Chapter 4: BRAKING SYSTEMS

Many car enthusiasts don't upgrade their brakes when they upgrade their car's power. BE AWARE! The original brakes are for the original performance and for road use and road speeds, not for high performance driving on a race track. 5 flat-out laps around a circuit in a typical family car and their are not much brakes left due to the heat build-up. Check out performance cars at a car show n' shine and you will see huge alloy wheels, big spoilers, loud stereos and other cosmetic bling bling....but rarely have the brakes been modified. This is OK if the car is a show car only (lives in a garage and only is driving at cruising speeds on rare occasions) but if the car is driven hard on a circuit the cosmetic bling-bling will not keep the car out of the sand-trap at the end of the main straight (or worse still out of the wall). HOW THE BRAKES WORK The specifications for a car braking system depend on the vehicle weight, load carrying area, performance characteristics and the price your willing to pay when buying a car. The disc rotors job is to absorb heat from the friction created during the braking process. As the heat builds the rotor must radiate heat away in order to stay under the maximum temperature for the continuous creation of friction. Disc brakes cool better than drum brakes. A cars braking system is designed on the principles of thermal dynamics, heat absorption and dissipation. This rotor is ventilated allowing cooler air to flow across both the inner and outer surfaces aiding the cooling process. In addition the vehicle designer has incorporated air ducts in the front of the car to force air into the front wheel arches. Finally alloy wheels allow more air to reach the brakes than for steel wheels. Disc Brakes Australia advises that if the temperature of the brake pads and rotors rise excessively through repeated heavy use of the brakes problems can occur such as; Excessive brake pad wear Brake Noise Brake Fade Rotor Damage such as: warping, cracking, heat spots Plus the increased possibility of boiling the brake fluid leading to a reduction in braking ability or even under extreme conditions brake failure. This reminds us that correct vehicle and braking system maintenance is vital and smooth use of the brakes means less heat and wear. One of the first things to consider in braking system maintenance is to check the remaining amount of friction material on the cars brake pads. This is a new brake pad, you will notice the metal backing plate. The car will not stop very well when your brakes are worn to this level. To check wear you can in most vehicles check through the relief hole in the disc brake caliper or you will need to remove the brake pads. If you do not know how to do this correctly leave it to the professionals. Considering the amount of energy that must be transferred from kinetic energy to thermal energy as the brakes are applied, to bring over 1.2 tonne of vehicle to a stop the brakes do a momentous job. As previously mentioned you often see ignorant drivers modify their car to make it go faster but do nothing to upgrade the standard braking system that may not be able to handle the extra loads. This is one of many reasons why modifying your car can void the manufacturers warranty. Sometimes drivers are shocked when their first set of brake pads lasts 60,000 km but the second set expires after 30,000 km. Whilst a mechanical problem could be the cause, another reason is that service shop machined the rotors to try a clean up the surface and assist your braking life but over-machined the rotor removing too much material. As such the rotors mass is now reduced. With a reduced mass the rotor can no longer absorb as much heat, therefore the rotor become heat saturated more readily and the brake pads run hotter, leading to increased wear. The brake rotor has a minimum thickness specification for a reason. In a front wheel drive car like a Proton, the front wheels and tyres are responsible for all the acceleration, all the steering and 80% of the braking, since under brakes weight transfers to the front, leaving the back brakes with little to do. Some service repairers ignore the rear brakes in front drive cars. At most having a cursory glance to check there is plenty of brake pad material left. If you have replaced your second set of front brake pads and there still is plenty of rear brake pad material remaining, chanced are the brakes are poorly balanced and the rear brakes are not doing the job they are supposed too. Just like tyres, if brakes are not evenly wearing out they are not working properly. Finally another cause of damage to a cars braking system is when replacing the wheel and tightening the wheel nuts. Using an impact (or rattle) gun can lead to large torque differences between each wheel nut, thus causing warping of modern lightweight alloy rotors. Wheels nuts should always be correctly tensioned in the correct order and with the correct torque. It is for these reasons and many others that you should find a reliable and trustworthy service shop and stick with them. GLOSSARY OF BRAKING TERMINOLOGY (Information courtesy of Disc Brakes Australia) ABS/ANTI-LOCK BRAKING SYSTEM Also known as anti-skid brakes. The modern ABS systems electronically monitor the speed of the wheel and regulate the hydraulic pressure accordingly. The aim is to maximize braking power while preventing the wheels from locking and the vehicle from skidding. BLEEDING The method of purging the air from the brake system's hydraulic lines and cylinders. Air is compressible and contaminates brake fluid. Air is released (bled) via a "bleeder valve" on each wheel cylinder. BRAKE ASSIST SYSTEM The Brake Assist system is designed to help emergency braking when the driver cannot apply the pedal force required to obtain the shortest possible braking distance. The systems monitors brake usage and interprets a quick, hard push of the brake as being an impending emergency. The system supplements the braking power by applying additional hydraulic pressure if the driver is not pushing hard enough. If the driver eases off the brake pedal the Brake Assist system reduces the amount of assistance. BRAKE BALANCE The ratio of braking force distributed between the front and rear wheels. In many race cars this brake balance is adjustable. BRAKE DISC (OR ROTOR) The basis of a disc brake system. A round metal disc rotates with the road wheel and in order to generate braking power, the disc