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Computer Overview

Why We Use Computers

Computers are to a cyclist, what a compass and sextant was to Renaissance oceanic navigators, it gives you a fix on your position at the moment. Cycling computers work using a ring with small magnets that is placed over the axle and hub of the front wheel. There is a magnetic sensor that is fastened to the fork in close proximity to the magnetic ring. As the ring revolves, the fork sensor notices each time a magnet goes past and tells the computer head that it just "saw" one. The computer has a look-up table of different wheel dimensions that equate to a specific circumference and the length of that circumference. Once you've keyed in your wheel diameter code the computer understands that each time it "sees" the wheel magnet that the wheel has revolved once and the computer multiplies the magnet RPM times the wheel circumference length times 60 (60 minutes to the hour), then divides that inches per hour by 63,360 (the number of inches in a mile) and arrives at MPH. Here's the expression: ((magnet RPM * wheel circumference) * 60)) / 63,360 = MPH Cycling computers with greater accuracy will have a large number of magnets in the ring that are seen and used in the calculation. With three times the magnets, in theory the information is three times as accurate. In practice, each of the makers claim to be accurate to within 0.5 MPH.

Computer Features and Functions

Let's discuss computer features and how they work. Understand in nearly every case the computer is just rapidly doing distance versus time calculations. Current Speed is simple enough, we also call it a speedometer. The speedometer merely displays the result of the above calculation each second.

Clock and Stopwatch

Each of these computers have an internal clock driven by a quartz crystal. Not all of them display the time, however many of them do. The internal clock is used to establish wait states and access times for the data bus to the microprocessor (MPU). Many of the makers use this internal clock to generate a 12 or 24 hour clock and stopwatch. The stopwatch is called "elapsed time" or "timer" depending on the manufacturer, but it's the same function.

Tripmeter or Trip Distance

The computer has a storage register that it saves for information regarding short term "trips". The register is resettable, and every time you press the button indicating that you are starting another trip, and the computer sets the trip meter to zero, you really just initialized the register. The trip meter keeps a record of how many times the magnet has been "seen" since the last reset and multiplies them by the wheel circumference (in inches) then divides that number by 63,360 (inches in a mile), rounds the nearest ten or hundredth and displays the result. The expression is ((magnet RPM * wheel circumference) / 63,360) rounded = distance in miles. This number is displayed, stored and updated continually. It is set to zero only on a reset or battery removal.

Odometer or Total Distance

The odometer takes the result of the previous calculation and adds it another storage register that is resettable only by battery removal, or when the mileage traveled exceeds the bit width of the storage register, or internal programing in reaching a specific number, when it resets automatically to zero. In this way it keeps track of the total number of miles or kilometers you have traveled with the computer.

Average Speed

which is really Average Speed for this "trip". This is a function of the clock and the trip computer. When you set the trip meter to zero on these computers, you also reset and start the stopwatch. The two functions work together. To arrive at your average speed (again, really average speed for this trip), the computer looks at the Stopwatch and divides that number of seconds into 3,600 (number of seconds in an hour) to arrive at a decimal hour quantity. This number is then divided by the distance in the Trip Meter (in decimal miles) and displays your average speed over the period of your ride. The expression is (Time Elapsed / seconds in hour) / trip distance = Average Speed

Maximum Speed

which is really Maximum Speed for this "trip", is an easy one. This is also tied to the Tripmeter. When the tripmeter is reset as you go on your ride, a storage register in the computer for Maximum Speed is reset to zero. The MicroProcessor Unit (MPU) looks at the result of the Current Speed calculation and asks "is this greater than the number in the Maximum Speed storage register already?". If the answer is "yes" the microprocessor updates the memory location with the new, faster speed. When you press for the Maximum Speed to be displayed, it shows you the number stored in this storage register.

Auto Start and Stop

Several of the computers have a switch built into the MicroProcessor Unit. When the MPU notices the magnet as moving and "wakes up" the computer. Many of them will also turn the computer off, if there is no magnetic pulse, after a period of time. In some instances, the MPU merely "suspends" processing and stops the computer, while you take a rest. When you get back on your bike and it notices magnetic activity, it activates processing once more, just where it left off.


