The ability of the drivetrain to supply force, is meaningless if this force cannot be applied unto the road through the tires' contact patches. The tires contact patch has to remain consistent via use of a suspension. This article will delve into the mechanical aspects in the operation of the chassis, suspension arms, springs, dampers, anti-roll bars, bushings, wheel alignment, wheels and tires. Sounds complex? Don't worry, that because it is!
Dynamic Coefficient
The dynamic coefficient of the chassis donates a number of generally static details about it's performance. These details include the gross weight, weight distribution, height of the center of gravity, track width and wheelbase.
The car's weight is the final limitation of the car's ability to generate a G-Force. Modern, non-aerodynamic cars, regardless of suspension and tire technology, can never go much beyond 1G (most cars don't even get so far) of a force in braking or turning, simply because of their weight.
The weight increases the downforce on the tires, which means more grip, but equally so it increases the force or the demands of this grip when the car is being turned, accelerated or braked. The problem is the weight also causes the tire's coefficient of friction decrease slightly so overall some grip is lost. Also, braking distances will increases and the car will not be as responsive and will perform badly when dynamic abilities are demonstated, like when changing directions into a corner or when switching quickly from left to right.
The center of gravity is another important feature. The lower it is, the more stable the car and the lower are the forces working over it. If you brake, you use the grip of your front tires to slow the car. At the same time, the force of inertia is trying to keep on pushing the car forward. This ressistance, between the tires friction pulling backwards at ground level and the inertia pushing forward against the center of gravity, creates a torque that rotates the car about the center of gravity, causing the "nose dive" under braking. This is a weight transfer. The lower the center of gravity, the smaller is the weight transfer and therefore it is also more immediate. The wheelbase and track width also work on a similar idea. Instead of lowering the car to contribute to it's stability, one can do the same by making the car's chassis longer or wider.
One issue is with cars that have such a wheelbase that leaves big parts of the car's body extending beyond the front or rear axle. The problem is aggrevated if there is much weight in that area, which might be caused by load which was in appropriately put in the trunk or an engine design (mostly Boxer) which puts the engine beyond the front axle.
If the grip of the tires and suspension is greater than the dynamic coefficient, than the car would roll over. If the opposite is true, the car would slide sideways instead. Modern passenger cars, including SUV's are made to slide instead of roll. In an SUV, where the center of gravity is placed higher, the solution is simply to reduce the grip offered by the tires and suspension.
When a car starts to slide, it can either slide it's front axle front and out of the corner (understeer) or slide the rear axle around and spin the car (oversteer) or briefly slide all four wheels aside (neutral-steer). Every car is said to have a natural "handling characteristic" which is determind when the car is held steady on constant throttle so it does not accelerate or decelerate, only turn. In this situation, any properly adjusted car will slide it's front tires first and understeer.
Understeer is safer because it is felt through the steering, scrubs off speed and corrects itself and is more natural to recover from, since your natural instincts of slowing down tend to resolve it. Also, in racing rear-wheel drive cars it allows you to keep some extra grip in the rear wheels through the corner and than accelerate out, which why professional race cars actually have quite a lot of understeer, not oversteer.
Chassis
The chassis is the metal body connected to the suspension. The chassis is flexible and it flexes when you go over bumps. If it weren't for the additional dampening of the chassis, the rocking over bumpy roads, even with a proper suspension and tires, would be unbelievably strong. Also, when you accelerate, decelerate or turn, you apply forces unto the chassis which also bend it and this is undesired because the chassis is the platform of the suspension, so any flexing moves the suspension and tires from the intended alignment relative to the road surface.
A softer chassis: Better ride comfort, more predictible responses, less grip, less responsiveness and less feel. A stiffer chassis: Less ride comfort, less predictibility but more grip and a more immediate response as well as a "feel" for the car's limits.
In old cars, a "ladder" chassis was made of a series of inter-connecting steel bars in the shape of a "ladder" that runs across the vehicle and is connected to a seperate body that holds the passenger's compartment, engine compartment and trunk.
