drive in a straight line.

On situations where the thrust line and geometric centerline are not identical, a thorough inspection of the rear axle and suspension system must be done. Replacing defective components should aid in positioning thrust angle close to the geometric centerline.

If the thrust angle is not identical to the geometric centerline and there are no defective components, align the vehicle using the thrust angle instead of the geometric centerline. Aligning the front wheels to the thrust angle is preferred to aligning to the geometric centerline. The ability to do this is a significant advantage of four wheel alignment. The result should be a straight steering wheel as the vehicle moves straight-ahead.

Adapted from http://www.specprod.com/
Copyrighted by Specialty Products Company, Longmont , Colorado, USA

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(Update: October 07, 2004)

Pointed the Right Way
By: John Hagerman

Camber, Caster and Toe: What Do They Mean?

The three major alignment parameters on a car are toe, camber, and caster. Most enthusiasts have a good understanding of what these settings are and what they involve, but many may not know why a particular setting is called for, or how it affects performance. Let's take a quick look at this basic aspect of suspension tuning.

UNDERSTANDING TOE

When a pair of wheels is set so that their leading edges are pointed slightly towards each other, the wheel pair is said to have toe-in. If the leading edges point away from each other, the pair is said to have toe-out. The amount of toe can be expressed in degrees as the angle to which the wheels are out of parallel, or more commonly, as the difference between the track widths as measured at the leading and trailing edges of the tires or wheels. Toe settings affect three major areas of performance: tire wear, straight-line stability and corner entry handling characteristics.

For minimum tire wear and power loss, the wheels on a given axle of a car should point directly ahead when the car is running in a straight line. Excessive toe-in or toe-out causes the tires to scrub, since they are always turned relative to the direction of travel. Too much toe-in causes accelerated wear at the outboard edges of the tires, while too much toe-out causes wear at the inboard edges.

So if minimum tire wear and power loss are achieved with zero toe, why have any toe angles at all? The answer is that toe settings have a major impact on directional stability. The illustrations at right show the mechanisms involved. With the steering wheel centered, toe-in causes the wheels to tend to roll along paths that intersect each other. Under this condition, the wheels are at odds with each other, and no turn results.

When the wheel on one side of the car encounters a disturbance, that wheel is pulled rearward about its steering axis. This action also pulls the other wheel in the same steering direction. If it's a minor disturbance, the disturbed wheel will steer only a small amount, perhaps so that it's rolling straight ahead instead of toed-in slightly. But note that with this slight steering input, the rolling paths of the wheels still don't describe a turn. The wheels have absorbed the irregularity without significantly changing the direction of the vehicle. In this way, toe-in enhances straight-line stability.

If the car is set up with toe-out, however, the front wheels are aligned so that slight disturbances cause the wheel pair to assume rolling directions that do describe a turn. Any minute steering angle beyond the perfectly centered position will cause the inner wheel to steer in a tighter turn radius than the outer wheel. Thus, the car will always be trying to enter a turn, rather than maintaining a straight line of travel. So it's clear that toe-out encourages the initiation of a turn, while toe-in discourages it.

The toe setting on a particular car becomes a tradeoff between the straight-line stability afforded by toe-in and the quick steering response promoted by toe-out. Nobody wants their street car to constantly wander over tar strips-then ever-ending steering corrections required would drive anyone batty. But racers are willing to sacrifice a bit of stability on the straightaway for a sharper turn-in to the corners. So street cars are generally set up with toe-in, while race cars are often set up with toe-out.

With four-wheel independent suspension, the toe must also be set at there are of the car. Toe settings at the rear have essentially the same effect on wear, directional stability and turn-in as they do on the front. However, it is rare to set up a rear-drive race car toed out in the rear, since doing so causes excessive oversteer, particularly when power is applied. Front-wheel-drive race cars, on the other hand, are often set up with a bit of toe-out, as this induces a bit of oversteer to counteract the greater tendency of front-wheel-drive cars to understeer.

Remember also that toe will change slightly from a static situation to a dynamic one. This is most noticeable on a front-wheel-drive car or independently-suspended rear-drive car. When driving torque is applied to the wheels, they pull themselves forward and try to create toe-in. This is another reason why many front-drivers are set up with toe-out in the front. Likewise, when pushed down the road, a non-driven wheel will tend to toe itself out. This is most noticeable in rear-drive cars.

