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Disk Brake

 

Synopsis

When in the modern technology of human transportation has increased, the mechanical devices used to slow down and stop vehicles has also grown more complex.

The early years of automotive development were an exciting time for the designing engineers, “a period of variation when there was no accepted practice, and essentially all ideas were new ones and deserving manipulation. Quite rapidly, however, the design of many components maintained in concept, and so it was with brakes; the preponderance of vehicles soon adopted drum brakes, each consisting of two shoes which could develop inside a drum.”

In this chaotic period is the first record of the disk brake. Dr F.W. Lanchester patented a design for a disk brake in 1902 in England. It was consolidated into the Lanchester car produced between 1906 through 1914. These new disk brakes were not as effective at stopping as the contemporary drum brakes of that time and were soon forgotten. Another significant development occurred in the 1920’s when drum brakes were used at all four wheels instead of a single break to halt only the back axle and wheels such as on the Ford model T. The disk brake was again utilised during World War II in the landing gear of aircraft. The aircraft disk brake system was adapted for use in automotive applications, first in racing in 1952, then in production automobiles in 1956. United States auto manufacturers did not start to incorporate disk brakes in lower-priced non-high-performance cars until the late 1960’s.

 

Advantages of Disc Brakes over Drum Brakes:

As with almost any artificial of technology, drum brakes and disk brakes both have advantages and disadvantages. Drum brakes still have the edge at a cheaper cost and lower complexity. It is why most cars built today use disk brakes in front, but drum brakes in the back wheels, four-wheel disks being an extra cost option or shouted as a high-performance feature. Since the weight shift of a decelerating car puts most of the load on the front wheels, the usage of disk brakes on only the front wheels is accepted manufacturing practice.

Drum brakes had another advantage compared to new disk brake systems. The geometry of the brake shoes inside the drums can be designed for a mechanical self-boosting action. The rotation of the brake drum will push a leading shoe brake pad into pressing harder against the drum. Early disk brake systems required an outside mechanical brake booster such as a vacuum assist or hydraulic pump to generate the pressure for first friction materials to apply the necessary braking force.

All friction braking technology uses the process of converting the kinetic energy of a vehicle’s forward motion into thermal energy: heat. The enemy of all braking systems is excessive heat. Drums are inferior to disks in dissipating excessive heat:

“The standard automotive drum brake consists primarily of two shoes which may expand against the inner cylindrical surface of a drum.

The more important part of the heat generated when a brake is applied has to pass through the drum to its outer surface to consumed to the atmosphere and at the same time subject to quite severe stresses due to the distortion induced by the opposed shoes acting inside the open-ended drum.

The conventional disk brake, on the other hand, consists mainly of a flat disk on either side of which are friction pads; equal and opposite forces may be applied to these pads to press their working surfaces into contact with the braking path of the disks. The heat produced by the conversion of energy is dissipated directly from the surfaces at which it is generated, and the deflection of the braking path of the disk is minimal so that the stressing of the material is not so severe as with the drum.”

The result of overheated brakes is brake fade the same amount of force on the pedal no longer provides the same amount of stopping power. The high heat decreases the relative coefficient of friction between the friction material and the drum or disk. Drum brakes also suffer another setback when overheating: The inside radii of the drum expand, the brake shoe outside spaces no longer matches, and the actual contact surface may decrease.

Another advantage of disk brakes over drum brakes is that of weight. There are two different areas where minimizing pressure is essential. The first is unsprung weight. This is the total amount of weight of all the moving components of a car between the road and the suspension mounting points on the car’s frame.

Auto designs have gone to such lengths to reduce unsprung weight that some, such as the E-type Jaguar, moved the rear brakes inboard, next to the differential, connected to the drive shafts instead of on the rear wheel hubs. The second “weighty” factor is more of an issue on motorcycles: gyroscopic weight. The heavier the wheel unit, the more gyroscopic resistance to changing direction. Thus the bike’s steering would be a higher effort with more massive drum brakes than with lighter disks. Modern race car disk brakes have hollow internal vents, cross drilling and other weight saving and cooling features.

Most early brake drums and disks were made out of cast iron. Current OEM motorcycle disk brakes are usually stainless steel for corrosion resistance, but after-market racing component brake disks are still made from cast iron for the improved friction qualities. Other exotic materials have been used in racing applications. Carbon fibre composite disks gripped by carbon fibre pads were common in formula one motorcycles and cars in the early 1990’s but were outlawed by the respective racing sanctioning organizations due to sometimes spectacular failure. The carbon/carbon brakes also only worked correctly at the very high temperatures of racing conditions and would not get hot enough to work in street applications.

A recent Ducati concept show bike uses brake disks of selenium, developed by the Russian aerospace industry(3), which claim to have the friction coefficient of cast iron with the light weight of carbon fibre.

Working with Disc Brakes

The calliper is the part that holds the brake shoes on each side of the disk. In the floating-calliper brake, two steel guide pins are threaded into the steering-knuckle adapter. The calliper floats on four rubber bushings which fit on the inner and outer ends of the two guide pins. The bushings allow the calliper to swing in or out slightly when the brakes are applied

When the brakes are implemented, the brake fluid flows to the cylinder in the calliper and pushes the piston out. The piston then forces the shoe against the disk. At the same time, the pressure in the cylinder causes the calliper to pivot inward. This movement brings the other shoe into tight contact with the disk. As a result, the two shoes “pinch” the disc tightly to produce the braking action

 

STAGES OF WORKING

The sliding-calliper disk brake is similar to the floating-calliper disk brake. The difference is that sliding calliper is suspended from rubber bushings on bolts. It permits the calliper to push on the screws when the brakes are applied.

Proper function of the brake depends on

(1) the rotor must be straight and smooth,

(2) the calliper mechanism must be suitably aligned with the rotor,

(3) the pads must be placed correctly,

(4) there must be enough “pad” left, and

(5) the lever mechanism must push the pads tightly against the rotor, with “lever” to spare.

In the modern cars have disc brakes on the front wheels. Other some vehicles have disc brakes on all four wheels. Itis the part of the brake system that does the actual work of stopping the car. The most common type of disc brake on modern vehicles is the single-piston floating calliper. In this article, we will learn all about this kind of brake disc design.

 

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