Modern cars have a uniform chassis that consists of all of the body of the car. This allows for more rigidity. Different makers put various innovations into their chassis, in an attempt to make them more rigid. Perhaps the most notable is Lotus with their unbelievably light and stiff chasis, costing in lack of space inside the small Elise and a very high door-seal which makes the access into and out of the car quite labored. In many of their modells, Porsche connected the engine to the chassis itself, increasing the ridigity in the relevant part of the chassis.
For performance driving purposes, drivers often choose to re-inforce the chassis by use of bars that usually run under the hood from one damper housing to another, a so-called strut brace (to be distinguished from a torsion bar or an anti-roll bar, both discussed later). A roll-cage which is a safety requirement in many such cars, also contributes to a more rigid chassis.
Suspension Type
The suspension has different prototypes that differ in the shape of the suspension arms and in the type of spring used.
The front suspension of most modern cars is a simple Mc'Pherson Strut. This is a simple an compact but effective design where the spring is put around the damper and both connect down to the wheel bearing and lower steering linkage. This allows the whole unit: Steering arm, spring, damper to operate as one. The disadvantage is that it simply isn't ideal: When a weight transfer occurs and the body of the car leans back, squats forward or "roll" sideways, the strut will move along with it and so move the tire from it's natural angle perpendicular to the ground.
Better cars with better suspensions have a dual-wishbone suspension, where the spring and damper assembly ins put in between two seperate arms, each looking like a wishbone, one above the other. This design allows the wheel to move relative to the car's body and remain flat against the ground. Even better cars, or race-cars, are equipped with a multi-link suspension that will involve three to six arms in an attempt to give the wheel all of the necessary freedom of motion or "program" a certain change of angles into it
Springs
The springs are the part the operates the suspension.
1. The Leaf-Spring: A primitive and simple design in the form of leaves of metal plates that hold the axles in place and offer dampening. They come in various forms, like dual leaves (in the form of an elipse) and others. They are cheap to make and can have varying levels of rigidity through their length. Their downside is that they allow the axle to move around in unwanted directions, allowing it to be pushed sideways out of the corner, or rotates into it. It also lowers the car beyond necessary near the wheels.
2. The Torsion Bar: Used in the rear suspension of many small modern cars due to it's small space occupation that allows to form a flat floor for the rear seats and trunk. This spring is simply a bar that runs across the width of the axle, and is twisted around (under torsion) when the wheels hit a bump or are acted by external forces. Nevertheless they are not as efficient as modern coil-springs.
3. The Coil Spring: The modern spring, made of a coil which compresses and decompresses under load. This is a comfortable design that allows to create a progressive spring with varying levels of stiffness across it's travel, as well as not being space consuming, especially in the popular Mc'pherson suspension type.
One thing to know about springs: Unlike with the chassis, a stiffer spring does not necessarily mean more grip. When the spring is harder it does not allow the car's body to "roll" as much, thus helping to keep the tires flat against the ground and maintaining the grip, bu this is also affected by the wheel arms and the wheel alignment.
If anything, a stiffer suspension makes the tires and car bounce over bumpy surfaces but furthermore - they "over-power" the tires: If the spring is stiffer, what force that isn't absorbed into it will simply work against the tires or chassis. Stiffer springs give better grip on smooth surfaces and where a rigid chassis and grippy tires are already i use, i.e. on track cars.
The main advantage of stiffer springs is in more feel and a more immediate response to driver's inputs. The car also break traction more sharply, but is easier to control even when sliding (which is part of why professional drifting cars are so stiff) and especially being more effective in the dynamic field, mainy while turning left to right or on a slalom course. A series of opposing steering inputs causes the BACK end of the car to slide out, where in a stiffer suspension in the rear this won't happen as quickly.
Dampers
The dampers or "shocks" are closed pistons that are supposed to dampen the operation of the springs. If the spring were to be free of a damper (as it is in most leaf-spring suspensions) it would osciliate back and forth when a force is applied unto it or removed from it. Push down on it and it will compress. Release and it will decompress, but also bounce back and forth needlessly.
