Mind Your Binders
Don't let your brakes break
Many different vehicle systems could be classified as important, but there is only one system that gets used all of the time and if it doesn’t work the way it’s supposed too only one time, serious property damage and injury is guaranteed to result. This system is the braking system. Think about how many times you have applied the brakes of your car as you are barreling off of the freeway heading straight towards another car, a pedestrian, or a concrete wall, and your 2 or 3 ton instrument of death and destruction stops effortlessly. The amount of dynamic inertia contained in the movement of the mass of your vehicle is tremendous but you are so used to the brakes working without even thinking about it, that it’s no big deal.
So what’s happening when you step on that pedal? Brakes are nothing more than energy converters. The vehicle moving through space is nothing more then kinetic energy, energy in motion. In order for the vehicle to stop we cannot simply take away the energy or destroy it, we have to convert it into something else which would be another form of energy. The easiest thing to convert the kinetic energy into is heat. To convert the energy into heat we take advantage of friction between the rotating wheels and the material that makes up the brake pads or shoes. These brake components get hot and then the heat is dissipated into the air surrounding the brakes.
Sometimes when the brakes get overused they loose the ability to convert the kinetic energy in to heat energy. This is called brake fade. When the brakes get too hot, the coefficient of friction between the brake pads and the brake rotors is decreased. If the components of the braking system do not dissipate the heat, then they cannot convert any more energy into heat, if this conversion does not take place then deceleration of the vehicle does not take place, or at least it does not take place as much as it should. This is the reason that big trucks take their time when driving down a steep canyon road. These trucks are so heavy that they have an enormous amount of kinetic energy so they use engine braking to keep their speed in check, because they are very susceptible to brake fade.
So how is the force of your little foot enough force to bring a 6000 lb. land yacht to halt? Several different things are in place that makes this possible. First of all a good old fashioned lever is attached to the brake pedal. When you push on the pedal you are pushing on one end of a lever. On the other end of the lever there is a mechanism called a brake booster. The brake booster uses vacuum force from the engine, or hydraulic power from the power steering pump to help the brake pedal lever exert force on a hydraulic piston inside of the master cylinder. The master cylinder is where we find the brake fluid that flows through a closed system to each one of the wheels.
An old dead scientist by the name of Blaise Pascal discovered that mechanical advantage could be gained when a small diameter piston pushes fluid through a closed circuit and pushes against a larger piston. This means that the small piston in the master cylinder can move a few inches and the pistons at the brake calipers increase that force greatly as they push against the pads. This concept is similar to the way torque is multiplied when a small gear turns a larger gear. The large gear turns with more force but it turns slowly. In the case of a hydraulic circuit, the small piston moves a longer distance than the large piston, but the large piston exerts tremendous force.
|A brake master cylinder attached to a brake booster|
On the other side of the large piston are the brake pads. The brake pads are pressed into the discs or rotors at the center of the wheels. In the case of drum brakes the piston or actuator that is located at the wheel is much smaller then the piston used for disc brake assemblies. The moveable friction units that are used with drum brakes are called brake shoes. When the shoes are acted upon by the hydraulic piston, or wheel cylinder as its called, they expand outward, away from each other and press with great force into the inside of the brake drum. So in the case of disc brakes the pads grab the rotors, and in the case of drum brakes the shoes push outward against the inside of the drums. This is so simple and so reliable that people never think about this action whenever they mash that wide pedal on the left to throw on the skids.
In order to meet safety standards all cars and trucks must have disc brakes in the front. Disc brakes aren’t necessarily more powerful than drum brakes but they are much more resistant to fading, and the application is more consistently smooth. Since the front brakes do the majority of the heat conversion that brings the vehicle to a stop, it is important that they be up to the task. Many vehicles will have disc brakes on the rear axle as well. These disc brakes will always have smaller pads and rotors than the disc brakes that are on the front of the vehicle. Drum brakes are not as common as they used to be but they are still common enough. Drum brakes are only found on the rear axle of cars and trucks and are usually used on lower end vehicles. These would be cars and trucks that are more entry level or less expensive. Usually disc brakes on the rear of a vehicle would be
considered an upgrade.
Brakes don’t require a tremendous amount of maintenance, but brake pads and rotors are perhaps the most common wear and tear items anywhere on the vehicle. Front brake pads can wear out under normal usage in as quickly as 25,000 miles. Rear brake pads or shoes will usually last 2 or 3 times as long as the front brake pads, because the rear brakes provide less of the stopping force. The speed at which the brakes will wear out depends on the habits of the driver and the conditions under which the vehicle is driven. If the driver lives at the top of a canyon and drives down the canyon everyday to go to work, they are probably going to have to replace the pads on a more frequent basis.
