Don Jones®

Tech | Career | Musings

Speedometers are fascinating devices. At least, the old mechanical ones used to be.

Basically, you’ve got a cable hooked up to part of the car axle. That cable runs through a kind of flexible conduit. As the axle spins, it turns a small gear, which basically “twists” the cable inside the conduit. So the cable is, in effect, a very thin, flexible shaft that can curve to a degree. That cable routes to the backside of the actual speedometer.

The cable connects to a magnet. As the cable spins, the magnet spins. The magnet “catches” (magnetically) a small metal bar, which is the backside of the speedometer needle. The needle is also attached to a very thin, coiled hairspring.

So, picture this happening in slo-mo: the magnet twirls. As it does so, the metal bar on the back of the needle is attracted to the magnet, and tries to spin with it. As the magnet completes its rotation, the needle loses its magnetic connection and the spring starts to return the needle to zero. But! Just then the ,agent comes around for another go, and catches the needle before it hits zero.

Basically, your “speed” is the balance point between how far the magnet can turn the bar and how far the spring can return the needle, before the magnet comes around again. The only reason the needle seems steady is because, even at slow speeds, this all happens so fast. On older cars where the magnet is wearing out, it’s not uncommon to see “needle jitter” on the low end of the meter, though, as the needle tries to swing between two points a bit more slowly than it should.

That same cable, coming from the axle, is what advances the mechanical odometer, too, and one some cars it feeds the cruise control mechanism as well.

Cool, right?

2 thoughts on “[tmyk] How Speedometers (Used to) Work

  1. AlfieJ says:

    Fascinating, but with so many rapidly moving parts I’m surprised that speedometers don’t fail on a more regular basis. I guess most are digital these days – how about posting an article on how those work?

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  2. Alfred DeFreitas says:

    I can give you an overview of how digital speedometers work. Somewhere in the drivetrain, (could be the driveshaft, or transmission) there is a sensor or “encoder” that indicates each time the shaft rotates. This can be a magnet rotating and triggering a sensor, or a beam of light being interrupted by a spinning encoder. The faster the shaft rotates the more pulses per second you get from the encoder. There is a direct correlation between the number of pulses per second and the speed you are traveling. These pulses are read by a circuit that counts the time interval between pulses and calculates the speed that you are traveling. This value is then converted to a format that can be displayed on the dashboard, usually a digital readout as well as some graphical representation. There is also usually some electronic “dampening applied, so that the display does’n get too jittery.

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