The amount of toe-in or toe-out dialed into a given car is dependent on the compliance of the suspension and the desired handling characteristics. To improve ride quality, street cars are equipped with relatively soft rubber bushings at their suspension links, and thus the links move a fair amount when they are loaded. Race cars, in contrast, are fitted with steel spherical bearings or very hard urethane, metal or plastic bushings to provide optimum rigidity and control of suspension links. Thus, a street car requires a greater static toe-in than does a race car, so as to avoid the condition wherein bushing compliance allows the wheels to assume a toe-out condition.

It should be noted that in recent years, designers have been using bushing compliance in street cars to their advantage. To maximize transient response ,it is desirable to use a little toe-in at the rear to hasten the generation of slip angles and thus cornering forces in the rear tires. By allowing a bit of compliance in the front lateral links of an A-arm type suspension, the rear axle will toe-in when the car enters a hard corner; on a straight away where no cornering loads are present, the bushings remain undistorted and allow the toe to be set to an angle that enhances tire wear and stability characteristics. Such a design is a type of passive four-wheel steering system.

THE EFFECTS OF CASTER

Caster is the angle to which the steering pivot axis is tilted forward or rearward from vertical, as viewed from the side. If the pivot axis is tilted backward (that is, the top pivot is positioned farther rearward than the bottom pivot), then the caster is positive; if it's tilted forward, hen the caster is negative.

Positive caster tends to straighten the wheel when the vehicle is traveling forward, and thus is used to enhance straight-line stability. The mechanism that causes this tendency is clearly illustrated by the castering front wheels of a shopping cart (above). The steering axis of a shopping cartwheel is set forward of where the wheel contacts the ground. As the cart is pushed forward, the steering axis pulls the wheel along, and since the wheel drags along the ground, it falls directly in line behind the steering axis. The force that causes the wheel to follow the steering axis is proportional to the distance between the steering axis and the wheel-to-ground contact patch-the greater the distance, the greater the force. This distance is referred to as "trail.”

Due to many design considerations, it is desirable to have the steering axis of a car's wheel right at the wheel hub. If the steering axis were to be set vertical with this layout, the axis would be coincident with the tire contact patch. The trail would be zero, and no castering would be generated. The wheel would be essentially free to spin about the patch (actually, the tire itself generates a bit of a castering effect due to a phenomenon known as "pneumatic trail," but this effect is much smaller than that created by mechanical castering, so we'll ignore it here). Fortunately, it is possible to create castering by tilting the steering axis in the positive direction. With such an arrangement, the steering axis intersects the ground at a point in front of the tire contact patch, and thus the same effect as seen in the shopping cart casters is achieved.

The tilted steering axis has another important effect on suspension geometry. Since the wheel rotates about a tilted axis, the wheel gains camber as it is turned. This effect is best visualized by imagining the unrealistically extreme case where the steering axis would be horizontal-as the steering wheel is turned, the road wheel would simply change camber rather than direction. This effect causes the outside wheel in a turn to gain negative camber, while the inside wheel gains positive camber. These camber changes are generally favorable for cornering, although it is possible to overdo it.

Most cars are not particularly sensitive to caster settings. Nevertheless, it is important to ensure that the caster is the same on both sides of the car to avoid the tendency to pull to one side. While greater caster angles serve to improve straight-line stability, they also cause an increase in steering effort. Three to five degrees of positive caster is the typical range of settings, with lower angles being used on heavier vehicles to keep the steering effort reasonable.

WHAT IS CAMBER?

Camber is the angle of the wheel relative to vertical, as viewed from the front or the rear of the car. If the wheel leans in towards the chassis, it has negative camber; if it leans away from the car, it has positive camber. The cornering force that a tire can develop is highly dependent on its angle relative to the road surface, and so wheel camber has a major effect on the road holding of a car. It's interesting to note that a tire develops its maximum cornering force at a small negative camber angle, typically around neg. 1/2 degree. This fact is due to the contribution of camber thrust ,which is an additional lateral force generated by elastic deformation as the tread rubber pulls through the tire/road interface (the contact patch).

