Speedometers

This article was first published in 2004.

The most important instrument in the motor vehicle has passed a remarkable milestone. Just over one hundred years ago, the eddy current speedometer was patented by engineer Otto Schulze at the Imperial Patent Office in Berlin. The invention heralded the introduction of measuring instruments in the automobile. Very soon, drivers were getting a whole lot more than just current speed. Driven by the growing demand for information at the wheel, and the dictates of public authorities, the speedometer developed from an interesting option to the heart of modern driver information systems.

At the outset of the automobile age, motorists had little need for a speedometer. Vehicles were so slow that drivers could literally sense how fast they were going and adjust their speed. The dangers associated with motoring were comparatively limited. Only with increasing horsepower and greater speeds did the early “horseless carriage” begin to travel faster than the driver’s brain was able to reliably gauge and evaluate. The speedometer, until this time a curious but dispensable gadget, then became much more useful. Motorists were able to control their vehicle speed precisely and reliably, with less worry about pushing a vulnerable machine beyond its limits. Government officials also heavily promoted the use of speedometers. They viewed with alarm the increasing speeds of vehicles, growing traffic density and the number of serious accidents as justification for the first speed limits on public roads. And motorists needed speedometers to avoid breaking the law.

The Technical History

Widespread interest in gauging speed did not appear until around 1835 with the emergence of the railroad. At the time, numerous speedometers were developed, but they were more or less unsuitable for use in the automobile. It was Otto Schulze, the inventor from Strasbourg, who finally succeeded in developing a system for road vehicles with his ‘eddy current speedometer’.

Schulze used a flexible shaft to transmit the rotational speed of the wheel or transmission to the speedometer, which was equipped with a permanent magnet in constant rotation. Above the speedo magnet – but not in direct contact – was a metal disc or metal cup attached to a pointer. The revolving magnet induced “eddy currents” which the metal disc and pointer tried to follow. However, because the metal disc was attached to a spring, it could only manage a slight turning movement instead of a complete rotation. With the increasing speed of the magnet, the force of the eddy currents rises so that the pointer on the metal disc indicates a higher speed.

It was not until 1910 that automobile manufacturers such as Ford Motor Co. began to include the speedometer as standard equipment. But the First World War and the world economic depression of the 1930s kept production down. Nevertheless, there were major advances. By the mid-1930s, an instrument cluster attached to the steering column grouped all the important gauges and indicators together, including those for engine revs, fuel, lights and turn signals. Although integration at that time was limited to a common mounting plate for separate scales and dials – a bracket really – it was nevertheless the first step towards combining several different instruments into a single instrument cluster.

It is not until the mid-1950s that automobile speed measurement enjoyed its next major technology leap with the introduction of the electric speedo - originally developed by VDO for city buses. Instead of communicating wheel revolutions by means of a lengthy shaft, the rotational speed of the wheel or transmission was transformed into an electric signal with a dynamo. An electric motor on the speedometer then used this current to move the needle. Parallel to this development, VDO developed a moving coil instrument in which the voltage generated by the dynamo is displayed as the current speed. Also, a stepper motor was developed to drive the odometer. It all fitted into an 80mm housing that remained the worldwide speedometer standard well into the 1980s.

The age of the electronic speedometer began in the 1980s. Roller counters for mileage were replaced by liquid crystal displays. Sensors in the vehicle network took over the role of shafts, dynamos and rotating magnets. Stepper motors again transformed electric pulses into pointer deflections using electronic control systems. All the components were grouped tightly together in the driver’s line of sight. At a glance, the driver could monitor complex information with a choice of classic round instruments, text-based displays, indicator lights and large monitors, or LCD for navigation and communication. The invention of the light emitting diode, or LED, opened up completely new solutions for the illumination of the instrument panel.

The graphic presentation of information has always been subject to certain fashion trends. Sometimes there’s a roller instead of the round dial. Perhaps only a section of the scale is displayed. There may even be a preference for speed to be displayed in large digital figures behind the steering wheel. Still, during the last 100 years, the instrument has retained its dominant position.

