Читать книгу Extreme and defensive driving - Dmitry Aleksandrovich Liskin - Страница 4

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 large angle of turn of the front wheels on entry to corner;

• 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.