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 similarly 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 fiber 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 molecules begin to separate to the a degree that threatens the integrity of the fiber 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 tire is always slipping slightly and even a tire rendered absolutely sideways has a bit of grip reserves.
So, how does this grip generates movement? Well, here’s the (basic) equation:
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 fiber of rubber when it is moving around when the tire is rolling, than it would be practically airborne until 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 fiber, 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 until a certain peak, after which it will begin to come away from the tarmac and be faced will less and less load, before it completely lets go of the tarmac again.
The point is that when the tire is rolling, this process is repeated continuously. A certain element grips the road, then lets go just as another one grips the road, pulling the tire forward step by step like someone rock climbing. 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 straight down against the road surface, it is providing the full extent of grip, and then 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, depending on it’s category and function, by layers of steel and/or kevlar 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 onto the road and will have less chance of creating separation 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 grips 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 onto the air inside the tire, the less force has to be applied onto 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 onto 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, encountered 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 (until 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 not designed to solely carry the load) to grip the road. An under-inflated tire is thus more likely to collapse (“blow-out”) than an over-inflated tire.
What effects a tire’s ability to develop 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 tire roll speed, 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 equations 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 affect this ability: First is a tire’s size.
A wider tire has more elements gripping the road surface simultaneously. 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 tires profile is more important than it’s width. we will discuss this later.
Now, what if we try to reverse this equation? 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 equation can also be flipped like this: A tire with more load pressing onto it, would squish against the road and create a larger contact patch, artificially increasing the amount of elements on the ground. Vertical load can be viewed negatively as “load” on the tire, but can also be seen positively 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 line, since the tire’s work during a turn increases.
When faced with vertical force, 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 immediately apply 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 then 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 omni-existant percentage 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 until a certain portion of the contact patch, for which it is “winded” back onto 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 actually makes the tire face the road completely sideways. This makes the tire unable to roll onto the desired direction and is in fact “dragged” (slides) sideways ever more roughly.
add image of front slip angle
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 described 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 of the front end, we must take into consideration the rear of the vehicle. 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 onto the car (having another effect to be covered later) as 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 laterally 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.
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 by 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 onto another wheel which is being loaded. This vertical loading has an effect on the tires. Now, whether this effect is positive or negative would depend on 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). When turning, the front “inside” wheel works hardest: 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 necessarily.
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 (until 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 overloads the tire would depend 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 circle. This friction 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) until the limit of grip is reached.
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 until 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 then 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 postpone the braking point down the straight and being able to accelerate earlier on corner exit.
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 drivers to gain more time accelerating in a straight line, rather than in the turn. But, it goes deeper than that:
By staying on the brakes 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 in the right spot, we are able to compensate for this “loss” of grip. If we get the front heavier, we compensate with the expansion 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 it to develop the slip angle fully in a faster rate (aided by the reduction of the tire’s rolling speed). The wider patch makes the slip angle shallower too. This effect is measured in what is called “cornering stiffness”, giving us both a faster response and more grip reserves. This cornering stiffness is defined as the car’s “cornering coefficient in vertical loading”. The transition into a corner generates a certain “rotation”. This rotation costs us 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 quickly.
All of these factors sum up to generate one of three handling characteristics: Grip
When all four tires experience a similar slip angle, the car is “behaving” neutrally. In the perfect world, this would be 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 for various reasons. The result of a larger slip angle in the front and a smaller 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 road cars are set to understeer by two intelligent 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 gets 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 onto it’s center. For example, a front drive car with an engine situated in the back (this has never been done to my knowledge, only used as an example) would therefore be quite unstable and it’s back will be “thrown” into every corner, provoking oversteer.
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