To optimize a tire's performance in a corner, it's the job of the suspension designer to assume that the tire is always operating at a slightly negative camber angle. This can be a very difficult task, since, as the chassis rolls in a corner, the suspension must deflect vertically some distance. Since the wheel is connected to the chassis by several links which must rotate to allow for the wheel deflection, the wheel can be subject to large camber changes as the suspension moves up and down. For this reason, the more the wheel must deflect from its static position, the more difficult it is to maintain an ideal camber angle. Thus, the relatively large wheel travel and soft roll stiffness needed to provide a smooth ride in passenger cars presents a difficult design challenge, while the small wheel travel and high roll stiffness inherent in racing cars reduces the engineer's headaches.

It's important to draw the distinction between camber relative to the road, and camber relative to the chassis. To maintain the ideal camber relative to the road, the suspension must be designed so that wheel camber relative to the chassis becomes increasingly negative as the suspension deflects upward. If the suspension were designed so as to maintain no camber change relative to the chassis, then body roll would induce positive camber of the wheel relative to the road. Thus, to negate the effect of body roll, the suspension must be designed so that it pulls in the top of the wheel (i.e., gains negative camber) as it is deflected upwards.

While maintaining the ideal camber angle throughout the suspension travel assures that the tire is operating at peak efficiency, designers often configure the front suspensions of passenger cars so that the wheels gain positive camber as they are deflected upward. The purpose of such a design is to reduce the cornering power of the front end relative to the rear end, so that the car will understeer in steadily greater amounts up to the limit of adhesion. Understeer is inherently a much safer and more stable condition than oversteer, and thus is preferable for cars intended for the public.

Since most independent suspensions are designed so that the camber varies as the wheel moves up and down relative to the chassis, the camber angle that we set when we align the car is not typically what is seen when the car is in a corner. Nevertheless, it's really the only reference we have to make camber adjustments. For competition, it's necessary to set the camber under the static condition, test the car, then alter the static setting in the direction that is indicated by the test results.

The best way to determine the proper camber for competition is to measure the temperature profile across the tire tread immediately after completing some hot laps. In general, it's desirable to have the inboard edge of the tire slightly hotter than the outboard edge. However, it's far more important to ensure that the tire is up to its proper operating temperature than it is to have an "ideal" temperature profile. Thus, it may be advantageous to run extra negative camber to work the tires up to temperature.

TESTING IS IMPORTANT

Car manufacturers will always have recommended toe, caster, and camber settings. They arrived at these numbers through exhaustive testing. Yet the goals of the manufacturer were probably different from yours, the competitor. And what works best at one race track may be off the mark at another. So the "proper" alignment settings are best determined by you-it all boils down to testing and experimentation.

Improved Handling with Anti-Roll Bars
by John Comesky

Any enthusiast worth his salt knows that tires have arguably the biggest impact on a vehicle's handling. Obviously, however, there are chassis dynamics that extend beyond the realm of tires. Once you increase the traction threshold at the road surface, then you may be ready to take the next step into improved vehicle handling: reducing body roll through the use of anti-roll bars.

Properly chosen (and installed), anti-roll bars will reduce body roll, which in turns leads to better handling, increased driver confidence and, ultimately, lower lap times.

What is Body Roll?

Chances are, you've experienced the effects of body roll every time you're behind the wheel. It happens during almost every turn when one side of the car lifts, causing the entire vehicle to lean toward the outside of the turn.

The cause of body roll is simple physics: An object in motion tends to stay in motion until acted upon by an outside force. So in practical terms, as you drive ahead in a straight line, you're allowing a couple of thousand pounds of vehicle, fluids and passengers to build momentum in that straight line.

When you tell everything to change direction suddenly, through input at the steering wheel, the front tires may change direction thanks to the mechanical advantages of the steering system, but the momentum of the vehicle, fluids and passengers continues in the original direction. The tires are the only element capable of generating an outside force that can act against this momentum and change its direction.

At this point, one of two scenarios is most likely to occur. If enough momentum exists in the original direction, and the tires lack enough grip to act against the original forward energy, then the vehicle will slide out of the turn as the tires lose traction. However, if the tires have enough grip at the road surface, then instead of sliding, the vehicle's traction at the road surface will overwhelm the original forward momentum and act upon the original forces to induce a change of direction. Hence, a cornering maneuver.

But what happens to that energy? Even though we may have had enough grip to hang on through the turn, we know that the momentum of the vehicle mass will continue in the original direction. The result is a weight transfer toward the new outside edge of the vehicle - the same direction as the original forward momentum.