We therefore need a shock that will be stiff enough to support the springs in our car from osciliating needlessly, but not so stiff as to over-power the spring and prevent it from operating freely, allowing for about one "cycle" of osciliation. The dampers also allow the spring to operate differently for "fast" dampening, as in cruising over a bump at speed, or "slow" dampening as in turning into a turn and having the car "lean" to the outside of the bend, so we can have the best of both worlds, so to speak.
Bushings
The rubber bushings that hold the parts into place also play a surpsingly big part in the operation of the suspension. It also bends about under the loads of bumps or turning/braking/accelerating. Many cars have their bushings to thank for a "passive rear steering" tendency which means that under strong cornering, these cars have their rear wheels "turned" into the corner so the car suddenly "kicks" the back end around, but does not actually cause an oversteer - it only kicks around momentarily and gets back into line.
Anti-Roll Bar
This suspension component is simply a bar that runs across the width of the axle, locking the left and right suspension together, and thus helping to keep the axle more flat when the car rolls, without sacrificing the car's ride comfort. However, if the spring is too soft and the anti-roll bar is too hard, it could "over-power" the spring and cause the inside wheel to "swing" up briefly in corners. Many people misinterprate this as a car that was close to rolling over, where in fact it is part of the function of the suspension in the car in question.
Wheel Alignment
As you can see, trying to keep the tires flat against the road is preety impossible. Everything moves around and changes the angle of the wheels. So, what do engineers do? Intentionally put the wheels off-set. If you turn left, the car leans right over the right, "outside" wheels. This causes these wheels and tires to lean also and contact the road with the right corner of the tread than with the whole tread.
So, the wheels are intentionally angled away from the sides of the corner down, so that this angle will cancel out the angle caused by the car's body roll and at the end - the tire will remain flat against the road. This angle is called a camber angle. It's disadvantage is that if you are driving straight or not cornering hard enough, you have your tires angled in the opposite direction and again - not having full grip.
Another important thing is that the angled tire rolls towards a certain direction which is not straight ahead, so when the wheels are cambered like this, they contribute to the car's stability and more so when braking or accelerating. Camber also contributes to more wear on the shoulder of the tire which is leaned unto the road.
Another angle is the Castor, which means that the wheels are "tilted" back and around from the straight up and-down direction of the strut. Look at the tilt on a shopping cart's wheels to get the idea. This angle gives a "heavier" steering feel, but will increase the "scrub" when you let off of the gas at speed.
The last angle is the Toe, which is simply a steering angle. Wheels are said to toe-in if they are angled towards the one another, or toe-out if angled away and to the sides. Toe-in gives more stability and creates a certain suspense across the axle which makes it more responsive, but also makes the car respond to steering inputs a bit later to a car with toe-out. Toe-out can make a car helplessly unstable in a straight line, but very responsive to turning.
Wheels
The dimater, weight and offset of the wheels or rims is important. Allow-rims are desirable for their lower weight. This reduction of weight is particularly substantail because the wheels, unlike the chassis, are not supported by the suspension. Each bump on the road bounces the wheel and all of it's weight up and against the suspension, so if you reduce this "unsprung" weight you make it easier for your suspension to operate.
This is the exact opposite of what people achieve by fitting rims with a wider diameter. All they achieve is a screwed-up speedometer with more unsprung mass, with the single advantage of having more room for bigger brakes. Wider rims, however, to support wider tires, are effective. A bigger offset is also effective because it creates a bigger wheel base. However, this requires use of spacers which can hurt parameters like the wheel aligmnet, steering angles and even braking performance.
Tires
The tires grip the road surface through the tread, but their sidewalls and air within them work as part of the suspension. Modern Radial tires have a flat, thick tread layer with two thin sidewalls. The air between these sidewalls keeps them stable as they come across bumps and flex up and down to dampen them. Lower-profiled tires will not do the best job in dampening, causing less ride comfort and even less grip on bumpy surfaces, but will create a more immediate response and more grip on reasonable surfaces. The same applies for tires which are "over" inflated, with the single exception that more air reduces steering feel, unlike a lower or stiffer tire sidewall or suspension component.
