Grip

From Trackpedia

Race driving is all about utilizing the grip you have in the most efficient way. Ideally, you should be at 100% use of grip constantly, or even 105%. Keep in mind that, under such conditions, any mistake possibly leading you to a momentary lost of control will be impossible to recover from, regardless of technique and skill. There is simply no adhesion left to use. If this does occur, you need to stomp the brakes and await the car to stop or go back under control as speed is wiped off.

First off, we need to distinguish between two different concepts: Road-Holding and Road-Handling. Road Holding is the term for Grip, while handling is generated by a combination of grip and balance. However, road holding does not refer merely to the size of the tire's contact patch, it refers to adhesion, which is the fricative force that has to keep the car on the road, accelerate or decelerate it, and move it sideways. Adhesion is generated by grip. The common term "Traction" is typically referring to adhesion used for acceleration in the straight. The term "road holding" has been criticized by some experts (like Race driver and Autocar Magazine writer Chris Harris), as a nickname for grip, but as a general term for both grip and the adhesion it generates, we find it to be fit.

The basic concept is that grip in an automobile is relatively limited in amount. Each tire has a contact patch not much larger than a medium shoe. The weight pressing it down is also not necessarily too large, and changes according to conditions. Grip => Adhesion + Balance = Handling.

Here Tiff Needell demonstrates this difference, along with some basic handling characteristics.

Grip Benefactors

Read this (brief) article before you proceed.

Grip and adhesion are not the same. The term Grip refers to the size of the tire's contact patch, and has the potential of generating a certain amount of adhesion, which is the actual tractive force, divided between the different applications of steering, acceleration and deceleration. Grip itself, however, is also not static and is effected by several means:

Smooth driving vs. Decisive driving
Balance vs. weight transfers
Suspension and chassis build-up
Aerodynamic Downforce vs. Drag
Road conditions
Physical forces vs. slip

While it would be possible, to maintain a similar amount of grip in different speeds, one must not take speed out of account. Many drivers do not consider speed itself to be a deciding factor in car handling and in driving in general. However, speed is a great factor in creating handling "characteristics" and in contribution to their severity. This would become clearer as you learn about cornering.

Smooth driving vs. Decisive driving

A key moral about driving in general, as in motorsport, is driving smoothness. The smoother you go, the faster you go. However, while smoothness is the key, it's also very important not to be "too smooth", but to be decisive and quick at the same time. In the balance point between the two, lays the greatest exploit of the available amount of grip.

Steering: Steering inputs must be made smoothly, in one accurate action, rather than by a succession of rapid motions. However, a quick steering input will allow to make the car more compliant to the driver's steering, and will be quicker. The sharper the corner, the quicker the steering. Because steering introduces side forces, the wheel is generally manouvered as least as possible and car control relies more greatly on the pedals. Additionally, with steering, the equastion can be fliped: "The faster you go, the smoother you need to be".
Braking: When braking before corners, it's best to apply the brake smoothly rather than stomping it. However, it is a mistake to "squeeze" down the brake pedal, since that way, low braking pressure is applied for quite some time. When the car is travelling at a faster speed, it requires the initial application to be stronger and faster, rather than slower and softer. Smoothness is more important during the gradual release of the pedal.
Accelerating: The accelerator should be handled smoothly, but without squeezzing on it too slowely, because than the acceleration potential is not maxed out. As speed increases, the engine's capability to spin the driven wheels grows smaller, hence the accelerator (like the brake) is treated more decisivelly when at speed. However, as speed increases, it is more important to be smooth with the release, as an ubrupt lift-off of the pedal can greatly upset the car's stability.
Shifting: Shifting should be smooth, for speed and gearbox lifespawn, but it should be carried out quickly, for the sake of speed and in order to maintain a normal grip of the steering wheel.

Balance vs. Weight transfers

Each car has it's own balance. Most modern front wheel drive cars are set with 60% of their weight over the front tires, which have the most demands to fulfill. Rear-wheel drive cars are usually more equally balanced, but not by much, as are all-wheel drive cars. However, this balance is not static. It changes according to factors working on the car. These are weight transfers. The weight of the car stays essentially the same, but dynamic forces acting on the car (inertia, centripetal forces), create different loads on different wheels.

Transfering weight is a method of putting more weight on the tire or pair of tires that demand more adhesion for the nessecary application. This allows, to a certain degree, to increase grip to the specific tire. This can actually be used to increase the overall grip. During braking, where does the weight go? To the front, because it needs more grip . Therefore, transfering virtually unused grip from the rear to the front, so that a very slight weight transfer would be advantagous.

