Introduction to Brakes and Road Safety
The ability to brake safely is an important requirement for safe driving. Safe Braking is especially important for the most vulnerable road users such as pedestrians, cyclists and motorcyclists, as crash data shows these categories are all too often the victims of inattention by car, van, truck and bus drivers.
Vehicle manufacturers are continuously striving to develop better braking systems and technology such as Autonomous Emergency Braking (AEB) in an effort to increase safety and reduce accidents.
Safe braking however involves more than merely knowing how much pressure to apply to the brakes - safe braking also involves working out the relevant stopping distances etc.
In this section we will provide an overview of the importance of safe braking for road safety.
What is a Brake?
The simplest definition would be that a brake is a mechanical device which inhibits motion. Almost all wheeled vehicles have a brake of some sort. Brakes may be broadly described as using friction, pumping, or electromagnetics.
When the brake pedal of a modern vehicle with hydraulic brakes is pushed, ultimately a piston pushes the brake pad against the brake disc which slows the wheel down. On the brake drum it is similar as the cylinder pushes the brake shoes against the drum which also slows the wheel down.
Brake pads are a part of the disc braking system, which is standard equipment for modern vehicles. In a disc braking system, a calliper is situated around the front wheels of your vehicle (most of a car’s stopping force comes from the front tires). The calliper is fitted with brake pads. When you press the brake pedal, the calliper squeezes the brake pads against the wheel and the resulting friction slows your vehicle to a stop.
It is also important that we are aware of 2 important modern brake systems and their impact on road safety.
- ABS [Anti-lock braking system]is an automobile safety system that allows the wheels on a motor vehicle to continue interacting tractively with the road surface as directed by driver steering inputs while braking, preventing the wheels from locking up (that is, ceasing rotation) and therefore avoiding skidding.
- Autonomous Emergency Braking (AEB) is an autonomous road vehicle safety system which employs sensors to monitor the proximity of vehicles in front and detects situations where the relative speed and distance between the host and target vehicles suggest that a collision is imminent. In such a situation, emergency braking can be automatically applied to avoid the collision or at least to mitigate its effects.
What is the required Following and Stopping Distance?
Stopping distances refer to the distance that it takes for your vehicle to stop. Many vehicle crashes can be prevented if drivers allow themselves enough time and distance from the vehicle in front. This is referred to as a safe following distance.
Drivers are advised to drive at a speed that will allow them to stop safely within the distance available to them. The recommended stopping distance measured in time is at least two seconds. Safe driving involves keeping at least two seconds behind the vehicle in front, and is measured using a fixed point - when the vehicle in front passes a fixed point, it should be at least two seconds before you also pass the same point.
It is also important that the reaction time of the driver remains uninhibited and non-distracted as this could further extend the stopping distance.
Which Factors are affecting the Braking/Stopping distance?
An effective braking system and driver ability behind the wheel are most important to ensure swift and safe braking. There are however also a few other factors to consider:
- Tyres and braking: Tyres are one of the most important elements when stopping a vehicle quickly and efficiently. One of the factors used in calculating braking distance is traction coefficient. The higher the traction coefficient is, the shorter the braking distance will be. Braking distance can change greatly based on the type and condition of the vehicle's tyres. Tyres with little or no tread will be more susceptible to skidding during heavy braking. When the tyres skid (a decrease of the traction coefficient), they lose traction and increase the braking distance. The tyre compound or makeup can also change braking distance. Some high performance tyres offer better adhesion under heavy braking and won't break loose or skid as easily as harder tyres.
- Braking and Suspension Systems: If any of these components aren't up to specification or in poor condition, the vehicle's braking distance can change. Worn brake drums, rotors, pads, shoes or leaky brake lines will have an adverse effect on braking. Since weight transfer has so much to do with braking, worn shock absorbers and springs will only add distance during braking. When these components are worn, the weight wants to move to the front of the vehicle when you hit the brakes hard. [See Shock Absorbers and Safe Driving]
- Road conditions: The condition of the road and our ability to adapt to these conditions will affect braking distance. Dirt and gravel roads don't offer good traction and increase braking distance. Freshly paved asphalt offers the best adhesion.