When turning, the tire's sidewals are also pushed out by the side force. This causes the car to go around a wider radius than desired and flexes the tire in a way that causes folds in the surface, preventing some of the rubber from touching the ground. In the extreme situation of old tires with very low air pressure, the sidewall could flex so much as to get the tires off of the rims, causing the latter to bite into the road and roll the car over.
The tire's tread is also important. It's important to know that if the tread is more or less grippy or worn, it will not change the car's natural handling characteristics. If the car is said to slide it's front tires first, it will do so even if the rear tires are worn. However, whatever action will be needed to slide the rear - it would make them slide ever more early and ever more aggressively.
Professional racing tires have no tread "pattern" therefore allowing for more rubber to touch the road (in dry conditions). The tires also have extremlly stiff sidewalls, soft rubber compounds that grip very well and reduce weight, but wear down faster and heat up faster. The lack of tread patterns makes these tires have a very specific range of effective temperature.
Road tires have a tread pattern for when the road is wet or even heavily coated in dust. The tread also changes the tire's response because the tread elements also deform under the load, and they also effect the tire's temperature: The tread elements - ribs, cubes and what not - flexes around and generates heat, but simultanously the pattern channels air that cools the tire down.
"Ribs" in the tire are used to reduce rolling ressistance and noise, while giving a sense of "directional stability" to the tire. Lugs offer higher rolling ressistance with categorily higher grip, especialy in the wet and ever more so on dirt where the tread pattern hooks into the dirt. Off-road tires also have the sides of their lugs at the corners of the tread going in an angle and not down along the sidewall, giving mud or dirt the open space to move out. They also have re-inforced lugs with little "ridges" inside the voids for added rigidity.
Snow tires also have sipes. These are little spews that soak in additional bits of water in road tires of bite more into the snow in snow tires, and than spew the water out as the tire continues to revolve. This increases grip in the relevant conditions.
Assymetric tires have a tread pattern that offers soft rubber and little voids in the outer shoulder, ribs in the middle and a lot of angled voids in the inner shoulder. The outer shoulder gives us the cornering force, the center gives us longitudinal traction and the inner side gives us our steering feel. These tires give good braking/acceleration performance in the wet, with good dry cornering abilities for the dry. Directional tires have crescent-shaped grooves that allow a very effective operation when driving at speeds over water.
Three note about tire wear:
1. Cupping: Suspension problems show on tires in the form of irregular wear that appears as "cupping" and in the tread lugs recieving a shape of a raised "leading edge" and low, rounded and worn "trailing edge" of each lug.
2. Cold Tear: If tires are over-inflated, under-loaded or labored when still cold, they will exhibit wear that will appear as deep cracks that run around the tire (mostly in the center) and angled towards the direction in which the tire rolls.
3. Hot Tear: If the tires are under-inflated, over-loaded or pushed too hard when hot - they will wear, mostly at the two shoulders, in the shape a layer of cracky, abrased rubber with another layer of rubber molten off of it and reformed over the very edge of the tire.
Directly in contact with the suspension system is the steering mechanism. It begins with steering rim, a thirty-odd centimeter-wide rim made of some kind of plastic. Rotating this wheel rotates a steering rod which connects to a steering box or rack. This box or rack turn the rotation of the rod into a lateral motion that pushes an rod from side to side. This rod is connected to a steering tie-rod which tilts the front wheels into the direction of the corner.
The rack and pinion system is prefered in road cars for it's simplicity. In bigger cars, it becomes hard to fit suitable rack, and a steering box is used. The rack and pinion mechanism is made of a pinion or gear at the end of the steering column, which rotates about a geared rack, pushing it left or right. The width the rack or the intrevals of the gearing change the steering ratio (i.e. the amount of steering to turn for a given turning radius) but also changes the weight of the mechanism. A shorter rack allows to turn the wheel less, but makes it harder. Also, a shorter rack makes it harder to cruise down the road, as each little numb at the wheel will divert the car from the desired course.