The problem with exaggerated weight tranfer is the intensive lost of grip to the unloaded tires, while actually overloading the loaded tire, actually reducing the overall amount of grip. This will become clearer when disscussed friction circles. This is the reason why the driving styler and car design during tarmac ("Grip") driving, is intended to minimize weight transfers, however utilizing the slightest of weight transfer, to our advantage.

Steering: the steering wheel creates a lateral weight transfer, which shifts weight over to the inside wheel, for it to cope with the Centripetal force acting upon the car. Additionally, friction created by the tilting of the front tires while steering, resultes in an effect of deceleration that causes an additional weight transfer to the front, again increasing or decreasing the car's ability to drive through the corner.
Braking': Slowing down by braking creates the most dramatic weight transfer, which is a forward weight transfer. This increases grip to the front tires, and can therefore aid steering. However, it is important to stress that the tire can only hold a certain amount of pressure and different forces. Additionally, hard or unsmooth braking will cause the rear-wheel to come loose, possibly causing oversteer in corners.
Accelerating: Acceleration allows for a rearward weight transfer. This helps rear-wheel and all-wheel drive cars to accelerate quickly as they get off the line, while creating the opposite effect in front-wheel drive. It also makes you recover from oversteer in all cars.
Shifting: Shifting down creates a forward weight transfer and shifting up shifts weight backwards.
Cruising: It is important to state that a car coasting without pedal work is slowing down and is therefore under a forward weight transfer. With certain cars, in certain corner, lifting off of the pedals just as you turn-in and coast through the corner might be advantagous. In the straight, however, any coasting time must be minimized.
Neutral throttle: It is possible to use the throttle lightly to keep the car at a constant speed. This is also when the car is most balanced. However, it still puts traction demands of acceleration on the tires, and is therefore less reconmended for turning into corners.

Smoothness is particularly important when several weight shifts (like left and forward in a moderate bend) are combined, or when transitioning between opposite weight shifts (from braking to acceleration and vice verse, when steering from left to right or vice versa). The reason is Oscillation, which is a spring's tendency to "bounce" when depressurized. This can create an exaggerated weight transfer. Remember, you need to minimize weight transfers, but it is a mistake not to utilize weight shifts to your advantage!

- Driving the limit

Suspension and chassis build-up: Stiff vs. Soft

See also: Suspension

Handling and Weight transfers are effected by suspension setups. The goal is to make weight transfers relatively small and predictable. A car's suspension is made to soften the weight transfers and to absorb shocks from the road. The effect of suspension setting on grip is actually secondary at best. The spring only changes the speed of the weight trafer, and theamount of load transfer that occurs as aresult of weight transfer.

To further comprehend this, the difference between shifts of sprung or unsprung weight must be understood. Unsprung weight is the weight of the components not held by the springs, which includes mainly the tire, wheel, brake and other components. This weight transfer is effected by the compunds of these components, like tire wear and tire pressure.

To differ from this, sprung weight is the weight of the car's body, loaded on the springs. This weight transfer is what we call "roll" (lateral), "Pitch" (Rear) and "dive" (Front), and their rate an speed are the ones effected by suspension setups. The importance of suspension geometry in that respect, is to minimize the weight transfer and soften them, while also keeping a good compromise with road isolation (reduction of bumpiness).

The rate of weight transfer impacts the responsiveness of the car to driver inputs. The faster the weight transfer, the quicker the response. This allows the driver to have greater control of the car. However, a faster weight transfer requires greater skill of the driver. Smoothness and quicker reaction sensitivity to the tire traction are needed. It turns out that shocks have the largest impact on rate of weight transfer. The stiffer they are, the faster the transfer.

The impact of weight transfer on suspension geometry has to do with maintaining as large and flat a tire contact patch as possible. When the body rolls, dives, or squats as a result of weight transfer, the geometric relationship of the suspension components to the body and the wheel changes the shape of the contact patch. For the unloaded tires, the patch size will be reduced. This effect must be minimized. Changes in shocks, springs, anti-roll bars, and wheel alignment are made to maximize the tire contact patches of all tires during the dynamic changes of weight transfer. The length of the wheelbase also has in impact on weight transfer. A longer wheelbase (or a wider track or lower car) creates a greater lack of discipline to weight transfer.

