Extreme and defensive driving. Dmitry Aleksandrovich Liskin
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steer to the side, opposite to direction of turning. It is important to do this during acceleration – if there is no traction on the rear axle when the steer deviates from the “straight” position, the car will start to turn around. Preliminary shift in the opposite side not only increases space for maneuver, but also intensify further sliding of the front axle due to rocking. Let us show at the figure performance of reverse turn with a preliminary deviation.
As you can see, we succeed to stay within the marked lane.
If a car does not respond well to performing police turn, you can use at one time pressing of the throttle and brake pedals. Engine torque, transmitted to the rear axle, will prevent the rear wheels from braking, but high braking effort on the front wheels will lock them, causing a car to turn around. Let us illustrate at the figure reverse turn perform using the throttle and brake pedals.
Front-wheel-drive
On a front-wheel-drive car engine braking when reversing creates oversteer. We should seize this. But so that the engine does not stall during perform the technique, it is necessary to press the clutch, when engine speed drops to idle. Let us show at the figure performing a turnaround on a front-wheel-drive car.
Preliminary deviation should be performed by a very short and smooth steer motion, as a car may start to turn around. On rear-wheel-drive traction on the rear axle protects from turning, which is not present on a front-wheel-drive car during acceleration.
Regardless of the type of drive, some machine settings contribute to perform of turnaround. Shifting the center of mass to the rear axle (a small proportion of the weight on the front axle) will make it easier turn of the front axle around the rear axle. Besides, the more a car shows understeer when driving ahead, the more oversteer will occur when driving reverse. Therefore understeer will be another positive point for performing a police turn. So, factors, that affect steerability when driving in reverse are as follows:
• shifting the center of mass to the rear axle;
• understeer when moving ahead.
Steerability when moving reverse affects on success of turnaround performance. A turnaround while reversing can be performed with help of a rightly set of the brake system.
Exercise 1. Try to perform a police turn on a rear-wheel-drive vehicle in all the ways, described in this chapter. Which of the ways to perform a turnaround is the most optimal for your car?
Exercise 2. Perform a police turn on a front-wheel-drive vehicle. Be careful when performing a preliminary deviation, as front-wheel-drive vehicle shows oversteer during reverse acceleration (opposite to a rear-wheel-drive vehicle, which shows understeer during reverse acceleration).
Exercise 3. What settings must be changed, to increase car’s steerability when reversing? Test steerability of reversing at different settings.
Return from an obtuse drift angle
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.