Analysing the vehicle braking system

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The braking system is one of the most essential functions to an automobile. Having been developed and innovated over the last century, it has become extremely dependable and efficient. Some of the vital features, which work together to ensure reliability and safety, include the master cylinder, disc rotors, and calipers.

Since 1967, DOT regulation, department of transportation decided that all vehicles must have a dual master cylinder rather than a single master cylinder, which would guarantee one-hundred percent failure. T­o increase safety, most modern car brake system are broken into two circuits with two wheels on each circuit. In most vehicles these circuits are located on one front wheel and one rear wheel although some vehicles have a split system which separates front and rear wheels opposing-side diagonally. With a dual system like this, if a brake fluid leak occurs in one circuit, the driver only loses fifty percent of their braking ability. With half the braking ability remaining, the driver would still be able to stop although it would take somewhat longer.

Housed in the body of the engine compartment bulkhead, and mounted on the power brake booster unit, is one of the most important components of a vehicle's braking system. The master cylinder is a hydraulic pump designed to supply hydraulic fluid under pressure to each of the wheels. From the driver's point of view the master cylinder is located directly in the front of the driver's seat inside the firewall. This control device converts physical pressure from a driver's foot into hydraulic pressure to operate other devices in the hydraulic system. As the driver applies the brakes in a vehicle, the master cylinder forces the brake fluid through the brake lines creating pressure through the calipers. The master cylinder has two separate pistons located in the same cylinder, supplying braking force through two circuits in order to ensure more reliability. There is a combination valve that warns the driver if there is a problem such as a leak with the brake system as well as alerting the driver about other things to take note of. When the driver presses the brake pedal, it pushes on the primary piston through a linkage. Pressure builds in the cylinder and lines as the brake pedal is depressed further. The pressure between the primary and secondary piston forces the secondary piston to compress the fluid in its circuit. If the brakes are operating properly, the pressure will be the same in both circuits. If there is a leak in one of the circuits, that circuit will not be able to maintain pressure. When the first circuit leaks, the pressure between the primary and secondary cylinders is lost. This causes the primary cylinder to contact the secondary cylinder. Now the master cylinder behaves as if it has only one piston. The second circuit will function normally, but the driver will have to press the pedal further to activate it. Since only two wheels have pressure, the braking power will be severely reduced.

There are many signs in detecting whether or not a master cylinder is faulty. Some of the symptoms include: jerky and creaking movements of the brake pedals when stopping, soft or spongy brake pedals, and brake pedals that won't hold pressure at a stop and sink slowly to the floor. Any time a part of a braking system begins to fail, it is a major safety concern. A mechanic will not normally observe the brake fluid leaking out of the master cylinder externally or from the wheel areas when the pedal softness occurs. Therefore, the fluid level alone isn't an indication of a good or bad brake master cylinder, and the fluid will usually not be low or in need of topping off. This means that the brake fluid is leaking past the internal O-rings, which is an obvious external sign that the master cylinder is bad; it rarely malfunctions.

An external brake fluid leak may be possible and would be visible as the mechanic begins to perform the usual under-hood inspection. The master cylinder usually has a plastic reservoir that holds the brake fluid and is mounted to the metal part of the master cylinder by rubber grommets. The grommets allow for some movement caused by brake pedal and fluid pressures. They may leak fluid which can be seen by the naked eye. Three things will cause the master cylinder to leak--old age, neglecting fluid changes, and using the pedal-pump method of fluid bleeding in a car that has not had the fluid changed regularly. Old age is simply caused by the fact that moisture gets into the seals, corroding them to the point that they are unable to hug the cylinder walls tightly. Neglected fluid changes will dramatically shorten seal life, since the fluid absorbs water. The fluid in the reservoir drops as the brake pads wear, and is displaced by air that seeps in from the outside through the reservoir cap. Air carries moisture, and water is heavier than brake fluid and sinks to the bottom of the master cylinder bore. Water blackens aluminum with a skunk-shaped corrosion along the bottom of the bore. This wear is rough and eventually tears the seals. For these reasons it's best to replace the master cylinder which, on the other hand, can be rebuilt. This action would be time consuming, however, because if a master cylinder has a problem it's usually internally leaking from the seal or cup. Replacing these seals and cups will bring the master cylinder back to life. Most mechanics will automatically replace the master cylinder with a new one.

