Here is a counter I cobbled up this week. It’s a chain-drive unit. The components were bought on the internet.
At middle left are Delrin gears and chain obtained from:
http://www.robotobjects.com/scripts/prodList.asp?idCategory=42
I bought the white nylon spacer washer at a hardware store.
I used the counter from an old Jones counter. As shown it is on the right side of the bike and it rotates backwards. A proper counter will mount on the left and will increase counts as the bike rolls forward.
I ordered a new counter with proper rotation (model 075845.001) from:
http://www.veeder-rootcounters.com/uploadedFiles/Downloads/7458_7461_Spec.pdf
I have not got it yet. When I do I will have a neater job. The bracket needs neatening too. However, as it stands it works. I have no idea of its longevity, but will report as I learn.
The counter was about $45, the gears and chain about $10. The chain has no master link. You just snap together the length you need. The big gear has 40 teeth, the small gear has 15. With the ten digits per revolution built into the counter the unit displays 26.6666.... counts per revolution of the wheel. Other ratios are available to suit individual taste.
I don’t know how to make this a universal application. I had to shop around to find gearing that would fit within the geometry of my wheel and fork. I had to cut away the central hub of the Delrin gear so it would fit around the inner bearing nut on my wheel. The gear and spacer are loosely held in place by the outer bearing nut.
I offer this just for what it may be worth. You can download a movie of it working at:
Counter Movie
Click on "CounterMovie.mpg"