Let’s take a look at how the speedo has visually changed over the last century.

Speedos Through the Years

The date of birth of the speedo is generally regarded as 7 October 1902, when the Imperial Patent Office in Berlingranted Otto Schulze a patent for the eddy current speed indicator, based on an electromagnetic principle. This image shows an O. S. eddy current speedometer from 1908.

In the 1920s the company OTA (predecessor company of Siemens VDO Automotive AG) advertised its instrument panels.

In the 1920s, not long after the speedometer became standard equipment on many cars, it got a brother: the tachograph. This instrument displays vehicle speed and makes a record of all speeds during an entire trip. The “Autograph” of 1926 additionally recorded the travelled distance and was therefore also knows as time/distance recorder.

The speedometer was driven by a flexible shaft that was initially connected to the wheel hub. For space reasons a further innovation in the design arrived in 1930 - speedometers with angled transmissions.

In 1933, Kienzle introduced the TCO2 tachograph, a model which formed the cornerstone of the company’s success for the next 30 years. The heart of the TCO2 was a clock, a needle of steel or sapphire and a wax-coated disc which acted as the recording medium.

By the 1940s a typical eddy current speedometer had already become a sophisticated instrument with some 140 parts, including many needing to be precision machined. This image shows a 1950s moped speedometer.

There have been numerous variations in the types of display since the 1950s that followed certain fashions that often did not last long. This is a bicycle speedometer with a “cheese corner” shape.

Horizontal and vertical indicating devices were extremely popular for a time, but later disappeared completely from the automobile. There were flat indicators in both horizontal and vertical form that rotated behind a slot in the indicator. This is a cylindrical speedometer from the 1950s for Opel.

This photo shows a Volkswagen Beetle instrument from 1953 – note the reverse sweep speedo.

On the vast majority of the eddy current speedometers – also on this example of recent times – this driveshaft came out of the back of the speedometer in a straight line with the pointer axis.

The age of the electronic speedometer began in the 1980s. Roller counters for mileage were replaced by liquid crystal displays. Sensors in the vehicle network took over the role of shafts, dynamos and rotating magnets. Shown here is a 1986 VW display.

The classic speedometer was - and still is - a round pointer instrument with either concentric or eccentric scale. Despite all the variations in type and form of the indicator, it is ultimately the original form of the analog round display that has proved to be most popular in the long term.

Only with the development of new cockpit information systems has it been possible to adapt to the changing demands of vehicles during the last 100 years. This graphic shows an exploded view of the BMW 7 Series instrument cluster.

Siemens VDO Automotive has created the first car instrument cluster with a TFT display for the new Audi A8 and thus opened up new degrees of freedom for graphics and design.

Although vehicles today generate more data, commands and messages that have to be transmitted, the driver is informed much faster and far more efficiently than in the past. Siemens VDO Automotive AG is at work on exciting new developments such as the colored head-up display, which will change information management behind the steering wheel dramatically.

This photo shows Siemens VDO Automotive’s “Breeze” cockpit study which – according to the company - integrates innovative operating elements into a design of maritime and Mediterranean flair.

Siemens VDO Automotive AG 1902 Patent application by Otto Schulze to the Imperial Patent Office in Berlin 1905 First production of instruments based on the Otto Schulze patent 1921 OSA Apparate GmbH, Frankfurtis founded by Adolf Schindling, Georg Häuβler and Heinrich Lang for the “manufacture and sale of speedometers for automobiles” 1925 Renamed OTA Apparate for name protection reasons 1928 Merger of the speedometer division of the Deuta Werke, Berlin with OTA Apparate in Frankfurt to form VDO Tachometer AG - Vereinigte Deuta OTA, Frankfurt 1929 Founding of the tachograph manufacturer, Kienzle 1992 Mannesmann-Kienzle is integrated into the VDO Group 1994 Complete takeover of VDO by Mannesmann AG 1997 Renamed Mannesmann VDO AG 2001 Siemens Automotive merges with Mannesmann VDO to form Siemens VDO Automotive AG