From Trackpedia
This article will speak about Grip and handling from more advanced aspects of car engineering and a bit of physics. This is not crucial for most intermediate track drivers, but will reward. all who are interested in the subject. This article deals first with the question "how". How does a tire grip the road? How does it get the car to turn?
How does a tire grip the road? The chemical nature of grip
A contact between two objects in our world results in "friction". The "tractive force" that grips a car to the road is "static friction". It is in fact the result of the rubber compounds of the tire, made of rubber polymers, create complex and fascinating chemical reactions with the road surface, particularly with tarmac, with great similarily to plain glue. While driving, several factors enhance these reaction and generate grip: Heat and pressure.
These two factors create a distortion of a given fyber of rubber (a "gripping element") against the road, and this generates grip: The distortion enables the rubber to squeeze into the bumps on the road surface. The problem is when this distortion and heat go over a certain limit (dictated by the type of rubber), where the tire mulecules begin to seperate to the a degree that threatens the intergrity of the fyber up to the point where it is ripped apart. At these levels, the static friction begins to be rendered into "kinetic friction" (your everyday normal friction) which is what we call "slip". In fact, looking deeper, in our unperfected world where "ideal" road surface never exists, slip and grip will always co-exist. A perfectly gripping is always slipping slightly and even a tire rendered absolutly sideways has a bit of grip reserves.
So, how does this grip generates movement? Well, here's the (basic) equasion: "Tire grip + Tire roll = Forward movement". It would not take much thought to understand that only a small part of the tire is on the road at any given moment. If we look at a certain fyber of rubber when it is moving around when the tire is rolling, than it would be practically airborne untill it gets to the bottom of the tire. At this point, the element obviously comes in contact with the ground but it also comes under the effect of weight: both the weight of the rim and the weight of the car. This "load" is referred to as "Vertical loading".
The combination of vertical loading starts to apply pressure onto the fyber, cramming it against the road, generating the distortion that creates the grip. This load does not fall unto the elements in one blow: When the element first comes in contact with the road, there is little load onto it. As it moves towards the very bottom of the tire, placed vertical to the road and car body, it will start to bare more and more load untill a certain peak, after which it will begin to come away from the tarmac and being faced will less and less load, before it completly lets go of the tarmac again.
Illustration of the deformation of a tread element's distortion
At stage one the element in the picture is airborne. From stage (1) and onward it begins to contact the ground and be squeezed against it by the car's weight. At stage 4 it is reaching it's topmost level of distortion and grip, and at stage 5 it begins to fold back from the road and return to it's natural form.
The point is that when the tire is rolling, this process is repeated ad infinitum. A certain element grips the road, than lets go just as anothe one grips the road, pulling the tire forward step by step like someone climbing onto a cliff by hooking onto the rock with his hook and than hooking his other hook further up. Physicists would rather present this as an "action and reaction". I.E. The element is rolling against the direction of travel, hence "pushing" back against the tarmac, which "pushes back" and takes the tire forward.
Getting back to how load falls onto the tire, we can present it roughly as a graph. When the tire begins to take in load (when it first contacts the road) it deforms and begins to generate grip. When under full load, being faced staight down against the road surface, it is providing the full extent of grip, and than it begins to lose grip faster as it reaches to "rear" part of the contact patch, before it is disconnected from the road beneath.
Hystersis: What happens within the tire?
A tire is much more than a single layer of rubber. It is built, depanding on it's category and type, by layers of steel and/or cevlar strings, fabric, nylon and rubber soaked in certain carbon-based chemical agents. As the tire faces load and deforms, the vertical loading and the loads generated in the contact patch area work within the tire to create "movement" similar to the distortion of the tread elements, in and between the layers of the tire.
This is called hystersis, and it effects the tire's durability and grip. A tire that "works" less can apply more tractive forces unto the road and will have less chance of creating seperation between the layers inside it. The main benefactors on this are the following two:
- The compounds of the tire: Especially the density of the rubber. Soft rubber can create more adhesion as it distorts, but is able to distort less before it's integrity is threatened. Hence it gripps better, but wears faster.