To ease the problem of the steering mechanism's weight, a power assistance is introduced by putting the rack in a closed cylinder with a lubricant circulating through it. A pump pushes the fluid across to aid the efforts of the driver in turning the wheel. The pump's operation changes with the viscosity of the fluid and the RPM or speed of the motor, creating a variable weight that also depends on other variables.
In this way, a steering wheel can be considered as "informative" since the changes in it's ressistance to turning "tell" the driver what is going on at the front tires. Race cars usually have extremlly heavy and informative steering systems, also due to stiff, short suspensions and low-cut tires with aggressive castor and camber angles.
Note: Car manufacturers do not specify a time limit for replacing power steering fluid, but it's preferable to check fluid levels (by eye) each week, and replace it after around 70,000 miles or four years, but no later than 100,000 miles or five years. By than it will be oxydiated and filled with metallic chips and rubber debries, as well as degrade from daily and annual temperature changes and exposure to heat in the engine bay.
Replacing the fluid will contribute to more reliability and more informative steering, which I tend to connect to safety. Restortive oil additives can be added to the steering fluid quite succesfully. The fluid will be less viscous.
Power steering systems are sensitive to hard steering. If you crank the wheel up to the "lock" at the far end of the rack and keep it there for a few tens of seconds, it will make all of the fluid build up on one side of the rack and apply too much pressure against the seals. Don't keep the wheel crancked up to the lock, ease it a little.
Steering angles and Ackerman Steering
Steering does not end with turning the front wheels. This action also changes the other steering angles, usually adding to the front wheels camber angle in the intention of keeping the tires flat against the road surface in all different kinds of corners. Most front suspensions add more camber depending on the castor angle, which also changes the weight of the steering mechanism and magnifies the "realigning torque" that pulls the wheel back to straight. Flick the car into the reverse and the castor angles effectively flip over so this tendency will be gone.
Another issue is that the inside tire takes a shorter route compared to the outer tire. This led to the invention of the Ackerman steering configuration, which is based on a more complex steering-related theory that ends with the notion that the inside wheel should be turned further into the corner. This contributes to the "natural" understeer tendencies of road cars. In race-cars, the tire operate at the optimal slip angles all of the time, just on the threshold of sliding away and therefore turning the inside wheel more sharply will cause unwanted sliding and excessive understeer in fast sweepers, so the angle is either removed or even reversed.
Many cars also include a passive rear steering tedency where the rubber bushing of the rear-outside trailing arm suspension arm twists in a way that turns the outside-rear tire into the corner (toe-in). This allows the rear wheels to contribute to the active steering effort and helps rotate the car around the corner.
In some cars, this is used to help get over the chassis' understeer tendencies. The car begins to develop an understeer where pushing harder at the wheel will initiate the rear steering and rotate the car about the corner and remove the understeer. In other cars, this helps to counter-act an initial tendency to oversteer. The rear steering angle pulls the wheel in as it begins to slide out, and overall it feels like the rear briefly "kicks around" and than returns into place.
Torque-steer and bump-steer
Torque steer is a known illness for front-wheel driven cars. You turn, apply a good deal of power and feel the wheel kicking back against your arms. The reason is simple, the drive force that pulls forward and the lateral force that pushes you around the corner are in conflict, but there is more to it.
When you apply the drive force, it applies this "kick" unto the steering mechanism and the steering tie-rods operate as a leverage. Front-wheel driven cars usually use the cheap and compact Mc'Pherson strut suspenion, which is connected directly to the steering mechanism. This create a certain "kingpin inclination" between the two ties (upper and lower) and the center of the tire's tread, which creates a bigger leverage and more torque steer, and further more of it for bigger tires or bigger tire off-set (by use of spacers).
Steering accuracy is another thing that relates to this, and it has to do with the alignment and the tires' sidewalls. Tires with a bigger aspect ratio and a taller and softer sidewall wil deform laterally in corners and make the turning radius wider as speed increases. This makes it harder to predict just how much steering is going to be required for the corner. Performance-intended cars have very accurate steering systems which require a very static amount of steering for a corner, no matter how quickly you go through it.
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