Car Setup

Knowing your car is usefull knowledge in the realm of driving. Understanding the effect of changes in car design and setup allows the driver to be more knowledgeable at acquiring a "feel" for cars, whether in comparing different cars (as journalists) or simply at driving them effectivelly, as a driver; Verily, this article is also usefull for drivers who want to change the setup of their car, even though this is not the primary purpose of the article. For those, I issue the following rules of thumb: First, remember that a car is an object. Changing one aspect of the car carries it's effect on other parts. This also dictates that changes must be done scientifically, with one little change at a time. The second rule is that tuning always begins with driver's skills, continues in driveability, follows in grip (starting with tyres) and ends with the engine.

As an article for drivers, and within the knowledge limitations of the author, this article will deal with dynamic aspects of car setup: Tires, Suspension, Steering and Chassis. These components create a car that works as one piece, but still grips the road at two axles: Front and Rear. This is crucial to understand because all of the changes below: Tire improvements, suspension setups, chassis bracing, etc -- can all be applied on different amounts on the front and rear.

Making adjustments to increase the grip of the front will make the front lose grip (=slide) at a higher limit relative to the rear. If the front breaks away before the rear, the car's handling is said to be of understeer. Sliding the front wheels, which turn the car into corners, make them skid forward and out of the corner, as if they refuse to turn-in, hence under-steering. However, if the rear breaks-away, it is said to oversteer. The rear sliding wheels can no longer keep in line with the front wheels, making them swing aside in corners and rotate the car "too much" into the corner -- thus over-steering. A car can also achieve neutral handling, where both ends slide simultanosuly. 

We measure a car's handling by taking a turn on the limit. We need to keep the right foot on the throttle with sensitivity to keep the car at a constant speed through the whole corner -- not slowing down or speeding up, just turning. This is the natural balance of the car. If this makes the car understeer, the car is said to naturally understeer. If the rear is sliding, it is said to naturally oversteer. If both ends slide symmetricaly, it is said to behave neutrally.

Unlike what some people think, ALL cars are set to understeer -- Front-Wheel Drive, Rear-Wheel Drive, All-Wheel drive; Road car, Race car (Saloon/Single Seater), World Rally Car,  Drifting car or Off-road car -- Based on the laws of physics, when you first turn the wheel, you fight against the inertia that keeps on pushing the car forward, hence generating understeer. Also, understeer allows to feel the slide through the steering mechanism, and feel your way when driving on the limit. Recovering from understeer is more simple and intuitive.

Oversteer is often looked upon as "quick" because it allows the car to change direction more quickely. This leads to the false notion that race drivers set their cars to oversteer while in fact the amount of understeer in professional race cars can be surprising! Understeer allows the driver to brake as late as possible and as hard as possible before the corners and into them, while keeping the car stable. Since professional race-cars are rear-wheel driven, setting them to understeer in the apex means that there still is grip in the rear wheels, and allows to commence acceleration out of the corner earlier.

Understand that a driver can make the car do anything: Understeer, Oversteer and Neutral-steer. However, if he just turns the wheel, without adding acceleration or deceleration, it would generate understeer. If he would do the same turn and brake with feel, he might get "over" the natural understeer tendencies and create neutral handling or even oversteer. Applying throttle could either increase the understeer or reduce it and even generate neutralsteer or oversteer again. 

Tires

Tires are always the most substantial aspect of automotive design. They form the actual contact with the road. It is only through that fragile contact, that the capabilities of the driver, suspension, steering, chassis and engine can be "put down" onto the road and generate performance. The tire is round, but it contacts the road through a flat contact patch at the bottom. Looking at that contact patch in magnitude tells us that the seemingly solid rubber is in fact bulit out of a collection of polymeric rubber fybers. Likewise, looking at the same magnitude on the road under the contact patch shows us that the seemingly smooth road surface is in fact rugged.

This combination gives us grip. Imagine a rolling tire. Focus on one fyber on the tire's radius. While the tire rotates, it eventually brings the fyber to contact with the road. The fyber is than crammed against the road by the downward pressure of the car's weight. This pressure makes the elastic fyber squeeze into the little undulations of the road and grip into the them, pulling the car forward. As the fyber distorts more and squeezes deeper into the road, it will produce more grip, up to a point where more bending of the fyber would threaten it's integrity. 

