CARES FOR THE BRAKING SYSTEM
Even if you are an excellent driver, you cannot rely solely on your skills. You want to drive safely – your vehicle’s braking system simply must not fail you. See what can affect braking efficiency below:
- Condition and type of friction materials – you should use friction materials tried and tested in a broad range of temperatures, pressures, vehicle loads and weather conditions. Apart from the right friction material, another factor affecting braking efficiency is the type and condition of brake discs.
- The configuration and number of pistons, disc type and diameter and brake type (drum or disc brakes). All these components are matched to the vehicle type. That is why it is best to test the car in various conditions – in city traffic, during sporty driving and in the mountains.
- Reliability and good condition of callipers, guide pins and springs – these components play a significant role in difficult weather and road conditions (dirty road surface, high humidity, salt, corrosion).
- Condition of the brake fluid – the most important parameters are its service life and boiling point. Minimum boiling points for various brake fluid types are shown below:
DOT 3 – min 140ºC,
DOT 4 – min 155ºC,
SUPER DOT 4 – min 180ºC,
DOT 5.1 – min 180ºC;
- Condition of a vehicle’s suspension – defects and irregularities in the suspension system increase braking time and distance. That is why we should inspect the condition of shock absorbers, arms, rods and components made of rubber and metal.
- Condition of a vehicle’s tires, including tire air pressure – uneven wear and insufficient tread height lower grip.
- Alignment of the steering system – improper steering alignment can make the car change direction during braking, while in extreme cases it can even lead to skidding.
- Tightness and bleeding of the braking system.
WHAT NEGATIVE PHENOMENA CAN OCCUR DURING BRAKING?
- Stick-slip – generated when static friction exceeds kinetic friction. As a result the pad and disc surfaces disconnect and start to “slip”. This lasts until both surfaces “stick” back together.
- Squeaks of various frequencies – low-frequency squeaks (1kHz – ~3kHz) are generated by, for example, rods, callipers and yokes. The main cause is play between various linkage components. While medium and high-frequency squeaks (4kHz – ~16kHz) are generated by discs and pads. The cause of vibration coming from the pad is mainly a change in its friction coefficient and improper geometry of the friction material. In the case of discs the causes are deviations in disc thickness (DTV), hot spots and improper cast iron.
- Groan and moan – generated when the vehicle starts moving, when the speed of up to 2 rpm is reached and when the pressure in the system is low. It is a series of fluctuations caused by sticking and slipping of the pad and the disc. Possible causes of groaning include deformation of discs and pads, transfer of the friction material to the disc surface and variation of the friction coefficient.
- Judder– is a result of changes in the braking torque. Judders can be subdivided into a number of groups, according to their cause:
Hot Judder – disc temperature >200°C
Cold Judder – disc temperatura <100°C
High velocity judder >130 kph
“Green” judder observed with newly installed discs and pads during braking-in
Every single case of a noisy braking system has to be dealt with separately and in depth. We can often hear a general statement that the only component responsible for the generation of squeaks coming from the braking system is the brake pad. However, putting all the blame on the pad is an oversimplification and indicates lack of expertise.
There is a pattern, however, that the greater the friction coefficient (the more effective the brake), the greater the risk of experiencing vibroacoustic phenomena (squeaks and vibration) emitted by the braking system.
CARES FOR THE BRAKE PADS
FIRST, CHOOSE WISELY
Reliable high-quality brake pads are one of the most important components of a safe car. What to look for when selecting them? They should feature high mechanical strength and resistance to high temperature. Such conditions are not usually met by low-cost pads. Not many manufacturers possess such sophisticated quality assurance tools as Lumag, the manufacturer of Breck products (like state-of-the-art dynamometers). It would also be wise to choose products with wear indicators and certification. An important feature is the so-called shim – a metal component coated with a special protective layer on both sides, which dampens vibration and prevents squeaking throughout a pad’s service life.
CARE FOR THEM LATER
Do you like to be proud of the car you maintain yourself? Brake pad maintenance is something you can deal with yourself. However, if you have any doubts, go to a garage. Below we present 4 steps of brake pad maintenance:
- Clean brake pads with a proper agent, removing residues, metal chips and stains resulting from the use of brakes. You will find it in every shop selling automotive parts.
- Check the condition of the brake disc. It has to be replaced, if it has deep circular notches, cracks, a step-shaped feature on the external rim or if it is too thin.
- Clean the calliper surface interacting with the pad.
Check the braking fluid level in the reservoir located in the engine compartment.
WHAT CAN BE A CAUSE OF CONCERN?
Lower-quality or improperly installed brake pads can cause you some trouble. And even the highest quality pads are not indestructible. See below what to look for. Here is a list of the most common issues with brake pads:
- Separation of the friction material – the cause is using pads without an underlayer or with low mechanical strength.
- Damaged (bent) back plate. It is a result of faulty pad installation.
- Uneven wear of pads in a single calliper. It can be caused by the seizure of a pad in the calliper yoke/plate resulting from e.g. corrosion.
- Dirty pad, soaked with oil, grease or brake fluid. This problem may occur, if a service technician does not handle the part with proper caution during installation. The pad can also get dirty after a leak of braking fluid or grease from a bearing.