- Weather Conditions: We always need to consider the weather conditions and how this impacts on the road surface and the vehicle. Wet roads increase braking distance. Snowy and icy roads decrease traction even more.
- Driver ability: Driver experience and knowing your vehicle are important for safe and fast braking. Drivers can only benefit from additional driver training and advanced driving courses to enhance their driving skills.
- The Braking system: Whether or not the vehicle is equipped with an anti-lock braking system (ABS) must also be taken into consideration in calculating stopping distances.
Formula to calculate Braking Distance
How do we calculate Braking distance? This distance refers to the distance a vehicle will travel from the point where its brakes are fully applied to when it comes to a complete stop. It is affected by the following:
- original speed of the vehicle
- the type of brake system in use
- the reaction time of the driver/rider and
- the coefficient of friction between its tires and the road surface.
The theoretical braking distance can be found by determining the work required to dissipate the vehicle's kinetic energy.
The kinetic energy E is given by the formula:
E = (1/2)mv2,
where m is the vehicle's mass and v is its speed.
The work W done by braking is given by:
W = μmgd,
where μ is the coefficient of friction between the road surface and the tires, g is the gravity of Earth, and d is the distance travelled.
The braking distance (which is commonly measured as the skid length) given an initial driving speed v is then found by putting W = E, from which it follows that
d = v2/(2μg).
The maximum speed given an available braking distance d is given by:
v = √(2μgd).
Note that these theoretical formulas do not take account of the driver's reaction time (an example is the two-second rule).
[Info from Wikipedia]
Advice and Tips on Braking Safely
We would like to share advice and suggestions with drivers on how to brake safely:
- ‘Normal’ Braking: Under normal circumstances (on a dry and relatively clear road), you should be able to brake early and apply gentle pressure to the brakes. As you feel the vehicle begin to stop, apply a little more pressure, but ease off as you come to a full stop so that you do not stop too abruptly.
- Emergency Braking: If an unexpected situation arises that requires you to stop, you should brake straight away, but try to avoid braking too sharply, as this can cause your wheels to lock up and your vehicle to skid. With ABS, depress the pedal all the way down. Without ABS, brake moderately hard (70%) while taking off a bit of steering. It's always better to brake too much rather than too little. In a real emergency, it's best to simply stamp on the brake pedal and let the car stop.
- Braking in Bad Weather: If the road is wet or icy, you will usually need to alter your braking technique. This is particularly the case in icy conditions. It is extremely easy to lose control when taking a bend. The safest way to negotiate it is to brake gently on the straight leading up to a bend, so that you are travelling slowly when you reach the bend. Avoid braking sharply as this can lead to a skid. If you are worried about the grip (or lack of it) on the road, you can test it by braking gently to gauge the effectiveness. Do not leave it until just before a bend to test the brakes, as you will not have enough time to dry them out before you need to break.
- Braking after driving through water: If you have to drive through a considerable amount of water, your brakes can lose their effectiveness for a while afterwards. Test the brakes by braking gently to see if they work properly, but make sure that you do this only when it is safe to do so. If you find that they do not work as effectively as normal, apply light pressure to the brakes while you are driving at a fairly slow pace, as this will dry them out.
- Braking and Turning /Changing Direction: Avoid braking and turning at the same time where possible. Turning while on the brakes can cause the vehicle to not turn as well or not slow down as much.
- Brake-Turning: Turning the wheel into the corner while still lightly on the brakes. This causes the car to lean forward, pressing the front tires to the ground, giving them more grip for steering. This is a basic technique, and no corner should be made without it.
- Trail Braking: This is a method of feathering the brakes while turning into the corner and it will provide the best and safest control over the vehicle as it is leaning on the front tyres, allowing for more traction to those tyres.
Drivers are advised in an emergency to focus on where you want to go, and not on what you want to avoid. It's very difficult to steer away from something that you're looking at directly, and many people have a tendency to focus on what they are worried about colliding with. Instead, concentrate on where you want the car to go (to the side of the object) and pay attention to how the car responds.
