Two wheels good, four wheels mostly unheard of…

As the second century of automobiles opened, a lot of the building blocks of the modern day industry had been established. Mass production had been established, opening up vehicle ownership to huge populations for the first time. The invention of the electric starter had seen Internal Combustion Engine become the front runner in terms of powertrain, and a burgeoning motorsport scene had been established.

In terms of braking, the first century had created the backbone of technology, with the first disc brake and drum brake designs, regenerative braking of electric vehicles, and tentative steps in hydraulic actuation. The typical vehicle of the early 1920s had rear brakes only, and in the case of the Model T even this was an optional upgrade.

Racing roars to the fore…

As the roaring 20s progressed, some significant motorsport events were being established, and the public interest in these events meant vehicle manufacturers were paying them particular attention. One such event was the Le Mans 24 Hour, and one such manufacturer was Bentley Motors. Bentley’s 3-Litre car won the second ever Le Mans, in 1924, despite only having rear wheel brakes, and not enjoying factory support (the inaugural race in 1923 was won by a Chenard and Walcker 3-litre sport, with front brakes only). By 1927, Bentley had improved its car, and was again victorious, this time with brakes on each wheel. The factory was now supporting the racing team, and over the next 3 years, engine and vehicle upgrades (including supercharging) were supplemented with bigger drum brakes, and eventually cooling fins.

By the end of the 1920s, four wheel braking has become the norm, and even mass market low cost vehicles adopted drum brakes at each corner. At the same time, power brakes were first debuted by the Hispano Suiza company, with a novel design for engine power being used to assist the driver in applying brakes to all corners – technology which was licensed to Rolls Royce and others. The chief designer, Marc Birkigt, also invented a two-piece brake drum arrangement, featuring a lightweight drum with ribs on the outer circumference – for cooling and strength, and a hard-wearing friction layer, which would accept the brake shoe.

1924 Hispano-Suiza
Fig. 1: An example of a 1924 Hispano Suiza H6, with air-cooled drum brakes visible. Photo used under CC licence

Two inventors, Caleb Bragg and Victor Kliesrath came up with a design for vacuum assistance for braking. They used engine vacuum to supplement the driver’s mechanical force, and thereby boosted the driver’s brake force. The Bragg Kliesrath Corp. initially sold the component under their own name, but the company was later bought by Bendix.

Fig. 2: The original vacuum brake booster, invented by Bragg-Kliesrath

At around the same time (1924), a Belgian engineer, Albert Dewandre, invented a mechanism to use engine vacuum as an energy source for brake actuation, which was sold by Robert Bosch. While it was originally envisaged to directly actuate the brake, it later became used in combination with hydraulic brakes.

Speed is the answer. Now what was the question?

By the 1930s, vehicles were beginning to look very similar to their modern day descendants. Wind tunnels were being used to influence styling, and so wheel arches became integrated into bodywork, and aerodynamic drag was considered an optimisation necessity. Overall, the industry was consolidating, both in terms of number of manufacturers, but also in terms of design solutions employed.

During the 1930s, a couple of key developments would impact the direction of travel for vehicle designs, and with it, brake requirements. First among these was the widespread popularity of the Grand Prix motorsport championship, with individual manufacturers competing under a national flag. While the idea of GP had been around, the 1933 format (timed grid, single driver, formula libre, weight limit) previewed what is still used today. National governments began to promote teams, keen to find a political advantage, and investment in motorsport increased. This trend culminated in the German manufacturers Mercedes and Auto Union creating some of the most powerful vehicles ever seen, and pushing the boundaries of aerodynamics, weight (missing out paint, for example), suspension design, , body stiffness, engine layout and capacity – and of course power and speed.

1937 Auto Union Type C
Fig. 3: Auto Union Type C Grand Prix car from 1938. Photo used under CC licence.

At this time, another German innovation was changing the face of vehicle design – the Autobahn, and it’s unrestricted Vmax. Keen to exploit the newfound national prowess in racing, both Auto Union and Mercedes modified their race cars for speed records – with the eventual crown going to a Mercedes W125 Rekordwagen, posting a speed in excess of 430km/h on the public road in 1938 (not beaten until 2017). A further attempt was planned (the T80), but the outbreak of war meant this vehicle never ran.

Mercedes Benz Classic (MB Museum Stuttgart)
Fig. 4 : Mercedes Museum T80 and W125 Rekordwagen display. Image used under CC Licence.

For brakes, this meant that a largely uniform system design was established – 4 wheel (drum) braking, with hydraulic actuation, and power assistance where commercially viable. The modern pedal layout (CBA) was standardised. Even at the extremes of the performance envelope, the drum design was the brake of choice, with greater thermal mass and cooling airflow being used to rein in the prodigious vehicle speeds.

In the 1950s, significant developments started to change brake designs. Chief among these was the arrival of disc brake designs that could outperform the drums of the day, and therefore unlock higher vehicle speed and power. Jaguar equipped their C-Type with disc brakes for it’s 1953 Le Mans entry, in a bid to save weight and deal with more engine power. The C-Type had already won in 1951, running on drums – and was itself an adaption of the XK120 – the fastest production car at the time. The disc brakes were carried over into the Jaguar D-Type, which went on to win Le Mans in 1955, 56 and 57.

