1.12. Brake Efficiency & Stopping Distance
“Braking efficiency” is the ratio between the
retarding force (or force of friction between the- linings and the drum) and
the weight of the vehicle. It is expressed as in percentage. [1] [3]
The braking systems efficiency depends upon friction
between moving parts and stationary parts for their stopping force. [1] [4]
The automobiles braking performance and efficiency is
a very important parameter, because on the efficiency of brakes depends the
lives and comfort, not only of driver and passengers but also other persons
moving on the road. [1] [2] [3]
Now, recent improvements in the braking mechanism it
has been possible to stop even increased speeds of the modern cars on the road.
The maximum retarding force applied by the brakes at
the wheels, F, depends upon the coefficient of friction between the road and
the tyre surface µ, and the component of the weight of the vehicle on the
wheel, W, i.e., [1]
F = µW ..... [1]
If unity coefficient of friction is achieved, the
total retarding force produced at the wheels is equal to the vehicle weight
itself, which is equal to the gravitational force experienced by the freely
falling body of mass equal to that of vehicle. If this be the case, the vehicle
experiences a deceleration equal to the acceleration due to gravity, `g', and
the brakes are said to be 100 per cent efficient. Thus theoretical limit for
brake efficiency is 100 per cent. [1]
In actual practice, efficiency of 100 per cent is
rarely used for ordinary vehicles. Requirements like safety of passengers in
public vehicles and safety of the body in case of heavy goods vehicles reduce
the brake efficiencies to be used. [1]
Highly efficient brakes giving large deceleration
might injure the passengers due to sudden stopping of the vehicle. Similarly in
the case of goods vehicles, an extra efficient braking system would cause the
load to slide forward when the brakes are applied, tending to break the
driver's cabin. [1]
Moreover with very highly efficient brakes rapid wear
of tyres and brake linings takes place and there is always a risk of losing
vehicle control when brakes are applied. The brake efficiencies in general use
vary from 50% to 80%, which enable the vehicle to stop within reasonable
distance. [1]
Table 1.1 gives approximate stopping distances at
different vehicle velocities for various conditions of brakes. However, the
minimum allowable limit of brake efficiency for any vehicle is 50% for foot
brakes and 30% for hand brakes. [1]
The distances given in the Table 1.1 are approximate only and
they vary with the type of the road surface and condition of tyre treads, etc. [1] [3]
However, during emergency braking, the reaction of the
driver and response time of the brakes also play an important part.
The total stopping distance in case
of emergency braking may be divided into three parts: [1]
(i) Distance transverse during the reaction time of
the driver.
(ii) Distance traversed during the time elapsed
between the driver pressing the brake pedal and the brake being actually
applied at the wheels.
(iii) Net stopping distance, depending upon the
deceleration.
A reasonable estimate for the distance travelled due
to (i) and (ii) above may be taken as 6 m for a vehicle speed of 30 km/h. Thus
for example, the total distances travelled when the vehicle is stopped from a
speed of 30 and 80 km/h would be 6 + 6 = 12 and 42 + 16 = 58 in respectively
(for brake efficiency of 60%).
Thus the actual stopping distances will be more than the values given in Table 1.1, which is based upon deceleration only.
1.12.1. Stopping distance is depends upon: [1] [2] [3] [4]
1. Vehicle speed.
2. Condition of the road surface.
3. Condition of tyre treads.
4. Coefficient of friction between the tyre tread and
the road surface.
5. Coefficient of friction between the brake drum and the brake lining (between the disc and the friction pad in case of disc brakes).
1.12.2. The capacity of a brake depends upon the following factors: [1] [2] [3]
1. The unit pressure between the braking surfaces.
2. The coefficient of friction between the braking
surfaces.
3. The peripheral velocity of the brake drum.
4. The projected area of the friction surfaces.
5. The ability of the brake to dissipate heat equivalent to the energy being absorbed.
1.12.3. Brake Effectiveness: [1]
The following factors contribute to the
effectiveness of the brakes
1. Amount of pressure applied to the shoe brakes.
2.
Area of brake linings.
3.
Radius of brake drum.
4.
Radius of car wheel.
5.
Coefficient of friction of braking surfaces.
6. Coefficient of friction between tyre and road surface.
Reference:
1) Automobile Engineering Vol-1 by Kripal Singh_Standard Publishers and Distributors Pvt Ltd (India)_ Brakes-I, Brakes-II.
2) Report on the Second International Technical Conference on Experimental Safety Vehicles: Held at the Stadthalle, Sindelfingen, Germany, October 26-29, 1971_Section 4 Part 5_A Procedure For Evaluating Braking Perfromance_By Mr. Francies A. DiLeronzo, ESV Programe Office_pp:4-48 to 4.52.
3) A Text Book of Automobile Engineering_ Braking System_ Stopping Distance & Braking Efficiency by S K Gupta_S Chand And Company Ltd (India)_pp:136.
4) A Text Book on Automobile Chassis and Body Engineering_(A text book for +2 Vocational and Diploma Students of Mechanical Engineering)_Author Sri. N.R. Hema Kumar_Chapter 3_Stopping distance and Braking efficiency_pp:19-20.
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