- The air inflation of the tire: A tire is filled with air because air is gas and is able to expand without "wear". As such, the more force applied unto the air inside the tire, the less force has to be applied unto the layers within it. However, too much air will not allow the tire to distort to a sufficient degree and will press against it's layers and deform the contact patch, so it's all good to a certain point.
If the tire begins to distort, it mirrors back unto the contact patch. Many people think of a tire that is distorted this way, or "scraped" due to under-inflation, as more grippy for keeping a larger contact area with the ground. Alas, this assertion is only true for a very limited amount of soft surfaces, encountred in specific situations of off-roading.
The reason is that when driving on sand or loose gravel, the earth tends to squeeze into the folds created in the rubber, giving more grip. With tarmac or bitumen, the road surface stays put, and as the rubber folds, less of it is being in direct contact with the road. This is particularly important with under-inflated tires, which most people don't care about (untill stopping 20 feet too late in an emergency...), which folds the tire directly over the contact patch, leaving only it's flanks or shoulders (which are very ungrippy and exposed to wear) to grip the road. An under-inflated is thus more likely to collapse ("blow-out") than an over-inflated tire.
What effects a tire's ability to develope longitudinal forces?
A longitudinal force is a force in a forward-backwards axle. I.E. acceleration or deceleration. Having understood how grip moves the car forward, how do we get it to move "faster" or "slower". The simple answer would be by changing the speed of the tire's roll to faster or slower, but it a bit more complex. Changes in roll speed results in changes in friction. Changes in friction results in changes in distortion. Hence, applying a "longitudinal force" -- Acceleration or deceleration -- would take it's effect as another force, distorting the given element forward or backwards. All forces developed by a tire are regarded in equasions as "a" -- acceleration. Constant speed or an increase in speed are positive acceleration, deceleration is negative acceleration and cornering is lateral acceleration. Vertical loading is downward acceleration and bumps generate upwards acceleration.
There are many factors that effect this ability: First is a tire's size. A wider tire has more elements gripping the road surface simultanously. Hence, each element has to carry less load and less distortion. When this happens, each element has more "reserve" left to distort under the longitudinal force, because less of it's ability to deform is being used for "vertical" distortion. This has a certain downside when wet surfaces are involved, and the tire floats more easily. In fact, a tir'es profile is more important than it's width. we will disscuss this later.
Now, what if we try to reverse this equasion? If a wider tire disperses load better and is able to perform better, would removing load from the tire give a similar effect? Answer: Yes, but only to a certain point. Why? Because this equasion can also be fliped like this: A tire with more load pressing unto it, would cramm against the road and create a larger contact patch, artificially increasing the amount of elements on the ground. Vertical load can be viewed negativelly as "load" on the tire, but can also be seen positivelly as "pressure" against the road surface (professionally: "downforce") which gives the tire more "bite".
What effects a tire's ability to develop a lateral force?
A lateral force is a "turning force", developed when turning. Getting around a turn is more complex than going down a straight, since the tire's work during a turn is more complex. When faced with vertical load, a tire would be crammed and distort downwards. When faced with a longitudinal force, it would also flex forwards or backwards. When a side force is applied, a tire would distort aside. What do we mean?
A tire's distortion is an effect of the flexibility of rubber. When we turn the wheel we tilt the front wheels in an angle to the road, but does the whole tire immediatly applies to this angle? No! The contact patch, which is crammed down against the road and gripping it, is obviously reluctant to move from it's straight forward position. Hence, when we first turn the wheel, we turn the rim and than the sides ("sidewalls") of the tire, but the tread is actually still facing forward and in an angle towards where the "body" of the tire is pointing.
