Originally Posted by HillRider
To answer the OP in better detail.
A wired cyclometer has a wheel magnet that passes a pickup placed near it on the fork. The pickup generates an electrical pulse every time the magnet passes it. That pulse travels up the wire to the cyclometer head where the pulse is counted and the interval between pulses timed. That information allows the cyclometer to calculate your speed and distance.
This is not even close to correct. Wired computers have a very simple magnetically actuated reed switch in the sensor. Every time the magnet passes the sensor, the switch is closed for that moment. There is no "pulse". It's a very straightforward open or closed circuit. It's a digital circuit in the very simplest form - on, or off. The computer simply measures the period between the "on" states (assume "on" = switch closed) and calculates the speed from the tire circumference you enter.
If you ever suspect your wired sensor, or the 2 conductor cable (i.e, the "wire"), is bad then all you need do is pass the magnet by the sensor while monitoring the two cable conductor ends (which normally terminate at two contact pads on the computer head mounting bracket) with an ohmmeter. The circuit will show "open" or "closed" as you pass the magnet by the sensor (remember, it's a reed switch).
In the case of a wireless computer, you DO have a pulse generated by the sensor. A carrier wave is established between the sensor and the computer head and the carrier is modulated (ok, if it's frequency modulated then you'll get a momentary change in the frequency rather than a pulse on the carrier wave) every time the magnet passes the sensor. The sensor contains a Hall effect device that senses changes in a magnetic field. When cycling computers go screwy at traffic intersections, it's most likely that it's the Hall effect sensor - not the carrier - that's being affected. Trip sensors at intersections are, in effect, like a very big Hall effect sensor. They set up a steady magnetic field and when a large metal object is passed over it, the field sensor detects this and says "car over trip sensor" to the traffic computer.
One of the disadvantages of wireless computers is that, when riding in a tight group, your computer can get false inputs from other riders sensors that may be on the same carrier frequency as what your computer is using. One of my wireless computers will read the sensor as long as it's within 70cm, or 27.5", so it's easy to see why you could get a false input from someone on your shoulder. This is precisely why you see a lot of pros riding with wired units. All that being said, the algorithms in the head units are averaging the inputs out so, over time, they are still very accurate even if they do get the occasional false input. If you're doing a lot of group riding and having computer problems, then a wired computer probably makes sense.