Original or classic cyclometers measure and display distance, time and speed. The monochrome displays were visible even in bright sunlight and under low light condition. The devices could be powered by button cell batteries.
These units have a magnet that clips onto a spoke which rotates the magnet past a sensor. They count rotations and process data to display speed, distance and time. The original models had a wired sensor; modern models have wireless sensors. These devices need to be programmed with the circumference of the wheel to calculate how far the bike moves forward each time the wheel goes through a full revolution. The tire will flex under load; the distance travelled is a little less than the circumference of the inflated tire measured unloaded. Cateye had a chart in its manuals, listing the circumferences for dozens of tires, including several 700c tires from 700c x 18 to 700c x 40. Similar charts are online in support articles by Sheldon Brown and by volunteers in the public knowledge base at newwheel.net. I had a Bontrager (Trek) computer on my Trek hybrid – it gave the user choices of tire size in a menu rather picking a circumference in the menu. These systems assume uniform tire sizes, inflated to the rated/marked maximum. The circumference of a tire on a wheel is affected by the tire pressure. It is small inaccuracy, only a centimer in 200 (½ of one percent). Most units could be calibrated to one bike; a few could be programmed to two bikes. They may pause and appear to “sleep” if the rider stops for longer than a couple of minutes. It depend on the device, default settings and user choices. Setting them up is time consuming and balky.
GPS was not available to cyclists until the US goverment allowed non military users, after the year 2000, to receive satellite signals and calculate position on the ground to within 5 meters. This provided enough accuracy for navigation and tracking distance and speed. A cycling GPS head unit will measure distance accurately and “save” the ride in memory. It may lose a few meters as the device may need a few seconds to recognize when the rider has started to move after halting. The device may lose satellites in tree cover, and falter in calculating velocity or elevation changes. The rider usually has to power the device on, and the device then usually starts to record the session as a new ride or a lap. There are some nuances to setting up a device. Setting up and learning the unit requires time and attention, as changing anything during a ride takes time and reference to manuals and resources that may not be available.
Garmin, having produced watch sized GPS units for runners in 2003, began to produce and sell the Edge GPS receiver for cycling in 2005. Garmin added functions including rear radar, lights, power meters, electronic shifter controls, touch screens, colour screens, maps, navigation and voice prompts in more evolved and expensive head units combined with peripherals. The units for sale bundle primary functions with functions used occassionally by some users, and with some specialized functions and features. Garmin has added GPS functions using the alternative satellites of the Russian Glonass system and the EU Galileo system. Competitors including Wahoo have entered the market.
Basic models do not display a map or provide navigational prompts. Some can be paired with smartphones which may, if they are using wireless data, be able to display maps. A large screen displaying a map is useful if the rider can stop and check, but can be a distraction. The marriage of the GPS cyclometer to cloud computing, big data and social apps means cyclists are sharing their location data with the device manufacturer network and its partners. If the device network servers are hacked, as Garmin was in July 2020, users can lose access to functions that depend on the servers in the cloud. It may not matter much if the cyclist is only using the head unit to display and record distance and speed.
It seems to be efficient to use a smart phone app on a phone that you already own, and to not acquire a head unit but there are trade-offs. Smart phone mapping apps will tell a user where the user is on a map but do not necessarily calculate distance and speed in real time. These apps use the GPS receiver and sensors in the phone, and the location services of the OS ecosystem. The GPS receiver is not as good as a dedicated GPS head set; these apps do not appear to record distance as accurately as GPS headunit or a classic cyclometer. Google had MyTracks, an app that ran in Google Maps, but killed it in 2016. The cycling, running, walking and hiking apps push ads, harvest data and self-promote paid apps with better “features”. I am not happy mounting a phone to handlebars or using battery power and cellular data.