Installing A Sigma Sport BC800 Bike Computer

Like most other V4s, the speedometer on my Magna is not very accurate, it appears to display a speed that is 5% - 10% faster than reality.  While this is good for the ego, it is not very useful for setting an appropriate speed on the highway.   My solution for this problem was to install a BC800 bike computer made by Sigma Sport.   While designed for "you pedal it" bicycles, these actually work quite well for motorcycles.  With a maximum speed capability of 183 mph, it is more than adequate for most mere mortals.

The BC800 consists of three main modules, the display, a sensor, and a magnet. The sensor connects to the display by a thin two conductor wire. Every time the magnet passes close to the sensor, another "count" is sent to the display where it is registered. The concept being that the sensor is attached to the bike fork and the magnet to the wheel. Each time the wheel rotates, it passes the sensor and the rotations are counted.

Installing the BC800 is fairly straight forward and pretty much follows the standard installation for a bicycle. This procedure just has to be modified to reflect the differences between a bicycle and a motorcycle: a bigger front fork, potentially a wheel without spokes, and a rougher operating environment. Also, the distance between a desired display mounting position and the front wheel is greater than the length of the sensor wire.

The biggest difficulty in the installation is deciding how to attached the sensor to the fork. I wanted a method that would allow me to adjust its position on the fork as well as remove it entirely if needed. My solution was to attach the sensor to a standard hose clamp. This clamp could then be put on the front fork and adjusted as needed. Metal epoxy was used to glue the sensor housing to the hose clamp at a slight angle. Care was taken to only put epoxy on the sensor mounting back. In this way, the sensor can be removed from the back while leaving the back and clamp attached to the bike. The following two pictures show how the sensor is attached to the clamp.

hoseclamp1_tn.jpg (10316 bytes) hoseclamp2_tn.jpg (9666 bytes)


The magnets that come with the BC800 are in a small tubular housing that is designed to clip to the bicycle spokes. This generally won't work for a motorcycle. I did what a number of others have done, which is to replace them with a set of magnets from Radio Shack. The magnets chosen are their part magnets2_tn.jpg (22455 bytes)#64-1895, "rare-earth super magnets". this part consists of two, very powerful magnets about 1/8" in diameter. One of the magnets was attached to the disk rotor using epoxy and the other was stuck on top of it without epoxy. The magnets are so powerful that there is no concern that it will come off. Attach the sensor/hose clamp to the fork before glueing the magnet to make sure a correct location on the disk is chosen.

The length of the wire from the sensor to the display module is long enough for use on a bicycle, but most likely is inadeguate for a motorcycle. The solution is to cut the wire and splice in another two conductor wire. Cut the wire somewhere in the middle rather than attempt to open either the sensor end or the display module end. I soldered the splices and put shrink tubing over each wire. I then put a larger piece of shrink tubing over the entire splice to ensure it would withstand the treatment it was to receive.

There is probably a number of places where the display module can be mounted.  I chose to mount mine on the handlebars, just left display_tn.jpg (20295 bytes)of the housing.  For now, I have used the supplied rubber bands but will replace this if it starts to appear that it isn't secure enough.

Because the sensor housing is able to unsnap from the sensor mounting back, the routing of the wires was left to last.  I routed the sensor wires through the steering housing, following the same general path as the speedometer cable and other cables.  sensor_tn.jpg (15279 bytes)I then used wire ties to attach the wires to the speedometer cable.  I somewhat out of focus picture below shows the final attachment of the sensor to the fork.

The final step of the installation is to "program" the display.   The process is to push a small button on the back of display module for at least 5 seconds.  This puts the display in a mode for entering the WS (or wheel size) parameter, which is the circumference of the tire.  To get this circumference, I drew a straight line on the ground and lined the front wheel up with this line.  I then marked the tire and the ground where the tire rested on the line.  I rolled the bike forward along the original straight line until the tire had made two complete rotations and the mark on the tire came back around to the ground.  I marked the ground again at this point.  Now, measuring the distance between the two small marks gave me a figure that was exactly two times the circumference of the tire.  Obviously I could have just made one rotation of the tire, but making two rotations gave me a lightly greater degree of accuracy.  Dividing the number y two gave me the circumference (78.5" or 1994 mm).  If I wanted to display my speed in KPH, I would have entered in this value of 1994.  Since I wanted to display the speed in MPH, I divided this value by 1.61 to get a value of 1238.  This is what was entered.

So far, everything seems to be working fine.  One of the additional functions that this module displays is the maximum speed reached since the last time it was reset.  Interesting information and perhaps useful in helping you keep your speed down to a reasonable level.  Nah, probably not.