With approximately 8.8 million registered motor vehicles in Australia as at 31 January 2017 the operating systems used to optimise user safety and the safety of other road users is becoming increasingly more sophisticated and necessary. One of these systems, the anti-Lock braking system (ABS), allows a car to stop in a shorter distance and maintain more control by applying the brakes in repeated bursts to maintain friction between the tire and the road. ABS is now prevalent in many motor vehicles, specifically the car.
In cars without ABS, to avoid collisions the break is applied onto the brake pads for a continuous period of time causing the wheels to stop rotating. When the wheels of a vehicle lock, they are no longer rotating along the road, loose traction and instead begin to slide, causing a decrease in friction between the wheels and road. Friction between two objects is a force that opposes movement of one object over the other. Under these circumstances, friction opposes the movement of the vehicle’s wheels along the road. Therefore, friction decreasing when the vehicle begins to slide means less force is opposing the vehicle’s movement to slow it down, increasing the vehicle’s stopping distance. The driver also becomes unable to steer as the wheels do not have enough grip on the road to turn, meaning the driver cannot veer to avoid collision (figure 1).
ABS allows vehicles to stop faster whilst still maintain control of the car (the wheel’s do not lock). The driver is still able to steer by using two braking techniques; threshold and cadence braking. ABS is a system that utilises two different breaking techniques in order to optimise friction between the road and the tire thus increasing control allowing a shorter stopping distance. Threshold braking is when the driver applies the right pressure on the brake pedal for maximum braking force, which is just before the wheels lock (Ziersch, 2010). Cadence braking is when the driver applies the breaks repeatedly for a short period of time to ensure the wheels do not lock. This sophisticated system is activated without the users need to implement these techniques. When ABS detects that the wheels are about to lock, it automatically performs cadence braking, applying and releasing the brakes 15 times per second. The pumping of the brakes occurs so fast that the wheel’s speed hardly has time to change and therefore keeps the wheels at the point right before they lock (threshold braking), to achieve maximum braking force. This maintains the existing static friction between the wheels and road, when the wheels are trying to move (rolling) and prevents the changing to kinetic friction, when the wheels are already moving (sliding). Due to ABS doing this automatically, the driver need only press the brake pedal and continue to steer for the shortest stopping distance to be achieved, as static friction between the wheels and road is maintained due to the wheels not locking.
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As the break is applied a hydraulic disc brake system inside the vehicle transmits this pressure to the brake pads in each wheel in order to slow or stop the vehicle. By using an incompressible fluid such as breaking oil the pressure is transmitted through the system by pistons which are short cylinders in a tube moving against a fluid (figure 2). This system is a result of foundation work done by Pascal. Pascal’s principle states that “Pressure applied at any point of a fluid at rest is transmitted without loss to all other parts of the fluid.”
The following equations outlines the change in force from when pressure is applied by the first piston to when it acts on the second piston.
Pascal’s principle states that pressure will not change and therefore area is the measurement that affects force. This equation shows that if area decreases the force applied must decrease in order for pressure to remain the same. Therefore, when the force is transmitted through the fluid from the master to the slave cylinders, area decreases, meaning force applied to the pistons in the slave cylinders is smaller than the force applied by the piston in the master cylinder. However, due to the decrease in both area and force, pressure remains the same, upholding Pascal’s principle.
ABS is a hydraulic systems containing speed sensors, an ABS control module, pressure release valves and a pump, which are all utilised to stop the wheels from locking. When on the verge of locking a single is sent via the speed sensors, upon receiving said signal the control module will then stop the wheel from locking opening the valve on that brake line to decrease pressure on the wheel until the wheel starts to slightly accelerate. This process is repeated until the cars stops or the desired breaking has been achieved by the driver.
ABS maintains static friction between the wheels and road by preventing the wheels from locking. When the wheels lock, static friction changes to kinetic as the wheels slide along the road. It takes more force to try and get an object to move before it reaches the threshold and starts to move then static friction turns to kinetic and less force is exerted, meaning kinetic friction is weaker than static (see figure 3). ABS utilises the stronger friction. Therefore, maintaining static friction reduces stopping distance by having a larger force opposing the vehicle’s movement. To find the force of friction the following equation is used:
The vehicle’s normal force does not change and therefore when the coefficient is larger, the frictional force is also larger. Due to static friction being stronger than kinetic, static friction has a higher coefficient and therefore higher coefficients give shorter stopping distances. The coefficient of static and kinetic friction on dry roads being 1 and 0.8 respectively (Boal, 2001) further proves that maintaining the existing static friction when attempting sudden stops is more efficient than changing to kinetic friction.
As the vehicles normal force remains constant the impact of a smaller coefficient means that there is less frictional force. Data provided by Ben Townsend shows that in many circumstances the coefficient of static friction is higher than that of kinetic friction (table 1).
For example let us assume that we have a car with ABS and a total mass of 1400kg travelling at 60km/h on a road needs to stop suddenly to avoid hitting a child.
(see attached working)
It is clear that in all circumstances ABS is beneficial by allowing the car to stop in a shorter distance when compared to no ABS. ABS decreases stopping distance minimising the impact of collisions and preventing most accidents.
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(Australian Bureau of Statistic , 2017)
(Nice, 2000)(Maggie, n.d.)
(Ziersch, 2010) (Nice, 2000) (Maggie, n.d.)
(Static & Kinetic Friction, 2013)
(Nave, n.d.) (Brain, 2000)
(Static & Kinetic Friction, 2013)