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.
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:
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.
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.
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!
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:
Aerodynamic Downforce vs. Drag
Aerodynamics are an important feature in every car. They are meant for three things:
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.
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.
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.
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.
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.
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:
We must understand grip and handling to understand what our car is going through.
An advanced driver will seek to use both elements to maximize his performance:
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