Braking Efficiency The function of the vehicle brakes is to control the speed of the vehicle on hills, to reduce the speed when required and to stop the vehicle altogether and hold it stationary. How well a set of brakes fulfills this function depends on many factors; one of which – road surface condition- is in no way under the control of the driver; other factors such as tyre condition and gross vehicle weight, are not directly related to the design and condition of the brakes although they are the responsibility of the driver.
The ability of the brakes to perform their function is popularly known as braking efficiency and in most countries, legally enforceable regulations require that all road vehicles have an efficient braking system. In assessing braking efficiency, it is usual to consider the effect which the brakes achieve when they are applied.
The action of applying the brakes sets up a force effective at the road surface, which acts in the opposite direction to the motion of the vehicle and causes it to slow down or decelerate. This deceleration is normally compared to a standard value (the acceleration due to gravity g) and reported as a percentage of “g”. By defining braking efficiency in such a way directly comparable standards of braking can be established of differing classes of vehicle. Dependent on whether metric or imperial units are used, “g” may be 9.81m/sec/sec or 32 ft/sec/sec in absolute terms.
STOPPING distance refers to the distance a vehicle will travel from the point when its brakes are fully applied to when it comes to a complete stop. It is primarily affected by the original speed of the vehicle and the coefficient of friction between the tires and the road surface, and negligibly by the tires’ rolling resistance and vehicle’s air drag. The braking distance is one of two principal components of the total stopping distance. The other component is the reaction distance, which is the product of the speed and the perception-reaction time of the driver/rider. A perception-reaction time of 1.5 seconds, and a coefficient of kinetic friction of 0.7 are standard for the purpose of determining a bare baseline for accident reconstruction and judicial notice; most people can stop slightly sooner under ideal conditions. The total stopping distance is the sum of the perception-reaction distance and the braking distance. A common baseline value of is used in stopping distance
Stopping distance The total stopping distance of a vehicle is made up of 4 components.
- Human Perception Time
- Human Reaction Time
- Vehicle Reaction Time
- Vehicle Braking Capability
The most important point for any driver to remember is that if you double your speed — say from 30mph to 60mph — your braking distance does not become twice as long, it becomes four times as far.