Suspension Geometry & Alignment 101

Alignment

Pointed the Right Way

story 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.

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

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-the never-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 the rear 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 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 straightaway 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, then 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 cart wheel 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.

Like a shopping cart wheel (left) the trail created by the castering of the steering axis pulls the wheels in line.

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 (see next page). 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. The illustration on the bottom of page 46 shows why this is so. 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.

(TOP RIGHT) Positive camber: The bottoms of the wheels are closer together than the tops. (TOP LEFT) 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.

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.

[link http://www.ozebiz.com.au/racetech/theory/align.html]

Suspension Geometry

Cornering Speed

1) Tire's Grip
Most obviously, the selection of tires is decisive to cornering grip. Car engineers have nothing to do with the friction of the tires, which is determined by the compound and texture. However, they can choose the most suitable tires for their cars.
In the past decade, increasing tire's diameter and width is a common trend shared by all car makers. Do you still remember the Lamborghini Countach employed 15-inch tires ? Today's most exotic Ferrari, Porsche and Viper have 18 to 19-inch rubbers! Larger diameter accompany with larger width increase the contact patch area (that is, the area of the tire contacts with the ground), thus result in more grip. However, this also result in poorer wet road grip because the pressure acting upon the contact patch (that is, the car's weight divided by contact patch area) is reduced thus the tire becomes easier to "float" on the water. Therefore the texture also need to be improved for better water clearance.

Low profile tires are also fashionable in these days. Since the thickness becomes thinner, it is more resistant to side wall deflection under substantial cornering force. However, this is not much related to grip.

It must be mentioned that wide tires are not always good. Especially are front tires, the wider they are, the more resistance generates when they are steered. This create a heavy and insensitive steering feel, also more tire roar and wear. If you want to modify your car by using wider tires, always consider the drawback first. In my opinion, most well-sorted production sports cars have already equipped with the most suitable tires.

2. Suspension Design
To maximize cornering grip, the suspension must keep the tires perpendicular to ground under all conditions such as bump and body roll so that the contact patch area remains maximum.
Generally speaking, double wishbones suspension does the best job to keep the tire perpendicular to ground. The below figure shows how the conventional double wishbones suspension deals with bump and body roll. You can see there's no camber change at all under bump.

But the scene changes very much under body roll - camber changes for the same degree as the body roll. Track width also increases. Camber change reduces the contact patch area thus grip, and also introduces non-neutral steering (we'll discuss this later). Track width variation forces the tires to slip thus also reduce grip.

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Therefore engineers invented unequal length double wishbones. As shown in the below figure, the variation in camber and track width are largely reduced under body roll, although there is a small trade-off in wheel control under bump.

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Unequal length non-parallel double wishbones (below) is even more impressive, whose camber angle at the heavy-loaded outside wheel is nearly unchanged, although it is less good under bump.

3. Weight Transfer due to lateral force
When a car is cornering at speed, the car's weight transfers from the inside wheel to the outside wheel. The rate of change is proportional to the height of center of gravity (CG), the lateral acceleration ( in g ) and inversely proportional to the track width. As this :

Weight transfer = ( Lateral acceleration x Weight x Height of CG ) / Track width
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For example, a Factory Five Roadster is cornering at 0.85 g. Assuming its track width is 1600 mm, height of CG is 500 mm and it weighs 1250 kg, then we can calculate the weight transfer is 332 kg. Assuming the car has a perfect 50 / 50 weight distribution between front and rear, then we can see each inside wheel takes 146.5 kg while the outside 478.5 kg. What a big difference! Therefore you can see the outside wheel has far more influence to handling than the inside wheel. This explain why we prefer unequal length non-parallel double wishbones, because it has the least camber change on the outside wheel.
If the car corners at extremely high g-force, our calculation may find the weight transfer approaching half the weight of the whole car, this means the outside wheels take all the load while the inside wheels are virtually unloaded! Then the car is going to roll over! Don't worry, this is almost impossible in reality, as it requires impractically high lateral acceleration. In our FFR Roadster example, that equals to 1.6 g. Before that, the tires would have already run out of its traction limit and slide.

However, if the car is the elk-freightening Mercedes A-class or a high center of gravity SUV, with their exaggerate high center of gravity versus narrow track width, roll over might occurs even at a leisure cornering speed.

* * *
We've discussed the properties of weight transfer, but how does it relate to grip ?
Look at the following graph. It illustrates the Grip - Load characteristic of a typical tire.

