Читать книгу Extreme and defensive driving - Dmitry Aleksandrovich Liskin - Страница 8

Police turn is familiar to many from films. It is rumored that drivers of the special services keep it on board. Police turn allows you to quickly turn a car around with help of front axle drift, which can be created while driving in reverse.

Despite seeming from the outside difficulty of the technique, it is quite simple to perform. Consider performing the technique on front-wheel-drive and rear-wheel-drive cars.

Rear-wheel-drive

Firstly it is necessary to select the reverse gear and accelerate along a straight section of trajectory. It should be noted, that during reversing after releasing the throttle pedal the engine starts braking. Engine braking – is a braking torque (some kind of braking effort). Braking on the rear axle while driving in reverse creates understeer, similar to how high braking effort on the front wheels causes sliding of the front axle when driving ahead. Therefore after performing acceleration it is necessary to disconnect the engine and transmission before performing the technique.

To produce drift of the front axle, you need to quickly turn the steer. After drift occurs, wasted no time, it is necessary to immediately return the steer to the straight position. When longitudinal speed will be zero, the first gear can be switched on. Let us show at the figure performance of turnaround.

Initially we were moving in a straight line in reverse, but deviated from the straight line and after performing turnaround left the marked lane. Drifting of the front axle did not begin immediately: at beginning turn of the steer the car deviates from straight trajectory.

To stay in the lane, during acceleration it is needful to slightly deviate from straight line movement with a short movement of the 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.