funcționarea odometrelor

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How Odometers Work

Image Gallery: Car GadgetsOdometers accurately count miles through a system of gears. Seepictures of car gadgets.

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How Speedometers Work

Are gas mileage monitors accurate?

What will speed mean in the future?

Introduction to How Odometers Work

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 trainwith an incrediblegear ratio.

The odometer we took apart for this article (pictured above) has a 1690:1gear reduction!That means

the input shaft of this odometer has to spin 1,690 times before the odometer will register 1 mile.

Odometers like this are being replaced by digital odometersthat provide more features and cost

less, but they aren't nearly as cool. In this article, we'll take a look inside a mechanical odometer, and

then we'll talk about how digital odometers work.

The worm gear reductions are visible in this picture.

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Mechanical Odometers
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Mechanical odometers are turned by a flexible cablemade from a tightly wound spring. The cable

usually spins inside a protective metal tube with a rubber housing. On abicycle,a little wheel rolling

against the bike wheel turns the cable, and the gear ratio on the odometer has to be calibrated to the

size of this small wheel. On acar,agear engages the output shaft of thetransmission,turning the

cable.

The cable snakes its way up to the instrument panel, where it is connected to the input shaft of the

odometer.

The Gearing

This odometer uses a series of threeworm gearsto achieve its 1690:1gear reduction.The input

shaft drives the first worm, which drives a gear. Each full revolution of the worm only turns the gear

one tooth. That gear turns another worm, which turns another gear, which turns the last worm and

finally the last gear, which is hooked up to the tenth-of-a-mile indicator.

The output of the last worm gear drives a shaft that turns the tenth-of-a-mile indicator.
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Each dial is then turned by pegs on the previous dial through a small helper gear (white).

Each indicator has a row of pegssticking out of one side, and a single set of two pegs on the other

side. When the set of two pegs comes around to the white plastic gears, one of the teeth falls in

between the pegs and turns with the indicator until the pegs pass. This gear also engages one of the

pegs on the next bigger indicator, turning it a tenth of a revolution.

On the white wheel between the "3" and the "4," there are two pegs. One time per revolution, one of the gearteeth on the white gear falls in between these two pegs, causing the black gear next to it to move one-tenth of arevolution.

You can now see why, when your odometer "rolls over" a large number of digits (say from 19,999 to

20,000 miles), the "2" at the far left side of the display may not line up perfectlywith the rest of the


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digits. A tiny amount of gear lashin the white helper gears prevents perfect alignment of all the digits.

Usually, the display will have to get to 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 car 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 to hook the

odometer's cable up to adrill and run it backwards to 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...

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Computerized Odometers

If you make a trip to the bike shop, you most likely won't find any cable-driven odometers or

speedometers. Instead, you will find bicycle computers.Bicycles withcomputers like these have

a magnetattached to one of the wheels and a pickupattached to the frame. Once per revolution of

the wheel, themagnet passes by the pickup, generating avoltage in the pickup. The computer counts

these voltage spikes, or pulses, and uses them to calculate the distance traveled.

If you have ever installed one of these bike computers, you know that you have to 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 to the

total distance and updates the digital display.

Many modern cars use a system like this, too. Instead of a magnetic pickup on a wheel, they use

a toothed wheelmounted to the output of thetransmission and amagnetic sensorthat counts the

pulses as each tooth of the wheel goes by. Some cars use a slotted wheel and an optical pickup, like

acomputer mouse does. Just like on the bicycle, thecomputer in the car knows how much distance

the car travels with each pulse, and uses this to update the odometer reading.

One of the most interesting things about car odometers is how the information is transmitted to the

dashboard. Instead of a spinning cable transmitting the distance signal, the distance (along with a lotof other data) is transmitted over a single wire communications bus from the engine control

unit(ECU) to the dashboard. The car is like alocal area network with many different devices

connected to it. Here are some of the devices that may be connected to the computer network in a car:

Engine control unit (ECU)

Climate control system

Dashboard

Power window controls

Radio

Anti-lock braking system
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Air bag control module

Body control module (operates the interiorlights,etc.)