If enough energy is behind the weight transfer, then this energy will cause the outside suspension (in this case, the spring and strut assembly) to compress while the other side lifts and extends. An engineer type likes to describe this by saying that one side moves into jounce while the other moves into rebound. The rest of us call it lean or body roll.

Why is Body Roll a Bad Thing?

We often hear that preventing body roll is "so important" that we must all rush out and buy this product or that product in order to prevent it. And many enthusiasts have consequently accepted that body roll is therefore bad. But what exactly does body roll do to negatively affect vehicle handling?

For starters, it disrupts the driver. This is probably the effect that most drivers can see and feel during their own driving experiences. And while this is not the most important negative effect of body roll, it is true that the car does not drive itself, no matter how many aftermarket parts you install. So keeping the driver settled, focused and able to concentrate on the task of driving is a foremost priority for spirited vehicle handling.

However, the most often misunderstood effect of body roll upon vehicle handling is the effect of body roll upon camber, and the effect of camber changes upon tire traction.

Put simply, the larger the contact patch of the tire, the more traction exists against the road surface, holding all else constant. But when the vehicle begins to lean or roll to one side, the tires are also forced to lean or roll to one side.

This can be described as a camber change in which the outside tire experiences increased positive camber (rolls to the outside edge of the tire) and the inside tire experiences increased negative camber (rolls to the inside edge of the tire.) So a tire that originally enjoyed a complete and flat contact patch prior to body roll must operate on only the tire edge during body roll.

The resulting loss of traction can allow the tires to more easily give way to the forces of weight transfer to the outside edge of the vehicle. When this happens, the vehicle slides sideways - which is generally a bad thing.

How to Prevent Body Roll

By definition, body roll only occurs when one side of the suspension is compressed (moves into jounce), while the other extends (moves into rebound). Therefore, we can limit body roll by making it harder for the driver-side and passenger-side suspension to move in opposite directions.

One fairly obvious method to achieve this is through the use of stiffer springs. After all, a stiffer spring will compress less than a softer spring when subjected to an equal amount of force. And less compression of the suspension on the outside edge will result in less body roll.

However, stiffer springs require the use of stronger dampers (struts or shock absorbers) and have an immediate and substantial effect on ride quality. So even though handling is improved, they may not be the easiest or most cost-effective way to achieve the objective of reducing body roll.

For many enthusiasts, the use of anti-roll bars, also known as anti-sway bars, roll bars, stabilizer bars or sway bars, provides a more cost-effective reduction in body roll with minimal negative impacts upon ride quality.

How an Anti-Roll Bar Works

Put simply, an anti-roll bar is a U-shaped metal bar that links both wheels on the same axle to the chassis. Essentially, the ends of the bar are connected to the suspension while the center of the bar is connected to the body of the car.

In order for body roll to occur, the suspension on the outside edge of the car must compress while the suspension on the inside edge simultaneously extends. However, since the anti-roll bar is attached to both wheels, such movement is only possible if the metal bar is allowed to twist. (One side of the bar must twist upward while the other twists downward.) So the bar's torsional stiffness, or resistance to twist, determines its ability to reduce body roll. Less twisting of the bar results in less movement into jounce and rebound by the opposite ends of the suspension, which results in less body roll.

Factors that Determine Stiffness

There are two primary factors that determine an anti-roll bar's torsional stiffness: the diameter of the bar and the length of the bar's moment arm. Diameter is generally the easiest concept to grasp, as it is somewhat intuitive that a larger diameter bar would have greater torsional rigidity.

Torsional (or twisting) motion of the bar is actually governed by the equation:

twist = (2 x torque x length)/(p x diameter^4 x material modulus)

And since the diameter is in the denominator, as diameter gets larger, the amount of twist gets smaller. This, in a nutshell, means that torsional rigidity is a function of the diameter to the fourth power. This is why a very small increase in diameter makes a large increase in torsional rigidity.

For example, to compare the rigidity of a stock 15mm bar to an aftermarket, 16.5mm one, simply use the equation 16.5^4/15^4 (Note that 16.5 and 15 are being raised to the fourth power). Some quick math yields the figure of 1.46. In other words, a 16.5mm bar is 1.46 times as stiff or 46 percent stiffer than a 15mm bar of the same design.