As you can see, as the load increases on the tire, the grip generated by the tire increases, but at a declining rate. This says, when weight transfer to the outside wheel, the grip on the outside wheel is increased, but not increase as much as the grip loss on the inside wheel.
Therefore the total grip decreases as weight transfer occurs. The more weight transfer, the less the total grip becomes.

Now can have some conclusions : to maximize the cornering grip, we must minimize the weight transfer. We can achieve this by lowering the CG, by reducing the weight of the car or by enlarging the track width. The first could be implemented by placing the heavy engine and transmission as low as possible, by using a wide V-angle or even boxer engine, and by lowering the seats. The second can be implemented by using lightweight materials and better chassis structure, and reducing the size of the car, but this seems to conflict with the third method. Therefore I don't recommend to increase the track width to as wide as Lamborghini Diablo. It won't help making the car nimble too. Another advantage of weight reduction is obvious: quicker to accelerate and to stop.

These are no secret. Any one interested in motor racing already knows them.

Weight versus Downforce

But then you may ask a question: reduce the car's weight also reduce the grip generated by the tires, so what's the advantage ?
Firstly, because the car is lighter, centrifugal force acted on it is smaller. In theory the reduced grip could exactly withstand the reduced centrifugal force. Secondly, we could use aerodynamic downforce to increase the grip without increasing the centrifugal force. As a result, the car can corner faster.

4. Weight Transfer due to body roll
Body roll also introduces weight transfer thus reduction of total grip. Let's see the following drawing :

The lateral displacement of center of gravity (CG) is d. If we again use the Factory Five Roadster example (track width 1600 mm, height of CG 500 mm, weight 1250 kg), if it rolls 10 degrees when cornering, d will be 500 x sin10° = 86.8 mm. Then the load of the outside wheels can be calculated as: ( 1250 x ( 800 + 86.8 ) ) / 1600 = 693 kg while the inside wheels take 557 kg. So there is 68 kg weight transfer. Although it is not a great amount compare with the weight transfer due to lateral acceleration, its influence should not be ignored because camber change exists in this case.

We want to keep the body roll to an adequate level. We can use stiffer spring and anti-roll bar to reduce roll in the price of ride comfort. We can move the roll center, which is determined by the suspension geometry, as close to the CG as possible so that the roll moment is largely reduced, but this has a very bad drawback - a large jerking force will be generated and jerk up the body thus raise the CG. Alternatively, we could leave the body roll alone and try to lower the CG, so the weight transfer is also reduced.

After all, I don't recommend to eliminate body roll, since it is an important signal to tell us how well the car enters a corner and how close it approaches its limit. Body roll is a kind of feedback.

5. Four-Wheel Drive
Finally, 4WD can maximize the total grip of the car, both in straight line and cornering. The former case is easier to understand: compare with RWD and FWD cars, 4-wheel drive cars distributed less tractive force to each of its driving wheels, so it is less likely that the tractive force exceed the frictional force generated between tires and ground. In other words, the driving wheels are less likely to slide. However, since we are talking about handling, straight line grip is not our interest.
For cornering grip, whose direction is perpendicular to the wheel's tractive force, the above mentioned theory is completely useless. The actual theory is quite complicated, it requires the concept of Slip Angle, which will be introduced in later sections. We will continue this discussion later.

Steering
Surprisingly, steering mechanism is not in our scope. In fact, most good cars today use rack-and-pinion steerings whose designs are more or less the same. What makes one car's steering superior to another is the weight distribution, drivetrain system and suspension geometry etc.

Steering Response

I always said mid-engined cars are superior in handling. Some ignorant auto journalists interpret as "because the heavy engine is placed in the middle of the car, it is easier to achieve 50 / 50 weight distribution between front and rear. In other words, the car is more balanced."
Wrong ! Most mid-engined sports cars have about 60% weight bias towards the rear, thanks to the engine, gearbox and differential are all located at the rear half of the car. In contrast, a Factory Five Roadster has the engine in front and the transmission mid mounted (right behind the motor), so it could actually achieve the perfect 50 / 50. Other good front-engined cars such as BMW 3-series and Honda S2000 also achieve 50 / 50, thanks to the lay-back engines.

The reason I prefer mid-engined cars is, instead of better balance, mid-engined cars have superior steering response. This is because they have lower polar moment of inertia. Considering the two system shown in below.

