Extreme and defensive driving. Dmitry Aleksandrovich Liskin
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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.
Rocking, rhythmic drift
In the chapter “Drift: causes of origin and methods of fighting” it was shown, that when passing an S-like corner drift can occur when moving to the second half of the corner, which is related to the rocking of car body. Let us understand, what causes occurrence of drift and learn how to fight with rhythmic drift.
When passing a corner, centrifugal force causes the center of mass of car to move to the wheels, which are located on the outside of a corner. Suspension springs of the wheels, which are loaded with car’s mass, are compressed and store mechanical energy. When changing the direction of corner the energy is released, the springs are discharged and push the center of mass in the opposite direction, and other two springs begin to shrink. Definite frequency of changing direction of corners is able to increase amplitude of movement of the center of mass. Resonant increase in movement of the center of mass of machine is called rocking. Rocking can be longitudinal and transversal.
Longitudinal rocking
Longitudinal rocking – is alternately moving the center of mass along the direction of car body (forward-backward) with increasing amplitude. During longitudinal rocking the front and rear suspension springs are alternately compressed and stretched.
Compressions and stretches of suspension springs during driving can be caused by road roughness and resonant processes, which are related to characteristics of suspension. In virtue of loading of the front and rear axles alternately changes, grip on the road of the front and rear wheels changes sequentially. On the one hand, when the front axle is partially unloaded, grip of the front wheels on the road worsen. For example, if the front axle bounces on a hillock in a corner, car will move almost straightforward during the front axle is in unloading state. On the other hand, unloading the rear axle can cause vehicle to drift. Unloading of the rear axle is well felt, when a descent begins.
Longitudinal rocking is an undesirable effect, since
• behavior of machine becomes less predictable;
• the best grip on the road is achieved, if load between the axles distributed evenly; longitudinal displacement of the center of mass redistribute load of axles, the total tyre grip on the road is reduced;
• efficiency of acceleration and braking is