Add just one more millimeter to the diameter of the bar for a total of 17.5mm, and the torsional strength skyrockets to 85 percent stiffer than the stock 15mm bar (17.5^4/15.0^4 = 1.85).

However, in addition to the diameter of a bar, there is another very important factor that determines an anti-roll bar's torsional rigidity. This factor is known as the length of the moment arm, or in common terms, the amount of leverage between the vehicle and the bar.

As with anything, an increased amount of leverage makes it easier to do work. This is governed by the lever law: force x distance = torque. As distance, or the length of the lever increases, the resulting amount of torque also increases. (This is why it was easier to move your big brother on the teeter-totter when he moved towards the middle and you stayed out on the end. You enjoyed increased leverage at the end, while he suffered from reduced leverage near the middle.)

Because an anti-roll bar is shaped as a "U," the ends of the bar that lead from the center of the bar to the end-link attachment serve as a lever. As the distance from the straight part of the bar to the attachment at the end link becomes longer, the torque applied against the bar increases, making it easier for a given amount of energy to twist the anti-roll bar. As this distance is reduced, torque is reduced, making it more difficult for a given amount of energy to twist the anti-roll bar.

It is this lever law that is applied during the design of an adjustable anti-roll bar. By using multiple end link locations, the distance from the point of attachment to the straight part of the bar can be altered. Or, in engineers' terms, the length of the moment arm can be increased or reduced in order to make more or less torque against the bar.

Using a setting farther from the center of the bar increases the length of the moment arm, resulting in more torque against the bar, allowing more twisting motion of the bar, creating more body roll. Using a setting closer to the center of the bar reduces the length of the moment arm, resulting in less torque against the bar, allowing less twisting motion of the bar, creating less body roll.

The actual impact upon torque can be compared by dividing the center-to-center distances of the end-link attachment points. For example, say the center-to-center distance of the stock rear anti-roll bar is 200mm. We can compare this to the 160mm distance of the firmest setting of a four-way adjustable 17.5mm bar by simply dividing the distances (160/200 = 0.. In other words, a 160mm center-to-center bar produces only 80-percent of the torque that would be produced by a 200mm center-to-center bar of the same diameter. Or simpler yet, by using the 160mm end-link attachment points, we increase the stiffness of the anti-roll bar by an extra 20 percent.

What the Heck is TLLTD?

TLLTD stands for Tire Lateral Load Transfer Distribution. While this term may sound complex, it simply measures the front-to-rear balance of how lateral load is transferred in a cornering maneuver. It is commonly used to compare the rate of lateral traction loss between the front and rear tires.

Put simply, there is only so much force that a tire can handle. When we ask more of the tire than the tire can deliver, it "saturates," or loses traction. If the front tires saturate before the rear tires, then we call this understeer or push, which means that the car tends to continue moving in the original direction, even though the wheels are turned.

If the rear tires saturate before the front tires, then we call this oversteer or loose, which means that the rear of the car tends to swing around faster than the front, causing a spin. When neither of these conditions prevail consistently, then we describe the chassis as balanced.

We can measure and compare the steady-state understeer and oversteer characteristics of a vehicle by assigning a lateral load transfer percentage of the front relative to the rear. A TLLTD value equal to 50 percent indicates that the chassis is balanced, or both the front and rear tires tend to lose traction at roughly the same time. A front TLLTD value greater than 50 percent indicates that the front tires lose traction more quickly than the rear tires, resulting in understeer. And a front TLLTD value lower than 50 percent indicates that the rear tires tend to lose traction more quickly than the front, resulting in oversteer.

It is important to note that our discussion of TLLTD only considers steady-state cornering maneuvers, such as a long 270-degree on-ramp or off-ramp. Moderate-to-aggressive throttle or brake application can upset this balance during a transient condition, briefly transitioning a vehicle from understeer to oversteer.

The Effect of Anti-Roll Bars Upon TLLTD

Ideally, you now understand how an anti-roll bar can be used to limit body roll, and you understand that reduced body roll can lead to a reduction in adverse camber changes for better tire traction. But what may not be obvious is the effect of anti-roll bar changes upon TLLTD (understeer and oversteer.)

In fact, given the above information, one might even assume that a firmer anti-roll bar, which leads to better camber control, would lead to better traction. If we add a firmer anti-roll bar to the front, traction loss diminishes, so understeer is reduced, right?