Inspecting the Brakes
We are all aware of how important it is to brake safely – yet we often fail to pay enough attention to the condition of our brakes! This is an important part of effective vehicle maintenance
If you do not feel comfortable with your own mechanical skills, it may be best to have a professional to check your brakes and to install new parts. Working on your own brakes is not the time to test your mechanical abilities. If your brakes are installed incorrectly or worn and the condition thereof not assessed correctly they may fail and cause a severe accident.
Brakes are pretty much the most important safety device on your car. Inspecting your brakes at least twice a year for wear and damage can protect you and your passengers while saving you money by catching any damage before it becomes too costly.
You are often able to inspect the brakes without even removing the wheel. If your car has alloy wheels with spaces in the middle, you can get a proper assessment done by just peeping through the hole! Whether you can peep through your wheel or you have to take the wheel off, be sure you have a clear view of the brake pads and the big shiny disc.
You may start by inspecting the disc first. It should be shiny from the inside to the outer edge, and fairly uniform. Do not be too concerned if you can see slight lines in it as this is normal wear. However, if there are any rough spots or pronounced grooves in the disc, you should replace your brake discs. Brake discs should always be replaced in pairs so that your car's driveability and safety are not compromised.
Now take a look at the Brake pads. You'll have to peek up to see them, but if you follow the surface of the disc to the top, you'll see the outside pad touching the disc. Brake pads must be replaced when the thickness of the friction material is at 2mm thick, or when uneven wear is evident on the brake pads.
Finally, take a look at your brake lines. Rubber coated lines should be soft and supple, not cracked and rigid. If you see cracks in flexible brake lines they will need to be replaced. Also be sure to inspect the hard, metal lines.
Brake Pads and Safe Braking
The brake pad is one of the most important components in your braking system; it is the part directly responsible for stopping your car. However, brake pads are not built to last - As you use your brakes in everyday driving, they will slowly wear down, and, eventually, need to replace the brake pads. Therefore, it’s important to understand how brake pads work and when to replace them.
Brake pads are soft (from an industrial standpoint), so that they don’t erode the wheel while they are in use. Instead, the friction wears away at the brake pad, which is an easily replaceable part. Over time, the brake pads erode away with use and must be periodically replaced. There are a number of variables that affect how often you need to replace your brake pads, but they basically boil down to the model of car you drive, your driving behaviour and how often/hard you apply your brakes. Generally, brake pads may need to be replaced every 30,000 kilometres. Best advice is to have them checked regularly.
How do I know the brake pads need to be replaced?
Mechanics will easily assess when the right time would be to replace the brake pads – but there are some other warning signs for the driver as well. A sure alert that you need new brake pads on older
vehicles is squealing.
Brake pads on older vehicles actually have a sort of built-in alarm system to alert you when they need to be replaced. There is a little bit of metal that pokes out of the brake pad. When the pad is worn down enough, this metal comes into contact with your wheel every time the brakes are applied. This creates a squealing or screeching sound every time you apply the brakes.
Take warning though: waiting until you can hear your brakes screeching to replace your brake pads is not the best idea. The bit of metal is not exposed to the wheel until there is very little brake pad left. Once you can hear screeching, it generally means that your brake pads have only three or four weeks left before they are completely worn away. Without the brake pad, you’ll be stopping your car with metal-on-metal friction, which can warp your wheel, damage your brake system, and cause an expensive repair.
Modern cars today come with an electronic sensor on the brake pad. When your brakes have worn down, you will see a warning light on your dashboard. When this dashboard light is illuminated, or when you start hearing a squeak upon brake application, it is time for you to have your brake pads replaced.
[There is much more to a braking system than merely the brake pads. See below as well the guides to changing brake discs, brake drums, brake shoes and brake pads.]
Never overlook the importance of safe braking. We need to focus both on driver and vehicle fitness. If we are not caring for the vehicle components required for safe braking we are not only causing damage to the brake as a whole, but we are taking a big safety risk.
Be sure to use a great deal of care when replacing your brake parts. It is recommended that you have your brakes checked each time your vehicle’s oil is changed – about every 10,000 kilometres. This quick inspection, paired with proper maintenance of your brake pads, will prevent problems down the road, ensuring the safety of you and your family.
What are the correct brakes for your vehicle? View: BrakeBook
Also view for Brake Safety the Guide to vehicle provided by AutoZone:
Also view video clips from "The Garage"