Replacing break pads and shoes is very simple and doesn’t require much in the way of special tools. When selecting new pads at the parts store many different options exist. These options are based on the composition of the friction material used on the pads. The three most common materials can be classified as semi-metallic, organic, or ceramic. Each one of these materials has different qualities that affect performance. Typically the ceramic pads are the best all around brake pad, but of course that means that they are also the most expensive.
Should the brake rotors be machined or replaced every time the pads are replaced? The answer to that is technically no. If the rotors are not machined to remove any warpage, or replaced with new ones when the pads are replaced, the brakes will still work fine. The problem is that many times the pads will not break in properly and they may begin to vibrate in the calipers when the brakes are applied. This will lead to a squealing noise. The other thing that may happen is the rotors may have a tendency to become warped. Warpage usually occurs when the brakes are overheated, and when this happens is causes a vibration in the pedal and a shimmy in the steering wheel when the brakes are applied. Mounting the rotors on a lathe and cutting them so they turn true, used to be the most common way to service the rotors. Most shops it seems now just recommend replacing the rotors. This is not a bad policy because it more often results in trouble free brakes on the customer’s car in the future. Sometime the cost of machining the rotors and cleaning them up is the same as the cost of replacement rotors.
A few other things related to the brake system that require periodic maintenance are things such as brake fluid, brake shoe adjustment, and parking brake adjustment. Brake fluid typically should be replaced about every three years. On 99.9% percent of the vehicles out there the brake fluid has probably never been replaced because it is one of the most forgotten and neglected fluids on the automobile. The reason that the service of brake fluid depends more on time rather than miles is because one of the jobs of the brake fluid is to absorb moisture. This will occur whether the vehicle is driven or not. If the brake fluid becomes saturated with moisture it causes the boiling point of the fluid to decrease substantially. If the boiling point gets low enough it will boil when the brakes get hot and the vapors in the brake lines will compress rather than transmit force. This will cause failure of the brakes. Moisture is also corrosive to the brake components, and corrosion can also lead to brake failure.
|A blown out caliper piston caused by brake pads wearing out and grinding metal on metal.|
A few adjustments still remain as far as the braking system is concerned. Even though drum brake assemblies have been fitted with automatic self adjusters for the last 30 years or so, they still benefit from routine adjustment. The automatic adjusters just keep the shoes somewhat adjusted. The shoes in the drum brake assembly need to ride very close to the drums to ensure proper application. If they are not adjusted then the drum brakes will not take their share of the stopping duties. This can lead to premature wear of the front brakes or even brake fade. The parking brake can still be adjusted on all vehicles as well. This may be a matter of adjusting the parking brake shoes (these are part of a separate braking mechanism at the rear wheels of some cars), or adjusting the cable. If you apply the parking brake and it will not hold the vehicle, or even if the lever has to move to far in order to apply the brake, it might be out of adjustment.
One more thing to look for is the brake warning light. This is not the ABS light, which is something different. The brake warning light comes on for one of three reasons. The parking brake is left on, the brake fluid in the master cylinder reservoir is low, or there is a failure in the hydraulic system. If the light comes on and you have a hydraulic failure, the pedal will feel very spongy and may sink to the floor. This is very bad, get your brakes looked at right away. If you have left the parking brake engaged than you can easily remedy that situation. If the light is on, and the previous two scenarios don’t seem to fit, then it is likely your brake fluid is low. Do not top it off. Brake fluid does not get consumed or run out. If the fluid is low it is likely that your brake pads are worn out and require replacement. As the pads wear thin, the pistons in the brake calipers move out from the housing. The space behind the pistons fills with fluid. This fluid comes from the reservoir, which causes the level in the reservoir to go down.
Pay attention to your brakes. If they don’t feel right they aren’t right. If you hear grinding, or any other noise that is abnormal, don’t delay, get your brakes checked. Many people wait way too long to check their brakes. Some minor squealing and chirping is normal for disc brakes, but if you are not sure then it’s best to have them checked. The best thing that results from such neglect is a much higher repair bill because worn out parts damage other parts. The worst thing that results is serious damage to property and person. No excuses are valid for neglecting your braking system. If they fail you not only hurt yourself but you could hurt others.