This angle is called a "slip angle", because it reflects the omniexistant precentage of slip, the often microscopic gap between our "request" and the "result". This slip angle, like distortion under vertical loading, can be viewed as a graph. The elements appear to be scraped more and more sideways untill a certain portion of the contact patch, from wherehence it is "winded" back unto a straightforward position, as it leaves the road surface.
In performance driving, we get the car to maximize it's grip. Hence, the tire will develop a greater and greater slip angle. As such, performance drivers should view a slip angle as "good", right? Well, not quite, and for a simple reason: Looking at a tire from in front, we can divide the "tread" into three portions: The center of the tread and the "shoulders". The latter have less grip than the former. If we were to develop a very large slip angle, the tire will be facing the road with it's side. Therefore, it will grip the road with it's shoulder. In an extreme situation, a slip angle would make the tire "reluctant" to turn and no changes to the steering wheel would carry an effect on the car's turning angle. In fact, it so happens that a panic attempt to "force" the car into the corner actully makes the tire face the road completly sideways. This makes the tire unable to roll unto the desired direction and is in fact "dragged" (Slides) sideways ever more roughly.
The development of the front slip angle
This image illustrate a mature slip angle. The rubber is scraped aside. This angle can be simply viewed as the tread refusing to turn aside and into the corner with the wheel (you can tilt your computer screen aside to see this more clearly) and keep pointing forward. The degree to which the tread is pointing aside, is described as the "pneumatic trail". "Fy" stands for the maximal adhesive force created by the tread elements of the distorted contact patch. In this case, it appears a bit before the point of maximum lateral distortion. After this point, the elements begin to retract from the road surface and lose all deformation and return to their normal shape (elastics at it's best). This elastic nature creates the force desribed as "Mz", which the force we feel working against us when we turn the wheel and also the one that gives us the feedback from the tire and it's "desire" to return to straight.
Having explained the subject of slip angles to the front, we must take considuration of the rear axle. While the rear tires are not normally "turned" into the corner like the front, they are still taking a certain part in the turning action. Having set the slip angle, the tread will begin to align itself, "closing" the angle somewhat. This is when the car first starts to turn. Once this happens, a Centripetal force begins working unto the car (having another effect to be covered later) and the body of the car (chassis) moves into the corner. The rear wheels thus have no choice but to join the party, generating a slip angle. This slip angle pushes the rear wheel lateraly across the road, making the car more compliant to the driver's steering, because it rotates the front deeper into the corner. The extreme situation is that the rear slip angle turns greater than that of the front, making it turn sharper than it's direction of heading and resulting in a spin.
But, wait! The contact patch is not the only portion of the tire to deform under lateral load. When we turn, the tire as a whole, distorts aside. The sidewall of the tire curves aside and this curvature will make the tire face the road in an angle that would lean it more over one of it's shoulders and less over the center of the contact patch. This effect should be minimized and this is achieved by making the tires' profile lower. The trade-off, is less space over which the tire can cramp to soften bumps on the road.
Combination of forces
This is important because it brings the driver to the understanding that the steering mechanism is a manner of "asking" the car to turn, rather than in fact getting it to turn. As the limits of grip are reached, the driver should rely less on steering and more on managing his slip angles with his other means of car control. What do I mean? I am talking adhesion management and weight transfers. We will begin with the latter: Weight transfers. During movement, the car undergoes dynamic changes that carry an effect on it's balance. Surely you have felt the car leaning sideways (as well as pushing you aside) when turning sharply, or "nose-diving" during sudden braking.
This is roughly what we mean be weight transfers: When we accelerate, the engine speeds up the front tires and pulls the car ahead. This finds resistance in form of friction with the air. This results in the weight being pushed somewhat towards the back of the car. When we brake, inertia gets in the way and the weight is thrown forward. What is the effect of weight transfers? Old-school track drivers would refer to it as the incarnation of evil in physics. Their claim is that it reduces grip and stability. However, there is an important flip-side to it: When weight shifts off a certain wheel, it loses downforce and grip, but it also shifts unto another wheel which is being more loaded. This vertical loading has an effect on the tire's. Now, whether this effect is positive or negative would depand unto us drivers.