The way to improve tire grip can be achieved in one of several ways: The first is to use softer rubber that allows more flexing to take place. The compromise here is that the tire wear out sooner. The second method is to change the amount of downward force ("Vertical Loading") by using a wider tire that distributes the same weight over more fybers. The disadvatage here is that this tire will be heavier and will be less effective on wet surfaces.

However, the tire's job does not end with the contact patch. The sidewalls also have a job. When turning, a lateral force is working on the car and tries to push it's components away from the corner. This force takes a toll on tires. It makes them bend aside and twist. This twisting changes the contact patch: It creates folds in it, retracting some of the rubber from the road. By use a lower-profile or a stiffer tire construction (regardless of how soft the rubber compound is), a tire is made to be less distortive and more grippy. 

Additionally, stiffer components make for sharper responses. The car reacts to the driver's inputs faster. The disadvatage is that if the driver makes an input that is above the car's ability to perform (above it's limits of grip), it will still give us a sharp response, so the transient from grip to sliding is more sudden and with less warning. However, once sliding, the car will also respond more sharply to the corrective input, making the slide more controlable. 

Stiffening the tire is also made possible by use of air pressure: More air makes a stiffer tire and less air makes for a softer tire. But air also has another effect, unlike the size of the tire, which effects the tire's shape. Low air pressure makes the tire recieve a convex shape where the inside of the tread folds away from the road, which remains in contact with the corners of the tread (which are less grippy in general) and denser air pressurs make it recieve a concave shape which orginates grip from the center of the tread. The goal of proper air pressure is to keep all of the tire flat against the ground as well as possible.

Wheels

The goal of the wheel (rim) is to provide a stable mounting for the tire. For performance, lighter alloy rims are used to substruct unnessecary weight. Removing weight from the wheels (unsprung weight) is crucial. Of course, the rim also dictates the radius of the tires. A greater radius of tires does not increase performance, it again increases the weight under the suspension, but it might be nessecary to host large brakes in performance cars. Off-road cars also need to the rims to be rigid enough to sustain blows.

Weight

More weight presses down on the tires to generate more grip, but also places more weight for those wheels to handle with. The graph of grip/weight is none-linear: As weight increases, the grip levels increase, but not parallel. Eventually, the tire reaches a point of "saturation" where more weight reduces it's grip levels. Additionally, a heavier vehicle -- while it will generally grip more -- will have isssues with driveability and with using the grip it has. The heavier weight will make it harder to accelerate and slow down. In the corners, the car will have trouble with direction changes, particularly fast or repititive ones. Ultimately, substructing weight will indeed make you "faster everywere."

However, looking at a car's static overall weight does not give us the full picture. When the car moves, it experiences weight transfers. You accelerate, and the wheels start rolling faster and try to get ahead relative to the car's body which is constrained by it's weight and aerodynamic drag. This makes more "weight" to work on the rear wheels and push them down. In reaction, the car's body will seem to squat. Under braking, weight is transferred from the rear wheels to the front, and all cars other than trucks will thus have larger front brakes. 

Weight transfers does not change the car's mass. It only changes the downforce on the wheels and their grip levels. This can help the driver but also carries negative effects on driveability: Every weight transfer disturbs the car's balance and successive weight transfers tend to create a resonance that intesifies the effect of the second weight transfer, and the third weight transfer will be even harder. 

Our goal in engineering is to reduce the car's weight transfers. We can achieve this by substructing weight, chaging the car's  weight distribution and ride height. We cannot achieve it through suspension design: Stiffer springs don't decrease the weight transfer, they only make it more rapid and create less body roll in response to it. On the other hand, lowering a car and making it wider, will evidentially make it more "stable."

One important aspect here is the wheelbase. A good car (dynamically) has its wheels at about the very corners of the car's body. The closer the wheels are to the corners, the better. If the wheels are closer to one another, they work like one big wheel instead of four wheels, making the car less stable. The areas of the body that protrude from the line of the front and rear axles would create a load that decreases grip, especially if it's heavy. Look under the hood of most Audi models, and you will see the engine sitting mostly in front of the front axle, generating understeer. 

A car with an engine mounted on the rear would therefore be more likely to oversteer, because the center of gravity is placed further to the back. Placing the center of gravity closer to the middle and lower helps in achieving balance.

