Installing a Sigma bicycle computer

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With the advent of GPS this article has somewhat less relevance than it used to. Given the limitations of a bicycle speedometer, you may want to consider searching the forums for opinions on GPS instead.

There are a couple things to remember before you get a bicycle computer for your bike. This article is a couple years old and, as you may have noticed, electronics companies change models with the seasons these days. So, the model mentioned is no longer being manufactured. Just go to Sigmasport US, look at the specs, and find the one that best suits your needs. The way you mount them hasn't changed. Just remember to get a wired version. Wireless won't work on the EX250.

The Sigma brand is recommended because many other computers stop at 100 km/h, which is fine if you never expect to go faster than 62 mph.

If you're going to be riding your bike after dark, please note that bicycle computers do not generally have lights. Some do, but they usually require you to reach out and push a button to turn the light on, and it won't stay on for more than a few seconds; not a great idea when you're tooling down the highway at night. Sigma tells us that if the light was on all the time, the battery would go out in about a week. There are a couple things you can do about this:

  • Rig up some sort of light that can mount close to and shine on your computer
  • Go to the streetfighter article and look at the companies that sell gauges that are wired into the bike's 12 volt system
  • Try to find a lighted speedometer that can be hard-wired to 12v
  • Figure out the battery it takes (probably a 1.5v if it's a single battery) and go to Radio Shack. They have preset output voltage regulators for a couple bucks. If you can figure out how to wire one of those from your speedometer to a 12v aux line, you'll be set. This will probably involve some disassembly and soldering, as in the garage door mod. You'll need to use one of the bike circuits that comes on with the key, such as the gauge cluster lights, and not the always-on accessory wires.

Materials Needed:

  • Sigma Sport computer (BC800)
SigmaBrent 15.jpg
  • Two-Part Clear Epoxy
SigmaBrent 9.jpg
  • Rare Earth Magnet(s) from Radio Shack (recommended but not absolutely necessary)
SigmaBrent 13.jpg
  • A tape measure.
Pink tape.jpg

Extras you may need depending on your situation

  • Extra wire if yours is too short.
  • Shrink wrap to cover up spliced wire.
  • Soldering iron and solder/crimp connectors
  • Zip ties

How you actually do it

First, you're going to have to determine if the wires are going to be the correct length. More than likely they'll be too short. Take your Sigma sensor to the bike and route the wire down the fork leg and up to where you would like to mount the computer. If it's too short, you'll need to extend the wiring. When checking for enough length, remember that you have to allow room for the forks to completely extend out.

My wires are too short

In this situation, you need to splice some new wire inline with the sensor wire. (Note: this will void your warranty, so you may want to test the computer on a regular bicycle first.) There is an option for a wireless sensor pickup, but it costs as much as the computer itself and receives interference from the bike's ignition, so that option is out.

To lengthen the sensor wire: Cut the wire in half somewhere near the middle of it and splice in the extra wire you need to make the lead long enough.

This is what your goal will be.

SigmaBrent 1.jpg

Follow these steps to splice the wire together.

  • Cut the wire in half near the center.
SigmaBrent 2.jpg
  • Split the two wires away from each other.
SigmaBrent 3.jpg
  • Strip the insulation off the ends of the wire about 1 cm.
SigmaBrent 4.jpg
  • Twist the ends of the wire together like this, although crimp connectors are more secure.
SigmaBrent 5.jpg
  • Bend the twisted wire over and heat shrink each side individually.
SigmaBrent 6A.jpg SigmaBrent 7A.jpg
  • Tape the two sides together to keep the rest of the wire from splitting apart.
SigmaBrent 8A.jpg

Now that the wire is long enough to use, you need to mount the sensor, magnet, and computer.

Mounting the sensor

To mount the sensor and the magnet, use epoxy. If you're using quick dry epoxy you will have a couple of minutes or less of work time, so be sure to have figured out how you want to mount the sensor and magnet first.

Mount the magnet on the inner part of the rotor.

SigmaBrent 11.jpg

You could also use more than one magnet. These are stacked two high:

Scea sigma-6.jpg

Line up the pickup sensor and epoxy it to the fork leg. For added security, you could also use zip ties to hold the sensor in place.

You should try to get the sensor as close to the magnet as possible, but not too close. Shoot for 2-3mm of clearance between them.

Route the wire up the fork leg and through the fairing to your desired mounting point. Try to tuck the wire behind the fender, if possible, to keep it from getting into the wheel:

SigmaBrent 14.jpg

You'll have to figure out where and how you want to mount the actual computer, as it comes down to personal taste and the type of mounting hardware your Sigma has. This one is attached to the handle bar riser with some more epoxy:

SigmaBrent 12.jpg

You may want to use some sort of zip tie arrangement instead.

Scea sigma-1.jpg


The best thing to do is make a mark on the bottom of your front tire and on the ground with a marker or chalk (something you can see on the tire and the ground). Then, roll the bike forward until the tire has made two complete revolutions. Mark the spot on the ground where the second revolution is completed and measure the distance between the first mark and the last mark on the ground. Divide the number by two to get the circumference of the wheel.

Click on this animation to make this more clear.

Rolling A.gif

How to determine your wheel setting

After you have measured the circumference of your wheel, you'll need to convert that number to millimeters if it's not already. If your measurement is in inches, multiply by 25.4 to get a millimeter value. Your millimeter measurement should be in the neighborhood of 1770 or so.

Once you have this millimeter measurement, you can use it directly as your wheel setting, if you'd like the readout to be in km/h. If you want mph, multiply the number by 0.62137 to yield a number around 1100 for your setting. Also, remember to change your Sigma to display mph. Once you are finished, you'll notice the speedometer will read around 10% higher than the Sigma.