Both of them have equal front to rear weight distribution. The one having the mass concentrating near the CG (in other words, lower polar moment of inertia) is easier to rotate about the CG. This could be easily verified by our experience. Applying the same steering force, the mid-engined car steers more quickly. The same for countering a steering action. This means it is responsive to steer and correct.
There is another advantage: since less effort is required to steer the car, we can reduce or even discard power steering, which always filter the feedback from the road thus downgrade the steering feel.

Dynamic Balance

Another reason I prefer mid-engined car is actually the slightly rear-biased weight distribution. In acceleration, we need more weight on the rear wheels to generate more traction for better launch. Obviously, FR cars are inferior in this respect. (FF cars, however, might be even better, but we shall see FF’s disadvantages later)

If acceleration is not much related to handling, braking must be very decisive. When braking into a corner, weight transfers from the rear to the front, hence actually creating unbalance to a car which achieves 50 / 50 in static condition. In contrast, a 40 / 60 mid-engined car may achieve a real dynamic balance under braking.

Neutral / Understeer / Oversteer

We often hear these 3 terms in car magazines. I think few people would argue if I say they are the most important elements in the study of handling.

What is understeer ? Basically, if you turn the steering wheel and find the car steers less than you expect, the car is understeering. This is not because your subjective judgment goes wrong, in fact any car must have some degree of non-neutral steering due to the weight distribution, suspension design, tire used, lateral acceleration and road conditions. Further more, a car could understeer in this corner and then oversteer in that corner. The whole picture is very complicated, so I'll spend more paragraphs to discuss this topic.

What do we need ?

It seems that neutral steer must be more desirable than understeer and oversteer, but in fact it is not.
In fact, when running in straight line, we want a little bit understeer to make the car stable. When the car is subjected to side force, probably due to cross wind or the road's irregularities, understeer could resist the force and avoid the car to be steered automatically, therefore the driver need not to correct the steering frequently.

When the car is entering a corner, we also need a light understeer to provide the stability while the driver is easing off the brakes and building up cornering force. In mid corner, we need neutral steer. In the exit phase, a slight oversteer will be welcomed as it helps tightening the path. However, the degree of oversteer must be progressive and easily controllable by applying and easing throttle. We call this "Power Oversteer". Without power oversteer, we have to ease the throttle (thus loss time) or the car will run out of the corner.

However, I must make clear that what I say "slight understeer / oversteer" is usually deemed to be "near neutral steer" by most car magazines. This is because in reality there are too many cars running on severe understeer thus they used to them. In other words, if a car magazine said the Porsche 996 has mild understeer, it probably equals to "medium understeer" in our sense.

Basic Concept : Slip Angle

Before going on our study, we must understand the concept of slip angle first.
When a car enters a corner, all the tires are turned with respect to the ground. Due to the elasticity of the pneumatic tire, the tread in the contact patch will resist the turning action because there is friction generated between the rubber and the road surface. As a result, the treads on the contact patch will be distorted, whose direction always lags behind the direction of the wheel ( See figure in below ). We call the angular difference between the treads and the wheel's direction as Slip Angle.

Note : the car is turning left
In which direction the wheel is running ? It is the direction of the tread, not the direction of the wheel. I am not saying the tread has any ability to force the wheel to travel in its direction. On the contrary, the tread is only a sign showing how an arbitrary point on the tire surface travels. If the arbitrary point travels in that direction, so does the wheel which is the summation of thousands of those points.
Now you must think the existence of slip angle must reduce the car's steering angle thus leads to understeer. In fact, it is not so if everything else are perfect. Because both the front and rear tires have more or less the same slip angles, they counter each other thus the resulting steering angle remains unaltered.

However, if the front and rear wheels have different slip angles, then we get understeer and oversteer :

Understeer : Front Slip Angle > Rear Slip Angle

Oversteer : Front Slip Angle < Rear Slip Angle

Neutral steer : Front Slip Angle = Rear Slip Angle

Non-neutral steer due to Tractive Force

Car magazines often prefer the handling of rear-wheel-drive cars. They say FWD cars usually understeer while RWD is easier to provide power oversteer. Now, we use the concept of Slip Angle to explain this.
Consider a driving wheel, which is under cornering and has created slip angle. If tractive force (that is, the pulling force from the engine) is applied, the slip angle will increase (See Figure in below). This is because the tractive force applied between the tire and ground will distort the tread on the contact patch further.

Now the scene is clear.

FWD cars has the front wheel's slip angle > rear wheel's. This result in Understeer.

RWD cars has the front wheel's slip angle < rear wheel's. This result in Oversteer.

4WD cars, if the front / rear torque split is equal, has equal F/R slip angles, thus result in Neutral steer.