The beauty of weight transfers is that weight and grip are transferred proportionally to the load. I.E. During braking, the front wheels work harder, and require more grip. During turning, the front "inside" wheel works harderest: It is both tilted into the corner (unlike the rear wheel) and also has the sharpest trajectory, and indeed it would carry more load. But! would this not only make the tire face more work? Not nessecarily. Instead of making each element face more load, it would make the contact patch larger, having more elements face a slightly increased load. The two effects (the positive and negative) are not parallel: You can transfer any given amount of weight (untill a certain point), but you cannot change the amount of elements as you wish (you cannot make half an element to come in contact with the road). Hence, you can make the weight transfer beneficial or performance-limiting, it all comes down to dosage.
If, by the manner in which we handle the car and the way in which we set it, we get the weight transfers to be very slight, it would be quite beneficial in making the car handle better. The point where more weight helplessly overloades the tire would depand mainly on speed, which relates to the second subject of adhesion management. If we dictate a given amount of tire elements would only sustain a given amount of distortion, we could roughly express this in the form of a circle, known as a friction cirlce. This circle (which is in fact egg-shaped) will demonstrate just how much "grip" we have for either braking, acceleration or sideways forces (alone or combined) untill the limit of grip is reached.
Transients
From this point of view, the driver must segregate his actions: Bring the car to maximum acceleration in a straight line. As a corner is coming, the driver hangs on to the accelerator untill the last moment to brake, braking to a maximum (to slow down in the shortest path and gain a few more feet in acceleration), and only than get the car to turn with all it's grip dedicated to this one task, rather than shared with applications of acceleration or deceleration. However, this "classic" style has been rendered archaic nowadays. By getting the tasks to overlap slightly, we would seemingly give up some of our ability to corner, but would earn a few more feet of braking, hence being able to pospone the braking point down the straight and being able to accelerate eariler unto the straight.
Why is this worthy? First because the brakes are the strongest means of car control, able to generate the greatest change in performance over a given time or space. Second, because the following straight is normally longer than the turn, enabling to gain more time in the the houndreds of meters available there, rather in a few meters in the turn. But, it goes deeper than that:
- By staying unto the brake as we go into a corner, easing off it symmetrically to the initial turning of the wheel, we fill a "dead zone" of grip that would otherwise be left unused as we transit to throttle on the straight.
- By getting the weight of the car unto the right spot, we are able to compensate for this "lost" of grip. If we get the front heavier, we compensate with the expension of the contact patch, over the lost of adhesion for the sake of a longitudinal effort. This gives us "rotation" when turning the car into the corner, because the front wheels (which face the major load) have more bite, and the rear is less resistant to turn, as it is lighter.
By having more load and a wider contact patch, we have increased the amount of tread elements on the ground, decreased the load on each element, and gave them more time in contact with the ground, enabling to develop the slip angle fully in a faster rate (aided by the reduction of the tire's rolling speed). The longer patch made the slip angle shallower too. This effect is measured in what is called "cornering stifness", giving us both a faster response and more grip reserves. This cornering stiffness is defined as the car's "cornering coefficient * Vertical loading". The transiet into a corner generates a certain "rotation". This rotation costs in friction both to initiate and to terminate, so our goal is to decrease it to a certain degree.
It should be stated in this regard that the mere turning of the wheel does not create a tractive force. That force is generated by the turning force (sideways acceleration) that comes when the tread begins to turn. This relates both to the ability to brake into the corner and in the ability to turn the wheel quickely.
