Speedometer, what is it and which one to choose?

November 2, 2021

The speedometer shows or indicates the speed at which the vehicle is running. It is usually done in combination with the odometer that records the total distance traveled. The speedometer is driven by a gear mounted on the rear end of the driveshaft, the movement is transmitted to the speedometer by means of a flexible cable.

Speed ​​is indicated in kilometers per hour. The odometer records the total distance traveled after the car leaves the assembly line. Some of the recent cars are equipped with two odometers, one of which is a small capacity type that can be reset to zero. This short distance odometer is generally called a trip meter.

Now universally equipped for motor vehicles, they became available as options in the 1900s and as standard equipment from about 1910 onward. Speedometers for other vehicles have specific names and use other means to detect speed. For a ship, this is a well log. For an aircraft, this is a speedometer .

Next, we will talk in detail about this equipment that, like many others, such as the Wattmeter or Voltmeter, began as a simple laboratory device of the brightest minds on the planet. Join us!

History of speedometers

Otto Schulze , an inventor from Strasbourg, filed the first patent for the eddy current speedometer in 1902. Schulze envisioned the revolutionary device as a solution to a growing problem. Cars were not only becoming more popular, they were traveling faster as well. The average top speed of a car just after the turn of the 20th century was 30 miles per hour, slow by today’s standards but very fast at a time when much of the world was still moving at the leisurely pace of a horse-drawn carriage. .

As a result, serious accidents began to increase dramatically. Schulze’s invention allowed drivers to see exactly how fast they were traveling and to make adjustments accordingly. At the same time, many countries set speed limits and used police officers to enforce them.

Early solutions required cars to have speedometers with two dials : a small dial for the driver and a much larger dial mounted so that the police could read it from a distance.

How is speed measured?

Speed ​​is defined very simply: it is the distance it travels divided by the time it takes. So if you go 200 kilometers and it takes you four hours to do it, your average speed is 50 kilometers per hour. Actually measuring your average speed after you’ve traveled isn’t much help.

What you need to know as a motorist is your instantaneous speed. The speed that is going at a given moment. Discovering that is much more difficult than you think. If you have seen traffic cops, or speed cameras on the side of the road, you will probably find that they use radar beams to check speeds.

The radar gun, either handheld or mounted inside the speed camera, shoots an invisible electromagnetic beam at your car at the speed of light . Your car reflects the beam back and modifies it very slightly. The weapon discovers how the lightning was affected and, from that, calculates your speed.

Now, in theory, we could all have radar guns mounted on our cars, shooting beams at lamp posts and buildings and waiting for the reflections to return.

What we really need is a way to find out how fast the car’s wheels are spinning. If we know how big the wheels are, we can calculate the speed quite easily. But how do you measure the rotational speed of a wheel? Even that problem is not simple.

Imagine how much more difficult it must have seemed in the early days of motorsport, in 1902, when the German engineer Otto Schulze invented the first practical solution: the eddy current speedometer.

How does a speedometer work?

Originally patented by Otto Schultze on October 7, 1902 , the speedometer uses a rotating flexible cable usually driven by gears linked to the output of the vehicle’s transmission. However, early Volkswagen Beetles and many motorcycles use a cable driven by a front wheel.

When the vehicle is in motion, a speedometer gear set rotates a speedometer cable, which then rotates the speedometer mechanism. A small permanent magnet attached to the speedometer cable interacts with a small aluminum cup, called a speed cup, attached to the pointer shaft on the analog speedometer instrument.

As the magnet rotates near the cup, the changing magnetic field produces eddy currents in the cup, which in turn produce another magnetic field. The effect is that the magnet exerts a torque on the cup, “dragging” it and, therefore, the speedometer pointer, in the direction of its rotation without any mechanical connection between them.

One of the key disadvantages of the eddy current speedometer is that it cannot display vehicle speed when running in reverse, as the cup would rotate in the opposite direction; in this scenario, the needle would be pushed against its mechanical stop pin at the zero position.

