How a Car Braking System Works

There is a great deal of rubbish spoken on the subject of brakes. Some people spend a small fortune having drilled discs, larger brake callipers, competition disc pads and special hoses fitted, but none of it will make your car stop more quickly, not unless there is a fault in the standard equipment you are replacing.

Perhaps you can understand the content of the above statement a little better if we explore together how a braking system works.

When a driver pushes the brake pedal down with his or her foot, the pedal linkage acts upon a mechanical plunger that is contained within a master cylinder. Above the master cylinder is a reservoir supplying hydraulic fluid to the system. Don’t glaze over as this isn’t going to become a deep technical explanation.

As we said, the brake master cylinder is a tube inside which there is a plunger that operates a bit like a bicycle pump, but with its handle connected to your brake pedal.

Whereas the bicycle pump expels air as you push the handle in, the brake master cylinder uses hydraulic fluid. This of course is not squirted out into the air, but pushed through two outlet points into two sets of very small tubes called brake pipes. These pipes lead from the master cylinder to the four braking units, one at each wheel, and are the bits that create a slowing effect on the wheels.

Going back to bicycle pumps, if you were to blow into the end where the air is pumped our, you would push the plunger backwards with the force of your breath and the handle would extend from the pump. In a similar way, this is how the brake units work at the wheels of your car, as each one has its own set of plungers, or pistons, in smaller cylinders that are actuated by the hydraulic fluid being pushed from the master cylinder. After all, that displaced hydraulic fluid has to go somewhere.

Two Types of Brakes, Disc Brakes & Drum Brakes

The brake units at the wheels come in two forms, and you may have heard of each as disc brakes and drum brakes. Disc brakes work upon a similar principle to traditional bicycle brakes, where a bicycle wheel rim is clamped between two blocks of rubber to get a braking effect – you may even see disc brakes on a bicycle.

In much the same way, a car disc brake has a steel disc that is clamped between two sets of brake pads and this creates the braking effect. Disc brakes are either fitted to all four wheels, which is the case of the more up–market vehicles, or at the front wheels with drum brakes at the rear.

Drum brakes are operated by two semi–elliptical brakes pads called brake shoes and each brake unit is covered with a steel drum. As the driver applies the brakes of the car, hydraulic fluid pumped from the master cylinder is forced into a slave cylinder, which pushes the two brake shoes apart causing them to come into contact with the inside surface of the drum walls. This creates the braking force upon the wheel.

When a brake shoe or disc pad is pressed onto the contact surface of the drum or brake disc this converts kinetic energy from the moving vehicle into heat, caused by the friction between the two contact surfaces. The heat is dispersed through the brake components and absorbed by the surrounding atmosphere.

For the production road car, heat generated by the brakes is the enemy of the system, as if there is too much of it, the efficiency of the brakes becomes diminished. That is why you will see ventilation slots in your wheel rims, and on some cars (if you remove the wheels) you will see the brake discs have grooves cut in them, holes drilled in them, and sometimes the disc itself appears to be two sandwiched together with webbed pieces of metal between, which are cooling fins.

Hydraulic Fluid
(Brake Fluid)

Brake fluid (hydraulic fluid) is special stuff. It has an extremely high boiling point and an extremely low freezing point, both being way outside the tolerances of water. It is also non–corrosive or degenerative to brake parts, particularly rubber, although it is quite good at removing paint from the car bodywork! However, hydraulic fluid is easily contaminated, and as part of a vehicle service routine, it needs to be replaced periodically.

For example, hydraulic fluid is hydroscopic, which means that if exposed to the atmosphere it will act like a magnet to particles of water vapour, absorbing it like a liquid sponge. If a mechanic, for example, is careless enough to leave the top off the fluid reservoir of the master cylinder, water from the atmosphere would contaminate the fluid within. Once you have water within the system, and if there is enough of it, there is a danger of experiencing a condition known as vapour lock.

Brake Failure Through Vapour Lock

Vapour lock occurs where the water within the hydraulic fluid will actually boil within the brake pipes, when subjected to the heat generated by the operation of the brakes. The effect here is that boiling water turns to vapour (steam), and unlike the fluid, vapour can be compressed. Therefore the brake pedal could sink to the vehicle floor whilst you are trying to stop and with no braking effect taking place at the wheels. This is the effect of vapour lock.

