From The Road Up – Braking
It must be fairly obvious what the braking system is for; it allows the vehicle to be slowed in varying degrees and also provides some method of holding a stationary vehicle without having to continually apply the service (foot) brake.
The braking system on any motor vehicle, whether it be the smallest glass fibre two seater city car or the fully laden heavy haulage tractor unit and trailer relies on the basic principle of friction. It must be mentioned at this point that the majority of heavy commercial vehicles and buses/coaches have supplementary braking systems which do not use friction, instead they have various devices/systems which have a direct effect on the engine or exhaust, alternatively a system utilizing the principles of electro-magnetism mounted into the drivetrain.
There Are 2 Systems On A Typical Braking Set Up
Returning to the average family car, two entirely separate systems are fitted; 1) the service or foot brake system and 2) the park or hand brake system. With the service brake, the brake pedal is connected to the brake master cylinder and when the pedal is pressed hydraulic pressure is applied through the rigid metal pipes and the flexible brake hoses to the brake cylinders or brake calipers mounted at each wheel.
The calipers/cylinders are absolutely no different in their principle or operation to the huge hydraulic rams seen on earthmoving machines or in the tipping gear of trucks it is only their appearance that is so different. Simply, hydraulic pressure exerted within a sealed space with a moveable section at one end (a piston) can be used to move or operate another mechanism, in the case of a brake system, a friction device. All that is needed is the friction device (a pad or shoe) to be pressed against a rotating body (a disc or drum) in turn attached to the wheel assembly, as the exerted pressure is increased so is the clamping effort on the disc or drum thus slowing its rotation.
Sturdy Materials Are Used
Obviously the metal construction of the pad or shoe will need to be faced with a material which not only provides an effective degree of friction and a high resistance to heat, which is an unavoidable by-product of friction, but have excellent wear characteristics without producing premature wear in the disc or drum. The maximum amount of hydraulic pressure generated will depend not only on how hard the driver presses the brake pedal but also the relationship between the diameters of the master cylinder and the cylinders/calipers at each wheel.
One of the most important features of the modern service brake system is the dual circuit. With the old single circuit designs all the hydraulic lines were shared so that if a leak occurred all brake effort was lost. The dual system offers split circuits so that in the event of failure two brakes will work albeit with much greater pedal travel. The split will normally be arranged so that the two remaining brakes will be one at the front and one at the back usually diagonally opposed.
Regulations and Cable Tension
With the advent of the dual circuit system less emphasis is placed on the second brake system fitted to the motor vehicle, the park or hand brake. Now the car has effectively three brake systems, two almost separate hydraulic service and one completely separate mechanical only park. Regulations dictate that the park brake must be purely mechanical in operation, the most common being a centrally mounted lever in the passenger compartment coupled to the rear brake assemblies via a cable arrangement. Pulling on the handbrake tensions the cable(s) pulling levers inside the brake assembly applying the shoe or pad against the disc or drum. As the park brake must be able to be applied and locked in the on position, a ratchet mechanism is incorporated in the handbrake lever so the brake can be set only releasing when required by the driver.