Handling
All of these factors sum up to generate one of three handling characteristics. The first is when all four tires experience a similar slip angle, so the car is "behaving" neutrally. In the perfect world, this would be both the existant and the desired situation. In our world, cars will be more prone to experience the following two:
- "Understeer": Happens when the slip angle to the front is the larger one. This is the normal calibration of all cars due to various reasons. The result of a big slip angle to the front and a little slip angle to the rear (or a lot of grip behind and little up front) is that the front wheels are not turning the car to the driver's satisfactory. I.E. The car is turning wider than where the front wheels are pointed and is pushed by the stationary rear wheels towards the outside of the turn.
- "Oversteer": This is when the rear slip angle is larger and the rear is sliding sideways, rotating the front so that the front tires turn tighter than the turning of the wheel and perhaps provoking a spin.
One of the greatest effects on these characteristics is weight distribution and weight transfers. Getting the front tires to be either too light (and the rear too "grippy" to experience a sufficient slip angle), or getting the front overly-loaded, would result in understeer. Most of our front-wheel driven roadcars are set to understeer by two inteligent ways: The first, is that the fact that they are front driven, makes the front wheels share both the cornering force and the acceleration, making it prone to understeer when the accelerator is used in a corner.
The second is the mounting of the engine in the front of the car. This not only makes the front wheels highly loaded, it get the force of inertia to work on the front axle. It is like putting a hammer with it's bulk on the left edge and trying to rotate it around it's center, when opposed to a hammer with the weight of the iron pressing unto it's center. A car with an engine situated behind would therefore be quite instable and it's back will be "thrown" into every corner, provoking oversteer.
Suspension
If we were to look over the wheels, but below the chassis of the car, we would detect the system of springs and dampers that sits in between. This system carries the weight of the car's body (referred to as "sprung weight") and gives the driver a certain amount of "road isolation", which gives the driver the comfort in not having to feel every bump of the road. When a tire moves at speed over a bump, it is pushed upwards, because some of it's forward motion has been rendered vertical (upwards acceleration). The goal of a suspension is to minimize that and keep the tire stuck against the road surface.
The above statement only refers to softening bumps on a straight road. The same task has to be performed over both big bumps and during weight transfers. A soft spring might be better in softening bumps, but when a big bump is contacted it would cramm to it's full length too easily. It takes a certain balance to keep the car grippy. Also, springs have to soften the bumps created by the weight transfers generated by driver's inputs:Springs tend to create an interesting effect, which is the isolation of the weight transfers working on the wheels, from the load transfers working on the body of the car. When we formerly expressed the weight transfer during braking as the "nose-diving" of the car, we were slightly misleading you: This is the reaction of the springs and the weight they carry, to the weight transfers acting upon the unsprung weight of the wheels.
Are load transfers (the movement of the car) desired? This is one of the complex questions of car engineering. Softer springs and dampers allow for better road isolation (the car "jolts" less over a bump), but react more dramatically to weight transfers (the car jolts more when braking hard or during sharp cornering). You should know this carries no effect to the amount of weight transferred, but it does change the speed in which weight is transitioned over the car. A faster weight transfer makes for a more "disciplined" and "quick" car, but overdo it and you will get tweachy and treacherous handling. So, like with managing your weight transfers while driving, body "roll" must be use in the correct dosage.
A tire must face a certain net sum of load. The spring cannot decrease it, but it can "share" some of it and make the transition more progressive and smooth. If the car is made to be overly stiff, it would suffer from less "body roll", but this does not change the amount of load, the load not supported by the spring has to go to the tire, which would have to sustain all too much too soon. So, we have illustrated what counts as "too stiff", but what about too soft?
The cons of body roll is that with large angles, the wheel cannot remain in the similar position. As the body of the car leans aside into a corner or nose-dives forwards when braking, the wheel is forced to change it's angle relative to the road too. This change is normally reducing the grip levels (making the tire grip the road with it's less-grippy "shoulder") and is unwanted. If the geometry, contact patches and wheel-arms are designed to compensate for this, the effect can be reduced, but never removed. Hence, performance cars are indeed stiff in general.