However, there is such athing too stiff. The advantage of softer springs and more roll, is the sharing of load by both the tire and spring, the superior road isolation, the gradual transfer of load, compliance , more feedback and better axle independence. With asofter spring bending about more, it takes a greater amount of load. Where does the rest go? to the tire! Additionally, if the two sides of the car were bound to eliminate body roll, turning somewhere would lift the inside wheel fully airborne.

Another important concept of controlling weight transfer besides minimizing it, is to control where it is transferred. Where weight transfer occurs is related to the static weight distribution of the car, the roll couple distribution of the car, the height of the roll center of the car, and the slope of the roll center in relation to the ground plane.

Roll couple distribution is the relative roll stiffness between the front and rear of the car, and the left and right of the car. In cornering, the front of the car may roll less than the rear of the car. This has impact on how the weight transfer is distributed.

The roll center is the line through which the vehicle rolls. It is not necessarily parallel to the ground. Weight distribution, and roll coupling distribution can create a roll point at the front of the car which is lower to the ground that the roll point of the rear of the car. This creates a sloped line. The angle of this line has influence on how much weight is transferred, and where it goes.

The rule of softening the weight transfer is done by the dampers. However, these consist of only one part of the suspension system. It consists of Wheel-arms, that control the angle of the wheels. This angle allows to increase or decrease the default amount of grip each tire has, by changing it's angle of contact with the ground. Than there are Springs: the job of the springs is to keep the wheel pressed against the road surface in spite of bumps on the road surface.

The springs are put on dampers (mistakenly called "shock absorbers"), which are bars with inside pistons and plugs operated by hydraulic pressure, able of expending and retracting. They are the ones doing the weight transfers, while the springs are the ones absorbing the shocks and retracting the dampers. A smooth application is important for the dampers. For an example, if you let go of the brakes aggressively, the front dampers will decompress quickly and the front tires will momentarily bounce up, and it will now be up to the springs to set them back and they will be occupied in balancing out the shocks that the driver puts on the car, rather than sticking the tires to the road.

The chassis of the car is also exposed to physical forces, being soft enough to bend, particularly in it's lower portion, in admittance to changes in the forces acting upon it. One method of minimizing this, to offer smoother and easier control, is to brace the chassis with special bars.

A much more effective way, however, is to change the car's Center of Gravity (CG). Relocating the CG to a more favorable position can also reduce weight transfer, load transfer and body bending. The effect is usually lesser than that changes in the actual weight of the car or other benefactors.

Without getting into the engineering of it all, the location of the center of gravity acts as lever handle. We know from basic physics that a lever can be used to increase force and work. If the center of gravity is very high, there is essentially a long lever in the car. During braking, accelerating, or cornering, the G forces are amplified by this lever created between the CG point and the tire contact patches. The further apart they are, the greater leverage, and the greater the weight transfer.

With a given car, you can't change the CG location dramatically, but you do have some ability to affect the center of gravity enough to make major improvements to the car's handling performance. If you're willing to sacrifice some comfort, convenience, and looks, you can subtract and relocate weight to affect the front to rear and the side to side weight centers. You can also alter the CG height by lowering the car with lowering springs, lower sidewall tires, and to a smaller degree by adding removing, or moving weight in the car.

Once you have selected your car, there's nothing you're likely going to do to change the wheelbase or track width. You might increase track width a little with wider wheels though. Remember, a rim of a wider radius, is better than a wider one, and rigid wheels can also be better than light ones. A wider wheel is at a greater risk of aquplaning.

Load transfers are the changes in the angle of the body of the car, as a result of weight transfer. Their disadvantage is the change in the form of the tire contact patch and more sidewall collapse resulting in less grip. Thier advantage: The springs take some of the load from the tire so that weight is transfered in a gradual manner. It depends on the shape and the tire contact patch and some other aspects of car design. However, since the effects of altering the shape of the grip patch are usually more grave and cannot be efficiently covered for, a stiff car with minimum load transfers is usually the best for the track.