When replacing the master cylinder, the person who's working on the vehicle will locate the brake fluid reservoir. Using a large syringe or equivalent, he or she will siphon the brake fluid from the master cylinder reservoir, disposing it properly. The next step is to remove the electronic fluid level sensor just below the fluid reservoir by disconnecting and releasing the safety clip. Then, one would disconnect all break fluid lines from the master cylinder, and then loosen the bolts in a counter clockwise motion with a line wrench. In this case some break fluid will leak out when the lines come off, but it's important to keep in mind that brake fluid will ruin the paint of the vehicle. To loosen the master mounting bolts, one would need to use a socket wrench. When both bolts come off, the master cylinder would be removed from the car. After the removing procedure is done, one would bench bleed the new master cylinder before installing it.  Next would be the attachment of a pair of master cylinder bleeder tubes to the outlet ports of the master cylinder. If the vehicle is equipped with a manual transmission, for example, the person would plug the fitting on the reservoir for the clutch master cylinder hose. The next step would be to fill the reservoir with brake fluid of the recommended type, slowly pushing the master cylinder with a screw driver repeatedly until air is out of the brake line. Then, one would place the new master cylinder in position and the installation would be the opposite by tightening all the mounting bolts to its torque. The brake fluid lines would be reconnected to the electronic fluid level sensor. The final step would be bleeding the master cylinder and the brake system until everything works properly.

 

Most modern vehicles are equipped with disc brakes on the front wheels, while others have disc brakes on all four wheels. Some of the different types of disc brake systems include: the single piston and floating caliper, which is the most commonly used today. Brake disc rotors are the circular, flat disk-like, plates which cause friction that converts the car's kinetic energy to thermal energy.  The design of the disc varies somewhat; some are simply solid cast-iron while others are hollowed out with fins or vanes joining together the disc's two contact surfaces. These contact surfaces are usually included as part of a casting process. This ventilated disc design helps to dissipate the generated heat and is commonly used on the more heavily loaded front discs. Disc brakes are commonly manufactured out of a gray material iron because the SAE maintains a specification for most cars and light trucks, which is J431 G3000; superseded to G10. Today, disc brakes come with other options such as anti-lock systems, and two different types of rotors and designs. One type of rotor is a solid rotor, which is simply easy to manufacture since it's a solid piece of metal with a friction surface on each side. The second type of rotor is a ventilated rotor and consists of cooling fans, which increases the cooling area of the rotor. Two types of designs are fixed or floating rotors, which prevent the tires from locking up. Fixed rotors are commonly known as a one piece and floating rotors cast as a separate part and fastened to the hub.

 

A disc brake for a vehicle, within the brake rotor, extends from the hub end of an axle inboard. This creates sufficient distance to place the friction surface and the brake caliper outside the envelope of a wheel mounted at the end of the axle. The placement of the rotor friction surface and the caliper outside the envelope of the wheel enhances brake cooling, which makes the brake more readily accessible for inspection and maintenance without significant vehicle disassembly. It also permits the rotor's outer diameter to be extended beyond the diameter of the wheel rim. This lowers brake component stresses and permits the use of a lighter, lower cost of brake components.

 

A moving car has a certain amount of kinetic energy. The vehicle's brakes have to remove this energy from the car in order to stop it. Each time the driver stops their vehicle; their brakes convert the kinetic energy to heat, generated by the friction between the pads and the disc. Most car disc brakes are vented and are able to recover quickly from immersion; wet brakes are less effective. Unlike a drum brake, the disc brake has no self servo effect and the braking force is always proportional to the pressure placed on the braking pedal or lever.

 

Disc brakes operate on a simple, but effective principle developed over a century ago. Improvements on the basic concept have increased safety and efficiency in many ways. There are three basic components making up the disc brake's rotor, the hub, the calipers, and the brake pads. The hub contains the wheel bearings and is the part where the wheel is mounted. The mechanical energy from the brake pedal, a hydraulic fluid, is passed into the calipers--a specific type of oil that most vehicles require in order for the brake system to work properly within a set of lines. These lines are the brakes lines that connect to the piston within the calipers. When hydraulic pressure is applied to the caliper, the brake pads, which are made of a softer material than the rotor in order to keep it free from surface damage, are squeezed against both sides of the rotor producing friction and heat. Friction and heat are created in order to slow the rotation of the rotor, and therefore the wheels--bringing the vehicle to a safe, controlled stop.