By now, disc brakes had also made a less than promising debut in Grand Prix racing – on the BRM Type 15. The rules around the newly established Formula 1 changed repeatedly in the 1950s, resulting in many short-lived design ideas (including a brief appearance of 4WD, courtesy of Ireland’s Greatest Transport Engineer), but by the end of the decade, disc brakes were being used by all teams.

Meanwhile, on road cars, discs and callipers had made a fleeting appearance on a small US vehicle, the Crosley Hotshot, which debuted in 1949. These were aircraft spec parts, and their adoption to road vehicles proved more problematic than the Crosley team could stand – they switched back to drums within 6 months of production!

Mainstream adoption of discs and callipers was achieved by the Citroen DS, which featured inboard front discs, with conventional drums on the rear. Launched in 1955 to global acclaim, it featured the stunning integrated hydraulic system providing power brakes and steering, gearbox and engine, as well as self-levelling hydro-pneumatic suspension. Oh, and the first radial tyres from Michelin.

Fig. 5: A Citroen DS

As the 1950s closed, disc brakes had gained acceptance in most mainstream racing, but were still growing in popularity with road cars. Over the next decade (1960s), the technology made its way into the large majority of road cars as well.

A pause for breath…

During the early 1970s, the automotive world experienced another political input, two significant crisis of oil supply – which for a while threatened to significantly retract the production and availability of petroleum-based fuels. This lead to a dramatic change in consumer preferences, and fuel efficiency became a significant topic of research. The initial effect was that vehicle power levels declined slightly, before eventually recovering, but vehicle weight declined rapidly, which in turn drove a spike in efficiency improvements.

Fig. 6: US average car weight (lbs) 1975 to 2010 – source EPA
Fig. 7: US average car fuel economy (MPG) 1975 to 2010 – source EPA
Fig. 8: US average car power (HP) 1975 to 2010 – source EPA

From a brakes perspective, this left little to be done in pushing the traditional boundaries – vehicle power and weight declined, meaning the brakes from the start of the 1970s were still capable a few decades later.

The arrival of smart brakes

At the same time, Anti-lock brakes were being researched in earnest across the globe. In a previous post, Harry Ferguson’s work in F1 and road cars was covered – the first recipient of this being the Jensen FF in 1965. At around the same time, Ford brought it’s Sure-Track system to market in the US (rear wheels only), followed by GM offering a similar, 4-wheel anti-lock system in 1971. These systems were followed by similar systems in Japan and Europe. In 1978, Mercedes and Bosch presented ABS2 – the first digital, multi- channel system – a precursor to today’s familiar systems. The significant changes here included induction-based Wheel Speed Sensors, and a single modulator unit controlling all 4 wheels. In less than a decade, this technology was to become standard on all Mercedes products, and most manufacturers followed suit in the 1980s.

While the performance and complexity of the control systems advanced greatly over the next decade, the 1990s brought some very interesting technology to brakes. First of these was Electronic Stability Control – introduced by Mercedes and Toyota in 1995. This was followed in 1999 by the first Adaptive Cruise Control system (again Mercedes with Distronic).

Fig. 9: Modern ice lake test facility, central to ESC development

Also during this time, there was a renewed interest in electric and hybrid electric vehicles. In particular, California’s ZEV Mandate created some small series EVs from major OEMs. In Japan, Honda and Toyota created popular hybrid vehicles – the Insight and Prius, respectively. These all featured regenerative braking, and the growing interest in fuel efficiency meant brake energy recovery came back into popularity (a full century after it had first been developed).

21st Century Stopping

As the 21st century dawned, road cars got a new choice of friction material with the launch of Carbon Silicon Carbide discs, on the Porsche 911 GT2 in 2001 (“quickly” followed by the Ferrari Enzo, Porsche Carrera GT and Mercedes McLaren SLR). While Carbon/Carbon discs had been in regular use in motorsport, these discs were unsuitable for road use. The original discs were developed for Concorde and TGV – another fine example of French braking ingenuity. The promise of long lasting, fade-free, braking meant the technology was adopted for many performance vehicles, but the high material processing costs means it still remains a rather exotic variant.

Carbon brake rotors glowing from heat generated on them under full braking at 130mph entry into Turn 2
Fig. 10 : Porsche 911 with glowing CSiC brakes. Image used under CC licence

Braking by wire – where a direct physical connection between the driver’s input and brake output – has been another significant technological addition to 21st Century brakes. In the case of conventional vehicles, such systems can offer advantages in hydraulic pressure control, and extra functionality (traffic assist, for example). In hybrid or electric vehicles, the brake system can switch between friction and electric braking – thereby recovering brake energy. The renewed interest in electrified road transport this century has lead to significant research in this area of braking, and seen new and complex actuation and control methods deployed. With ever greater levels of regenerative braking, the ability and capacity to deploy an indirect brake response to the driver’s input will become a central tenant of braking system design.

So that brings us pretty much up to date. The second century of automotive braking can be summarised in two distinct phases for hardware; the 1920s to 1950s gave a standardised approach centred around drum brakes, and from the 1960s discs move into the mainstream. All the while, engine power is gradually creeping up, feeding the need for better (or at least bigger) brakes. The 1970s sees the advent of intelligent brakes, and lays the foundations for a blossoming in new brake features and functions. Brakes become disconnected from the driver, and a layer of software is added in between to achieve a variety of different aims; safer vehicles, less accidents, less driver fatigue, and of course, greater vehicle efficiency.

Next time, the focus will be on where the third century of braking will go, with a view of the context over the first 200 years, and projections for what might lie ahead in the decades to come.