Using a Bicycle Computer for a Speedometer
Shown is the Sigma Sport BC 401. I have documented what I did to install and use this device as a backup speedometer. My speedometer though rebuilt still did not seem accurate, so I bought the 401. It is a simple procedure when you can get by the misspelling and poor language conversion in the instructions that came with the computer. These instructions are for the Sigma, but should be close for other makes.
The base unit attaches directly to the handle bars. This system has tape applied to the back and also a rubber band to hold it into place. Make sure you have enough wire to run from the handlebar unit to the counter holder.
The Magnet attaches directly to the wheel spokes. Since the spokes on a motorcycle wheel are larger than a bicycle you will have to increase the diameter of slot made for the spoke. I held the magnet with a vise, then used 1/4 in drill bit to increase the size. Be careful, you are working with plastic and the heat created by drilling will melt the plastic. A metal clip will then hold it into place.
The hard part is where to locate the count holder. A requirement to be extremely close to the magnet attached to the spokes makes it difficult to locate. On a bicycle it is attached to the front forks, but on a motorcycle that leaves too much of a gap. What I did was take a bolt used to hold on the rear springs, it has a big flat head that I could attach the counter to. I used epoxy to glue it to the head. I had to remove some of the thread from the bolt to shorten it up. The bolt goes into the backside of the front brake caliper. The bolts that hold the caliper on the fork slider does not go all the way through so there is enough room to thread the count holder bolt in. Cut the bolt in order to make the appropriate clearance with the magnet.
The final part is to set tire size value in the computer. Most computers require the wheel circumference. I used a fabric tape measure (temporarily borrowed from my wife's sewing kit, shhh!) You use that information and a simple math formula to calculate the input value. The instructions for the computer will tell you how to set the date and time if required. I love the device. I get a real close reading to the exact speed. Every model has different options. Mine has time, total distance, trip meter. Other models have additional items like, top speed. One problem, you cannot see it at night. They do sell a funky looking lamp that actually shines a light on the face of the computer. Also there is a wireless conversion you can install and it would eliminate the wire. So give it a try and let me know your results. Email Jeffrey Potter with any comments.