Pockets of air within the fluid will produce a spongy feel to the brake pedal, and therefore make the brakes less effective. Air should be expelled from the system regularly, and this is achieved by a process known as bleeding the brakes, and should occur at vehicle service intervals.

If too much heat, even for well maintained brakes, is generated under braking it can cause something that is referred to as cooking the brakes. This causes the friction material of the brake pads or shoes to become glazed. Glazing reduces the properties of the friction material, which in turn reduces the efficiency of the braking system.

Typically, this might occur when a driver does not drive the vehicle with empathy, as planting the accelerator flat to the floor, and then at the latest possible moment, stamping on the brake pedal to slow or stop. This type of driver behaviour will not be tolerated for long by standard braking equipment, as it is not designed to put up with acute levels of abuse. Incidentally, we refer to this type of person as a bore and brake driver.

Does it Give Better Performance Fitting Bigger & More Powerful Brakes?

Vehicle manufactures are not daft. There is a great deal of research and development that goes into the build of a vehicle, and one such consideration is that of the type of braking equipment each model is to be equipped with. Once that decision has been taken there follows many thousands of hours and miles of development and testing to make sure that everything functions correctly.

All the various parts of a vehicle are manufactured, fitted and chosen to harmonise with each other, so everything is balanced. Therefore, the brakes on your car are intended for your car, engineered for your car and therefore will provide the correct degree of braking force for your car — providing that braking system is maintained in good working order.

However, you will often read and hear of people saying how more powerful brakes do a better job at stopping the car than the original manufactures equipment. With that in mind, let us consider the following.

The Braking Effect of The Brakes Acts on The Wheels

If you press hard enough on the brake pedal that will cause enough resistance against the rotation of the discs at the wheels to get to a point where that force becomes greater than the rotational force of the wheel. When this happens the rate of wheel rotation will be retarded. As the disc is directly attached to the road–wheel, that road wheel has no choice other than to maintain the same rotational speed as the disc (Although we are talking of brake discs here, the same action applies to brake drums).

If that sequence of events causes the rate of rotation in the road–wheel to be reduced to the point where it stops, then the brakes have completed their ultimate job. Standard braking equipment will achieve this, but bigger and more powerful brakes WILL actually do this better.

However, that is a description of the braking effect upon the disc⁄drum⁄wheel and that is the easy part. To cause a braking effect upon the vehicle itself, you still need to slow the rotation of the road wheels, but what you have to rely upon to actually slow or stop the vehicle is adhesion being maintained between the tyres and the road surface. In other words, to slow or stop the car you need tyre grip, and upgraded brakes WILL NOT improve this.

We know this to be true, because if we could make a car stop more quickly by spending more money on brake parts, driving with snow, ice, oil on the road, or wet roads would not cause any variation in our overall stopping distances.

Standard Brakes Are All You Need on Your Cars

Under public road driving conditions the standard braking equipment will not produce any less braking efficiency than the after–market add–ons you can buy in the name of improving vehicle performance. What the aftermarket products do achieve is to make it easier to operate the brakes, which means having the capability to create the maximum degree of braking power at the wheels with less leg power being required from the driver at the pedal.

The ability to reduce speed by braking is still limited by the maximum available grip of the tyres in contact with the road. All modern cars, and almost all obsolete ones as well, have brakes that are powerful enough to either lock wheels into a braking induced skid, or to activate the ABS from any speed throughout the vehicle’s available speed range, and on a dry road. Therefore, braking performance is all down to the friction properties (coefficient of friction) present within the road surface upon which you are driving, and the ability of your tyres to grip it.

If You Can’t Get What You Want From Your Brakes,
Maybe it Isn’t Your Brakes That Are at Fault

So, you may spend your hard earned cash on four–pot callipers, drilled discs and competition brake pads, but unfortunately all you will be buying is a false sense of well–being. Automotive jewellery, we call it. If you do decide you must and need to fit what you believe to be better equipment, before you do, just ask yourself what exactly is it you are compensating for? Because there is one thing that high quality after market braking equipment will do brilliantly, and that is withstand the abuse given by a poor driver.

Julian Smith
Ride Drive Limited

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Part–1 Effective Braking Techniques
Part–2 How a Car Braking System Works
Part–3 Vehicle Stopping Performance
Part–4 Effective Braking Techniques

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This page was last updated
Tuesday, 15-Feb-2011

How a Car Braking System Works