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                                        VEHICLE SKID CONTROL

1. INTRODUCTION

    Vehicle skid can be defined as the loss of traction between a vehicle’s tyres and the road surface due to the forces
acting on the vehicle. Most skids are caused by driver error, although only about 15% of accidents are the direct result of
a vehicle skidding. Skids occurring in other accidents are usually the result of last minute action, by the driver, when
faced with a crisis ahead rather than actually causing an accident. Skids can occur both in the dry and wet as well as icy
conditions, however, the chances of losing control and having an accident increases by 50% in the wet. The most
common type of skid we will be confronted with is when the rear end of the car slides out, causing an oversteer or when
the front of the car plows toward the outside of a turn without following the curve of the turn causing an understeer.
Usually, oversteer occurs as a result of going into a corner too fast or incorrectly hitting a slick area, causing the rear
wheels to oversteer. A third skid called the four wheel skid can also occur, where all the four wheels lock up and the
vehicle slides in the direction where the forward momentum is carrying it, with no directional control.


        To counter these skids and to prevent accidents from happening, Vehicle Skid Control (VSC) is incorporated in
the vehicle. Vehicle Skid Control (VSC) takes the safety aspects of the driver and the vehicle to the next level. It comes
under the category of “Passive Technology”, which helps you to avoid a crash. Vehicle Skid Control (VSC) senses the
onset of traction loss and helps the driver stay on track. This is achieved via the system's ability to reduce engine power
and to control the brake actuator. VSC helps the driver maintain vehicle traction under demanding conditions by
detecting and helping to correct the wheel spin. VSC uses a variety of sensor input to determine if the car is losing
traction, then applies the brakes to individual wheels to help correct for discrepancies. The system will also back off the
throttle to reduce power. VSC integrates traction control to limit rear wheelspin on slippery surfaces. The VSC system
electronically monitors speed and direction, and compares the vehicle's direction of travel with the driver's steering,
acceleration and braking input. VSC can help the driver compensate for loss of lateral traction, which can cause skids and
loss of vehicle control.


2. CAUSES


The main causes of skidding are as follows:



a) Harsh or sudden acceleration.
b) Excessive or sudden braking.
c) Coarse or jerky steering movements.
d) Oversteer and understeer.


           The effects of the above will be enhanced by speed. Combining these effects with non-
recognition of adverse road and weather conditions will create problems for the driver.


3. TYPES OF SKID
      The main types of skid that a driver could encounter on the public highway fall into three categories.
1) The front wheel skid.
2) The rear wheel skid.
3) The four wheel skid.


3.1. The Front Wheel Skid

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                       Figure 1:FRONT WHEEL SKID



3.1.1.Characteristics
        The car tends to take a course outside of the expected course that the driver has steered
(understeer); see figure 1. If the front tyre approaches the traction limit more rapidly, the effect is that
the front of the car takes a wider radius curve than the driver intended. The car is said to understeer.


3.1.2. Cause
       Excess speed on entry to a hazard i.e. a corner or bend, or sudden braking to reduce the speed
when negotiating the hazard. Both of these actions will have the effect of destabilising the vehicle making
it more vulnerable to a loss of control.


3.2. The Rear Wheel Skid




                                        Figure 2:REAR WHEEL SKID



3.2.1. Characteristics
        The rear of vehicle swings out of line and gives the impression of trying to overtake the front
(oversteer); see figure 2. If the rear tyres approach their traction limit more rapidly than the front, then
the effect is for the rear of the car to steer a wider path than the front wheels. This rotates the car more
than the driver intended and, if nothing is done, leads to the car turning a smaller radius corner. When
this occurs the car is said to oversteer.


3.2.2. Cause
        As with the front wheel skid, excessive speed into the hazard and sudden braking or acc-eleration
with a rear wheel drive vehicle, destabilising the vehicle, are the main causes of this skid.


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3.3. The Four Wheel Skid




                            Figure 3:FOUR WHEEL SKID




3.3.1. Characteristics
        All four wheels have locked up and the vehicle is sliding in the direction that the forward
momentum is carrying it, with no directional control; see figure 3. Both front and rear wheel skids, if
unchecked sufficiently early, can develop into four wheel skids.


3.3.2. Cause
        Harsh or sudden braking has caused the wheels to lock. A sensation of increase in the vehicle’s
speed often occurs.




                                                 Figure 4




4. UNDERSTEER AND OVERSTEER


4.1. Understeer


     As the name implies, understeer occurs when the front slip angle is greater than the rear and the car
goes straighter rather than following the intended turn. The slip angle, or yaw angle in technical

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terminology, is the angle between where the car is pointing and the intended path. The yaw moment is
the rate at which the yaw angle is changing. The higher the yaw moment, the more likely it is that the
driver is losing control. At the same point, the front wheel may start to grip less even when the steering is
turned sharply and as a result the car continues in more of a straight line than a sharp turn. Here in this
case, the skid control system brakes the inside rear wheel, effectively tightening the car’s line. By applying
the brakes, the car slows down which further helps stabilise it.


