How do speedos work

Speedometers are innovative devices that use electromagnetism to send signals from the physical wheel speed to the dash display. The how and why behind this is quite interesting from legal and technical standpoints and to learn the reasons, we need to go back a little in history.

Standard speedometers have been keeping a check on driving speeds since the early s. The very first standard mechanical speedometer, also called the eddy-current speedometer was patented and later developed by Otto Schulze, an inventor from Strasbourg. Speedometers in the 20 th century used to have two separate dials.

While the smaller dial was for the motorist to check and adjust speed accordingly, the bigger one was for the police for monitoring purposes. Today, we have moved to digital speedometers — an instrument cluster or a digital dash that shows an electronic readout of the speed and various other elements on the dashboard screen. In a mechanical setup, a speedometer uses analogue devices that attach a drive cable directly from the transmission to the dial.

By: William Harris. Photo courtesy of Dreamstime. Cite This! Print Citation. Try Our Sudoku Puzzles! More Awesome Stuff. The other end of the cable fits to a drive shaft leading into the speedometer. On the end of this shaft is a magnet. Positioned close to but not touching the magnet is a cup-shaped metal drum that is attached to the needle giving the reading on the dial.

A small coiled hair spring holds the needle at zero. The drum is attracted by the magnet so, as the magnet turns, the drum turns too. The faster the car is travelling, the greater the pull of the magnet on the metal drum and the further the needle moves round the dial. But the restraining force of the hair spring also increases as the needle moves round the dial.

At a certain point the forces of the spring and the magnet balance out and the needle steadies to give a reading. The two other common types of mechanical speedometer give the reading by a bar or a mark moving along a straight calibrated scale. Both are roughly similar in operation to the round dial type of speedometer - a cable driven by the gearbox output shaft turns a magnet which causes some sort of indicator to move against the force of a restraining hair spring.

In one type the indicator consists of a moving ribbon attached at each end to a spool. The magnet causes the ribbon to reel off one spool to the other against the force of the hair spring. As the ribbon moves, a mark on it lines up with the calibrated scale to give a reading. The other type of indicator using a straight scale rather than a dial has a barrel marked with a line.