It's a certain compromise: Stiff means the car bounces over bumps. Not only is this unnerving for passengers, it unnerves the tires contact patches (when it exceeds a certain point). However, when the car is turned, accelerated to slowed-down it's body tends to move less (albeit more sharply). And this relates to springs only. What about the damper?
The damper is made to stop the spring from osciliating. I.E. from sawing and moving up and down after decompressed. e.g. If you brake hard the front dives down. That, lift off quickely and the front will push back up. An undampened spring would bounce all the way up, than back down, up, down, and so on. This effect is not wanted but, on the other hand, nor do we want a damper that will brake the spring and make the car irresponsive.
Other suspesion changes include an anti-roll bar that ties both ends of the front or rear axle to one another and makes them stiffer, but here the trade-off is for suspension independence: The ability of each side of the car to contact bumps without pressing the other side down. Too stiff a bar and you might lift a wheel in the air when one of your wheels goes onto a curb mid-corner.
The role of the suspension:
- To be comfortable
- To keep the tire pressed against the ground when driving over bumps and when making inputs of steering, brakes and throttle.
The parts of the suspension and their role:
- Springs: Soften the bumps on the road; sustain blows from large bumps without compressing fully; allow the body of the car to move during weight transfers, making the weight transfer (and the response of the car) gradual; Allowing for feedback to the driver through the body movement of the car; change the vehicle ride height.
- Dampers: Stop the spring from osciliating, which makes it bounce back when depressurized and over again.
- Anti-roll bar: Reduce the leaning of the car (and hence the leaning of the tires) during a corner by locking both sides of the car to one another.
All in all, we want the car stiff to:
- a) Avoid the soft spring being thrust by a big bump
- b) Avoid the movement of the car's body, which changes the angle of the tire, when the spring is too soft
- c) Avoid the soft damper from letting the spring bounce up and down after depressurized.
- d) Avoid the car having a slow response due to soft springs.
- e) Avoid the car from having bad feedback, through soft springs.
But, we don't want it to stiff. We want it as stiff as possible without yield to any of the following:
- a) The car bouncing over bumps because of overstiffened springs.
- b) making it be too sharp in response to inputs by overstiffening the springs.
- c) making it irresponsive by overstiffening the dampers.
- d) making it too unstable when only one side of the car goes over a bump, by overstiffening the anti-roll bar.
A nicelly-stiffened race car would:
- Would be very grippy, but could use and overcome it's grip levels sharply.
- Not bounce over little bumps, but not feel very smooth over the surface.
- Not be dramatically shocked when going over small curbs, but will jolt noticeably.
- Not lean too much into a corner or nosedive under braking, but will lean faster
- Not nessecarily react instantly, but feel responsive and quick
- Not bounce back when you lift off of the brakes, but will respond quickely.
- Will give you the feel of the car, but might be harder to sense where the limit actually lies
Suspension geometries
Remember me saying how a tread element reaches full distortion when at the bottom of the tire, vertical both to the ground and the car's body? Well, that is only partially true. Modern suspensions are built so that the wheels are placed in an angle, and this effects the shape of the contact patch. Tilt the tire forward and what have you got? A given tread element would contact the ground later, reach the peak of vertical load (along with longitudinal and lateral loads) faster, and than begin to wind off the ground earlier, giving you more "returning torque", with a harder steering and more feedback. Tilt a tire slightly aside (upside of the tire facing towards the car's body) and you will get more of the tire's shoulder to contact the road. These settings are known as Camber, Castor and Toe.
- Camber is the angle in which the contact patch "attacks" the road. In most cars, the tire's contact patch is facing outwards from the car's body and the tire's upper portion is pointing towards it. When the body of the car leans sideways in a corner, it will change the angle of the wheels. When going straight, it would make the tire contact the road somewhat with the "shoulder", but when you turn and the car "leans", the tire will also "lean" and the inside tire will be flat against the road.
