Speedometer

[SteveD]

How does a speedometer work?  What is the mechanism it uses to convert the continuous uni-directional rotation of the cable to a bi-directional movement of the needle as the vehicle accelerates and decelerates?

[john91]

I'm not a physics teacher but i'll do my best, on the end of the speedo cable in the speedometer is a magnet, very close to but not touching the magnet is an aluminium disc mounted on a spindle with the speedo pointer on the other end and a hair spring that tries to keep the pointer on the zero mark, when the car moves and the cable revolves the magnet, it induces an electric current in the aluminium disc, just the same as a dynamo, this electric current creates a magnetic field in the disc which is attracted to revolving magnet and turns with it, the faster the magnet turns the more it will drag the disc and pointer, but is prevented from revolving it completely by the spring as there is no mechanical connection between the cable and the disc, just magnetic attraction, thus indicating the correct speed, as the car slows down the magnetic drag reduces and the spring pulls the pointer bact to zero. A similar principle is used in a domestic electric meter, an aluminium disc revolves between a magnet to brake the disc and stop it spinning too fast. The term used for this principle is the "eddy current effect", hope that all makes some sense.

[Hippy]

The mechanism at the speedometer end that john91 describes so well relies on the "Hall Effect" named after scientist E. H. Hall who discovered it in 1879.

Some speedos and tachometers use pulses of electricity generated in a fixed coil from magnets rotating at the geabox output shaft (or camshaft for tachos). The rate of these pulses is directionally proportional to the speed of rotation and can be converted electronically into a varying voltage. This is then displayed in what is no more than a damped voltmeter that you see on the dash. Advantages? No expensive mechanical torsion transmitting cables to wear out, electronics are relatively cheap to mass produce, and the electric wiring can be adapted to any model as it can be routed to suit the car rather than the constraints of the mechanical torsion cable.

[john91]

You've brought me up to date Hippy, I should have said that the last time I saw inside a speedo was about 40 years ago. Thanks.