In terms of driver handling, it would be wrong to avoid weight transfer. It is imperative to utilize weight transfer for your advantage, depanding on the situation:

Steering: As said, smooth steering allows for the dampers to compress smoothly, so weight is evenly transfered from one side to the other. Jerking the wheel would cause the damper of the "outside wheel" to de-compress violently and the outside wheel will bounce. However, a quick input must be made, in order for the response from the dampers and springs to effectively lean the car (as you lean a bike), to aid the car somewhat in changing the direction of travel. In the wet, it's even harder for the car to change the direction of travel, so the importance of a quick application is greater, not smaller. However, this action will take longer on slippery conditions, thus it might appear as though if the car is responding in a delay. The car must be let to get around without additional steering inputs. Stiffer suspension will be more sensitive, and has to be dealt with more smoothly, while a softer one can be handled more decisively. Note that in the WRC, when driving fast on rugged terrain, the driver constantly has to struggle the wheel and even manhandle it, even without turning. This is the result of a highly stiff (4-5 times) suspension, with steel dampers filled in gas and oil.
Braking: Braking gives more grip to the front tires, allowing for improved turning into corners. However, braking too hard will put too much effort on the dampers, and they will not be able to support the wheels as they go over the surface. Additionally, the rear of the car may become loose.
Acceleration: Accelerating will cause a rearward weight transfer. Going off of the brakes and on the throttle might unsettle the car, unless done smoothly.

Aerodynamic Downforce vs. Drag

Aerodynamics are an important feature in every car. They are meant for three things:

Air intake: Air is being mixed with fuel inside the intake manifold. In rally cars, extra air is nessecary to fire up the turbocharger in slow speeds (Anti-Lag System), or to keep the car running in mountain tracks with thin air.
Reduction of drag: Air which is either static or running in every direction other the straight forward, will damage the car's steering and acceleration. The body of the vehicle and even the air intake systems, are made to minimize friction with air, allowing the car to run freely.
Aerodynamic downforce: The idea is to shape the car and add "accessories" to it, in order to create an aerodynamic downforce, applying more pressure on the tires. This is better than mechanical downforce (weight transfer) because it is consistant. For this sake spoilers (rear wings), side-skirts (low side wings) are installed and even the air intake system is made to create downforce by releasing air through roof and bonnet scops and vents. The idea is to create maximal downforce with minimum drag at high speeds. Unfortunatly, most aftermarket spoilers don't supply the goods.

Road Conditions: Grip vs. Slip

A wet road is not essentially different from a dry road. The idea to be slightly smoother and slower. However, actions still have to be made decisively.

Steering: As said, quick steering is important in the wet more than on the dry. A delayed response is natural and the car must be let to turn without turning the wheel even more. The idea is to be accurate with the initial input. On gravel, it should be stated, the suspension has to cope with hard conditions, and it is why WRC driver constantly have to move the wheel about.
Braking: With threshold braking, the braking distance in the wet is quite like in the dry. However, the point of threshold itself will require less braking pressure, and will be harder to maintain. It should be stated that on gravel, locking the wheels is a good means of a quick slow down, as the locked wheels dig into the soft terrain.
Acceleration: Accelerate smoothly, but decisively. This specific action should be the smoothest one.
Shifting: Traditionally, corners in wet racing events are engaged one gear lower than on the dry.

Further reading: http://www.redlinerennsport.com/DrEdEd9Rain.html

Physical forces: Lateral vs. Longitudal

As said, the adhesion generated by the grip is very limited, and is divided between the different applications: Acceleration, deceleration, steering. The first two actions are longitudinal forces (working in a straight line along the contact patch), and steering is a lateral force (side to-side). For now, we will describe it as 100%. If 50% is used for braking, how much is left for steering? 50%, right? Well, not quite. It goes deeper than that.

When steering, the inertia of the car is striving to keep pushing the car forward (Yaw), while the direction in which the front tires are pointing is forcing the car to turn (Grip). The force working on the car in the turn is called a Centripetal force (not to be confused with a centrifugal force), while the force keeping it in is actually the lateral adhesion generated by the grip of the front tires. If you add too much longitudal forces (Accelerate or decelerate) or put in too much steering too sharply, and it will come on the expense of lateral (or Latitudinal) adesion and the car will slip from the corner. This is known as "Slip angles": The difference between the direction the tire is facing, and the direction it is turning the car through.

When braking, adhesion is being used to wipe off speed. Too much braking, and you won't have any adhesion left for lateral applications. Practically speaking, the front tires (and than the rear tyres) will tend to stop revolving, thus "locking up", and being "dragged" over the road, increasing braking distance as well.
When accelerating hard, the demands from the driven wheels, especially in a front wheel drive, will be too much. The wheels will spin and will not be able of accelerating or steering the car.

For more information on slip angles, read this short article

This term must be put into consideration with weight transfers. In the event of light braking, the forward weight shift gives the front tires a certain addition of grip, which has the potential of generating more adhesion. Yes, some of this adhesion is used to decelerate the car, but if the brake pressure is light enough, there will still be extra adhesion left for cornering, more than it would without the weight transfer.