- Castor is the angle when looking at the tire's profile. When the tire is leaned backwards (clearly visible in the wheels of a supermarket trolley), it will change the shape of the contact patch and the location of the point where a tread element is under maximum load. This changes the amount of grip supplied by the front of the contact patch versus the amount of aligning torque produced by it's back. Castor also effects camber: When turning the front wheels, their camber angle changes and this change relates to the amount of castor.
- Toe: Tires can be tilted to face "away" from the vehicle with their front, or towards the vehicle. When the front tires are tilted towards one another, they want to roll towards one another and this gives more straightline stability, but make it more resistant to turning aside from that straightforward direction.
Chassis movement
The chassis is a part of the cár's body (in modern cars, an integral part) that, like the tires and springs, also has to endure some loads. Being the main mass or the holder of the car's mass, most of the force of inertia and Centripetal forces (with rotation and other forces that go along with it) works on it. A tire's grip (and slip) therefore has to "drag" or "push" the car during acceleration and "force" it into corners. As these opposing forces take their toll on the chassis, the softer portions of it's metal begin to deform also.
Like with everything we witnessed before, this movement creates angles that change the angle of the springs and of the tires. This effect is wanted (as it gives less resistance from the body to turn), but only when decreased to a minimum. Chassis bracing are thus used to give it additional resistance.
Another important thing besides the stifness of the chassis, is the dynamic coefficient of the car. On an equally long car, it would be best for the wheels to be placed as apart from one another as possible. The closer the wheels are toghether, the more unessecary load works unto them, the more unessecary body roll is being generated by the protruding parts of the the car's body and the less stable the car is. It can be said that the closer are the wheels from one another, the more they operate as a single wheel with more load and less grip. This is worthy as long as the car's body itslef does not have to be strectched, like more downforce is worthy without making the car heavier.
Driver's issues
This relates to drivibility. This is a concept formed by the car's handling, when combined with it's feedback. Good drivibility is when the car does what the driver wanted in a quick but predictable manner, and gives him the nessecary feedback. Drivibility, grip and power are what makes a car.
The first thing a driver should learn when driving a car is that creating friction is harder than maintaining it. Hence, if we were to record one's use of his tires' grip in a graph, it would have certain "spikes". When we turn the wheel, the levels of friction (also experssed as grip of slip angles) would increase parallel to amount of turning. When he would get the wheel turned to the nessecary degree, the graph would go yet higher and than settle back down to a static position as long as the wheel is still turned in the same angle. When the steering is winded off the side force will progressivelly decline and, upon returning to the straight forward position, there will be slight side-force generated towards the opposite direction (one of the variations of the "pendulum effect").
The morale here is that being smooth and progressive with your inputs allows for these "spikes" to be minimized. Getting to the limit of grip is often not worth it if reached too quickelly. This of course does not mean slow inputs that fail to trigger the nessecary response. Decisiveness is crucial when driving. In some sharp slow turns, the car must be made to change direction faster and "take a set" towards an upcoming straight or a sucessive turn, in which case you might find "quick" steering input, with little "progressiveness" to be better than your normal progressive input. This gets the front wheels turned to the nessecary degree before the rear wheels begin to experience a slip angle, resulting in an often delayed response and yet with better handling, allowing to get back a stable state with the throttle eariler. Accuracy is crucial here, to have the patience (derived from knowledge) to let the car turn without adding more steering (increasing the slip angle), rather than force it in due to eagerness.
One of the means of giving a driver the confidence and knowledge as to what the car is due to do, is seemless feedback from it. We have talked about light steering as generally "incompliant", and of heavier steering is more "grippy". This is just that feedback. It is related to a force created in the back of the front contact patches. This is the place where tread elements begin to wind away from the road-surface and the slip angle is thus being "winded" back. Because of this, that portion has has more grip and it wants to return to it's staight-forward position. This is what gives both the feedback and retractes the steering if we let go of it. The rear slip angle is only felt by the movement of the rear axle (through the seat) or by it's effect on the front slip angle, making the steering heavy and the "aligning torque" larger. In fact, if the wheel is let go in that phase, it would turn the front tires the other way!