Springs

Besides the little undulations, that allow the tire to grip the road, there are also larger undulations that shock the tire. The tire is moving forward as it hits a bump, making some of it's forward movement turn into upwards of downward acceleration and bouncing it off of the surface, aside from making it uncomfortable inside the car. The springs are made to negate this phenomena by absorbing the bumps and keeping the tires against the ground. A softer spring absorbs more energy and allows to keep the tire against the road.

However, the springs also have to carry the car's body. This gives them another task, which is to control the body movements of the car. When weight transfers occur, the car's body moves with it. This can be seen and felt as a nosediving of the car under braking and "leaning" towards the outside of the bend during cornering (called "body roll"). Contrary to what some believe, body roll has both a positive and a negaitve influence. The negative influence is that moving the car's body in an angle moves the tires in an angle, and changes the way they contact the road. You turn right, the car rolls left, so the wheels are forced to contact the road with their left shoulder and less with the center of the contact patch, not to mention the right corner.

However, it is custom to speak of stiffening the suspension as a move that decreases grip levels. First, because too stiff a suspension will fail in the original task of keeping the tires flat against the road but also, because a soft spring "absorbs" some of the cornering energy instead of the tire. If you force the car not to lean on it's side,  you force it to slide laterally. This returns to the point of viewing the car as a singular object: To benefit from stiffening the car, all parts must be stiffened, begining with the tires, through the springs and coming up to the chasis. If you just stiffen the springs relative to the chassis and tires, they will become the "weak" links of the combination.

Under the conditions of a stiff chassis and stiff, grippy tires, stiffening the suspension helps in reducing the roll angles, and keep the car and the tires flat against the ground. Since the car is stiffer, it will also have all the other aspects of a stiffer vehicle: A sharper response, including a sharper lost of grip, but also more control on slides and weight transfers. On the track in particular, we also assume well-paved tarmac, so we can stiffen the car quite a lot to reduce the roll angle and not suffer from a car that bounces over the surface.

In road cars, one of the solutions is to apply progressive springs, that are not as stiff across their whole length. The are softer at first to absorb little bumps, but becomes stiffer as the driver pushes them harder, to restrain the roll angle. Another solution is coilover springs, which allow the driver to personally configurate the ride height, which is also a function of the springs.

Old cars, and many of the modern trucks as well, don't have coil springs, but rather have leaft springs, made out of "leaves" of elastic metal. The advantage of these leaves has to do with simplicity, and they also have advantages in requiring less rubber bushings to connect them to the chassis, and the ability to make them progressive by fitting different layers with different thickness and stiffness. However, they reduce the ride height near the wheels, they have a problem when they reach for compression, and they cannot hold the axle in place, so that the axle itself swings around and moves laterally under cornering forces.

Another type of spring is the Torsion Beam. In your everyday family car, the rear axle usually hosts such a suspension, which consists of a beam stretched between the two wheels and connected to them on special arms. When the wheel hits a bump, the wheelarm rotates and "twists" the beam. The force of torsion than twists the beam back to push the wheels back unto the road surface. The advatage is mainly in space management, because the beam that actuates as a spring is placed across the car, the dampers can be placed in very flat angles, opening space for the back seats or trunk.

Dampers

Mistakenly called "shock abosrbers", the dampers provide resistance to the spring. They are brakes that stop the springs from osciliating. When you press on a spring to compress it, letting go will make the spring saw back and forth before returning to it's original form. This motion is called osciliation and it is measured in cycles. The task of the dampers to ensure just a single cycle of osciliation. Dampers allow enable the spring to compress differently at different rates. When a spring hits a bump, it generally compresses quickely, so dampening the road bumps is a task for a function known as "fast bump", whereas restraining the movement of the car's body is done more slowely, thereby being under the slower movement of the suspension (slow dump). Dampers allow to have springs that are soft enough to grip the road surface and provide road isolation, and still stop the car's body from rolling too much.

Anti-Roll Bar

To differ from the torsion beam, and from the tower strut which will be disscussed later, the anti-roll bar (mistakenlly called a "sway bar") is meant to enable a reduction of body roll, without damaging the ride. Stiffen the springs to reduce body roll and your make a harsher ride over bumps, but if you take a stiff bar and use it to bind both sides of the car toghether, you achieve this reduction of roll angle, without interfering with how the springs dampen the bumps.