An opposite example is when wheelspin occurs during acceleration. In this case, any rearward weight transfer will be minimal, because the power is not being put down on the road, thus not creating acceleration and not generating a weight transfer. However, if we add high speed to the equasion, we decrease the engine's ability to transmitt power to the wheels, and the effect of weight transfers becomes more crucial than than of friction distribution.

This article from Ross Bently summerizes the friction circle concept quite nicely.

Distortion:

Slippage has an advantage, though. Under high tractive demands -- particularly in two directions -- the tire and it's gripping elements tends to melt or deform. A certain amount of slippage limits distortion to a level where it actually spreads the tire's contact patch and the face of each of it's gripping elements on a bigger surface, optimizing grip. This is turn, spreads the (unsprung) weight over a larger surface, and allows each gripping element to carry less load.

In fact, at the correct slip angle, the rear end of the contact patch, where the slipping tread elements are regripping the surface, is where the most grip lays. The point of distortion will begin at the crossing point between the wheel alignment axle and the phyiscal force vector. This, in combination with the moment of inertia, generates feedback from the tires to the steering wheel. It is also the reason why the wheel "self-centers" itself if left freely. Slip angles effect this too, and can result in changing the speed in which the wheel returns to straight, or even make it spin to the opposite lock (under power oversteer).

It is important to state that the distortion and slip are gradual effects, and require the wheel to revolve somewhat before the optimum angle is reached.

Slippage:

What we have just seen is that too much of any application will result in some sort of slippage: steering will result in a "slip angle", braking in "wheel lockup" and accelerating in "wheel-spin". This will both hinder the effectiveness of the original application (steering, braking or accleration that are not ideal) and will also not allow for any other application to be put into the mix. It is important to state that a minimal amount of slip is always present in every direction.

Another term to be understood regarding this is the friction circle ("Traction circle") and types of friction. The adhesion that we spend so much time describing, originate from the rolling action of the tire, generation "static friction". When grip limits are reached and the tire starts to slip, it means that adhesion has been maxed out and that tractive demands exceed the radius of the friction circle of the tires' grip. However, what happens to a slipping tire? Instead of acting within the boundaries of the static friction circle, it moves into the realm of the kinetic friction circle. Rolling tire=Grip=Static friction; Sliding tire=Slip=Kinetic friction.

Keep in mind that each tire (or at least each axle) has it's own slip angle/rate and friction circle. This is also the reason why slippery conditions require greater slippage rates (E.G. Rallying), because, in such conditions, there is less possible static friction than kinetic friction, so the driver controls the car within the boundaries of the kinetic friction circle, rather than the static one. Friction is a closed circle, the amount of friction decrease from the first circle, moves squarely to the other.

In relation to friction circles, it is important to stress that in our physically unperfected world, a certain amount of kinetic friction (slip) will always exist alongside a certain amount of static friction (grip). Therefore, the very concept of illustrating the amount of adhesion as 100% is simplistic, because one might conclude from it that handling characteristics such as understeer and oversteer appear only on the final limit. However, they exist permenantly, and the driver must manage his tires' friction and weight distribution to control it.

According to this concept, if the driver gets the tire (ideally all four tires) to the very edge of the static friction circle, and actually passes the line into the kinetic friction circle slightly, he will be benefiting from both. Just like being smooth and quick as one, the driver should actually be slipping slightly with his car. If the car feels "planted" it is not being driven fast enough or is being handled too smoothly. It is nessecary to reach the threshold of slipping. However, that threshold is not reached just before slipping, it is actually reached at about 6-8% slippage. That's the key: not to use 100% of the adhesion, but actually reach 105% of the adhesion you have. Not to push the car to the limit, but on the limit itself.

The ideal situation is slight neutral-steer or neutral handling, which is where all four tires slip in unison. It is sometimes regarded as a point between oversteer and understeer but it's actually both of those combined. It is a complex and rare handling characteristic that depands on weight and load transfers, car balance, tractive demands, speed and the car's CG, which is the coordinate between the slippage rates of the front and rear. Therefore, rarely would a car be naturally tuned to this, and the goal of many car modifications and driving techniques are to induce it. The idea is to make the back axle slightly lose and immediately and very accuratly use the throttle and a slight reduction of steering lock to "drag" the car through.

