Whenever we admired a car or wondering about which care to
buy, we often ask many technical queries related to power, torque, mileage,
engine capacity, turning radius, 0 to 100 km/s and etc. But have we ever
wondered how well it has done the 100 to 0 km/s. Yeah, you are right! How well
does it stop.
We all have a perception that the vehicle will come to a
halt immediately after we apply the brakes. It’s not your fault; it’s due to
relative motion. Anyway, the vehicle brought to a complete stop due to our
necessary evil called Friction. Frictional forces include the losses between
tires and road, wheel bearings and aeronautical drag.
The total stopping distance comprises of 4 components-
- Human Perception Time
- Human Reaction Time
- Vehicle Reaction Time
- Actual Braking Time
Human Perception time
is the time taken for perceiving the hazard by our sensory organ (eye),
transmitting it to the brain and processing the data. The human perception time
is .25 to .5 secs.
Human Reaction time
is the time taken for the legs to apply brake after the brain sends signal for
lifting the leg from the accelerator to the brake pedal. It varies from .5 to
.75 secs.
Vehicle Reaction time-
As soon as we apply force on the brake pedals, there is a delay for the actual
brake to be applied. This is due to the play on the brake pedal,
compressibility of the brake fluid and efficiency of the braking system.
Actual braking time is the time taken
for the frictional forces to decelerate the vehicle from its running speed to
Zero. They are affected by many practicalities like tyre pressure, surface
finish of the road, grip of the tyre, wheel alignment, brake pad material,
stiffness of suspension, presence of anti skid braking, slope of the road.
These features will only increase the braking time, therefore let ignore all
these features and stay with an ideal condition. The main feature that
determines the braking time is the coefficient of friction, µ.
Let’s go to the physics part.
Kinetic Energy= ½ mv2, where m is the mass of the car and v
is the velocity
Work done by brake due to friction = µmgd,
Where µ is coefficient of friction, g is the acceleration due to
gravity and D is the braking distance
Lets calculate the actual braking distance of a heaviest Hyundai
i-10 weighing 961 kg travelling at a speed of 100 Km/hr (28 m/s) assuming µ to be .9 at its maximum value.
(Refer: http://hyperphysics.phy-astr.gsu.edu/hbase/mechanics/frictire.html )
Kinetic Energy= ½ x 961 x 28 x 28 = 376712 J
Work done by brake= .9 x 961 x 9.8 x
d.
Equating both the energy equation the
actual braking distance is 45 m. For my convenience, I will convert all the time
factor to distance traveled.
Average perception reaction time can
be summed up to 1 sec. The vehicle will travel through a distance of 28 m in
that 1 sec. And 7 m in the vehicle reaction time.
So the total braking distance is
45+28+7= 80 m. Which means, for a safe
driving, we need a clear vision of the road upto a distance of 80 m or in terms
of time, it will be 4.17 secs, including the deceleration time and perception-
reaction time.
We need to develop a habit of
maintaining a hazard free road for the next 4.17 sec at each moment. This figure is set to increase to 200
m or 12.4 sec, when it rains. Imagine its takes more than 10 sec!
The Perception reaction time is a human factor. It is effected, actually “increased” due
tiredness, vigilance of the driver, intake of alcohol, use of mobile phone and age. Perception-
reaction time of 3 to 4 secs is always possible. Which gives us a worst case
scenario of 150 m (total braking distance)
“Keep your eyes on the road and drive safe!”
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