The easiest way is to not use a sensor at all. Using the Garmin head unit alone, it can use GPS to detect the position and therefore estimate the speed and the distance. Although using GPS works pretty well for overall distance, it is not accurate for instantaneous speed readings. Also, it does not work properly when cycling in between tall buildings, under bridges or anywhere where there is no good GPS signal.
A more accurate way to get speed readings is to get a second sensor for the bike. I don't really need cadence readings, and so a speed sensor will be sufficient. With speed, the distance and other parameters can be calculated.
What I need is actually just an ANT+ speed sensor, which can be found quite easily. However, all these speed sensors rely on a magnet on the spoke and a sensor tied to the fork. Although this works well, it also means that some setup is required and it is difficult to move the sensor from bike to bike.
Garmin recently launched a new type of speed sensor that comes with the Garmin Edge 1000 cycle computer. This speed sensor does not require a separate magnet, and is just a sensor on its own. Best of all, it attaches to the bike with just a rubber strap, which makes it easy to transfer the speed sensor across different bikes.
The new Garmin speed sensor
Easy-to-install, as stated on the packaging
The speed sensor embedded in a rubber cover, with a rubber strap for attachment around the wheel hub.
The battery compartment (CR2032 battery) is hidden at the bottom of the sensor. The hook for the rubber strap is located on the other side of the sensor.
Installation instructions for the new Garmin speed sensor. It basically tells you to wrap the sensor around the wheel hub, and that is all that is required!
I think this speed sensor works by using a gyroscope to detect its orientation. Every time the sensor flips from right side up, to upside down and then back up again, it will detect this as one revolution. Together with the wheel circumference entered in the Garmin head unit, this will give the speed of the wheel. It is a very simple concept that should work quite well. Mounting the sensor on either the front wheel or rear wheel will do, as long as it is on the wheel hub that is spinning.
Although installation is supposed to be very straightforward, it is not as simple to install on a small wheeled bike such as my Dahon Boardwalk.
I tried to install the sensor on the Novatec front hub, but the narrow 74mm OLD front hub means that the hub flanges are very close to each other, and the sensor cannot fit in between the hub flanges.
Next, I tried the rear hub. However, due to the larger diameter of the Chris King R45 rear hub, the rubber strap is not long enough to stretch around the hub and onto the hook.
In the end, I used a rubber band from the Garmin mount to extend the rubber strap of the speed sensor. Now it fits nicely around the rear hub of the Dahon Boardwalk.
With this new Garmin speed sensor on the Dahon Boardwalk, I am now able to move the Garmin head unit between the Merida and the Boardwalk easily, since both bikes have their own sensors. Also, this new speed sensor can be moved to another bike very easily. An example would be the Dahon MuEX. In just 1 minute, I can attach a Garmin mount to the bike, and also attach the speed sensor on the MuEX. The Garmin Edge 510 can then be used on the MuEX to track the cycling activity.