The drawback of the anti-roll bar is that it limits the abilities of the suspension. Once again we need to remember that the suspension is one big organ, and changing one element changes another. Just like reducing the roll angle of a car with a soft chassis and tires would reduce it's grip levels, applying too stiff a roll bar unto too soft a spring, will turn the spring into the "weak link." This means that under heavy cornering, the body roll will result in the stabilizer bar forcing the spring and wheel to lift and swing in the air. This does not mean that the car is anywhere near to rolling over, but it does not do good to performance.

Chassis

The Chassis is the car's body. In older cars the chassis was only the lower part of the vehicle (usually in the form of a ladder chassis). In modern cars, the chassis and body are one. The chassis, like all other parts viewed so far, also endures the forces working on it and twists somewhat. This is not the body roll, but an actual twisting of the elastic metal parts. 

Different manufacturers design the chassis of their cars with different technologies, offering various levels of stiffness, which also carries an effect on safety. Porsche invented a chassis that is also at one with the engine, which contributes a lot to it's stiffness, while Opel and BMW use hydralic pressure to create the metal work of their chassis (in the Astra and M-3), leading to more stiffness in the corners of the tin. Improving the chassis stiffness can be achieved by means of struts that connect both front wings over the damper housings, and by the safety installement of a roll cage inside the car.

Bushings

The rubber bushings connect the suspension to the chassis. The amount, formation and compounds of bushings carry an effect because they too have to face the loads working on the car. As the bushings twist, they can move the whole suspension from it's original alignment. They also create frequencies of noise as the suspension under them moves. The first solution is to install stiffer bushings, usually made out of materials such as polyorithane. Another solution is to include a sub-frame which provides extra isolation from the noise.

Suspension Types

The same suspension can be placed in different ways. Three main ways are the Mc'Pherson strut, Double-Wishbone/Multi-link, and Trailing arms. The Mc'Pherson strut is the most simple of the three, and is used as a cheap and compact solution in the front of many cars today. It consists of a damper mounted inside the coil spring. This saves room, but also makes the damper support the spring and allows to design the coil of the spring differently. 

The Double-Wishbone is more complex, but yields better results. It consists of placing the wheel, with the spring and damper, clanched between two A-shaped arms, one above the other. This gives the wheel itself a certain freedom of motion. This freedom of motion helps avoiding something we mentioned earlier which is the interference of body roll. Turn the car and you will make the car's body roll, but when the car's body rolls, the wheels do not have to tilt also. With the double wishbone formation, even when the wheels hit a bump or when the car's body rolls in an angle, the tire can remain flat against the road, straight up-and-down and vertical against it, maintaining grip.

This designed can be changed by altering the formation of the arms. The popular way is to make the upper arm shorter. The result is that while the wheel is pushed up by a bump it does recieve a slight angle, but it recieves no change of angle over body roll. A multi-link suspension is an elaboration of this concept. By adding even more arms it allows the wheels to be even more independant or to "program" them intentionally to recieve a certain angle during a certain situation.

In professional racecars, the multi-link setup also allows to change the angle of the springs and dampers. When the wheel hits a bump, the upward movement is turned into a lateral movement of the wheelarms, so the springs can be placed horizontally, allowing to reduce the ride height.

The trailing arms are a different story. They are a more simple suspension, used in the rear of some car, especially with a torsion beam spring, where each wheel is connected to an arm that runs longitudally under the car, and hangs unto a pivot. When the wheel hits a bump, the arm pivots up and back down to keep the tires flat against the road. The problem is that when the body rolls, the arms and wheels roll just as much. To reduce this effect, the Semi-Trailing Arms have been invented. These arms are assentially the same, but their pivots are placed in an angle that makes it pivot in an angle that allows the wheels to stay flat against the road even when the car's body begins to roll.

One shortcoming of both trailing and semi-trailing arms has to do with braking. When you brake, the bushings of the arm and pivot would twist as to change the angle of the wheels so that they are turned away from the car. When this happened in the rear of the cars with this suspension, the result was lack of stability. The solution was to split the links of the arm into two bushings, so that the movement of each cancels the movement of the other, keeping the wheel static or even turning it towards the body of the car. This is known as the "Weissach" formation and it was the first installment of passive rear steering.

