Brakes

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About Brakes

The purpose of a braking system is to slow and stop your vehicle. It is controlled by a foot (service brake) pedal and by a hand or parking brake.

The brakes also allow you to make an emergency stop if you need to.

It is therefore important that you ensure that your braking system is maintained and operating at optimum levels. It is a legal requirement that your braking system performs as intended and your brakes will be tested as part of your MoT.

Like all components, brakes and indeed braking performance deteriorate over time and because it happens gradually, many people don’t notice the difference in their braking system. Having your brakes checked regularly could also save you time and money by preventing damage to other components.

How do brakes work?

We all know that pushing down on the brake pedal slows a car to a stop. But how does this happen? How does your car transmit the force from your leg to its wheels? How does it multiply the force so that it is enough to stop something as big as a car?
When you depress your brake pedal, your car transmits the force from your foot to its brakes through a fluid. Since the actual brakes require a much greater force than you could apply with your leg, your car must also multiply the force of your foot. It does this in two ways:

- Mechanical Advantage (Leverage)
- Hydraulic force multiplication

The brakes transmit the force to the tires using friction, and the tires transmit that force to the road using friction also.

Disc Brakes

A disc brake is a wheel brake which slows rotation of the wheel by the friction caused by pushing brake pads against a brake disc with a set of calipers. The brake disc (or rotor in American English) is usually made of cast iron, but may in some cases be made of composites such as reinforced carbon–carbon or ceramic matrix composites. This is connected to the wheel and/or the axle. To stop the wheel, friction material in the form of brake pads, mounted on a device called a brake caliper, is forced mechanically, hydraulically, pneumatically or electromagnetically against both sides of the disc. Friction causes the disc and attached wheel to slow or stop. Brakes convert motion to heat, and if the brakes get too hot, they become less effective, a phenomenon known as brake fade.

Disc-style brakes development and use began in England in the 1890s. The first caliper-type automobile disc brake was patented by Frederick William Lanchester in his Birmingham, UK factory in 1902 and used successfully on Lanchester cars. Compared to drum brakes, disc brakes offer better stopping performance, because the disc is more readily cooled. As a consequence discs are less prone to brake fade; and disc brakes recover more quickly from immersion (wet brakes are less effective). Most drum brake designs have at least one leading shoe, which gives a servo-effect. By contrast, a disc brake has no self-servo effect and its braking force is always proportional to the pressure placed on the brake pad by the braking system via any brake servo, braking pedal or lever, this tends to give the driver better “feel” to avoid impending lock-up. Drums are also prone to “bell mouthing”, and trap worn lining material within the assembly, both causes of various braking problems.

Drum Brakes

A drum brake is a brake that uses friction caused by a set of shoes or pads that press against a rotating drum-shaped part called a brake drum.The term drum brake usually means a brake in which shoes press on the inner surface of the drum. When shoes press on the outside of the drum, it is usually called a clasp brake. Where the drum is pinched between two shoes, similar to a conventional disc brake, it is sometimes called a pinch drum brake, though such brakes are relatively rare. A related type called a band brake uses a flexible belt or “band” wrapping around the outside of a drum.

Advantages

Drum brakes are used in most heavy duty trucks, some medium and light duty trucks, and few cars, dirt bikes, and ATVs. Drum brakes are often applied to the rear wheels since most of the stopping force is generated by the front brakes of the vehicle and therefore the heat generated in the rear is significantly less. Drum brakes allow simple incorporation of a parking brake.

Drum brakes are also occasionally fitted as the parking (and emergency) brake even when the rear wheels use disc brakes as the main brakes. The vast majority of rear disc braking systems use a parking brake in which the piston in the caliper is actuated by a cam or screw. This compresses the pads against the rotor. However, this type of system becomes much more complicated when the rear disc brakes use fixed, multi-piston calipers. In this situation, a small drum is usually fitted within or as part of the brake disc. This type of brake is also known as a banksia brake.

In hybrid vehicle applications, wear on braking systems is greatly reduced by energy recovering motor-generators (see regenerative braking), so some hybrid vehicles such as the GMC Yukon Hybrid and Toyota Prius (except the third generation) use drum brakes. Disc brakes rely on pliability of caliper seals and slight runout to release pads, leading to drag, fuel mileage loss, and disc scoring. Drum brake return springs give more positive action and, adjusted correctly, often have less drag when released. It is however possible to design special seals that retract the piston on a disc brake. Certain heavier duty drum brake systems compensate for load when determining wheel cylinder pressure; a feature rare when discs are employed (Hydropneumatic suspension systems as employed on Citroën vehicles adjust brake pressure depending on load regardless of if drum or discs are used). One such vehicle is the Jeep Comanche. The Comanche can automatically send more pressure to the rear drums depending on the size of the load. Most other brands have used load sensing valves in the hydraulics to the rear axle for decades.

