Brakes

Contents

Brake systems

Modern cars are equipped with a hydraulic dual circuit brake system. That the brake system is hydraulic means that the braking force is transferred from the brake pedal to the wheels by means of brake fluid, which is transported via brake lines. The fact that the brake system is two-circuit means that the brake system has two independent systems operating on separate circuits. If one brake line is damaged or starts leaking, there is still another brake line which allows braking on some of the wheels.

All motor vehicles in Sweden must also be equipped with two mutually independent braking systems: service brakes (also known as foot brakes) and parking brakes (also known as hand brakes).

Wheels are equipped with either drum or disc brakes, which brake the wheels when you push the brake pedal.


In the picture you see a wheel equipped with disc brake

Disc brakes have better braking capacity and are today the most commonly used brake type. However, less powerful cars with drum brakes on the rear wheels are still manufactured.

Most cars are also equipped with a brake servo (also called a vacuum servo) which makes it easier to brake when the engine is running. If the brake servo is not working properly, or while being towed with the engine turned off, you have to push much harder on the brake pedal to achieve maximum braking power.

Check-ups

You can check the different parts of the brake system yourself. These check-ups are easy to do and should be performed at regular intervals.

Check the brake system by pushing hard on the brake pedal for 20 seconds while the car is stationary. The pedal should stop about halfway down and offer firm resistance.


Push hard on the brake pedal for 20 seconds when checking the brake system

Check the brake servo (vacuum servo) by pushing the brake pedal repeatedly and then starting the engine with the brake pedal pushed down. If the brake servo works as it should, the brake pedal should move downward slightly as you start the engine. If it does not, something is wrong with the servo, which means you have to push much harder on the brake pedal to achieve maximum braking power.

Check the brake fluid level from time to time, it should be at the max mark. If it is not, there is a leak in the brake system or the brake pads are worn. The brake fluid should be replaced every two years as it absorbs moisture, which lowers the fluid's boiling point, resulting in a diminished braking effect.


The symbol in the centre of the lid indicates that the container contains brake fluid

If the brake warning light comes on, you have either forgotten to release the parking brake or the brake fluid level is too low, probably due to a leak. If the brake fluid level is too low, the brakes will not work properly. If this is the case you must stop the car immediately and have it towed. You must never drive without fully functional brakes!


The brake warning light

Test braking

Every now and then, and when you have borrowed or rented a car, you should test brake on an unpaved gravel road at low speed to ensure that the car's brakes work properly.

When test braking, make sure the car does not pull diagonally or make any metallic scraping sounds. Also make sure the front wheels brake harder than the rear wheels so that in the event of a wheel lock, the front wheels lock before the rear wheels. If you find something wrong with the brakes, it must be fixed immediately.

It is also important to test the brakes after they have been exposed to moisture, for example after you have washed the car or driven through a deep puddle of water, because moisture diminishes the braking effect. When you brake, heat is developed which causes the moisture to dry.

Parking brake

The parking brake (hand brake) is a separate brake that usually operates only on the rear wheels. If, for any reason, the service brake stops working while driving, you can try to stop the car using the parking brake, but you must never drive a car if the parking brake is the only working brake.

Engage the parking brake when you park your car, even if you have not parked on a hill. Also leave the car in a low gear, in the event of a parking brake failure. If you never use the parking brake it will eventually seize up and stop working.


If the parking brake is never used it eventually stops working

It is not recommended to use the parking brake when it is cold and damp outside, as it can freeze. In such circumstances you should just leave the car in a low gear.

You can check the parking brake by starting the car, engaging the parking brake and first gear and then releasing the clutch slowly. The car should not move forward.

When you do this, the brake warning light will come on, as one of its tasks is to remind you that you have forgotten to disengage the parking brake.

ABS

When braking in an older car that has regular brakes, the wheels can lock causing you to lose steering control of the car. More modern cars, however, have an anti-lock braking system (ABS). With ABS, the wheels do not lock when braking, which means that you can continue to steer even when braking hard. ABS also reduces the risk of skidding and can often shorten the car's braking distance.

However, the car's braking distance will not always be shorter with ABS. On gravel, ice and snow, the braking distance may even be longer. The biggest advantage of ABS is that it helps you maintain steering control of the car, not that the braking distance becomes shorter.


ABS helps you maintain steering control of the car, reduce the risk of skidding and sometimes shortens the braking distance

ABS operates by automatically reducing the braking force on wheels that are about to lock. When there is no longer any risk of the wheel locking up, the system increases the braking force again. This process is repeated several times per second, which helps you maintain steering control of the car.