Transmission control module

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

The engine control unitcounts all of the pulses and keeps track of the overall distance traveled by

the car. This means that if someone tries to "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 and the

data. The header is just a number that identifies the packet as a distance reading, and the data is a

number corresponding to the distance traveled. The instrument panel contains another computer that

knows to 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.

For more information on odometers and other often-overlooked car parts, check out the links on the

next page.

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Lots More Information

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More Great Links

Odometer history

Speedometer/Odometer Calibration Program
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Automotive 101: The Electrical System (An Overview) - Has info on speedometers and odometers

The Federal Odometer Tampering Statutes

Odometer Effect for Serial LCDs

The History of the Odometer

An odometer records the distance that a vehicle travels.

Aspeedometermeasures the speed of a moving vehicle. A tachometer indicates the speed

of rotation of the engine.

Vitruvius

According to Encyclopedia Britannia, "About 15 BC, the Roman architect and engineer

Vitruvius mounted a large wheel of known circumference in a small frame, in much thesame fashion as the wheel is mounted on a wheelbarrow; when it was pushed along the

ground by hand it automatically dropped a pebble into a container at each revolution,

giving a measure of the distance traveled. It was, in effect, the first odometer."

Chang Heng

Chang Heng the inventor of the knownseismograph,also invented an odometer that had a

figure that struck a drum as each li or 0.5 km went by to measure distance.

Blaise Pascal

Blaise Pascal(1623 - 1662) invented a prototype of an odometer, a calculating machine

called a pascaline. The pasacaline was constructed of gears and wheels. Each gear

contained 10 teeth that when moved one complete revolution, advanced a second gearone

place. This is the same principal employed in the mechanical odometer.

Thomas Savery - Odometer used on Ships

Thomas Savery(1650 - 1715) was an English military engineerand inventor who in 1698,

patented the first crude steam engine, among Savery's other inventions was an

odometer for ships, a device that measured distance traveled.

Ben Franklin - Odometer used to Measure Postal Routes

Ben Franklin(1706-1790) is best known as a statesman and writer, however he was also

an inventor who invented swim fins, bifocals, a glass armonica, watertight bulkheads for

ships, the lightning rod, a wood stove, and an odometer. While serving as Postmaster

General in 1775, Franklin decided to analyze the best routes for delivering the mail. He

invented a simpleodometerto help measure the mileage of the routes that he attached to

his carriage.

William Clayton, Orson Pratt, Appleton Milo Harmon - Odometer called the

Roadometer

An odometer called the roadometer was invented in 1847 by the Morman pioneers

crossing the plains from Missouri to Utah. The roadometer attached to a wagon wheeland

counted the revolutions of the wheel as the wagon traveled. It was designed by William

Clayton and Orson Pratt, and built by carpenter Appleton Milo Harmon.

William Claytonwas inspired to invent the roadometer by his first method of recording the

distance the pioneers travelled each day. Clayton had determined that 360 revolutions of

a wagon wheel made a mile, he then tied a red rag to the wheel and counted the

revolutions to keep an accurate record of the mileage travelled. After seven days, this

method became tiresome and Clayton went on to invent the roadometer, first used on
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the morning of May 12, 1847. William Clayton is also known for his writing of the pioneer

hymn "Come, Come, Ye Saints."

"I walked some this afternoon in company with Orson Prattand suggested to him the idea

of fixing a set of wooden cog wheels to the hub of a wagon wheel, in such order as to tell

the exact number of miles we travel each day. He seemed to agree with me that it could

be easily done at a trifling expense."

"Brother Appleton Harmon is working at the machinery for the wagon to tell the distance

we travel and expects to have it in operation tomorrow, which will save me the trouble of

counting, as I have done, during the last four days."

"About noon today Brother Appleton Harmon completed the machinery on the wagon

called a 'roadometer' by adding a wheel to revolve once in ten miles, showing each mile

and also each quarter mile we travel, and then casing the whole over so as to secure it

from the weather." ~ From William Clayton's Journal

Samuel McKeen - Odometer used on Carriage

In 1854, Samuel McKeen of Nova Scotia designed an early version of the odometer, a

device that measures mileage driven. His was attached to the side of a carriage and

measured the miles with the turning of the wheels.

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