Due to the fact that a drum brake’s friction contact area is at the circumference of the brake, a drum brake can provide more braking force than an equal diameter disc brake. The increased friction contact area of drum brake shoes on the drum allows drum brake shoes to last longer than disc brake pads used in a brake system of similar dimensions and braking force. Drum brakes retain heat and are more complex than disc brakes but are often the more economical and powerful brake type to use in rear brake applications due to the low heat generation of rear brakes, a drum brake’s self-applying nature, larger friction surface contact area, and long life wear characteristics (%life used/kW of braking power).


What is brake fluid?

Brake fluid is a type of hydraulic fluid used in hydraulic brake and hydraulic clutch applications in automobiles, motorcycles, light trucks, and some bicycles. It is used to transfer force into pressure, and to amplify braking force. It works because liquids are not appreciably compressible — in their natural state the component molecules do not have internal voids and the molecules pack together well, so bulk forces are directly transferred to compress the fluid’s chemical bonds. In addition, the brake fluid serves as a lubricant of all movable parts and prevents corrosion. It has to be compatible with rubber seals and hoses thus allowing braking systems to achieve long service and optimal performance. Most brake fluids used today are glycol-ether based, but mineral oil (Citroën liquide hydraulique minéral (LHM) and silicone (DOT 5) based fluids are also available.

Why does brake fluid deteriorate?

A brake fluid has hygroscopic properties – meaning that it absorbs moisture during its life in your car through the pipes, hoses and joints that it lubricates. As the water content in the brake fluid increases, the temperature the liquid boils at decreases from when the brake fluid is ‘dry’ (ie no water content when new) to when it is ‘wet’ (contaminated with water).

The boiling point can be significantly reduced by water contamination below 5%. See below the difference in boiling temperatures for DOT 4 ESP Brake Fluid and how it exceeds the international standards for performance (known as the DOT standard).

Why change brake fluid?

Many experts have long recommend changing the brake fluid every year or two for preventative maintenance. Their rationale is based on the fact that glycol-based brake fluid starts to absorb moisture from the moment it is put in the system. The fluid attracts moisture through microscopic pores in rubber hoses, past seals and exposure to the air. The problem is obviously worse in wet climates where humidity is high.

After only a year of service, the brake fluid in the average vehicle may contain as much as two percent water. After 18 months, the level of contamination can be as high as three percent. And after several years of service, it is not unusual to find brake fluid that contains as much as seven to eight percent water.

As the concentration of moisture increases, it causes a sharp drop in the fluid’s boiling temperature. Brand new DOT 3 brake fluid must have a dry (no moisture) boiling point of at least 401 degrees F, and a wet (moisture-saturated) boiling point of no less than 284 degrees. Most new DOT 3 fluids exceed these requirements and have a dry boiling point that ranges from 460 degrees up to over 500 degrees.

Only one percent water in the fluid can lower the boiling point of a typical DOT 3 fluid to 369 degrees. Two percent water can push the boiling point down to around 320 degrees, and three percent will take it all the way down to 293 degrees, which is getting dangerously close to the minimum DOT and OEM requirements.

DOT 4 fluid, which has a higher minimum boiling temperature requirement (446 degrees F dry and 311 degrees wet) soaks up moisture at a slower rate but suffers an even sharper drop in boiling temperature as moisture accumulates. Three percent water will lower the boiling point as much as 50%!

Considering the fact that today’s front-wheel drive brake systems with semi-metallic linings run significantly hotter than their rear-wheel drive counterparts, high brake temperatures require fluid that can take the heat. But as we said earlier, the brake fluid in many of today’s vehicles cannot because it is old and full of moisture.

Water contamination increases the danger of brake failure because vapour pockets can form if the fluid gets too hot. Vapour displaces fluid and is compressible, so when the brakes are applied the pedal may go all the way to the floor without applying the brakes!

In addition to the safety issue, water-laden brake fluid promotes corrosion and pitting in caliper pistons and bores, wheel cylinders, master cylinders, steel brake lines and ABS modulators.

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