A recent trip to the bike store revealed dozens of units and it's nearly impossible to evaluate every one. Suffice it to say that from ten dollars to more than a hundred, there's a cycle computer for every possible need.
How It Works
At the most basic level, a cycle computer functions by measuring the rotations of the wheels. This measurement translates into any amount of information depending on how sophisticated the unit is, and whether the unit gathers data from any other inputs.
On most computers, a magnetic attachment is placed on the spokes of the front wheel and then a sensor is placed on the fork. Even wireless units — very much in vogue and at the top of the totem pole of price points — still stick the sensor on the fork and spoke but send the information to the computer via radio frequency, removing the need to run wires up the fork and to the handlebars.
In all cases, the circumference of the wheel is programmed into the computer (a process that is as simple as pressing a few buttons or as complicated as a lunar landing, depending on the unit). As the magnetic unit passes the sensor an electrical impulse is sent to the computer. The faster the wheel goes, the more frequently the sensor receives the impulse. More advanced units use sensors on other locations to determine cadence (pedaling speed) and gear position that can provide valuable information to the cyclist.Basic Features
All cycle computers are capable of providing speed measurements, and most will compute a ride's average speed, maximum speed, and total distance. As units increase in price and sophistication they begin to offer trip distances, total distances, stopwatches, lap counters, and more. Most units are controlled by buttons located in (hopefully) convenient locations near the display. Large, easily actuated buttons are preferable to small ones. Trying to change a setting on a bike using a computer that requires the simultaneous pressing of several small buttons can be dangerous.
A clear and large display area is another key feature of a computer. The device may be able to accurately predict winning lottery numbers and calculate pi to the 30th digit, but if you can't read the display while on a bumpy trail, it's useless. Many computers display the most important information in a large space, with less frequently used data in smaller regions. This works well, until the cyclist happens to need one of those small bits of data, at which point the rider is relegated to bending over and squinting at the display.
Most computers have no backlighting, owing to the fact that many cyclists only ride during the day; those riding at night tend to be very busy with the task of seeing more important things. I've found that a backlit display is a godsend at night, which is when I spend a lot of my time on my road bike. Without backlighting the low-contrast LCD displays become invisible, requiring frequent stops under streetlights for readings.
Some units allow for removal of the computing device from the handlebar-mounting bracket. Removable computers can be taken off to prevent rain damage while on a roof rack, and for storage to prevent damage in transit. On the downside, they are more prone to fall off or be stolen.
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