By Roger Hughes – RQA Principal Consultant.
This is part of a new series of articles where we ask our experts to provide an overview of how specific products have developed over time. Understanding how things work and how they have developed, can help with understanding risks today and into the future.
Have you ever wondered how, with a very little effort from your foot pressing on the brake pedal you can quickly bring a vehicle weighing over 1 tonne to a stop?
Well hopefully by the time you’ve finished reading this short article you’ll have the answer.
A short physics lesson to begin with…
All automobile mechanical brakes work by converting the kinetic energy of the moving vehicle into heat by forcing two surfaces together. One, the brake shoe or brake pad, fixed to the structure of the vehicle, and the other, usually a disc or a drum, driven by the motion of the vehicle itself. The quicker the vehicle travels, the more kinetic energy it has. The faster kinetic energy can be converted to heat, the quicker the vehicle can be slowed down. Heat can be generated faster by increasing the clamping force between the two surfaces and also by increasing the friction between the two surfaces.
And now for the history…
In the early days of vehicle development, brakes were rudimentary devices whereby a driver simply pulled a lever which then pressed a wooden block against the rim of a wheel or a pulley attached to the axle. How long the lever was and how much effort the driver could pull it with determined how much braking effort could be generated. However, the wooden blocks wore out very quickly and were prone to catching fire. The saving grace being that the vehicles didn’t travel too quickly and were not too heavy.
As the motor vehicle developed and became faster and heavier, more efficient methods of slowing the vehicles down needed to be developed. Pulling a lever by hand was not going to cut it going forward.
The solution was to introduce a pedal that the driver could push with one foot, replacing the hand operated lever, which was then connected to the brake blocks by metal rods or cables. The wooden blocks evolved too, they were often covered with materials such as leather which offered better friction properties and could be more easily refurbished by simply by hammering on a new piece of lining once the previous piece had worn out. But these developments brought new issues, the rods and the cables needed constant adjustment, especially as the brake shoes wore out, and were not really compatible with the up and down movement of the suspension or the steering of the front wheels as the vehicle went round a corner. So, as the vehicles themselves become faster and heavier, further evolutions were required.
The invention of hydraulic brakes…
In 1917 Malcolm Loughead patented a hydraulic brake system, all the rods and cables were replaced by hydraulic pistons and tubing. A small piston, operated by a driver’s foot pedal would push oil through a series of pipes and hoses and that oil would then push out another cylinder at the wheel end of the circuit. The cylinder at the wheel end of the circuit would then push the brake block against a drum attached to the wheel. By this time the linings used in the brake drum had moved on from the days of wooden blocks covered in leather, and instead harder wearing and heat resistant materials, often asbestos based, were being utilised.
The mechanical advantage offered by the original lever was replicated by varying the sizes of the pistons. A small piston operated by the driver would push oil into a larger piston at the wheel end of the circuit, and the mechanical (now hydraulic) advantage achieved simply being the ratio of the working diameter of the two pistons. The development of a hydraulic brake system made it easy to distribute braking effort to all the wheels of the vehicle at the same time. The brakes themselves had also developed so that rather than acting on the rim of the wheel, pads were pressed against the inside of a drum that was attached to the axle of the wheel and rotated with the wheel. Hence the phrase drum brakes.
Then brake discs…
The constant thirst of the buying public for faster vehicles and the automotive manufacturers ability to meet that need, identified one of the main failings of the drum brake system. Despite the use of heat-resistant linings on the brake shoes inside the drum, heat still built up in the drum and the brake shoes to the point where they overheated and the friction levels would drop dramatically and the brakes would no longer slow the vehicle down. There was simply insufficient air flow to cool the brakes down, either more air flow was required, of the surface area of the brake itself needed to be bigger, or both.
The solution came in the 1950’s with the development of the brake disc, a metal disc attached to the wheel hub which rotated with the wheel, replaced the drum. The shoes were replaced with pads that operated on either side of the disc and were clamped against it by a calliper which housed the hydraulic pistons. This design change offered a significantly larger surface area from which the heat could now dissipate and was more exposed to the airflow through the wheels as the vehicle travelled along so helping keep them cool.