4.2. Oversteer


      Oversteer, on the other hand, occurs when the rear tyres have a greater slip angle
than the front tyres and the back threatens to overtake the front, causing the vehicle to
spin. In other words, if the rear tyres approach their traction limit more rapidly than the
front, then the effect is for the rear of the car to steer a wider path than the front wheels.
This rotates the car more than the driver intended and, if nothing is done, leads to the car
turning a smaller radius corner. When this occurs the car is said to oversteer. Here the
skid control system brakes the outside front wheel to reduce oversteer, effectively pulling
the tail back into line.

5. SKID CONTROL

   Stability control systems or skid control systems with names like StabiliTrak, Dynamic Stability
Control, Stability Management, and Vehicle Skid Control are the latest advancement in vehicle safety.
Regardless of the different names, they all perform the same task – to sense the onset of traction loss and
keep the driver on track. These systems are designed to deliver transparent intervention the moment the
situation becomes unstable. A vehicle skid control system actually detects when a driver has lost some
degree of control. It then automatically stabilizes the vehicle to help the driver regain control. Vehicle Skid
Control (VSC) takes the safety aspects of the driver and the vehicle to a completely new level. These skid
control systems are often integrated with the engine management system to cut power in even more
tricky situations. This scenario is a complex system of sensors and microprocessors that continually
monitor the vehicle for any signs of instability. Once detected (usually in the form of a slide or skid), the
system automatically applies selective braking to specific wheels thereby stabilizing the vehicle. This split-
second intervention often happens so quickly that it is over before drivers even realize they were in
danger of losing control. By gently stabilizing the car at the critical moment, control is returned to the
driver with minimal fuss and alarm. Luxury cars, such as the Mercedes Benzes, BMW, Lexus, etc. now sold
in India, have stability systems installed that are designed to remove oversteer or understeer.


6. COMPONENTS


    The Vehicle Skid Control (VSC) is made possible by the combination of different electronic and
mechanical components. Some of the components are those used in Anti-lock Braking System (ABS), and


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an electronically controlled engine throttle, as well as a dedicated computer and sensors, providing
information to the VSC system. These include:




                                                          Figure 5



   Yaw rate sensor.
   G-sensor.
   Steering angle sensor.
   Electronic throttle control.
   Slip indicator.
   Computer.

           Yaw rate sensors detect changes in the car's rotation in a left or right direction. It keeps track of
the direction in which the car is moving relative to which way the driver is




           turning the steering wheel. When the sensors detect understeer or oversteer, a computer takes
over and applies brakes or controls power to one or both the drive wheels, so that the car comes under
control.


           The system is programmed to respond to a wide variety of scenarios and is so
selective that it can apply only the brake on one specific wheel if that's what is needed to
regain control. The G-sensor or gravity sensor determines if the car is accelerating or
decelerating, cornering and braking forces simultaneously while the car is on the move
and accordingly controls the throttle. Steering angle sensor evaluates the direction and
rate of change in steering wheel movement. Electronic throttle control reduces the throttle
for 1/7th of a second, to control the wheel spin, when the front or rear wheels lose
traction. Slip indicator alerts the driver that the tyres are about to exceed the grip limit.
The central processing computer monitors the steering movement together with either
taking over and applying brakes or controlling the power to one or both the drive wheels.
           7. WORKING



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    The heart of all these systems is a central processor that takes information from a number of sensors,
and then determines whether the car is in a stable or unstable state. By combining the datas from ABS
sensors (for wheel speed), steering angle sensors, yaw sensors (measuring the amount a car fishtails, or
rotates around its vertical center axis), and lateral force sensors (measuring the amount of sideways g-
force generated by the car), the central processing unit can actually detect when a vehicle is behaving in a
way contrary to how the driver intends. VSC also includes a slip indicator with a warning sound and light
to alert the driver that the tyres are about to exceed the grip limit.


