Extreme and defensive driving. 2nd expanded edition

First part of the book is independent and is mainly aimed at mastering of defensive techniques. To bring the techniques to automatism, at the ends of the chapters exercises and tests are offered that simulate situations that may arise on the road. In the text of the first part of the book there are practical parts “test” for testing car and the defensive techniques and “requirements” for successful completion of the defensive techniques. Driving techniques are shown in the form of flowcharts and marked with the letter M.

Front wheels sliding

The phenomenon when car not answers steering wheel turns, moves along straight trajectory (or trajectory begins to straighten) due to loss of grip of the front wheels on the road, is called understeer. Car shows understeer (low steerability), when the front wheels slide on the road.

The figure shows, how we entered a corner at a fairly high speed and pressed the brake pedal to the stop, which caused the front wheels to lock and, as a result, front wheels sliding.

Consider what happens with a contact patch of the front tyres to the road when understeer due to heavy braking.

The figure schematically shows deformation of a tyre during front wheels sliding, when the front wheels are turned by the steering wheel at a large angle. Contact patch is marked gray. When braking, load on the front axle increases, so contact patch also increases. When understeer, the tyre deforming, contact patch rotates to direction of movement, and so-called tyre slip occurs. Tyre slip is angle of twisting of contact patch relative to direction of movement of the wheel. Steerability factor is defined as ratio of rear wheels contact patch to front wheels contact patch. If front tyres deformation exceeds rear tyres deformation, steerability factor is less than one. In this case, they say understeer or low steerability.

Sliding of the front axle is reflected in feedback of the steering wheel. If at entry to a corner, the steering wheel turned with some effort, and after car stopped obeying the steering wheel and the steering wheel began to rotate more easily, then sliding of the front wheels began. After the front wheels regain traction, feedback of the steering wheel is also restored.

Sliding of the front wheels is a cause of accidents. Let us list causes of understeer, which are related to driver errors:

• too high speed or too large angle of turn of the front wheels at corner entry ;

• excessive braking during cornering (including wheels lock).

Common mistake: a driver over speeds at entry to a corner and, fearing that trajectory would not fit into borders of the corner, turned the steering wheel deeper into the corner and pressed the brake pedal. If the front wheels are turned too much, they slide on the road. After pressing the brake pedal, the front wheels lock. In the end, trajectory becomes straight, car go out of the corner or collides with a barrier.

When entering a corner at high speed you may feel that trajectory will not able be laid within the corner. When it seems that we do not fit into a corner – it is important not to lose your composure. In no case you should sharply hit the brakes and continue to turn the steering wheel in direction of the corner, as these actions will only worsen the situation, there will be sliding of the front wheels. It is necessary to depress the throttle pedal and start to slow down smoothly in the corner. Let us consider examples of trajectories of passing through a corner.

1 – is “ideal” trajectory of sports style cornering, 2 – entrance to the corner was a high speed, 3 – speed on the corner entry was so high that no chance to avoid way out of the corner.

While driving along trajectory 2, speed of car gradually decreased with help of soft braking. Note that radius of trajectory 2 gradually decreases, as with smooth braking speed of car gradually decreases. It can also be seen that when driving along trajectory 2, car moves along outer edge of the road, unlike trajectory 1. If the front wheels began to slide, trajectory 2 would straighten out and not fit into the corner. If at entry to a corner it is found that initial radius of trajectory is too large, this does not means that it will not be possible to lay trajectory within the corner.

What can be methods of dealing with front wheels sliding, if it still occurred?

1. If sliding is caused by braking, the first step is to depress the brake pedal. This should put control back in our hands.

After the wheels have regained their grip on the road, you need to start soft braking if you need to slow down.

2. If the steering wheel is turned at a too large angle, straighten it to restore grip of the front wheels on the road. Speed of car, grip of the tyres on the road, turn angle of the wheels are interrelated parameters. There is no point to turn the steering wheel at too large angle if car is not able to move along trajectory of desired radius: instead of small radius we get sliding of the front wheels and straightening trajectory of movement.

3. Drifting is controlled process, unlike sliding of the front wheels. Therefore, it is possible to stop sliding of the front wheels by drift. By causing the rear wheels to slide, we regain control of car. In addition, speed of vehicle will fall during side sliding. There are various ways to cause the rear axle to slide. Let us look some of them.

a) Sliding of the rear axle can be triggered by using the handbrake. After sliding occurs, you need to turn the steering wheel against of drift direction (against of direction of car rotation), so that car does not turn around. After speed of movement drops, we restore trajectory.

b) A small slide of the front axle that occurs at exit of a corner can be extinguished by sharply pressing of the throttle pedal (this is used for a rear-wheel drive car only, sufficiently high engine torque is required).