Types of Speedometers

There are two types of speedometers: electronic and mechanical . Because the electronic speedometer is actually a relatively new invention, the first fully electronic speedometer didn’t appear until 1993.

Electronic speedometers

Many modern speedometers are electronic. In designs derived from earlier eddy current models, a transmission mounted rotation sensor delivers a series of electronic pulses the frequency of which corresponds to the rotational speed of the driveshaft and thus the speed of the vehicle, assuming that the wheels have all-wheel drive.

The sensor is typically a set of one or more magnets mounted on the output shaft, differential crown wheel, or a toothed metal disc positioned between a magnet and a magnetic field sensor. As the part in question rotates, the magnets or teeth pass under the sensor, each time producing a pulse on the sensor as they affect the intensity of the magnetic field it is measuring.

Alternatively, particularly in multi-wired vehicles , some manufacturers use the pulses from the ABS wheel sensors that communicate with the instrument panel. Most of the modern electronic speedometers have the additional capability over the eddy current type to show the speed of the vehicle when moving in reverse.

Mechanical speedometers

The centrifugal type uses the centrifugal force of two ball weights. As the shaft rotates, the balls move outward and lift the toothed tube, which rotates the pinion and pointer to indicate speed.

As the speedometer cable rotates, the magnet rotates at the same speed . The rotating magnet creates a fluctuating magnetic field inside the speed cup, and according to the laws of electromagnetism, that means that electrical currents also flow inside the cup. In effect, the speed cup becomes a kind of electricity generator.

But, unlike a proper generator, the kind that generates electricity for your home at a power plant, the currents in the speed cup have nowhere to go. There is nothing to take away their energy. Then the currents just swim uselessly in eddies. Since they are electrical currents and move on an electrical conductor within a magnetic field, another law of electromagnetism says that they will create motion.

Other mechanical speedometers

In addition to eddy currents and rotating cables, inventors in the late 19th and early 20th centuries tried other ways of measuring speed using ingenious mechanical methods. There were governor speedometers, for example, that functioned a bit like centrifugal governors on steam engines. With weights that rose higher as the axle extended faster. The weights were connected to a lever that pushed a needle up and down on a dial to indicate speed.

Speedometers on bicycles

The speedometers measure the time between each revolution of the wheel and read on a small digital display mounted on the handlebar. The sensor is mounted on the bike in a fixed location, pulsing as the spoke mounted magnet passes.

In this way, it is analogous to an electronic car speedometer that uses pulses from an ABS sensor, but with a much cruder resolution of time and distance, usually one pulse and display update per revolution, or as rarely as once each. 2-3 seconds at low speed with a 26 inch wheel.

However, this is rarely a critical issue, and the system provides frequent updates at higher road speeds where information is most important.

Speedometer calibration

All speedometers must be calibrated to ensure that the torque created by the magnetic field accurately reflects the speed of the car. This calibration must take into account several factors, including the gear ratios in the transmission cable. The final drive ratio in the differential and the diameter of the tires. All of these factors affect the overall speed of the vehicle.

Speedometer Accuracy

No speedometer can be 100 percent accurate. In fact, most manufacturers build speedometers to fall within a fairly tight tolerance range. No more than 1 percent to 5 percent too slow or too fast. As long as a car is kept to factory specifications, its speedometer should continue to record the vehicle’s speed within this range. But, if a car is modified, its speedometer may need to be recalibrated.

Dr. Loony Davis5
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Born and raised in Brussels in an English family, I have always lived in a multicultural environment. After several work experiences in marketing and communication, I came to Smart Water Magazine, which I describe as the most exciting challenge of my career.
I am a person with great restlessness and curiosity to learn, discover what I do not know, as well as reinvent myself daily, someone who is curious about life and wants to know. I enjoy sharing knowledge.
This is my personal project but I also collaborate in other blogs, it is the case, the most important web on water currently exists in the US, if you are interested you can read my articles here.

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