          If the processor does detect instability such as a slide produced by a sudden swerve, it
automatically applies light brake pressure to a select wheel (or wheels) to maintain or restore control.
Here, the VSC computer uses engine throttle control and individual wheel braking to help counteract
skidding and spinning.The high-speed computer constantly compares the driver's intentions, as indicated
by steering wheel, throttle and braking activity, with the car's actual motions measured by the various
sensors. If they do not correlate, the VSC computer selectively applies individual wheel brakes and/or
momentarily reduces engine power as necessary to help the driver regain the intended direction of travel.
For example, if the car were tending to continue straight rather than responding to the driver's right turn
of the steering wheel, VSC would typically reduce engine power and would apply the right front brake
momentarily to help the car follow the intended path. Once proper vehicle attitude is restored, VSC
returns to a standby state. When VSC is active, a warning beep tone and instrument panel warning light
indicate that the system is functioning. In many cases, VSC reacts well before the driver is aware of a loss
of lateral traction. A VSC shutoff button deactivates VSC and electronic traction control for use. At all
other times,VSC remains on and functioning. VSC differs from Anti-lock Braking System (ABS) technology.
ABS prevents vehicle wheels from locking, decreases the distance required to stop and improves a driver's




         control during emergency braking on wet and slippery roads whereas VSC is intended to help a
driver maintain the intended direction of travel, even when the brakes are not applied. However, VSC and
ABS compliment and work in close coordination with each other in stability control system, providing
enhanced driver control in a broad range of situations.VSC can help provide a measure of control in real-
world situations faced by even the most careful and experienced drivers. VSC senses the onset of traction
loss and helps the driver stay on track. This is achieved via the system's ability to reduce engine power
and to control the brake actuator.


8. WHEN DOES IT HELP?


     Like the safety systems that preceded it, Vehicle Skid Control is designed to step in when human
input is incapable of effectively controlling the vehicle. In most cases, critical situations are the result of
human error in the first place-driving too quickly, inattention, misjudgment or simply panicking in an
emergency situation. In these situations, everyone can benefit from a safety system that occasionally
helps regain vehicle stability, while never taking full control out of the driver's hands.


                   After the introduction of ABS, no safety advancement has added such a high level of
driving security as VSC. When used with ABS and traction control, Vehicle Skid Control significantly


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increases a driver's chances of recovering from potentially dangerous situations. But no matter how
advanced the safety aid, the ultimate fate of a vehicle and its occupants remains in the hands of the
driver. No safety system should ever be expected to protect unconditionally. So while the latest
generation of stability control systems offer drivers increased protection from both themselves and the
unexpected, they can never overcome poor judgement or the laws of physics.


9. REMEDIAL MEASURES


     In each case, the cause can be removed by taking the foot off the accelerator or brake and
depressing the clutch. The reasons are as follows:-


    By decelerating, the vehicle’s speed is lowered, which in turn will start to reduce the magnitude of the
     skid.
    Relaxation of the pressure on the brake pedal will unlock the wheels and allow the tyres to regain
     traction, enabling the vehicle to be steered.
    Depressing the clutch pedal has 3 beneficial effects:


     (i)   The engine will not stall, enabling the vehicle to be moved quickly from the danger area.


     (ii) The link between engine (providing power) and transmission is broken; there is no drive to any of
             the wheels, therefore the vehicle is no longer a front, rear or four wheel drive model.


     (iii) A very slippery surface can cause the drive to lock up which in turn causes the wheels to lock,
             keeping the vehicle in a skid situation.


10. ADVANTAGES AND DISADVANTAGES


10.1. Advantages


1)         Monitors each wheel independently maximizing the performance of the car.
2)         Increases comfort, both physical and psychological.
3)         Improves safety aspects of the car and the driver.
4)         Helps save money long term.
5)         Enhances the ability to dodge a renegade object in its pathways.


10.2. Disadvantages


1)         High initial costs.
2)         Overdependence.
3)         Not perfect.
4)         Repairing cost may be high.
5)         11. CONCLUSION




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      Driving has become more and more dangerous with the ever increasing population of man and
vehicles. It is estimated that 25% of all accidents are caused by driver distractions. Automotive
technology is being developed everyday to make our lives on the roads much safer. Vehicle Skid Control is
one such instance. Safety is the principal benefit of this technology.


              But no matter how advanced the safety aid, we should never forget that the ultimate fate of a
vehicle and its occupants remains in the hands of the driver. No safety system should ever be expected to
protect unconditionally. So while the latest generation of stability control systems offer drivers increased
protection for both themselves and the vehicle, they can never overcome poor judgment or the laws of
physics.


             When we drive, it not only affects our safety but the safety of everyone around us whether
driving or not. With increasing development in the field of automobiles, it is only imperative that we go for
vehicles that have these technologies installed in them. Vehicle Skid Control would not, in anyway,
eliminate all road accidents; however it would lower the percentage of crashes thereby lowering the
number of fatalities.


12. REFERENCES


1.         www.experiencemad.co.uk


2.         www.audidrivingexperience.com


3.         www.trailer-bodybuilders.com


4.         www.graham-sykes.co.uk


5.         www.lexus.com


6.         www.howstuffworks.com


7.         Heitner Joseph, Automobile Mechanics,2nd ed ,East West press, New Delhi, 2001


8.         Sing Harbans,The Automobile,1st edition,S Chand,New Delhi,2001


               9.   SEMINAR TOPIC FROM      :: www.edufive.com/seminartopics.html
10.




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