Because all four tires are at their optimum slip angles, the pedals are going to control the car better than the steering, so you "breath the throttle" (mistaken for "throttle maintenance, which is used in fast curves where there is understeer rather than neutral-steer) and actually steer the car with the throttle dosage, which clearly makes you faster. This situation is hard to achieve and requires skill, and is not going always going to be the practical way around. Keep in mind that this limit should be reached quickly but gradually and smoothly. Also remember not to exaggerate, as it takes only a bit of slip, not an actual slide.

However, since our driving conditions are never ideal, we will not and should not be handling our car neutrally all of the time. Sometimes, a slight degree of underster or overster is actually prefered, depending mainly on the car, corner and phase of the corner. Very fast curves are normally driven through completly under power understeer, by keeping a certain amount of acceleration all throughout, weight transfers backwards and resultes in understeer. Even in a RWD, the speed causes a reduction of power, so the weight transfer is more effective than the effect of power on slip angles. This way, once the Apex is clipped, the driver can quickly regain full throttle status, because of the excess weight on the rear axle.

Sharper corner, are normally negotiated in slight oversteer. By braking slightly, the front grips more than the rear, allowing the car to rotate better into the corner. Getting on the gas neutralizes the slide and causes the car to drive neutrally through the corner and exit it in a situation of slight power understeer/oversteer, depending on the car, with the car driven through the corner with the throttle.

In this regard, it is worth mentioning that any car can be made to illustrate all sorts of handling characteristics (Understeer, oversteer, etc...), unlike categorily saying that Front-wheel drive cars understeer and Rear-wheel drive cars oversteer. Additionally, most cars, when maltreated, will understeer rather than oversteer, as it safer, more predictible, easier to detect in an early stage, and results in a frontal collision rather than a side hit and potentially a tip-over.

The key is for the car to be at 105% of it's traction, with the driver at 99% his capability. Remember, though, that from the point onward, by a mere increase of 5% to tractive demands, no matter if the driver increases his skill by 200%.

Here you can see an autocross being pulled-off just on that limit:

Further reading:

Summary

We must understand grip and handling to understand what our car is going through.

Road holding: Includes two concepts:
1. Grip: The size and compound of the rubber patch in contact with the road.
2. Adhesion: The friction created by grip. It is divided by all of the different application: Steering, braking and accelerating. The more adhesion used for braking/accelerating results in less adhesion left for steering. The higher precentage of adhesion occupied, the bigger is the precentage of slip and the closer the car is to the limit. The best performance is achieved slightly beyond the limit.
Road handling: A certain amount of slip is always present alongside grip. While driving, the balance of the car changes and different amounts of slip can be generated in different tires, making one of the axles, front or rear, more loose. (Road holding = Grip + Adhesion) + Balance = Road handling.

An advanced driver will seek to use both elements to maximize his performance:

Grip: The driver will effect the grip levels of the tire by making more weight act upon it, either by car setup or by using weight transfers. When accelerating/braking/steering, the weight of the car "moves", pressing one pair of tires against the road on the expense of the other tires. "So?", you are probably asking, "If grip moves from one tire to another, how can the overall amount of grip be effected?". Well, sometimes one tire might needs more grip than another, so shifting a tiny bit of extra weight (and grip) to that tire helps.
Adhesion: The driver needs to utilize as much of his available adhesion to brake, turn and accelerate to the maximum. Reaching the limit and passing it slightly, must be done quickley but cannot be done instantly.
Road handling: The driver will get his car to handle as he needs: Sometimes it's better for the rear to slide a bit more than the front, sometimes it's the opposite and sometimes both should slide symmetrically.

One example is the combination of braking and cornering. When you brake, weight transfers to the front. Most cars are guided in a corner by their front tires, so giving them a bit of extra grip can be beneficial. "But", I hear you shout, "will braking not eat away adhesion otherwise used for cornering?". Yes, but if you brake very lightly the beneficial effect of the weight transfer over-compensates for this.

The factor that prescribes the balance between the two is speed. The faster the car is going, the bigger is it's inertia. Therefore, it is both harder to turn it aside and harder to slow it down as effectivelly. However, the two are not effected by speed to the same degree. With the brakes, the faster you go, the harder it is to slow down the car, thus the stronger must the brakes be applied. With steering, the opposite holds true, which why we do not see people yanking the wheel around at 90mph, no do we?

So, at a very slow speed, the brakes are strong enough to overload the tire and eat a great amount of adhesion for braking, while not compensating for it by the weight transfer.

If you managed to wrap your heads around this, and you are interested in deeper and more scientific aspects of grip, you might want to consider reading our section on advanced grip