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

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.

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 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 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

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 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

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.