Car speedometer: how does it work and how much does it really tick?

What is a speedometer?

The speedometer measures the speed of the car (from the Greek tachys, speed, and then other terms such as tachograph, tachycardia and many others) and is thus one of the most important components of vehicle instruments. This is why the law has made it mandatory for all vehicles whose speed exceeds 25 km/h. In fact, the obligation was staggered by European standards: the last categories to have the speedometer by law were 2 and 3 wheeled vehicles (July 1, 2001) and motorcycles, as of January 1, 2002.

What is a car speedometer and how does it work?

This tool, which is mandatory by law, allows the driver to know what speed the car is traveling at. We remind you that speed is expressed as the ratio between the distance and the time it takes to travel: for road vehicles, the unit of measurement in kilometers per hour (or miles per hour) is generally used, and is abbreviated to km / h. Speedometer is a deeply passed tool technological development, since the first main series was built in 1923 by Otto Schulze Autometer and its basic design has not changed significantly for 60 years. The principle of operation is to directly calculate the revolutions of the transmission at the output of the gearbox or wheel revolutions: knowing the number of revolutions and the circumference of the wheels, it is possible to extrapolate the vehicle speed from these quantities.

Speedometers have been magnetic for a long time: A flexible wire attached to the transmission rotates a magnet facing a metal disc integrated with both a speedometer needle and a helical spring. The metal disk tends to ‘follow’ the magnet with a force proportional to the speed of rotation of the magnet itself – and thus the speed of the car – but the spring opposes it and so the disk (and the index finger it is attached to) stops in a position balanced between its tendency to rotate and the force of the spring. The higher the speed, the greater the force of the disc and the greater the pressure of the spring, as the pointer moves more and more on the disc.

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The electronics development It contained lines, magnets and stopped springs, which were replaced by pulse generators – connected to the transmission or wheels – and counters for these pulses: the higher the speed, the greater the frequency of the pulses. In this case, the speedometer under the driver’s eyes is an instrument that calculates the pulses per unit time (their number in one second is proportional to the revolutions of the wheel) and calculates the corresponding speed to show the driver, Driving stepper motors Capable of rotating by small fractions of a degree rather than rotating continuously. This data can be displayed in analog form, with a needle moving on a disk (perhaps visualized by default on the screen, picture below), numerically in numerical digits or in both directions.

What quantity does the odometer put?

The electronic speedometers They can be a lot precise, but they are not because their semantics must be greater than the actual speed. In fact, European authorities have made it clear that the speed indicated by the speedometer should not be It will never be lower than the actual speed So that the speed limit cannot be exceeded due to an error in the speedometer. However, there is an upper limit: the indicated speed cannot exceed 110% of the actual plus 4 km / h. If we go to 100 km / h, then the indicated value cannot exceed 114 km / h while at 50 the speedometer can mark no more than 59 km / h.

What to do if the car speedometer does not work?

If the mechanical speedometer no longer works or the needle moves erratically, the first suspect is the elastic cord that is causing the magnet to die. If this is not damaged, the fault may be with the hardware itself. Electronic gadgets have to last longer because they don’t have the right place for the elastic cord. In the event of a malfunction it is though complicated to get your hands But doing an electronic diagnosis (or done, if you are able) will be able to understand the origin of the error.

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