Other benefits were also provided by this new braking system design; the callipers could be easily enlarged to allow for multiple pistons to be incorporated into them, significantly increasing the clamping force that could be applied to the back of the pads. Making the callipers bigger also allowed for large pads to be utilised thus making use of that extra clamping load and ensuring the pads didn’t wear out as quickly. The design also allowed for easier replacement of the pads themselves.
But how do brakes stop a 1 tonne car?
So back to our original question, how can a driver, simply by pushing a brake pedal slow a vehicle down so effortlessly? Well, it is all to do with the cross-sectional area of the piston connected to the driver’s brake pedal, known as the master cylinder, relative to the combined cross-sectional area of all the pistons in the brake callipers, known as the slave cylinders. If the cross-sectional area of the master cylinder is, say, 500mm2 (based on a master cylinder of approximately 25mm in diameter), and the combined cross-sectional area of all the brake calliper pistons (slave pistons), is 23,500mm2 , assuming there are four pistons in each of the two front brake callipers and two in each of the rear ones, so 12 in total, and each of nearly 50mm in diameter, then the hydraulic ratio is 23,500 / 500. In other words, 47:1. So a driver pushing the brake pedal with a force equivalent to 25kg, would create a total braking force of equivalent to 1,175kg! Add to this the high friction levels offered by modern brake pad materials, we can begin to see how a driver can slow a modern vehicle down.
Did I mention brake boosters?
But the good news doesn’t stop there, the driver is further assisted by their effort on the brake pedal being amplified by a brake booster. This can either be in the form of a vacuum powered booster that helps push the brake pedal down when the driver pushes the pedal, or a small hydraulic pump that pressurises a sealed reservoir with hydraulic fluid which is then released into the brake circuit when the driver presses the brake pedal and adds to the brake fluid pressure that the driver develops through the brake pedal and the master cylinder. Either of these systems can further increase the brake fluid pressure generated by the driver alone by up to a factor of 4, so our 1,175kg of equivalent braking effort now becomes 4,700kg of equivalent braking force. During emergency braking a driver can easily generate up to 50kg of equivalent braking force on the brake pedal.
So, a combination of hydraulics and boost assistance amplifies significantly the efforts of the driver and that is how by applying a load equivalent to 25kg to the brake pedal, a driver can easily stop a modern vehicle.
Why don’t brakes get more attention?
We all rely on our brakes working efficiently for us 100% of the time, so it is surprising that brakes are seen as one of the most neglected systems on a vehicle. We have all become so reliant on warning lights and messages telling us about the health of our vehicle before we set off, yet on most cars the hydraulic aspect of the braking system only has one warning feature, and that is to tell us if the brake fluid level in the reservoir is low. Yes, some vehicles also have a device to warn if one of the brake pads is worn, but this is not a mandatory requirement.
Given where they are located, the brake discs, pads and callipers are exposed to the very harsh realities of vehicle life, water, dirt, salt, cold, ice, sudden stops from high speed, repeated stopping in traffic, constant use when descending a steep road, the list goes on. So, it is surprising then that the brakes are so neglected by many drivers and the only time they are inspected is at the annual UK MOT and only then once a car hits its third birthday. Japan’s Shaken vehicle inspection is every two years. But some countries such as the USA, do not have a nationwide, regular car testing requirement at all, and so brakes may be checked even less frequently.
During the UK MOT the brake pads are visibly checked for wear, the discs checked for cracks and corrosion and to see how worn or distorted they are, the brake fluid visually inspected to see if it is dirty or cloudy, and then the efficiency of the brakes checked during a brake rolling road test.
It’s now up to you…
So, the next time you set off in your vehicle for a trip, spare a thought for your brakes. Consider giving them a break by getting them inspected, or better still, learn how to check them yourself. It’s not hard and you don’t need special tools other than a jack and a wheel brace. There is plenty of online material on how to perform a simple brake system check. That way you can be sure that the next time you need to perform an emergency stop, your brakes will be there for you.
Happy driving.