how do speedometers work?
In: Technology
I think cars measure the speed of the tires spinning. if they spin X amount of times in a Y amount of time, then you get your speed. That’s why it’s important to get the proper tires fitted for your vehicle.
Traditionally they’d be made with something called a d’arsonval meter. Basically a metal coil, a magnet, and a spring. Put more current through the coil and it rotates against the spring, put less and the spring pulls it back. So the angle of the turn correlates to the amount of current applied.
Then you have a wired coil on your axle spinning in a magnetic field and it will generate a current that corresponds to your speed (or at least the speed of your axle). Put the two together and you have a surprisingly accurate measurement of speed.
Today, they probably do something similar but with an analog to digital converter in there somewhere that digitizes the axles speed and sends it to the dash as a digital signal.
The wheels spin a flexible shaft that looks like a cable. This shaft has a magnet on the other end that sits inside another circular magnet that, via a spring and a couple of gears, is attached to the needle in the speedometer. When the shaft spins, the other magnet wants to spin and twist the spring/gears, but it’s designed with some resistance to stop it from spinning; spinning the shaft faster lets the other magnet overcome that resistance. Because the resistance has been specifically designed, the speed the shaft and, therefore, the wheels, need to spin at correlates directly with the numbers written on the speedometer, giving you your speed.
Note that because of this, the wheel is effectively a gear, giving the reason why you need to put the right sized wheels on your car; if you change the size of the wheels, the original speedometer gives the wrong reading.
There’s a ring behind the brake that rotates and the car computer watches that speed and calculates the speed and displays it on the speedometer readout.
It’s all computer maths.
Most of these comments are about old school speedometers, built before about 2000 for the most part. Any modern car, specifically any car with ABS braking, measures the speed with what is called the ABS ring. A toothed ring mounted just behind the brake assembly on each wheel. The car computer constantly monitors the speed that the ring turns, by counting the teeth as they pass a special sensor. This both provides the speed for the speedometer, and also let’s the computer know if one wheel is either skidding, to active the ABS system, or spinning, to activate traction control.
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