Mechanical odometers have been counting the miles for centuries. Although they are a dying breed, they are incredibly cool because they are so simple! A mechanical odometer is nothing more than a gear train with an incrediblegear ratio.

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­The odometer we took apart for this article (pictured above) has a 1690:1 gear reduction! That means the đầu vào shaft of this odometer has khổng lồ spin 1,690 times before the odometer will register 1 mile.

Odometers like this are being replaced by digital odometers that provide more features & cost less, but they aren"t nearly as cool. In this article, we"ll take a look inside a mechanical odometer, & then we"ll talk about how digital odometers work.


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On the white wheel between the "3" và the "4," there are two pegs. One time per revolution, one of the gear teeth on the white gear falls in between these two pegs, causing the đen gear next to lớn it lớn move one-tenth of a revolution.
You can now see why, when your odometer "rolls over" a large number of digits (say from 19,999 khổng lồ 20,000 miles), the "2" at the far left side of the display may not line up perfectly with the rest of the digits. A tiny amount of gear lash in the white helper gears prevents perfect alignment of all the digits. Usually, the display will have lớn get khổng lồ 21,000 miles before the digits line up well again.

You can also see that mechanical odometers like this one are rewindable. When you run the car in reverse, the odometer actually can go backwards -- it"s just a gear train. In the movie "Ferris Bueller"s Day Off," in the scene where they have the oto up on blocks with the wheels spinning in reverse -- that should"ve worked! In real life, the odometer would"ve turned back. Another trick is lớn hook the odometer"s cable up khổng lồ a drill and run it backwards khổng lồ rewind the miles.

While that does work on older mechanical odometers, it does not work on the new electronic ones, as we will see in the next section...


If you make a trip to lớn the bike shop, you most likely won"t find any cable-driven odometers or speedometers. Instead, you will find bicycle computers. Bicycles with computers like these have a magnet attached khổng lồ one of the wheels and a pickup attached to lớn the frame. Once per revolution of the wheel, the magnet passes by the pickup, generating a voltage in the pickup. The computer counts these voltage spikes, or pulses, & uses them to lớn calculate the distance traveled.

If you have ever installed one of these bike computers, you know that you have khổng lồ program them with the circumference of the wheel. The circumference is the distance traveled when the wheel makes one full revolution. Each time the computer senses a pulse, it adds another wheel circumference lớn the total distance & updates the digital display.


Many modern cars use a system lượt thích this, too. Instead of a magnetic pickup on a wheel, they use a toothed wheel mounted khổng lồ the output đầu ra of the transmission and a magnetic sensor that counts the pulses as each tooth of the wheel goes by. Some cars use a slotted wheel & an optical pickup, like a computer mouse does. Just lượt thích on the bicycle, the computer in the oto knows how much distance the car travels with each pulse, and uses this lớn update the odometer reading.

One of the most interesting things about car odometers is how the information is transmitted to lớn the dashboard. Instead of a spinning cable transmitting the distance signal, the distance (along with a lot of other data) is transmitted over a single wire communications bus from the engine control unit (ECU) to the dashboard. The car is like a local area network with many different devices connected to lớn it. Here are some of the devices that may be connected lớn the computer network in a car:


Many vehicles use a standardized communication protocol, called SAE J1850, to lớn enable all of the different electronics modules to lớn communicate with each other.

The engine control unit counts all of the pulses & keeps track of the overall distance traveled by the car. This means that if someone tries to lớn "roll back" the odometer, the value stored in the ECU will disagree. This value can be read using a diagnostic computer, which all car-dealership service departments have.

Several times per second, the ECU sends out a packet of information consisting of a header và the data. The header is just a number that identifies the packet as a distance reading, & the data is a number corresponding khổng lồ the distance traveled. The instrument panel contains another computer that knows to lớn look for this particular packet, and whenever it sees one it updates the odometer with the new value. In cars with digital odometers, the dashboard simply displays the new value. Cars with analog odometers have a small stepper motor that turns the dials on the odometer.

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For more information on odometers và other often-overlooked car parts, check out the liên kết on the next page.