(Remind you, understeer, oversteer and neutral also depend on suspension design, weight distribution etc. So we cannot say all FWD cars must understeer or all RWD car must oversteer. In fact, car makers usually design the suspension geometry to compensate the non-neutral steering generated by FWD / RWD and weight distribution.)

Power Oversteer and Lift-off Oversteer

The more tractive force we apply, the larger slip angle is created in the driving wheel. Therefore, for the RWD cars, we can use the throttle to control the degree of oversteer. When the car is entering a corner too fast and seems likely to run wide, we can correct its direction by increasing the throttle (not to do this before reaching the mid corner !), then the car oversteers. If we find the correction is too much, we can ease the throttle and let the car returns to neutral steer or even mild understeer, depends on the suspension design and weight distribution.
Only RWD cars or rear-biased 4WD cars can do this ! In the same situation, the driver in a FWD car has nothing to do other than easing the throttle, slow down the car thus reduce the centrifugal force, and hope the car can overcome the corner. There are many disadvantages :

You lose time during slow down.
You lose engine rev during slow down, thus the engine takes longer to rise back to the useful power band once you exit the corner.
Very often, if you miscalculate, you are unlikely to have sufficient road ahead for you to slow down, especially in tight corner.
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Therefore we always say RWD car is superior than FWD car in handling. There are, however, some well-sorted front-driver (especially some GTi) can play "lift-off oversteer", which is actually the reverse of "power oversteer" - a degree of permanent oversteer is built into the car but is only accessible when the car is pushing to the limit and with throttle disengaged. Step down the throttle again will reduce the oversteer and even back to understeer. Anyway, obviously this is still not as controllable as "power oversteer". While power oversteer can extract a lot of oversteer - actually depends on throttle - lift-off oversteer is rather limited, simply because it is impossible to build a lot of permanent oversteer to the chassis without deteriorating handling in lower speed or straight line.
Once again I have to emphasis that the power oversteer must be highly controllable by the driver, otherwise the car may lose control and spun. To make a good power oversteer car, the secret is to match the power and cornering limit perfectly at the speed concerned. If the cornering limit exceeded the power, the rear wheels will grip hard and refuse to slip. In contrast, if the cornering limit is too low or the engine torque is too high at the speed concerned, the rear end will slide severely once the throttle is pressed. Therefore, the cornering limit must be set at a level where the engine output, at the speed and road we normally want the car to power oversteer, has just sufficient power to exceed. To implement it , choose a suitable set of tires, applying suitable amount of downforce and an adequate front / rear weight distribution is very crucial.

RWD versus 4WD

Basically, 4WD does not introduce power oversteer. However, most people still prefer it simply because it provides superior cornering grip thus improve cornering speed. As I have promised earlier in the Cornering Grip section, here I'll explain how 4WD improve cornering grip :
Consider a driving wheel running in a corner. Due to the frictional force applied from the road surface, the tread in the contact patch distorts and creates slip angle. The faster the car corner, the more centrifugal force generates thus the larger the slip angle becomes. You can interpret this as the elastic distortion of the tire generates a counter force to keep the car fighting with the centrifugal force. When the car is accelerated fast to the extent that the elasticity of the tire reaches its limit, it could not distort anymore, thus more speed will lead to the tire slide, and the car lose grip. This point is what we call "Cornering Limit".

A FWD or RWD car has already a lot of tire distortion (slip angle) in the driving wheel because the tractive force is shared by only two wheels. Therefore there is not too much space left before the tires running into their cornering limits. On the contrary, 4WD cars distribute tractive force to all wheels, thus each wheel shares considerably less tractive force thus create smaller slip angle in cornering. The car can corner at higher speed before the slip angle reach the cornering limit. * * *

Grip aside, we concentrate back to our current topic - steering tendency.

There is always argument that whether the neutral steer of 4WD is better than RWD's oversteer. Although neutral is more favorable in the entry phase and mid corner phase during cornering, it doesn't provide the "correctability" of power oversteer in the exit phase. Remember, no driver could avoid miscalculation, no matter Mrs. Robinson or Michael Schumacher. Normally we need to feel the car's attitude and the road condition every moment before deciding how to control the car in the next moment. In this sense, RWD's controllable power oversteer is what we want.

Moreover, power oversteer of RWD ask the driver to intervene the throttle during cornering. This let him feel more involving and that he is mastering the car. In contrast, 4WD cars let the tremendous grip, the limited-slip differential and even the computer to rule the car's cornering. Therefore we always hear road testers said RWD is more fun to drive.