This is the number of revolutions per minutes that the cranks revolve. It is counted by the MPU using a similar magnetic detection system. In this case a magnet is fastened to your crank arm and a detector of fastened to the seat tube above the front changer. Each time the magnet passes the sensor the microprocessor counts "1". The microprocessor measures the time interval between each two pulses in decimal seconds and then divides 60 by that time interval to arrive at Cadence (revolutions per minutes). The expression is 60 / (pulse B time - pulse A time) = RPM


There is a least one computer that keeps altitude information. The unit uses a sensor that detects air pressure and notices it's change relative to initial calibration. To be useful, you would have to know the elevation you start from on each trip, to calibrate the unit against that day's atmospheric barometric pressure. The computer with the altimetric functions will keep a record of total elevation gained or lost, (the odometer of the altimeter set), and has a trip altimeter, which is storage register that is updated and compared by the MPU to see if you've added or subtracted elevation against the number already held in storage.

Splits and Memory Locations

There is at least one computer that will keep stored for you multiple "splits". If you were to take a mile run around a quarter mile race track, you would have to do four "laps", (remember?). What you've really done is "split" the mile in 4 quarter mile segments. A "split" is a segment of a total course, and in racing it refers to the time taken to cover a known segment. In a foot race, the segment is a lap, in a bike races it's a "stage" or "lap". This information is kept in a storage register, or multiple storage registers. The ultimate reset, to zero everything in the computer, for all these models is the removal of their battery. Fork or Chainstay mounting? Most of these computers come with a front fork mounted magnetic sensor. If you want to use them while on a trainer in the winter time, it would be difficult, because many of the trainers work only with the front wheel removed. Avocet makes a separate wire harness for the rear wheel that can used with all their computers, however you cannot use the cadence function on the model 50 with it. We include the wire length with all the computers, so you can measure your bike to see if the sensor will reach the rear wheel, should you want it rear mounted. Naturally, the wireless models can be mounted with no difficulty on the rear wheel. Also, practically speaking, front wheel mounting increases the accuracy of the computer, the driving forces on the rear wheel, and rear braking, (skidding), will decrease, on the computer, the actual distance traveled. Several of the more complex computers have more than one line of information at a time. Using one to three lines, on the display, it's possible to monitor several functions simultaneously. When a computer has more than one line of display, we mention, and the height of the characters on the display line so you can get a feel for it's readability at a riding distance. We describe these multi-line display as having an upper, middle and lower display line. More than one of the Liquid Crystal Displays in our graphic images of the computers appear "fuzzy". This is because all of these computers and heart rate monitors use older "passive" LCDs, rather than the newer "active-matrix" LCDs. Passive LCDs keep all the elements operating continuously with pulses at half voltage, stepping up to full voltage on the display element when the element is switched "on". In active matrix LCDs the two outer sheets of glass in which the Liquid Crystal is trapped, carry thin films of connecting transistors etched on to their inner surfaces. Each of these transistors is, in effect, a high speed switch that rapidly turns on, or off, to polarized the liquid crystal at that moment and thus lets light through the display. Passive LCD displays under close examination are not visually as "stable", they are really pulsed several times a second, with a small electric half or full voltage charge to turn "on" the element in the display, which creates the character or numeral. This pulsing, or flickering, of the character element is picked up by our electronic camera, (remember all of these images or "photographs" are really digitized broadcast video images captured in full color), and delivers an LCD character that is not as sharp as you are used to seeing on an LCD display. All of the displays on these computers really give the same crisp, sharp character you are used to seeing on a common LCD, we just couldn't capture them with the clarity that we are all used to seeing. Where this fuzziness exists, please use you imagination and realize that they are, in fact, well defined crisp characters. Home hobbyists or mechanical engineers might find the cadence funtion on these cyclometers useful for other counting applications. If you can fasten the "wheel" magnet to something that will pass in front of the cadence sensor, on each "count", these cadence computers can used toi count finished goods, work in progress, interim production rates. The magentic switch can even be replaced with a leaf, roller, or reed switch. Wild Huh?

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