To eliminate understeer at exit of the corner, we sharply pressed the throttle pedal. When the rear axle start drifting, we had to depress throttle pedal and turn the front wheels to the left to stop increase of the drift. After that movement speed decreased.

c) Drifting of the rear wheels on rear-wheel drive car can be caused by following method: press the clutch pedal, press the throttle pedal and gain high engine speed, then depress the clutch pedal without depressing the throttle pedal. After that, if engine torque is enough and it is sufficiently inert, the rear wheels should start slipping and the rear axle begin drifting. This technique can be used if there is shortage of engine torque or time to perform reception b).

Jerk traction created by a method b) -c) will cause drift of the rear axle, slightly speeding up car. After drift occurs, you may depress the throttle pedal and try to stabilize car, winning distance from edge of the road. It should be borne in mind that drift of the rear axle is no less dangerous than slide of the front axle. Therefore, you need to apply all the techniques consciously, after training and careful preparation.

During cornering there is a roll of body and the wheels on outside of corner are loaded with mass of car, and on inside, on the contrary, they are unloaded.

Hereafter for brevity wheels, on which vertical load increases, we will call loaded, and wheels vertical load on which decreases – unloaded. When wheel is loaded, its grip on the road (friction force) increases, and when wheel is unloaded, its grip decreases.

Important note: limited slip differential may be required to cause rear axle drift due to the engine torque. To provoke drift, the engine torque must be transferred to both loaded and unloaded wheels, which can be only by limited slip differential. If motor has low torque and differential has no internal friction, there may be difficulties in creating drift.

Consider the demonstration of understeer on example of emergency maneuvering, this is relevant for city conditions. Let us say that obstacle has appeared in our path while we are moving. Our task is to avoid obstacle and come to the stop without incident. To simulate such situation, we will use a test in the form of special markings and placed cones on the test ground.

Along the strip of cones we will accelerate the car. After we achieve fixed speed (in this case, we will gain speed of 75 km/h), we will move strictly straight along the strip of cones to the vertical marking, without taking any action. As soon as we achieve the line, our hands will be untied, we will be able to perform any actions with the controls. Shaded area – is an area where may be obstacles. Hitting the shaded area or turn the car around will be considered as a failure. We need to avoid sector of expected location of obstacles and stop. Let us call this test “understeer test”.

So, the first test. We gain 75 km/h, after the line we brake, trying to go left the leftmost cone.

Pressing the brake until it stops caused the front axle to slide, and the car hit into the shaded area. If there was an obstacle, there would be collision. Aggressive braking during maneuvering is unacceptable. Now we are going to maneuver without braking. As in the previous test, we gain speed of 75 km/h and move strictly in straight line, after the marking we try to perform the maneuver.

When we achieve the line, we depressed throttle pedal and turned the steering wheel sharply to the left. The car started drift and almost turned around. This is due to our car has high steerability. Return to the starting position and repeat the test. Now we work with the steering wheel more precisely and composedly.

We successfully went around the shaded area by depressing throttle pedal and working with the steering wheel only. After bypassing the danger zone, we braked to the stop.

In this test you can select geometric parameters: distance from vertical marking to the shaded zone and its length from the center of the acceleration band.

For fixed dimensions, there is maximum speed at which car can pass the test. In this case speed was 75 km/h. At higher initial speed, the car inevitably hits into the shaded area.

Understeer and sliding of the front wheels occurs, as rule, when driving cars that have low steerability. For vehicles with low steerability, sliding of the front wheels is quite often.

Exercise 1. Provoke sliding of the front wheels by locking the wheels with heavy braking, when moving around a ring or passing a corner. Depress the brake pedal and regain control of car.

Exercise 2. Repeat previous exercise, but now press the brake pedal not to the end, a little weaker. Gradually reducing effort on the brake pedal, find optimal pedal position when the front axle does not slide and trajectory is maintained, but car reduces speed as quickly as possible.

Exercise 3. Provoke understeer by turning the steering wheel too much. Take control of car back by slide of the rear axle with handbrake.

Exercise 4 (for rear-wheel drive). Accelerate smoothly at exit of a corner to get sliding of the front wheels. Suppress understeer by causing the rear axle to drift with help of engine power. This may be sharp press on the throttle pedal or spin of engine with subsequent depressing of clutch pedal.

Exercise 5. Place cones on a ground to perform the test described in this chapter. What is maximum speed at which car can pass your test?

Exercise 6. Moving at constant speed, turn the steering wheel to find position of the front wheels, at which radius of trajectory is minimal, but the front axle does not slide.

Exercise 7. Place cones on a ground that simulate a corner. Make long distance behind the cones, ensuring safe exit from the corner. Enter corner at speed at which the front wheels slide and car go out of the corner. With help of previously practiced techniques, extinguish speed at entrance to the corner to overcome slide of the front axle.

Steerability

Steerability is one of the most important characteristics of car. In many cases behavior of the vehicle can be described by the term of steerability. Steerability – is speed of change the longitudinal direction of the body when the steering wheel is turned while driving. Steerability shows how car will behave while moving after turning the steering wheel.

Steerability may be low, neutral, or high. Steerability affects the behavior of car in corner and, as a consequence, the technique of passing corner.