I am not saying 4WD cannot have power oversteer. Bugatti EB110, with its 30/70 front-to-rear torque split, did that beautifully while providing tremendous grip. Even though a 50/50 4WD car like Mitsubishi Lancer Evo V could achieve slightly power oversteer by means of well-sorted suspension geometry. For example, if the suspension is setup such that to introduce rear outside wheel positively cambers when subjective to body roll, the contact patch area decreases thus slip angle increases, then power oversteer is also available. However, you cannot set the suspension to provide power oversteer as much as RWD car since there is a trade-off in total grip and straight line stability.

New Trend for RWD cars

In the past 2 decades, we saw car makers gradually increases understeer in RWD cars, making them more "secure" to drive. Porsche 996 is a good example. Its predecessor 911 used to offer hell a lot of oversteer, now the 996 becomes a very civilized GT.
This is partly due to the market orientation ( it seems the wealthy customers tend to love secure rather than excitement), partly due to the use of wider tires. In the past 2 decades, tires of sports cars had been widened for about 50%, in addition to the growth in diameter, the contact patch area had been largely increased. Of course this is intended to increase the grip. However, increased contact patch area means every square inches of the contact patch carries less cornering force, so the tread distort less and the slip angle is reduced.

It is known that for the range of slip angle we concern (normally less than 20°), tractive force has less influence to the narrow slip angle than the wide slip angle, as illustrated in below :

Therefore, when apply the same power, the rear wheel slip angle increases in a lesser rate in wider tires. In other words, power oversteer is less obvious.
This explain why the 115 hp version BMW Z3 1.9 has virtually no power oversteer ability. Its engine lacks the power to generate sufficient slip angle to the wide 205 rear tires.

If it get considerable more power, like the M Roadster, power oversteer would have come back. But then again the car maker is very likely to install even wider rear tires in order to cope with the increased performance, as did in the M Roadster. So once again the power oversteer is quite limited.

In my opinion, this trend is quite frustrating to the front-engined RWD cars. It makes them having less and less fun to drive, although the increased grip will ultimately improve cornering time. To mid-engined cars, whose rearward weight bias used to create some undesirable oversteer, the adoption of wider tires could actually improve the handling and driving fun.

Non-neutral steer due to front / rear weight distribution

Here we are going to discuss the theory behind front-heavy cars tend to understeer and rear-heavy cars tend to oversteer.
When a car is cornering, its CG is subjected to centrifugal force. The tyres generate slip angle thus frictional force to counter the centrifugal force, so the car keeps cornering without slide. (See figure in below)

If the car is heavier at the front, that is, the CG is near the front, obviously the front tyres shares most of the centrifugal force thus they have to generate larger slip angle thus larger frictional force to counter the centrifugal force. As a result, the front slip angles exceed the rear's, and understeer occurs.
On the contrary, rear-heavy car has larger slip angle at the rear, thus introduce oversteer. Similarly, we can find a 50/50 balanced car having neutral steer. This is our choice for optimum handling. We don't really need oversteer in this case, because such oversteer is not controllable, unlike power oversteer which we have found in RWD cars.

The result favours front-engined, RWD cars (FR), which is easiest to achieve 50/50 F/R weight distribution.

Mid-engined, RWD cars (MR), with its slight rearward weight bias at about 40/60, is slightly inferior in here. But remember, its superior steering response, steering feel and dynamic balance are probably more than enough to compensate.

Front-engined, FWD cars (FF) is the worst in here, and far worst. As all the heavy mechanical parts - engine, transmission, differential - hang over the front end, the front axle normally takes up to two-third of the weight. This tends to create heavy understeer. In addition to the understeer generated by the FWD configuration, the result is even worse. This require a lot of work to do in the suspension geometry and steering mechanism for compensation. And there must be some trade-off. Take an Alfa GTV as an example. It has to install an ultra-quick 2.2 turns steering to counter understeer, thus requires quite a lot steering effort. If power steering were increased, steering feel must be deteriorated. The multi-link rear suspension was also probably chosen for compensating the understeer because the geometry is more tunable than the original MacPherson strut.

There is another problem troubling the Alfa - the 3.0 V6 version, which is intended to be the range-topper, found its even heavier front end leads to inferior handling than the cheaper and slower 2.0 version. This is a headache to the marketing personnel.