When you brake in an emergency with a car that has ABS, you should brake as hard as possible and steer to avoid the obstacle or hazard. When the system is active, the brake pedal vibrates and pulses and make a certain sound. This is normal, and means that the system is working as it should.

If the ABS warning light comes on while driving, there is something wrong with the ABS system. Seek out a workshop immediately if this happens.


The ABS warning light

Stopping the car

Stopping a moving car often takes longer than you might think. Even if you react faster than the average driver and drive a new car with good brakes and tyres, the stopping distance is always long, especially at high speeds.

Reaction distance + braking distance = stopping distance

The reaction distance is the distance the car travels from when you discover an obstacle or a hazard until you apply the brakes.

The braking distance is the distance the car travels from when you first apply the brakes until the car comes to a complete stop.

The stopping distance is the entire distance the car travels from when you discover an obstacle or a hazard until the car comes to a complete stop. The stopping distance is made up of two parts: a reaction distance and a braking distance.


The illustration shows what the reaction distance is, what the braking distance is and what the stopping distance is

Reaction distance

The reaction distance is the distance the car travels from when you discover an obstacle or a hazard (for example, an elk crossing the road) until you apply the brakes. It is only your reaction time and the car's speed that determines how long the reaction distance will be.

The reaction distance increases proportionally in relation to the speed. For example, if the speed triples, the reaction distance also triples.

The easiest way to calculate the approximate reaction distance is by rounding the speed to the nearest ten, taking away the zero and multiplying by three.

Examples

The formula is based on a driver who reacts in one second, which is the average reaction time. If the reaction time is two seconds, the reaction distance becomes twice as long.


Most drivers react about one second after they have discovered an obstacle or a hazard

Braking distance

The braking distance is the distance the car travels from when you first apply the brakes until the car comes to a complete stop.

Speed is the factor that mostly affects the braking distance. All moving objects possess a kinetic energy that increases or decreases quadratically in relation to the speed. The braking distance increases and decreases in exactly the same way. For example, if you double the speed, both the kinetic energy and the braking distance will quadruple. If you triple the speed, both the kinetic energy and the braking distance will be multiplied by nine.

Other factors that also affect the braking distance are brakes and brake technique, tyre quality and condition, road grip and slope, as well as the position and weight of the load. In poor conditions, the braking distance can be many times longer than in ideal conditions.

Braking distances in ideal conditions:

The simplest formula for calculating the braking distance is ((v/10)^2)/2 where v is the car's speed in km/h. In words, the formula is: speed divided by ten – to the power of two – divided by two.

This is how you calculate the braking distance at 30 km/h using the formula: ((30/10)^2)/2 = 4.5.

You do not need to know the formula to pass the theory test. But you need to know the approximate braking distances at different speeds, both in order to pass the test and for your own safety.


Snow and ice reduce grip and extend the braking distance

Stopping distance

The stopping distance is the entire distance the car travels from when you discover an obstacle or a hazard until the car comes to a complete stop.

You calculate the stopping distance by adding the reaction distance and the braking distance (reaction distance + braking distance = stopping distance.)

The illustration below shows how the stopping distance, that is, the reaction and braking distance, increases in relation to the speed.


The reaction distance increases in proportion to the speed, while the braking distance increases quadratically

Stopping distances in ideal conditions:

If the reaction time is longer than the average (one second) and/or if the braking occurs in other than ideal conditions, the stopping distance can be many times greater.

Collision speed

The higher the speed, the more serious an accident will be. In the figure below you can deduce the stop and collision speeds at different initial speeds and distances. The coloured lines show how the speed drops in a fast (one second reaction time) and a hard braking scenario on dry asphalt.

For example, by following the red line, you can deduce that the reaction distance at 110 km/h is just over 30 metres, that the stopping distance is 90 metres and that the collision speed with an object 50 metres in front of the car is approximately 90 km/h.

By studying the figure you can see that a small increase in speed can result in a much higher collision speed. A person driving at 70 km/h can, for example, avoid a collision with an elk crossing the road 43 metres in front of the car while another person in the same situation, yet driving at 90 km/h, collides with the elk at almost 70 km/h.

Kilometre/hour to metre/second

To respect and really understand the tremendous speeds at which you often travel when driving a car, it is good to know how many metres per second (m/s) different speeds correspond to.

Divide the speed by 3.6 to convert kilometres per hour to metres per second. But if you just remember that 10 km/h equals about 3 m/s, the other speeds are easy to estimate.


100 km/h corresponds to almost 28 metres per second