Читать книгу Extreme and defensive driving - Dmitry Aleksandrovich Liskin - Страница 9

Turnaround of a car on the road is unlikely, but it can be quite dangerous, if it happens for some reasons when moving at high speed. When sliding at a large angle (90—180 degrees), it is impossible to control trajectory of movement and predict behavior of machine. An untrained driver will not be able to take any correct actions during turnaround of a car to overcome drift.

It is known, that on a front-wheel-drive car it will not be difficult to get out of almost any drift angle – it is just needful to press the throttle pedal and turn the steer. For a rear-wheel-drive car there is an effective “throttle+brake” technique, but drift angle, which can be overcome by this technique, is limited. Regardless of the type of drive, machine can be quickly returned, using dosed braking effort in definite range of drift angles.

Let us try an experiment. We will move around a ring in reverse. While moving, press the brake and clutch pedals all the way down.

The sports car’s braking system is very powerful and can lock both the front and rear wheels. After pressing the clutch and brake pedals all the wheels locked, and the car continued to slide move in a straight line. The front wheels locked a little earlier than the rear ones. The final position of the body is at small angle relative to the initial position. Let us repeat the test with less effort on the brake pedal.

When the brake pedal was pressed, both the front wheels and rear left wheel locked. The car almost turned around. If initial speed of movement was greater, the car would most likely perform a turnaround before stopping. Braking efforts on both rear wheels are the same, but at the beginning of braking weight of the car puts more pressure on rear right wheel, than on rear left wheel. Therefore braking effort, which should lock right rear wheel, must be higher, than braking effort on the rear left wheel. In the next test we will push the brake pedal even more weakly, but we will try to provoke lockage of the front wheels.

After pressing the brake pedal both front wheels locked, both rear wheels rotate, until the car stopped. The results are similar to the previous test: the car began to turn around after pressing the brake pedal. On all cars, as a rule, brakes of the front wheels are much stronger than brakes of the rear. If the brake system is correctly set, when the brake pedal is fully pressed when moving on an asphalt road in reverse in a corner or in an obtuse slide angle both front wheels must lock, and the rear – continue to rotate.

Based on the previous experiments, let us formulate test for performing a car turnaround with help of the brake system.

Turnaround test when reversing with help of the brake system. While reversing around a ring, abruptly push the clutch and brake pedals to the end. Both front wheels must lock, rear loaded wheel must not lock. After pressing the pedals car should turn around.

Locking of the front wheels when the brake pedal is pressed must occur for any level of wheel loading. For example, when a car move in a drift, the body rolls, and some wheels are loaded, others – are unloaded. If the braking system is weak, front loaded wheel may not lock.

The test must be performed in clockwise and counter-clockwise move directions, since the braking system can work “criss-cross”, creating slightly different braking efforts for wheels on the same axle (depending on correct setting of the braking system).

Practice shows, that turning around with help of the brake system is not effective, if drift angle is close to 180 degrees or equal to 180 degrees (reverse moving). When pressing the brake pedal the front wheels lock, there is braking effort on the rear wheels, but car will not turn around. Therefore firstly it is necessary to deflect trajectory by turn the steer, after which already lock the front wheels with the service brake. In addition, drift angles in range of approximately 90—140 degrees are also unsuitable for performing turnaround using the brake. While moving in drift at these angles rotation speeds of wheels is low and their locking will not significantly change angle of drift.

On a rear-wheel-drive car you can increase angle of drift, if you cause the rear wheels to skid (gain engine speed in first or second gear and release the clutch pedal). On a front-wheel-drive car wheels skidding should reduce drift angle. On both front-wheel-drive and rear-wheel-drive vehicles the torque and inertia moment of the engine should be enough to cause the drive wheels to skid. In addition, to create skidding of driving wheels, anti-skid system (traction control) should not be installed on car.

The figure shows performing turnaround using the brake, when the car is moving in deep slide. In the chapter “Turnaround while reversing (police turn)”, it was noted, that the success of turnaround is affected by reverse steerability, namely – it should not be too low. The same requirement applies to performing turnaround with help of the brake.