However, once again I have to point out that everything must have exception, especially when all mass production cars are also limited by other factors such as packaging, requirements for refinement and cost etc. When both under these limitations, a well-sorted Alfa 156 could outhandle an ill-fated BMW 3-series. Although recently RWD luxurious / sports sedan / compact elegant sedan seems to be reviving, FF is still the main trend for the majority budget cars due to its lower cost and space-saving advantage.

Non-neutral steer due to Suspension Geometry

We've said a lot suspension geometry can alter the steering, and it is usually used to compensate the undesirable steering tendency due to uneven weight distribution and FWD / RWD. Now I'll briefly go through this.

Camber - Decisive to understeer and oversteer

As shown in below, if a wheel is not perpendicular to the road, then it is cambered. If it leans towards to the center of the car, then it is negative cambered. (or " toe-in"). If it leans outwards to the car, it is positive cambered (or " toe-out", as shown in the following picture.)

When a wheel has positive cambered, due to the elasticity of tyres, the wheel will be reshaped to something like the base of a cone. It will have a tendency to rotate about the peak of the cone, as shown in the picture. Now, you will see the wheel tries to steer away from the center of the car.

If both the right and left wheels are positive cambered (that means they leans to opposite direction), the steering tendency will be cancelled so that the car remains running in straight line. If the car is turning into a corner, weight transfer put more load on the outside wheels than the inside wheels, that means the outside wheel's steering tendency will have more influence to the car. As the positive-cambered outside wheel tries to steer the car to the outside of the corner, the car will be understeered.

On the contrary, if both wheels are negative cambered, the car will oversteer.
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For FF cars, we could introduce some negative camber to the front wheels to reduce the understeer. Similarly, more positive camber could be employed to the rear-heavy 911.

We may deliberately need positive / negative camber, but we don't want the camber to be changed when the wheel meets bump or when the car body rolls into a corner, otherwise the handling will be very unpredictable or even uncontrollable. Therefore we prefer a suspension geometry whose camber varies little under all conditions. As said many times in before, double wishbones, especially is non-equal length, non-parallel double wishbones, is generally regarded to do the job best. Therefore from sports car to Formula One, all the high performance cars use it. For other kinds of suspensions, you can read the previous chapter about Suspension.

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Steering Feedback and Torque Steer

The steering must offer enough "feel" to the driver so that he can sense what's happening as he approaches the cornering limit of the tires. It must also have some self-returning action, but it cannot be so heavy as to cause fatigue or loss of sensitivity. This feel, feedback and self-returning action is a function of kingpin inclination, steering offset and castor angle :

The more the steering offset D, the more self-returning effort generated. Similarly, the larger the castor angle, the more self returning action.

If the car is FWD, the steering offset D will introduce torque steer. This is because the tractive force will try to pull the center of contact patch of the front wheels forward, thus the wheel will rotate about the point the kingpin axle projected to the ground. The torque steer moment is the product of D and the tractive force. Therefore the amount of torque steer is proportional to D. The solution is to build more inclination to the kingpin so to reduce D. This is easy to be implemented in double wishbones suspension which is shown in the picture, but not MacPherson strut, whose kingpin also serves as spring and shock absorber. If we incline the kingpin too much, there will be too much lateral force transmit via the spring / shock absorber to the car body, thus causing shake and instability.

Therefore we say MacPherson strut is not very suitable for FWD cars having a powerful engine. Alfa Romeo 164 is one of the examples, whose torque steer ruined the otherwise brilliant handling. No wonder its successor, 166, has switched to double wishbones front suspensions.

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Chassis Rigidity

The last method to improve handling is to strengthen the chassis. Since the late 80s, we saw chassis rigidity of new cars have increased a lot. Whenever a new car is launched, the manufacturer must claim its torsional rigidity has been increased by at least 20%. This is partly due to the requirements for crash protection, partly in order to improve handling.
Consider a car with a very weak chassis which is easy to flex and twist under force. If it employ stiff springs and dampers to the suspension, the shock cause by road irregularity will be transferred to the chassis directly. The weak chassis will be twisted and bent, thus the suspension geometry will be reshaped, creating non-neutral steer and other side effects that is not the original suspension design intended to cope with. Therefore a weak chassis must ride on softer spring and dampers.

For the benefit of handling, we always want stiff spring and damper as long as ride comfort is acceptable. So we need a rigid chassis which could cope with the stiff suspensions without flex or twist.

Credits: Mark Wan, Bill Peirce

[link http://www.ffcobra.com/FAQ/handling102.html]

Hope this compilation helps you tune your car's handling to your needs as well as give you a basic understanding of FWD, RWD and AWD handling characteristics.

Information was transfered from Gen7Accord.com.