Brake efficiency and stopping distance

Braking efficiency : 

The maximum retarding force applied by the brake at the wheels F depends on the coefficient of friction between the road and the tyre surface µ. The weight of the vehicle on the wheel is W. 

F = µ F 

If the coefficient of friction achieved unity, the total retarding force produced at the wheels is equal to the vehicle weight itself, which is equal to the gravitational experienced by the falling body of the mass equal to that of the vehicle. 

If this is the case, the vehicle experiences a deceleration equal to the acceleration due to gravity g and the brakes are said to be 100 % efficient. Thus theoretical limit for brake efficiency is 100%.

In actual practice, the efficiency of 100% is rarely used for ordinary vehicles requirements like the safety of passengers in public vehicles. 

The brake efficiency usually varies from 50 to 80 % which enable the vehicle to stop within a reasonable distance. 

Stopping Distance : 

Approximate stopping distance at different vehicle velocities for various conditions of brakes are different. However, the minimum allowable limit of brake efficiency for any vehicle is 50 % for foot brakes and 30% for hand brakes. 

However, during emergency braking, the reaction of the driver and response time of the brakes also an important part. The stopping distance in case of emergency braking may be divided into 3 types : 

• Distance traversed during the reaction time of the driver. 
• Distance traversed during the time elapsed between the driver pressing the brake pedal and the brake being actually applied at the wheels. 
• Net stopping distance, depending upon the deceleration. 

Stopping distance mainly depends upon following factors :

• Vehicle speed
• Condition of tyre
• Condition of the road surface
• Coefficient of friction between tyre and road 
• Coefficient of friction between the brake drum and the brake lining 
• Braking force applied by the driver