Wrong. Let's evaluate more closely the meaning of TLLTD - tire lateral load transfer distribution. Stated another way, we might describe TLLTD as the relative demand of side-to-side energy control that is placed upon the tires. Because a firmer anti-roll bar allows less deflection, it will transfer side-to-side energy (lateral loads) at a faster rate.

As the rate of lateral load transfer increases, additional demands are placed upon the tire. So if we install a firmer anti-roll bar in the front, then we increase the distribution of lateral load transfer toward the front tires. This increases the front TLLTD value, which will result in additional understeer, holding all else constant.

The same logic also holds true in the rear. A firmer anti-roll bar in the rear will increase the rate of lateral load transfer, placing more demand upon the rear tires, accelerating lateral traction loss and creating more oversteer, holding all else constant.

This is why blindly adding parts to your car may not produce the desired results. A wise consumer consults with, and buys from knowledgeable experts that have the tools to make informed tuning recommendations.

I Want a 50 Percent TLLTD On My Car, Right?

Since on paper a 50 percent TLLTD indicates a balanced chassis, many enthusiasts are tempted to jump to the conclusion that this is therefore desirable. They may think that all cars should obviously come this way from the factory. Unfortunately, this is not the case and the considerations are not that simple.

In reality, a car with a 50 percent TLLTD is literally on the constant brink of oversteer. And there are many factors that can quickly and easily take the car from the brink into a full-scale, out-of-control, spinning-in-circles disaster.

For starters, consider the effects of weather conditions that might create a wet or icy road surface. Or imagine that the driver happens to apply too much brake late into a turn - a common mistake among novice drivers. Or consider the effects of varying tire temperatures, tire pressures, or tire wear, all of which will have major impacts upon lateral traction thresholds. And of course, varying weight distribution, as a result of changing fuel tank levels, passengers, or the number of subwoofers in the trunk, will also impact TLLTD.

With all of these things to consider, automotive design engineers are forced to create a more conservative TLLTD. As a result, they intentionally target higher front TLLTD values so that stock vehicles will be prone to understeer, the assumption being that understeer is safer and more predictable for the average driver.

For example, a stock DOHC Saturn is tuned to produce a front TLLTD of approximately 63.4 percent - a relatively conservative target. (But give Saturn some credit, as this is on the aggressive end of the conservative spectrum, especially compared to other front-wheel-drive economy cars.)

As a general rule, an average street-driving enthusiast is probably willing to accept some compromises, within reason of a more aggressive TLLTD in exchange for better handling. A suitable target is probably a front TLLTD value of approximately 58 percent, a value that is considered aggressive, but suitable for street driving.

How do I Create the Right Handling Balance?

Since most enthusiasts do not have the knowledge or software needed to calculate chassis characteristics such as TLLTD, the responsibility falls upon knowledgeable tuners.

Obviously, TLLTD and body roll will both be affected by changes to springs and anti-roll bars. While understanding the effects of multiple changes can get confusing, the answer is usually only a phone call away.

Damm, this is an awsome thread, thank you very much for sharing this information, i'l be reading this when i've got more time.

I disagree with the last article on the following points:
Anti roll bars are not the way to go when increasing roll stiffnes. ARB's reduce your cars overall cornering limit by lifting the inside wheel up. Bumping springrates does not have this drawback.
ARB's reduce your suspensions ability to work independently. They are sort of a solid axle conversion set.
Higher springrates do increase ride harshness, but only to a extent. Softer damperrates can make a higher rate spring quite liveable. Bumpdampening is the single most important influence on ride quality.
TTLD, or FRC= front roll couple. Setting FRC at 60% (60/40 TTLD) is a good thing on a RWD car. A high hp car may need up to 70% FRC. Yes it understeers, but it also gets the power down much much earlier. Understeer through FRC settings are more a sign of poor driver controll over the cars rotation then poor suspension design.
On a FWD car you'd want the opposite, so the remarks in the article are correct. Though I disagree that the settings of 60+ on a fwd car are used because of safety. The dynamic balance of the vehicle is never mentioned. A 65% FRC car in a 90 mile an hour corner has an effective 50% dynamic FRC. Its not a question of whether you want oversteer or not. Its where you want it, at 30mph or 90+.

There are a lot more of these small errors in the articles, so i wouldn't use them as a guide. Try to understand the principles behind them and you won't have to carry around a booklet with you to every trackday.