is clamped by a caliper holding 2 friction linings (pads). BRAKE FADE A reduction of braking performance, usually caused by too much heat. BRAKE FEEL The sensation transmitted to the driver via the brake pedal during brake application often dictated by brake pad material and brake booster efficiency. BRAKE FLUID A water soluble liquid formulated specifically to be used in hydraulic brake systems. BRAKE HOSE A flexible rubber (or synthetic) hose used to join the hydraulic brake components. BRAKE LINE A rigid tubing which links various hydraulic components in a braking system. BRAKE PAD/BRAKE LINING Brake Lining is the common name for the friction material or brake pad. Can contain asbestos fibres. The pad is produced by bonding friction material to a backing plate. Under brakes the pad is clamped by hydraulic pressure to the disc rotor to produce friction and slowing forces. Sometimes non-asbestos pads are more abrasive, accelerating rotor wear. CALIPER A type of clamp which grips a disc rotor to create friction and thereby generate stopping power. CROSS DRILLED DISCS (see picture above) Disc rotors with friction surfaces which have been drilled with rows of holes to improve cooling, reduce weight and provide an escape route for dirt and gas wedged between the brake pads and the disc. High performance rotors can be both cross-drilled and slotted. CURVED VANE DISC Ventilated rotor in which the cooling channels (or vanes) have been curved to increase their ability to pump out hot air and cool the disc. Curved vane rotors are more efficient than conventional ventilated rotors and, as a side benefit, tend to be stronger. DISC THICKNESS VARIATION A variation in thickness between two points on the friction surface of a disc rotor. Usually caused by poor manufacture, poor machining or the rubbing of the rotor against the caliper when the brakes are off. The thickness variation creates a harsh vibration when the brakes are applied. DUAL CIRCUIT BRAKES This is a safety design incorporated on modern cars which ensures there are two largely independent hydraulic brake circuits. If there is a failure with a brake line, the other circuit still provides braking force. GLAZING The process whereby a brake lining or disc rotor becomes smooth and glossy due to excess heat resulting in reduced braking efficiency. HEAT DISSIPATION Process whereby braking components rid themselves of heat caused by friction. Most heat is dissipated into the surrounding air or through mating metal components such as the wheel. Dissipation can be accelerated by various forms of ventilation. HEAT SPOTS Shiny dark areas on a rotor caused by extreme heat. MANUAL BRAKES A (old technology) braking system which does not use power assistance to magnify the pedal effort. MASTER CYLINDER The engine room of a brake system, where the force applied at the pedal is converted into hydraulic pressure so that it can be sent to each wheel cylinder. MINIMUM THICKNESS Through wear and machining a disc rotor becomes thinner over time; as a result the disc becomes less able to dissipate heat and more prone to warping and other problems. The minimum thickness is the disc rotor is usually determined by the vehicle manufacturer. Once minimum thickness is reached the disc must be discarded. OUT-OF-ROUND Effect where a disc rotor is no longer true to its original shape; as a result of either warping, inconsistent wear or other damage. This can cause pulsing, grabbing, additional noise and lowered performance. PAD KNOCK-OFF A condition experienced by some racing cars during hard cornering when the stub axles flex and the disc pushes the brake pads (and piston) away from the disc. When the brake pedal is reapplied, there is a longer pedal travel. PARALLELISM A term which refers to the relationship between the two friction surfaces on a disc brake rotor. It is critical that the surfaces are parallel, particularly with ABS, as the slightest shudder can confuse the anti-lock system. PISTON The piston is the moving part of the brake caliper. Upon receiving increased pressure from the brake fluid, the piston is forced outwards and against the back of the brake pad, which is forced against the disc. POWER-ASSISTED BRAKES Brakes which use power-assistance (usually from engine vacuum) to magnify the pedal force, thereby reducing driver effort and increasing braking power. PROPORTIONING VALVE Hydraulic proportioning valve is designed to stop the rear wheels from locking up (since the rear wheels become light under heavy braking and more likely to skid). PULLING Tendency of a vehicle to pull to one side under braking. FLUID RESERVOIR A chamber connected to the master cylinder (usually by hoses) and used for storing brake fluid. SLOTTED DISC Type of disc brake (as shown above) where the disc rotor has a series of slots or grooves across its friction surface. These are designed to improve the bite of the pads and break down the build-up of gas and dirt which can occur between the pad and rotor. High performance brakes can be ventilated, slotted and cross drilled. SPONGY PEDAL When the brake pedal feels spring like, perhaps due to the presence of air in the hydraulic system. SQUEAL PLATES Metal plates which are placed on the brake pads which are designed to audibly squeal when the brakes are almost fully worn. SWEPT AREA The total friction area contacted by the pads during one revolution of the disc rotor. VENTILATED ROTOR (see picture above) Disc rotors which have a series of fins (or cooling passages) between the two friction surfaces to aid in heat dissipation. WARPING A condition experienced by the disc when it becomes out of round. Often caused when the brakes are used excessively then the vehicle is stopped and heat from the pads/caliper dissipates unevenly, through the rotor. Rotors which are warped or out-of-true have excess run out, meaning the surface varies or wobbles as it rotates around a fixed point. WEAR SENSOR A device to warn the driver (usually via a dashboard light) that the brake pads (linings) need replacing. WHEEL CYLINDER A small cylinder located at each wheel to convert hydraulic pressure back to mechanical force in order to apply the brakes. WHEEL SENSOR A device which electronically monitors the speed at which a wheel is rotating. Usually it forms part of an anti-lock braking system, though nowadays wheel sensors increasingly supply information for traction control systems as well.

(Written by Joel Neilsen, Managing Director, Safe Drive Training)