Now, what about speed? When people learn about grip and vision in performance handling courses, they tend to develop a certain doubt to the effects of speed on safety. This doubts are only half true. As we have seen, grip and adhesion are effected by changes in friction, I.E. By speeding up, slowing down or turning, and not by the speed. A car driving 60mph in a constant speed, would have the same grip levels in 40mph in a constant speed. So what does speed effect?
Speed effects the above: the ability of the car to accelerate/decelerate and/or change direction. How? An increase in speed is an increase of the moment of inertia working on the car. This force effects all three elements:
- Acceleration: Torque vanishes with speed. I.E. A car can accelerate faster at a slower speed, than it can at a high speed. In a standard manual car, reverse gear gives you the slowest maximum speed, but the highest amount of toruqe, enabling the car to accelerate faster, carry more load, "feel" faster and more powerfull (more weight transfer), handle better uphill, and also cause more engine wear. As such, at a slower speed, the accelerator is capable of generating a greater tractive force unto the tire, making the potential of overcoming a tire's total grip level, greater. You cannot spin the driven wheels in fifth gear.
- Deceleration: When people need to stop from a high speed (e.g. 60mph, or 90 on the track), they tend to hesitate. The car's response to the brakes (in the form of weight transfer) would appear to most people as frightening, dangerous and -- at a high speed -- hazardous. They would therefore, instictivelly, apply the brakes weakly to begin with and, as the hazard they try to avoid grows nigh, their fear of impact would increase and the pressure on the pedal would increase progressively.
- A skilled driver, however, would know that at a higher speed, when the car carries a greater moment of inertia, the brakes work harder and their ability to slow the car down effectivelly is decreased, not increased. This is ought to be compensated by braking faster and harder at a higher speed, and progressivelly release the pressure as speed speed decreases, and not the other way around.
- This human conclusion is not entirelly off, because if the tire does slide and does lose lateral stability and seek to rotate, speed will increase this rotation.
- Cornering: With acceleration and deceleration, the forces are longitudinal and work parallel to the moment of inertia, which makes their effect smaller and requires compensating by giving it more gas/brake pressure. With cornering, the cornering force ("Lateral acceleration") works to divert the car from where the moment of inertia is pushing it (straight on), so the effect is multiplied by speed.
- In this case, we want to make a smaller and smoother input, as speed increases, in order to avoid a response stronger than required, which will overload the tire's grip.
- This knowledge is important because, as people enter a corner too fast, the body of the car is reluctant to turn and this effects the tires. In each cornering scenario, a certain wheel works harderst. Normally, it's the front inside wheel (it is both tilted into the corner and has the sharpest route). This causes the front axle to slide (have a larger slip angle) and, since this axle has to turn the car into the corner, it results in the car not responding accordingly to the turning of the wheel. The car would thus run wide and ""under-steer"(and not oversteer and spin or/roll, as most people think would happen when turning at coarse speed).
- The normal response would be to think of cornering like a longitudinal force. I.E. We drove into a corner too fast, and the speed (inertia) antagonized the car's ability to turn, and we must therefore compensate by turning more. This is not true. The speed increases the Centripetal force and makes it use more of the car's grip and an addition to the turning degree of the front tires only gives it more force to handle, and less grip.
- Again, this instinct is not fully wrong: Understeer is selected as a car's natural handling, because it restricts itself: It makes the turning arch wider and wider, so eventually the force is decreased and the car gripps the road surface again. More steering = more sliding = more understeer = more deceleration = more load being transmitted forward = potentially more grip. This equasion is not always true, sometimes the car has went over the limit of grip and an addition of steering would only helplessly overload the front tires.
