Who said, “I cannot change the laws of physics!”? Yes, it was Montgomery Scott, the actor who played Scotty in the original ‘Star Trek’. And he was right, you can’t change the laws of physics. Another unchangeable law is the first law of thermodynamics, which states that energy can neither be created nor destroyed. Fortunately for us, the form of energy can be changed. Why is this fortunate? Because braking systems are one of the mechanisms that change the form of energy, which we need them to do so we can stop our cars.
We don’t need to talk complicated science here. Energy has a number of forms, some more obvious than others. Sound energy, for example is easily understood. So is electromagnetic energy, when referred to by one of its other names, light energy. In a moving car, there is another kind of energy. This, kinetic energy, is the energy of motion and to slow the car down, we need a means of converting this into another form of energy. We can convert it into heat energy, and converting the energy of motion into heat energy that can be dissipated is exactly what brakes do.
The first kind of brake will be familiar to many. Drum brakes of the kind with expanding internal shoes were first used on horse-drawn vehicles. Car drum brakes were actuated using cables, or rods and levers, and early competition cars had finned brake drums, the fins added to encourage cooling. The first hydraulically-operated drum brakes were invented in 1918 by Malcolm Loughead, of the Lockheed family.
Disc brakes first appeared in the 1890s. The first caliper-type disc brake was patented by Frederick William Lanchester in Birmingham, in 1902. However, though disc brakes were (and are) remarkably efficient, their level of efficiency wasn’t really needed at first, and drum brakes won out, partly because of their ‘self-servo’ effect. This effect meant that a lesser pedal pressure was needed to operate the brakes.
Disc brakes became the desirable option in the 1950s, when Chrysler became the first major manufacturer to use them widely. Cars were fast enough and heavy enough to need disc brakes by then, and the adoption of the servo made them usable with a reasonable degree of pedal pressure. A servo simply uses the vacuum created within an engine’s inlet manifold to give the brakes power assistance. Studebaker led a renaissance in disc brake development, though the refinement of the brake servo. Now, disc brakes are the norm; there are very few, if any, vehicles still using drum brakes on the front wheels.
So, how do disc brakes work? To discover this, we have to get down and dirty by looking inside a car wheel. The wheel will be mounted on a hub, which will itself have a brake disc mounted on it. The hub, disc and wheel all rotate together. Embracing the disc is a cast metal housing, the brake caliper. The caliper has one or more cylindrical internal bores, each of which houses a piston.
When the brake pedal is pressed, hydraulic fluid pushes the piston towards the surface of the brake disc. Between the open end of the piston and the surface of the disc, a brake pad with a metal backing is pushed against the disc. Where there is just one piston, the caliper can slide, trapping the disc between the piston pad, and another pad, held in the caliper so it sits opposite its counterpart.
So, the disc is squeezed between the operating faces of the two brake pads. The pads create friction as they squeeze the rotating disc and this, in turn, creates heat. As you might expect, a lot of heat is created and, just to give you a measure of this, the typical temperature at the interface between a brake disc and brake pad during braking can reach 450°C on a road car. But this is what we want, the heat has originated in the car’s kinetic energy, that energy of motion we want to dissipate.
The ‘floating caliper’ described above is common enough but it’s the simplest kind of brake caliper. The next kind up the scale is the ‘fixed’ caliper. This doesn’t slide to let the pads grip the disc but has two opposing pistons. As hydraulic pressure is applied, the pistons move towards one another, causing the brake pads to grip the disc. High performance and competition cars can have more than two caliper pistons. Four-cylinder calipers became common, on later Jaguars for example. Some high-performance braking systems have as many as eight caliper pistons. Unsurprisingly, disc brakes are self-adjusting; when the hydraulic pressure is released, the hydraulic seals pull the pistons away from the disc by a tiny amount.
The development of brake discs has also continued over the years. The theory of improving braking harks back to finned brake drums of yore. Brake discs obviously have to have smooth, parallel braking surfaces. But it was possible to increase the brake disc’s surface area, just as fins increased the sruface area of a brake drum. By making the disc double-skinned and having its two elements separated by ribs, you are effectively putting the fins between two, siamesed discs. This is why a lot of cars nowadays have items like this, called ventilated discs.
There is another method of increasing the surface area of the disc, making it better able to radiate heat away. Look carefully at a high-performance car and you might notice that there are holes in its brake discs. This is also offered as a modification to existing conventional discs and it works. The downside is that, sometimes, drilled discs can be prone to premature failure through cracking.
Can we go further? Yes, and brake manufacturers have done. Brake discs are made of cast iron. Motor racing demanded a better material for discs and they found one. One problem with cast-iron discs is that they can reach a temperature high enough to cause the hydraulic brake fluid behind the caliper pistons to boil. This meaans the brakes suddenly become far less efficient, in a phenomenon called brake fade.
The motor racing fratenity found that carbon-ceramic brake discs were sufficiently thermally efficient to prevent brake fade altogether. In fact, carbon-ceramic discs shrug off extremes of temperature and weather, and they’re lighter and stronger than conventional discs. Such discs can be found on many track cars and are appearing on some high-performance cars. However, light weight and a long service life equate to a heavy price. When such brakes are offered as an option, it isn’t unknown for the price tag to be £6,000 or more.
As you can see, brakes have developed over the years but their operating principle remains unchanged. Is there an ulitmate? Currently, carbon-carbon brake discs are used on Formula One racing cars. They’re used exclusively on F1 cars for a reason. Which is? They start to become efficient at about 400°C, just 50°C below the temperature at which road car brakes are heading towards the upper end of their temperature range. Under extreme braking, carbon-carbon discs can hit 1,200°C. In practice, a road car braking system can’t be expected to even approach such operating temperatures in use. But no road car braking system can reduce a car’s speed by a fivefold factor in a matter of seconds.