My commuting bike with a "cadence" computer mounted on the toptube
I find that cadence is the information most useful to me while riding. Knowing how fast I am pedaling is important both for making the most efficient use of my body and for protecting my knees from extra wear and tear. I know most people don't share these concerns, but, since you are looking at this page, you probably do.
I've been using Cateye Astrale computers on all my bikes for many years. They are one of the best bike computers with cadence, and have generally worked well for me. Unfortunately they have not been reliable on my last two tours in Europe - it isn't the computers themselves, but the wires and the electrical contacts that have caused me problems - so I started looking around for a simple way to provide a cadence backup. I looked around at what I had to work with and decided to use an existing inexpensive computer to display cadence.
The display mounted on the top tube
Any bike computer can be used to display cadence instead of speed, just by programming it with the right "wheel diameter" and placing its sensor so it is triggered by a crank mounted magnet each time the crank arm passes it. The magic number is 1667 for most cycle computers set to display kilometers per hour. Then the kph display will show cadence divided by ten., eg.. 80 rpm is displayed as 8.0 kph. The magic numer is 1667/.62 or 2689 for mph.
The sensor mounted on the chain stay with a magnet mounted on the crank arm
Since computers without cadence are normally mounted on the handlebar and have their sensor mounted on the fork, the wiring that connects the sensor and the computer is fairly short. Because of this, I mounted the computer on the top tube of my commuter bicycle. After riding with it there for most of a year, I decided I liked that location. It is out of my normal line of sight, but easy to see when I want to check my cadence. I have another computer, without cadence, on my handlebar. Since both computers were designed to mount on handlebars, the display on the computer on the toptube is rotated 90 degrees which is a minor pain.
The "cadence" computer on my touring bike is designed to mount on the handlebar stem
Since I liked having the "cadence" computer on my top tube on my commuter bike, I decided to put on on my touring bike. This time I bought a cheap ($9 at Nashbar) computer which, because it was designed to mount on a handlebar stem, doesn't have the rotation issue. It has another issue - it doesn't display fractional mph. This means 80 rpm would be 8 and 90 rpm would be 9. That isn't quite enough resolution, so I used a wheel size of 3334 or twice 1667. Now 80 rpm displays as 16 and 75 rpm displays as 15. This is as much resolution as I need. The Astrale displays cadence to four digits, but, when I try to hold a steady cadence, I can't even keep the first two digits constant!
The sensor for the "cadence" computer on my touring bike
Since the sensors were designed to mount on a fork blade, the normal mount will work pretty well on a chainstay. You can see in the image above that I used the normal mount for the Sigma computer I use on my touring bike. The old Vetta computer had lost its sensor mount, so I improvised - with Goop, a plastic tie and a nut - a new mount. It is important that, if you ever ride a bike in long pants, to mount the sensor so your cuff can't catch on it! I tore pants cuffs and damaged sensors before I learned this!
The "cadence" setup on my touring bike
The cable was too short to follow the tubes
The cables that come with cyclecomputers are too short for this method to work on a large frame - my frames are 54 cm - so you'd have to splice some extra length into the cable or get a special longer harness assembly. Those assemblies, made for using a handlebar mounted computer with a rear wheel pickup, are available for some - I've seen Sigma and Cateye versions - computers. If you are good at soldering, you can extend the existing cables, but its is tricky on the ones, like Cateye, with small, very flexible, wires.
Now that I've discovered how easy it is to have a separate cadence display, I'm getting a computer with an altimeter for my touring bike. I used to ride with a Vertec altimeter on my handlebar, but, since I now have a camera mounted on one side and a computer mounted on the other, I had to give that up. With toptube mounted cadence and time display I can, once again, have speed, cadence, time of day, and altitude all displayed ;-}.
For touring, I like to have redundancy in the stuff that I need. This setup, with two cycle computers, gives me redundancy. I can, if I need to, make either computer display cadence or speed. I just have to move a sensor and maybe the wheel magnet and recalibrate the wheel size.