- “Vitrification” of the friction material. It is a result of continuous braking with very hot brakes or the fact that pads were made of an improper friction material.
- Excessive pad wear. Its cause is the lack of proper inspection of a vehicle’s braking system during routine maintenance checks or low quality of the friction material.
- Cracked friction material. It results from bending of the brake pad caused by e.g. seizure in the calliper yoke. Another cause may be low mechanical strength in high temperatures.
- Charring of the friction material caused by extreme heat. Excessive braking, overheating the pads and “racing” are the main reasons of pad “burning” by drivers.
- Uneven pad wear caused by its improper interaction with the disc. It is caused by excessive brake disc wear.
- Irregular wear (diagonal wearing out of the material). Results from cuts on the calliper’s sliding surfaces and excessive calliper play.
- Excessive corrosion. It affects pads made of low quality materials. Corrosion may also damage pads in cars stored in a corrosive environment after a long period of disuse.
HOW OFTEN SHOULD BRAKE PADS BE REPLACED?
It is natural for friction materials of the braking system to wear with use. After some time they have to be replaced or regenerated. Friction elements of good quality with the optimal friction coefficient wear much slower than parts made of worse quality materials. Brake pads have to be replaced when the thickness of friction linings drops below a specified minimum level or if you can reasonably suspect that the linings will not last to the next overhaul. Another reason for replacing a pad or lining is a change of the friction element’s structure caused by overheating or mechanical damage (fracture, cracking, chips, crumbling). The functioning of front brakes has to be checked every 15 000 km, while every 30 000 km the quality of both – front and rear – brakes has to be inspected. The difference in the distances given above results from the fact that the load on front brakes is usually higher, so they wear quicker than the rear brakes.
Even the best car equipped with the most advanced safety technologies poses a threat to the passengers and the surroundings, if the driver does not comply with traffic regulations and drives carelessly. It is also important to learn the basic rules of using brakes according to conditions. Be safety-aware and read on.
Staying alert behind the wheel, by closely observing the road and its surroundings or even keeping one foot close to the brake pedal, has the greatest impact on a vehicle’s braking distance. The average reaction time of a driver, measured from the moment when an obstacle appears to the moment of real and full use of the braking system with full efficiency, is around 1.25 s. A vehicle travelling at 60 kph covers over 21m during this time, while a vehicle moving at 100 kph covers more than 35m in this time.
PRICE OF LIFE: 50 KPH
Police statistics prove that an unprotected road user (a pedestrian or a cyclist) stands a chance of surviving a collision with a car moving at the speed of 30 kph. However, when the speed reaches 50 kph, the odds of surviving the crash drop to 50%. In turn, at the speed of 60 kph the chance of survival is close to zero. That is why the speed limit in urban areas has been set at 50 kph, and it is not a coincidence. It is a compromise of sorts between smooth flow of traffic and relative safety of these unprotected road users.
SPEED AND BRAKING DISTANCE
When speed doubles, braking distance and energy of collision nearly quadruple. When a car travelling at 50 kph stops, a second car, moving in parallel and starting to brake at the same time, but from the speed of 60 kph, will be still travelling at 44 kph.
BRAKING DISTANCE IN WINTER
When driving in winter we have to remember that road surface covered with snow roughly doubles the braking distance, while a road surface covered with ice quadruples it.
THE BEST WAY OF USING BRAKES – IN SEARCH OF THE GOLDEN MEAN
Braking system manufacturers recommend avoiding long and soft braking cycles with low pressure in the hydraulic system, as this leads to heating of braking system components. Also “sporty” driving with frequent braking and keeping high temperature of discs and brakes has a negative impact on the durability of these components. On the other hand, delicate use of brakes, with frequent use of engine braking, is also bad when you want to take advantage of the right braking efficiency. When brakes are used in this way, layers of oxides, which are constantly building up on the outside surface of the friction material, are not removed. Considering long life and efficiency of brakes we recommend using brakes with medium intensity and braking hard from time to time. Furthermore, if possible (considering road conditions and traffic), you should brake over a short time. This type of brake use will ensure that they stay effective and will increase brake and pad replacement intervals.
BRAKING WITH AN ABS
When braking with an ABS system you should always fully depress the brake pedal and keep it pressed until the car comes to a complete halt. If you need to avoid an obstacle during braking, you should do it while keeping the brake pedal fully pressed.
You have to be aware that a car with an ABS system does not always stop faster than a car without it. During braking on snow or loose surface a car without an ABS will actually stop faster, as full wheel locking on such surfaces is an advantage.
You also need to remember that ABS does not function properly on pot holes and washboard-like ripples on the road surface, so on such surfaces you need to remain especially alert.
SAFE DRIVE DOWN A STEEP SLOPE
Prolonged application of the main brake while driving down a steep slope is dangerous. In such operating conditions braking systems quickly overheat and lose their efficiency. To prevent that, trucks use a so-called engine brake which throttles the flow of exhaust gases or increases compression pressure in cylinders. Sometimes a retarder (hydrodynamic or electromagnetic) is installed next to the gearbox, which causes a braking effect by generating additional resistance. In passenger cars the best way to safely drive down a steep slope is to use engine braking. You can also follow the popular rule of thumb saying that you need to go down the hill using the same gear you would use to go up the hill.