Drivers hardly notice when the brake disc rotors are functioning properly. If the steering wheel vibrates when the brakes are applied, this may be a sign of a disc problem. As such, excessive heat is an issue as well; it reduces the auto part's ability to grip the pad and may result in a reduced braking ability. The first thing a mechanic would do in this situation is start by loosing the wheel lug nuts; raise the vehicle. The next action would be to remove the caliper bolts and bracket bolts; suspend the caliper out of the way with a piece of wire because hanging the caliper from its brake hose might damage it. Before loosening the bolts from the caliper bracket, the disc should be marked in relation to the hub so it can be installed in its original position on the hub. Next, one must visually inspect the surface for score marks, cracks and other damages around the disc rotor. Check the disc run-out with a dial indicator at a point about half an inch from the outer edge of the disc. If an indicator reading exceeds 0.003 inch this could cause pulsation upon the brake application and will require disc refinishing or replacement. In most cases a disc must not be machined to a thickness less than the specified minimum refinish thickness and that's when a micrometer comes in handy. The rotor may be measured about half an inch from the edge. Rotors should always be resurfaced when new pads are installed but refinishing is not required every time the shoes pads are replaced. When shoe pads are replaced, the rotor only needs to be resurfaced if they are warped or badly scored. In fact, refinishing the rotors more often than is necessary will reduce their life. If worn beyond safe limits, cracked, or severely warped, the rotor must be replaced. Sometimes, deep scores get worn into brake rotors due to a worn-out brake pad that may have been left on the car for too long. Brake rotors can also warp, losing their flatness. If this happens, the brakes may shudder or vibrate when the driver stops.

 

Now days, high performance disc brake rotors can disperse heat caused by friction and are a little larger in size. On the road, drilled or slotted discs still have a positive effect in wet conditions, because the holes, or slots, prevent a film of water building up time between the disc and the pads. Their downfall is that in the long run cross-drilled discs will eventually crack at the holes due to metal fatigue. This is largely due to poor manufacturing. Also, being subjected to high stresses will cause cracking much sooner and more severely. There are 120 types of high performance discs rotors and most of them are made from aluminum.

 

Since the master cylinder is the heart of the brake system which plays a major role in a vehicle. As drivers apply the brakes, the master cylinder forces the brake fluid through the brake lines creating a hydraulic pressure at the calipers. Brake calipers are essential to a vehicles' ability to stop and are arguably one of the most important automobile brake parts also. Today most cars have disc brakes in the front wheels, and drum brakes in the rear. But now a lot of cars and trucks consist of 4 disc brakes, two in the front, and in the rear.  In a disc-braking system the vehicle's wheels are attached to metal disc rotors that spin along with the wheels.

           

A disc brake caliper includes inboard side housing and an actuation element movable relative to the inboard side housing for direct application of an inboard side brake pad. It also includes a bridge extending from the inboard side housing to an outboard side for direct application of an outboard side brake pad. In other words, the brake calipers are usually in the form of a U-shaped metal bracket that fits over the rotor like a clamp holding brake pads, which is a pair of metal plates bonded with friction material, inside each side. The outboard brake pads are usually on the outside of the rotors and the inboard brake pads are on the inside. The disc rotor is located between the calipers and the brake pads as a hydraulic cylinder presses toward the metal brackets and squeezes the rotor in doing so.

 

            In a disc braking system the purpose of the caliper is pretty much to slow the car's wheels by creating friction with the rotor. When a driver applies the brakes, the hydraulic fluid pushes into one or more pistons, in two directions. The pressure pushes on one side of the rotor but also pushes the calipers the other way against the outer pad. This in turn applies pressure on the opposite side of the rotor as well. Since the brake pads have high-friction surfaces and serve to slow the rotor down or even bring it to a complete stop. When the rotor slows or stops, so does the wheel, because they're attached to one another.

There are two main types of calipers in a vehicle--floating calipers and fixed calipers. Floating calipers are unattached, and as the name suggests, float around within its track in order to center itself on the rotor. In most cars, floating calipers are single pistons, though some may be found in expensive vehicles with two pistons on the same side for added braking amenities. Fixed calipers normally consist of four pistons connected to the system's support. Since they are fixed, they stay firmly in place and do not move. Among the four pistons, two are located on each side of the support, pressing the pads against the rotor. Out of the various types of calipers, the most common ones are single piston floating calipers. Four piston fixed calipers are more expensive and usually found in high performance luxury cars.

