Thursday, 27 July 2017

Mind Your Binders

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Many different vehicle systems could be classified as important, but there is only one system that must work perfectly every time or else serious property damage and injury is likely 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 two or three ton instrument of death and destruction stops effortlessly. The amount of dynamic inertia contained in the movement of the vehicle’s mass is tremendous, but you are so used to the brakes easily burning off this energy, that it’s no big deal.

The braking system on the average vehicle is more powerful than the engine on that same vehicle, but we never talk about this. Consider a normal car with 200 horsepower that takes 10 seconds to accelerate to 60 MPH. This is a fairly normal thing for a normal car by today’s standards. Now, imagine if that car took 10 seconds to stop from 60 miles per hour. That would be terrible performance.

For a car to overcome static inertia (the tendency of the car’s mass at rest, to want to remain at rest) and achieve a given speed, in a given amount of time, a given amount of horsepower is required. In order for the same car to overcome dynamic inertia (the tendency of the car’s mass in motion to want to remain in motion) present at that same given speed, it needs almost as much power in the brakes as it has in the engine. the issue here is that 10 seconds is way too long for bringing a car to a stop from 60 MPH. taking that long would be dangerous. In order to make the car stop much faster the brakes must use force that is much greater than 200 HP the engine has, in order to get the car to stop in 4 seconds instead of 10. Brakes are very powerful.

So what’s happening when you step on that pedal? For starters, brakes are nothing more than energy converters. The vehicle moving through space contains kinetic energy (that dynamic inertia thing), or 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 another form of energy. The easiest thing to convert the kinetic energy into is heat.

brakes-you-mean-coward-pedal-580x606

To convert the energy into heat, we take advantage of friction between the rotating wheels and the material that makes up the brake pads. When brake pads squeeze brake rotors, the friction turns the vehicle motion into heat. The friction between the tire and the road surface will also increase dramatically and the vehicle slows and stops. These brake components get hot and the heat is dissipated into the air surrounding the brakes.

Sometimes when the brakes get overused they lose the ability to convert the kinetic energy into 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 fast enough, then they cannot convert any more kinetic energy, if this conversion does not take place then deceleration of the vehicle does not take place. The driver freaks out, pumps the brakes to no avail, and goes flying off the road into a tree. Or maybe they just gear down and take it easy on the brake pedal for a minute or two until they cool down.

So how is the force of your little foot enough force to bring a 6000 lb. land yacht to a 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. A lever will multiply mechanical force. This happens by trading distance of travel, for force of travel.

On the other end of the lever there is a mechanism called a brake booster. Most brake boosters use vacuum from the engine, along with atmospheric pressure, to help the brake pedal lever exert more force on the hydraulic pistons inside of the master cylinder. When force from your foot is applied via the brake pedal lever to the brake booster, a valve opens in the booster that allows engine vacuum to pull on a large diaphragm. This creates a negative pressure on one side of the diaphragm which allows atmospheric pressure to push on the other side of the diaphragm.

brakes booster and MC

The brake pedal lever, the brake booster, and the brake master cylinder all mounted in series through the firewall of your car.

The amount of boost will depend on a few things, the amount of atmospheric pressure, the total surface area of the diaphragm, and how much vacuum your engine produces. A diaphragm that is 12 inches in diameter will have a surface area of about 113 inches. If we multiply the surface area by the atmospheric pressure pushing on it, we can see that at sea level we get approximately 1661 pounds of extra force helping us apply the brakes. in most cars the force will not actually be that high because there are all sorts of variables that we are not considering. However, it is because of the brake booster that we are able to use a light touch of our foot on the brake pedal to drastically affect the force applied to the braking system. In another article maybe we will address a much less common type of booster that uses hydraulic pressure, but we don’t need to get into that here.

booster vacuum

A common brake booster

The master cylinder is where we find the brake fluid that flows through a closed hydraulic system to each one of the wheels. This unit is acted upon by the brake booster. The master cylinder contains two pistons that build pressure to transmit force. Each piston sends pressure to the actuators at two wheels. The pistons and their systems are independent so that if you lose hydraulic pressure in one half of the system, you still have pressure in the other half, and you won’t lose all braking ability.

master

Master cylinder

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 against a larger piston. Because of this we can further multiply braking force at the wheels. Once again we trade distance of movement for force of movement. The pistons in the master cylinder are small, and the pistons at the brake calipers are large. This means the small pistons move a greater distance with a small amount of force, and the large pistons at the calipers move and smaller distance with a greater amount of force. This is why you commonly see brake pads that seems to ride against the rotors even when the brakes are not applied.

drum brakes

The mechanisms that make drum brakes work are complicated and heavy.

The actuators at the wheels come in two different flavors. Disc brakes and drum brakes. Both contain movable friction units that act against something that rotates with the wheel. The movable friction units that are used with drum brakes are called brake shoes. Drum brakes are not very common anymore and are only found on some entry level cars, but then only on the rear axle. Drum brakes use two brake shoes inside a rotating drum. When the brakes are applied, the shoes push outward into the drum. This works well enough but drum brakes do not dissipate heat very well, have a relatively small swept area on the drum, and are heavier and more complicated than disc brakes.

HPIM2039

Disc brake assembly with a common floating caliper.

Disc brakes use two brake pads mounted inside a caliper to grab both sides of the rotors when the brakes are applied. This is a very simple mechanism that easily dissipates heat because the entire swept surface of the rotor is exposed to airflow around the wheels. This means that disc brakes have much better fade resistance. Disc brakes became the standard for the front brakes on all cars back in the 70’s because safety standards were tightened to a point where drum brakes couldn’t cut it anymore.

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 consistent. 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. When the brakes are applied, a tremendous amount of weight shifts to the front of the car. This happens as the nose of the car dives, and the center of gravity changes. For this reason the front brakes do most of the work, most of the time.

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. The best result of neglect is a much higher repair bill. The worst result is serious damage to property and person. No excuses are valid for neglecting your braking system. If they fail, you could not only hurt yourself, but you could hurt others.

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A modern brake system might integrate the master cylinder, the brake booster, and the anti-lock braking mechanisms and control units, all into one assembly.

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