Let us run a couple of tests. We will use two cars. The first is a retro car, a 1970 rear wheel drive sedan. Like many cars of those years, it has a big and heavy engine, which is located in the front. The second is a modern 2011 front wheel drive hatchback with a lighter engine, which is also located under the hood in front of the front axle. Fuel injection on the hatchback engine is carried out by injectors controlled by an electronic engine control unit. Torque of this engine is lower and shifted towards higher rate (compared to the sedan engine), but the engine develops more power at high rate.

The tests will be carried out on a single corner. We will enter the corner at over speed, at which a car does not “build in” the corner a little.

At the entry to the corner depress the throttle pedal and press the clutch pedal, after which we quickly turn the steering wheel all the way to the corner and keep the steering wheel in the full turned position. When speed drops below 10 km/h, press the brake and stop. First, we test the retro car.

After turning the steering wheel the car did not move along trajectory of desired radius. The front wheels began to slide. You can see the tyre trace of the left front wheel unloaded in the corner. Eventually, the car flew out of the corner. When trajectory is straightened, there is slide of the front axle and car cannot move along trajectory of desired radius, it is said that car shows low steerability or understeer. Car with low steerability when maneuvering is disposed to sliding of the front axle. When driving car with understeer, the body unwillingly turns after turn of the steering wheel.

Now we mount the hatchback and repeat the same experiment.

We got trajectory with smaller radius. If you compare the final positions of the bodies, the sedan body turned at angle less than the hatchback body.

In the modern car, in addition to sliding of the front axle, began drift of the rear axle, after which the car lost speed in drift and went inside the corner. This means that the car’s steerability – high. Oversteer is accompanied by rear axle drift. When you turn the steering wheel the body of car with high steerability begins to turn quickly in the direction of corner, and drift of the rear axle occurs.

Let us compare the characteristics of the cars. The old sedan front track is less than the rear track, the hatchback has the conversely. The sedan has engine at the front, like the hatchback, but the base is longer, which means the center of mass is moved away from the rear axle. The automobiles have different body types – sedan and hatchback. Thus, the steerability is affected by width of the front and rear tracks, mass and its spacing. Let us look at some examples.

In race cars prepared for drag racing, the front track is less than the rear, and the heavy engine is at the front, at large distance from the rear axle. All this creates desired very low steerability. Besides, on the front axle of these monsters – are very light wheels with small diameter compared to the rear. The huge wheelbase allows you to accurately adjust trajectory.

In the rally hatchbacks with front and all wheel drive are popular. The front track is usually wider than the rear. The steerability of the rally cars with these types of drive is often high.

Cars prepared for oval racing – are front engined sedans with large wheelbase and less the front track than the rear. Their steerability – is low.

In circuit racing cars with different steerability are used, depending on type of race track, driving style and preferences of racer. Beginning drivers who participate in circuit races usually prefer stable and predictable car behavior and tend to choose low steerability. High steerability allows you to enter corners faster, but driving car with high steerability requires the racer’s driving skills and special car settings.

Let us sum up the results of our tests and reasoning.

• on car with low steerability it is easier to fly out of the turn;

• hatchbacks have higher steerability than sedans;

• if the motor is distanced from the rear axle we get less steerability;

• low steerability is priority for drag and speed racing, such as oval;

• high steerability is well suited for non-speed tracks with lot of corners.

Settings that result in reduce of steerability:

• narrow of the front track and widen of the rear track;

• increase the front clearance and decrease the rear clearance;

• increase deflection rate of the front suspension springs and decrease deflection rate of the rear suspension springs;

• increase the bump and rebound resistances of the front shock absorbers and decrease the bump and rebound resistances of the rear shock absorbers;

• increase the stiffness of the front antiroll bar and decrease the stiffness of the rear antiroll bar;

• use of more hard front tyres and soft rear tyres;

• increase pressure in the front tyres and decrease pressure in the rear tyres;

• increase camber of the front wheels and decrease camber of the rear wheels;

• increased toe in of the front and rear wheels;

• increase aerodynamic downforce of the rear of car and decrease aerodynamic downforce of the front of car;

• increase side aerodynamic drag of the rear of car and decrease side aerodynamic drag of the front of car;

• move the center of mass to the front axle (distance the center of mass from the rear axle);

• decrease inertia moment (mass or diameter) of the front wheels and increase inertia moment of the rear wheels.

Opposite actions lead to increase of steerability.

Influence of the aerodynamic elements on car behavior strongly depends on speed of movement. At low movement speeds effect of the aerodynamics on behavior of car is negligible. But at high speeds, when heavy air flows, influence of the aerodynamics becomes significant.

In practice, as a rule, it is difficult to talk about neutral steerability – this term exists in theory and mathematics. After tests, you can only tell what kind of steerability appropriate of car – understeer or oversteer and how pronounced oversteer is. Only theoretical calculations (we are talking about so-called steerability factor), which take into account large number of variables, can accurately tell what kind of steerability car has. Oversteer can be estimated qualitatively by angle of drift or by distance between traces of the unloaded in corner wheels of the front and rear axles.