Installing a Cycle Computer Submitted By: Trail Tech JustKDX : Trail Tech has developed a more robust cycle computer sensor and wiring harness systems to allows cycle computers to better withstand the demands of off road use. They supply ready built kits for a number of off road applications. There are many cycle computers out there that will work just fine on a motorcycle; cost starts below $20. However, since almost all are geared toward the bicycle market, it will be necessary to do some modifications; first to make them functional, and second to make the setup sturdy enough to withstand the abuse a trailbike takes.
To provide the most protection to the wire that connects the sensor and the computer mount, following the brake line routing is advised. Optimum wire length is about 60 inches (on the Yamahas it would be nice to get it even longer, about 72 inches. This is because the brake line routes around and under the fork leg). If the desired bicycle computer has a shorter wire, be prepared to lengthen it. Twenty-two gauge wire (e.g. speaker wire from Radio Shack) works well. The wire should be unshielded, unjacketed, and paired. To further protect the wire, it is a good idea to sleeve it with ¼ inch polyethylene tubing. There are no polarity requirements on the wires, and solder is definitely recommended to join them together. Shrink tubing is also a nice touch that will add an extra level of protection.
Once the wire has been lengthened appropriately, the sensor needs to be attached where the magnet can trigger it. Assuming the sensor housing is basically cylindrical, use a cable stay (available at electrical supply stores, hardware stores etc.) to attach the sensor housing to the top surface of the brake caliper mount. The cable stay will need to be fastened to the caliper mount with a small screw. Once the sensor has been mounted and tested satisfactorily, apply epoxy around the cable stay and sensor, encapsulating the setup in a protective cocoon.
For anyone interested in building a sensor and sensor housing from scratch, magnetic reed switches can be found at www.digikey.com (call and ask for a Coto switch 23AAA or 23AA).
Depending on where you choose to mount the sensor, you will need find the optimum location for the magnet, either on the wheel hub or on the rotor itself. Remove the magnet from whatever holder it is housed in. The best shape is a circular plug, about 0.35 inches in diameter with a thickness of about 1/8 inch. Modify the magnet as necessary (e.g. sand it down). If the bicycle computers magnet is too far out, it might be a good idea to purchase a rare earth neodymium magnet (maybe Radio Shack. On the Internet, check out www.buntingmagnetics.com). The magnet can be placed into one of the open spaces on the rotor. Once the right location is determined, use epoxy (e.g. JB Weld) to fix the magnet in place. To determine the optimum location for the magnet, set the motorcycle on a stand so that the front wheel rotates freely. Plug the computer into its mount. Use a piece of tape to hold the magnet in place on the rotor where it will pass the sensor. While someone watches the computer, hold the sensor in place on the caliper mount by hand, and roll the wheel. The sensor will be active at both ends (where the cable enters and at the extreme tip). Don't place the magnet so it passes the center - there is a dead spot in the center.
Some of the bicycle computers come with handlebar mounts that are very compatible with motorcycle bars. Some do not. The Sigma, for instance, comes only with a rubber band style of attachment, and this is inadequate for a dirtbike. If you plan to mount the computer directly to a stock odometer plate, then it wont matter. If attaching the computer mount directly to the odometer plate, it will be necessary to use flat head screws to allow the computer to slide on and off the mount freely.