Ackerman Steering

When the car turns, the outside wheel takes a wider radius. When we turn the wheel, we tilt both front wheels into the corner. However, with Ackerman steering, the inside wheel is turned more than the outside wheel, because it takes a tighter circle. On professional race-cars, this fucntion is removed because the cars function at the maximum slip angles that allows to maintain grip, and therefore having the inside wheel turned more (in some race cars it's even turnd less) will make it go over the limit.

Rear Steering

Rear steering is the tendency of the outside-rear wheel to be titled when faced with a certain load. Active rear steering systems allow for the rear wheels to turn into the corner in slow speeds, which helps in turning the car more tightely, and turn them away from the corner at a high speed. Passive rear steering is a function of the rear suspension, and usually performs the latter action. In some cars (Like the Volcan ZX) turning the wheel away from the car helps in reducing the risk of oversteer. An attempt to oversteer the car would only generate a so-called "Cynical oversteer" that begins and ends with a momentary wiggle of the back-end.

Other cars, to name the new Fiat Abarth or Clio, use the same tendency to make up for understeer. The rear suspension is set to create a steering angle under a load greater than what the front can take. I.E. The front of the car starts sliding (understeer) and than the rear wheel is turned and rotates the car around the corner. Again, this feels like a momentary oversteer: The back of the car swings around, but just for a moment, before going back into line. It's important to understand that this is NOT oversteer! Neither of these situations is desired in setting up a true race-car.

Bump steer

Changing components in the suspension does not merely carry an effect on other parts of the suspension, it carries an effect on the steering mechanism. Under pressures such as a bump or under body roll, the steering tie-rod and the suspension needs to move symmetrically. If a wheel hits a bump that makes it swing, and the suspension swings more than the steering arm, it will make the suspension turn the wheel. This happens in road cars, but must be minimized in a serious race-car design. One way to reduce bump steer is to change the connection of the steering tie-rod to the wheel, by applying so-called "hi-steer", where the steering arm connects to the wheel from above rather than from below.

Kingpin inclination

The Kingpin inclination is the inclination between the the nuts that connect the wheel-arms to the wheel. The upper nut will be typically further from the tire than the lower one. If you draw an imaginary line between them, you recieve the kingpin inclination. This creates a steering offset. Draw the imaginary line all the way down untill it contacts the tarmac, and you will see that it in an offset relative to the center of the tire. This offset creates steering feel, but in a front-wheel-driven car, it will create torque steer (where the torque of acceleration "fights" with the turning of the wheel and tries to set the wheel straight). 

Reducing the Kingpin inclination in a Front-Wheel driven car is impractical with a Mc'Pherson strut, which must be placed vertically (because it's kingpin is the spring and damper), and adds yet another advantage to front double-wishbone suspensions.

Camber

We said we need to reduce body roll in order to keep the wheels straight up-down and flat against the road. If the wheel is tilted in an angle relative to the road it is cambered. Because cars naturally have body roll, the wheels are intentionally cambered. They are placed in an angle towards the road surface, leaning against it with their inside corner. Under body roll, this negative camber cancels the positive camber created by the body roll, and the tire is placed straight and vertial against the road.

Of course, stiffening the suspension reduces body roll, so we can reduce the camber. Camber changes at the front as the front wheels are turned, and also changes at any wheel when the suspension is compressed over a bump. Multi-link suspensions reduce the the camber changes over bumps.

Castor

Castor is the inclination of the wheels when you look at them fron the side. A negativelly castored wheel is titled backwards, like a supermarket trolley's wheel. This places the grippiest point of the contact patch further back. The effect is for negative castor in the front wheels to make the steering sensitive, but heavy to turn. Flick it into reverse, and the car will be moving in the other direction, so that the castor will be as if reversed, so you will suddenly have a lighter steering, which will even not turn back to straight when you let go of it.

Toe

Toe is the steering angle. A wheel that has toe-in is turned towards the car itself. A wheel that toes-out is turned outside and away from the car. Having the wheels aimed for toe-in and towards one another, makes it stable in a straight line, but also stable when turning into corners, making it harder to change direction. Toe-out makes the car unstable over the road, but more compliant in turning in.