returned to the “straight” position, after the throttle pedal was released.
The psychology of an untrained driver forces him to release the throttle pedal when a drift occurs, regardless of the drive type. This is a common error in driving a front-wheel-drive car. Finding himself in an unfamiliar situation, overcame by sense of fear, the driver releases the gas pedal. So the drift increases. Eventually, driver loses control over the car, the car continues to move by inertia, as if there is no driver in it.
For successful exit from any angle of drift on a front-wheel-drive car engine torque must be enough, to keep skidding of the front wheels at least on the first gear, otherwise the engine will fail. If drift angle of 90 degrees or more occurs it is likely, that you will have to lower the gear, to increase torque on the front wheels.
Rear-wheel-drive
On a rear-wheel-drive car the throttle pedal must be released when exiting from drift. Let us take a rear-wheel-drive understeer car. We will provoke drift of the rear axle by pressing the throttle pedal roundly when passing a corner. To stop drift, depress the throttle pedal and turn the steering wheel against of drift direction. When drift begins to decrease, we will immediately return the steering wheel to the “straight” position.
When exiting a drift with help of steering the maximum drift angle from which you can exit depends on the maximum turn of the front wheels. Note that during exit from drift, the front wheels of the car are directed along the direction of movement. If a zero-friction differential is installed on the rear axle and no friction is created inside the differential when negative load, then braking by the engine will create noticeable oversteer. Therefore, in the case of zero-friction differential, it is better to squeeze the clutch during fight with drift, to exclude engine braking.
At next, we will try an experiment. Let us see, what happens if press together the throttle and brake pedals, while the car is in a drift.
During pass the corner we provoke drift by throttling. Then we hit the throttle and brake pedals at the same time. The front wheels were locked, and there was traction from the engine retained on the rear wheels. Rear unloaded wheel began to skid. Trajectory straightened and drifting stopped. There are visible tyre traces, left by locked front wheels and unloaded rear wheel, which slipped on the asphalt.
Consider what happened to each axle when the throttle and brake pedals were pressed together. Since the drive is carried out on the rear wheels, the front wheels will only be affected by the braking effort, which causes the front wheels to lock. Locked front wheels cause understeer. Now let us look at what happens to the rear axle. For now imagine, that the rear axle has a zero-friction differential. The rear wheels will have braking efforts and equal engine torques.
The left figure shows equivalent forces, created by braking (black arrows), and equivalent forces, caused by engine torque (white arrows). At the right figure the resulting equivalent forces on the rear wheels are shown in white arrows. High braking effort on the front wheels locked them. On the rear axle a small traction was retained. As you know, traction on the rear wheels leads to loading the rear axle and unloading the front. To lock the front wheel is easier, when the front axle is less loaded. As a result, conditions for understeer are created.
The torque on the rear axle can be adjusted so that the braking effort is compensated: neither traction nor braking effort will act on the rear wheels. In this case only the front wheels will brake, which is similar to a strong shift of brake balance to the front axle. As for power, it is distributed unevenly between the wheels: zero-friction differential transfers the main part of engine power to the unloaded wheel.
At the figure the arrows show the power transmitted to the rear wheels.
The torque on unloaded rear wheel causes it to spin rapidly and skid on the road surface. When the trajectory gradually straightens and roll of the body decreases, the unloaded wheel regains grip on the road, speed of rotation gained by it decreases sharply. At that there is a push against the direction of drift, which causes the trajectory to straighten even more.
If a high-friction differential is installed on the rear axle, more engine torque will be transmitted to rear loaded wheel, than to rear unloaded wheel. When the throttle and brake pedals are pressed at the same time, there will be less understeer, than in the case of a zero-friction differential, since the increased torque on rear loaded wheel will push the car in the direction of increasing drift.
The figure shows the total equivalent forces, when a high-friction differential is installed on the machine. A high-friction differential transfers more than half of engine’s torque to rear loaded wheel. The increased traction on rear loaded wheel pushes the car even deeper in the direction of drift (to the left in the figure plane). An addition increase of the torque on rear loaded wheel leads to an increase of engine power transmitted to this wheel.
Thus, the same time pressing the throttle and brake pedals on a rear-drive car creates understeer and allows you to get out of drift. In the future this method of fighting with drift on a rear-wheel-drive car will be called a defensive “throttle+brake” technique. As a rule, at drift angle of approximately 45 degrees (the angle must be checked for each car) and more “throttle+brake” technique does not work, since traction on rear loaded wheel causes car to turn around.
It is important to understand, that “throttle+brake” technique forced a car to move straight. When performing the technique drifting stops, the machine continues to move in a straight line in the same direction, in which it was moving before performance the technique. Therefore it is necessary to calculate trajectory and duration of “throttle+brake” pulse. If a driver has reacted to drift in time, a short impact on the brake pedal and steering correction may be enough. Note, that “throttle+brake” technique allows you to get out of drift, without turning the front wheels.
Since the front wheels are locked, it does not matter, which way they are turned. But to successfully take control at the end of the technique, when the front wheels are in locked condition, you should put the steering wheel straight. There are two reasons for this:
• the suspension geometry is such, that when the front wheels are turned by the steering wheel, the front of car come down; the straighten wheels will lift the front of car back; the greater front clearance – the lower steerability;
• if the front wheels are set in the straight position, car will remain neutral behavior after releasing the throttle and brake pedals (the car’s behavior will not change).
At the end of “throttle+brake” technique it is enough to align trajectory with a corrective steering. If it is necessary to avoid an obstacle or bend trajectory because of other reasons, you should turn the front wheels previously, during perform the technique, when the front wheels are locked.
The torque transmitted to rear loaded wheel depends on engine torque. If engine torque is high enough, efficiency of “throttle+brake” technique may fall: when together pressed the throttle and brake pedals the rear wheels start to skid. In this case to exclude skidding of rear loaded wheel, you should limit effort on the throttle pedal during performance the technique.
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