Requirements for successful return from an obtuse drift angle with help of the brake system

• when moving in an obtuse drift angle after pressing the brake pedal to 100%, the front wheels must lock, rear loaded wheel must maintain rotation;

• drift angle should be high enough, but should not be close to 180 degrees (reverse moving), approximate angle range – is from 140 to 160 degrees;

• steerability of reverse moving should not be too low.

Let us try another experiment. When the car is in drift (angle of about 150 degrees), turn the steer to stop in direction of increasing drift angle. We will assume, the car is moving in neutral gear or with pressed clutch pedal, and the front wheels are in the “straight” position.

As we can see, after turning the steer the car began to slowly turn around. Trajectory was slightly curved. Tyre traces were interrupted, as the car moved almost in reverse for short distance. Steer turn allowed not only to turn around, but also to bend trajectory against the direction of drift. We observed similar trajectory bending, when performing a police turn (trajectory bending was undesirable, we tried to eliminate it with help of preliminary deviation).

Let us compare two ways of return from drift with an obtuse angle: by locking the front wheels and turning the front wheels.

Left part of the figure shows turnaround, made by locking the front wheels, and the right part – shows turnaround, accomplished by turning the steering wheel (police turn). Turnaround of the car with help of the brake system is faster and trajectory is generally close to straight line. The second method, unlike the first, allows you to deflect trajectory in the direction, where rear part of car looks when moving in drift (when the front wheels are locked, there is also deflection of trajectory, but it is very small). Moreover the higher steerability when reversing, the faster turnaround will occur after turning the steer and the less curvature of trajectory will be.

Aerodynamic elements, which increase side resistance to air movement of rear part of the body, can contribute to passive safety of a car.

At the figure hatch marks the rear antiwing. When drifting occurs when moving at high speed, such aerodynamics on the rear of a car can create an effort, which prevents increasing of drift. Like tail of an arrow, which stabilizes it in flight, aerodynamic elements on the rear of a car prevent occurrence of drift, but, at the same time, reduce steerability. And the higher speed of movement – the stronger effect. At low move speed and correspondingly low air flow rate the aerodynamics will not affect behavior of the vehicle. Using such of aerodynamics is justified: to pass high-speed corners it is not needful for high steerability, as angles of corners are small, but for passing slower and sharp corners (for example, hairpins) need high steerability.

Notes on performing turnaround with help of the brake system

• if car has a powerful braking system, which can lock all wheels or road surface has a low grip on tyres (wet asphalt, snow-covered road), it is necessary to limit effort on the brake pedal, so as not to lock rear loaded wheel;

• if car has already started to turn when performing turnaround, you may release the brake pedal;

• when the front wheels are in locked condition, you should to straighten them – so that car remains neutral behavior after releasing the brake pedal;

• after drift angle becomes small (approximately 10—30 degrees), you may switch to tackle control by working the steer;

• if machine has received a large moment of rotation and drift angle increases rapidly, you should not wait for drift angle reaches 140—160 degrees and lock the front wheels to slow down the rotation; but after stopping the increase of the drift, drift angle should hit into range of approximately 140—160 degrees to complete turnaround.

Exercise 1. Provoke a drift with an amplitude of more than 90 degrees on a front-wheel-drive car. Create skid of the front wheels in the first or second gear to get out of the drift.

Exercise 2. Provoke a drift with angle greater than 90 degrees on a rear-wheel-drive car. To increase drift angle, cause the rear wheels to skid in the first or second gear.

Exercise 3. Provoke a slide angle of 140—160 degrees. Return car, using the brake system to lock the front wheels. As soon as car starts to turn around, the brake pedal may be released.

Exercise 4. Provoke a sliding angle of 140—160 degrees. Turn the steer in direction of increasing drift to turn car around. Straighten the steer before completing turnaround.

Which of the ways to get out of drift, used in exercises 3 and 4, allowed you to perform turnaround faster?

Exercise 5. Accelerate car in reverse. Perform a small turn of the steer to bend trajectory of movement. Then perform turnaround by locking the front wheels.