It is uncommon that one would have a problem that entails changing or replacing only the calipers in a vehicle. Usually, the dominant underlying problem rests in a different part of the braking system, which the calipers happen to be a part of. When changing or replacing brakes, sometimes redoing the calipers is necessary though. One of the problem signs with calipers includes slicking, which often reveals it self in harsh weather conditions when the roads are slippery.  A sticking caliper may be dangerous causing the wheels to slide when a driver tries to brake. If replacement is indicated usually because of fluid leakage, it's recommended that the calipers be replaced, not overhauled. It may be rebuilt but it would take time. When repairing a caliper a technician would loosen the wheel lug nuts; raise the vehicle. Then the person would start by checking for any leaks, and corrosion. The next step would be to use hose clamping pliers, and plug the brake hose with caps to keep it from contaminating the brake system and prevent loss of any more brake fluid than is necessary. One would then remove the inlet-fitting bolt and disconnect the brake hose from the caliper: Discard the sealing washers from each side of the hose fitting. If a contamination occurs the first step would be to evacuate all the brake fluid from the system with a vacuum bleeder tool. If the person is working alone and don't have a vacuum bleeder then a helper would come in handy as he or she would open the bleeders on each wheel and have catch basins available to catch the fluid as it runs out. Then the helper would help pump the pedal until the procedure would be finished. After the bleeding is complete, one would inspect the bushings for corrosion cuts and nicks. If any brushings happen to be damage, they would have to be replaced with new ones. When the brake hose is disconnected from the caliper, the caliper can be removed from the car. Installing a replacement caliper is the opposite of removal: connect the brake hose line to the caliper, then mount the caliper and secure it with its two mounting bolts, and torque them to the factory spec. One would install new brake pads in the caliper, remount the wheel and secure it with the lug nuts. When changing calipers, it is important to note that one would need to keep the depression of brake paddle at an even level in order for the pads to be able to brush across the disc. Finally, when the new calipers are installed, one would have to fill the system with brake fluid and bleed it to get any trapped air bubbles out. Again, a vacuum bleeder is the best tool for the job, although manually pumping the pedal will also purge the air from the fluid. When the fluid runs clear without any bubbles, tighten the bleeder valves should close them. For bleeding the air from the system, a person would work on one wheel at a time, with the other 3 bleeders closed.

There are certain limitations to just how much the vehicle's brakes and its calipers can do to stop a vehicle; while they may be able to bring the wheels to a stop, it's up to the gripping power of the tires to do the rest, and improved brake parts can't help beyond a certain point. However, there are several ways in which brake calipers have been improved. Some common features found in performance brake calipers include: Bigger pistons, more pistons, less heat retention and differential bore calipers. The larger the pistons are, the greater the area over which they come in contact with the brake pads, and the more clamping force they have on the rotor. Low-end floating brake calipers have a single piston, on the inboard side. Low-end fixed calipers have a single pair of pistons, flanking the rotor disc. High-performance calipers can have multiple pins or pairs of pins, mounted on opposing sides of the rotor. Six-piston models are increasingly common and even 12-piston models are not unheard of. Increasing the number of pistons also serves to increase the clamping force of the caliper. In a sense, brakes can be thought of as a device for converting movement into heat. As the vehicle slows down, all of that kinetic energy has to go somewhere and most of it ends up as heat. As the surface of the rotor heats up, the clamping force of the pistons has to be increased to avoid brake fade. If the caliper has multiple pistons or multiple pairs of pistons, the brake rotor surface is initially heated by the pistons pushing against the brake pad at the leading edge of the caliper, making the rotor surface hotter when it rotates back to the pistons closer to the trailing edge of the caliper. Therefore it helps if the pistons closer to the rear edge of the caliper are larger. Differential-bore calipers use smaller pistons up front, larger pistons toward the back. All of these technologies can increase the braking power provided by a caliper. For smaller cars that typically aren't driven at high speeds, this extra braking power isn't really necessary. However, the faster and more powerful a vehicle is, the more it will benefit from high-performance calipers.

            In conclusion, brakes are designed to slow down a vehicle and will always tend to do so in a simple matter. Since the master cylinder is the heart of the brake system, it plays a major role in a vehicle. As drivers apply the brakes in a vehicle the master cylinder forces the brake fluid through the brake lines creating pressure to the calipers where two pads sit and create friction at the disc rotors. Finally, most people may want all the horsepower in the world but when it comes to speed good brakes can really come in handy in order for the vehicle to stop.

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