Extreme and defensive driving. Dmitry Aleksandrovich LiskinЧитать онлайн книгу.
the engine torque is divided between the front wheels by the differential.
On the front axle of the car a zero-friction differential is installed, which transmits half of the engine’s torque to the each wheel. To explain the test results, we assume that the engine torque is enough to maintain traction on the front wheels when the brake pedal is pressed full way down.
For demonstrativeness of illustrations we introduce the concept of equivalent force. Equivalent force is the force that must be applied to the top of wheel to obtain the torque on wheel (this may be engine torque or torque generated by a braking system).
The figure shows the force was applied to the top of the wheel and compels the wheel to rotate faster clockwise.
Let us draw equivalent forces that occurred when one time press the throttle and brake pedals in the previous test.
At the left figure black arrows indicate equivalent forces created by braking system, white arrows – equal equivalent forces corresponding to torque produced by the engine and is divided between the wheels by differential. The right figure shows the resulting equivalent forces on each front wheel, which corresponds to the resulting torque (difference between engine torque and brake system torque). We did not press the brake pedal to the end, the braking effort on rear loaded wheel was not enough to lock it.
If braking effort is high enough, the rear wheels may become locked. Let us imagine a case when the rear wheels are locked. Note the locked wheels with crosses.
Direction of the equivalent forces corresponds to direction of movement of the front wheels. Front unloaded wheel has worse grip on the road than the loaded one, and there is a chance that it starts to slip. If the unloaded wheel starts to skid, the main part of the engine’s power will be transferred to it.
Although pressing the throttle and brake pedals creates oversteer on a front-wheel-drive car, steerability may be reduced by engine’s torque. If the rear wheels are locked, but the engine has an enough high torque, car will not turn around in a drift due to high traction on the front wheels. At the same time, if braking effort on the front wheels is fully compensated by the engine torque, maximum steerability will be achieved.
Increased friction of differential allows you to transfer more torque to loaded front wheel and thus more efficiently implement engine power. Let us represent equivalent forces, when a limited slip differential is set on the front axle.
The left figure shows equivalent forces generated by braking (black arrows) and engine torque (white arrows). Figure on the right shows resulting equivalent forces on the front wheels after subtracting the braking effort from engine torque. A greater amount of torque on loaded front wheel means that more engine power will be transferred to the wheel compared to situation, when a zero-friction differential was installed.
Thus, traction on the front wheels when braking creates oversteer. Presence of a small traction on the front wheels during braking is equivalent to shifting balance of braking effort to the rear axle. But too much traction on the front wheels can lead to straightening of trajectory (understeer).
In addition to the previous reasons, drift can be caused by a sharp change in the road surface. For example, if a driver inadvertently drove a rear wheel to wayside of the road on which there is ground or snow, car can drift. An inexperienced driver, as a rule, does not operate with information about condition of the road surface and how it should affect the braking path and control technique. If you move in the “normal” tempo, familiar for dry weather, a rear-wheel-drive car can easily start drift at exit of corner on a wet road. Another danger may be invisible ice under the snow. Unsuspecting driver moving quite calmly on the road or passing a corner, can hit into emergency situation.
You can create oversteer in all the ways that you can create rear axle drift. For example, shifting the brake balance to the rear axle will increase steerability when braking at entry to a corner. On a front-wheel-drive car steerability can be improved by using the throttle and brake pedals together.
Ways to go out of drift
Front-wheel-drive
Consider ways to get out of drift. At first consider the front-wheel-drive type. We will provoke drift by the hand brake. When the rear axle begins drift, we must press the throttle pedal to 100% and direct the front wheels against direction of drift to stop the drift.
There are noticeable traces left by locked rear wheels during the action of the handbrake. The first was locked the unloaded wheel. After the end of handbrake action, the throttle pedal was pressed and the steering wheel was turned against of the drift direction. It may be seen unloaded front wheel began to grind after pressing the throttle pedal. This is explained by the fact the machine has a zero-friction differential. Before the end of the drift the steering wheel was 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