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Mounting A Bicycle Computer

I put on a lot of miles in the U.S. but my speedometer only shows kilometers per hour (Km/h ) and of course my odometers are in kilometers too. I've gotten pretty good remembering what speeds in Km/h equal the posted speeds and my mental math for distance conversion is pretty good but for $25 or so, I thought I'd give this a try. Don't think this is only good for us riders from North of the border. If your situation is the reverse, this little bugger can be set for kilometers.

If you're into Iron Butt type rides, then the average speed feature could be invaluable and it has one regular and two trip odometers.

The only thing that I didn't like was running the wire down the fork leg. You can get wireless models for about 3 times the price but the batteries don't last nearly as long and the supplied magnet module may not work on a motorcycle.
 

 

 

 

Lengthen The Sending Unit Wire:
The wire from the sending unit to the computer is just a bit too short. I got a length of speaker wire from a car stereo shop for free and spliced in about 18". Before you solder the connections, remember to slide the heat shrink tubing over the wires first. I used two layers of tubing due to the exposed nature of the connections. I used smaller tubing over the individual connections, then I put a larger piece over the whole connection.
 

Magnet:
The sensor is triggered by a magnet. The magnet supplied is held in a bracket and is made to mount on the bicycle spokes, so unless you've converted to spoke wheels, you're going to need a different magnet.

Before you start, head down to the hardware store or Home Depot and find some rare earth magnets. You can also try Indigo Instruments. They can be very small and are quite powerful, come in all shapes & sizes, and are cheap like borscht. Mine cost $1.20 Cdn.  and is 1/2" in diameter x 1/8" thick. It has 9 pounds of attractive force. The 1/4" magnet I first tried  had only 2 pounds of force. Also, with a larger diameter, it makes mounting more forgiving as you don't have to be as precise in lining it up with the sensor.

I just stuck the magnet to the brake disk and didn't bother with any glue (yet). Some use JB Weld to stick the magnet on. I've had the bike up to about 180 km/h (about 100 mph) and that little magnet stuck like shit to a blanket.
 

 

 

Mount:
Since I had my CB radio mounted on the center of the Heli-bars, I decided to go with a simple home made bracket from 1" x 1/8th inch aluminum. Drill two holes, bend in vice, tiddly up the rough edges and voila. Or as the French would say, "voila". Paint it black if you're so inclined. If it's goin' on your Harley, chrome it.
 

 

Mount the computer:
Here it is nicely mounted. (This is the old Cateye that I tried first.) Not quite out of the rain, but close. If it turns out not to be waterproof, I'll either relocate it or buy a little cycle condom for it.

If you don't mind drilling holes in your bike, you could mount it to that flat surface between the gauges and the mirror. In either case, it's at about the same eye height as the rest of your gauges. I have an idea how to mount it under the stock gauges. That may be next. I found that after using it for a day, I quite like the digital readout.
 

Mount the sensor:
The sensor is a two piece unit. The part closest to the fork has two square notches which are perfect for a couple of zip ties... almost. The longer ties are a little bit too wide so I opened up the notches very carefully with a hack saw and Dremel tool. A file would work fine as well for those with more patience than me.

The sensor & magnet should be about 2 - 5 mm apart. That's awfully darned close at 70 mph so make it secure. The rare earth magnet is so powerful, that you really don't need to get things that close.

I'll replace the zip ties with black ones if I decide to keep the rig.

Remember, when you mount the sensor, the forks are slightly compressed. When they extend, you better have a little slack in the wire or something is gonna break. Put the bike on the center stand, get some weight on the back to extend the forks, and secure the wire. I secured mine to the brake line with some small zip ties.
 

The magnet is the round silver disc on the
brake rotor just to the right of the sensor 

 

Calibration:
Not much I can tell you about this. You'll just have to do the inevitable and read the damn instructions like I did. The Sigma isn't anywhere near as easy as the Cateye. If you want to switch between MPH and Km/H, you have to change to tire size setting. The Cateye would do the arithmetic for you. I calibrated my Sigma at 1948mm for my Metzeler ME880's. Divide this by 1.61 if you are running it on the MPH setting and you would set it at 1210. (See table below)

Here are a couple of tricks. Let's say your speedometer is in mph and you're confident that it's accurate. Ride at a steady speed and compare the two, then adjust if necessary. If the speedo is reading low, then increase the tire size setting; if it's reading high, then decrease the tire size setting.

Now for your trip across the border, multiply the tire size setting by 1.61 to go from MPH to Km/H, or divide by 1.61 to go from Km/H to MPH. It has settings for two different wheels in case you move the computer back and forth between bikes.

Alternatively, find one of those roadside radar operated signs that tell you how fast you're going. Hold a steady speed and compare and adjust as necessary.
 

 
Before you take off on that long trip...
And finally, make a photocopy of the directions on setting up the computer and keep it with the bike. Sure as shit, when your battery finally gives up the ghost in three years, and the Rexall in Outer Moosomin, Saskatchewan does in fact does carry the right replacement, just how are you going to get the odometer readout to be just like it was before the bloody thing crapped out ?
 

Sigma web site

Features

  • Current speed
  • Odometer
  • Trip distance
  • Clock
  • Riding distance
  • Maximum speed
  • Average speed
  • Completely watertight 
NOTE: I found that my Cateye Enduro would not cope over 112 km/h (about 65mph). I contacted Cateye and all they could tell me was that was the max for that unit and to watch out for their new computers in 2003 which would cope with speeds over 200 km/h. I have now rewritten this article and installed the Sigma BC-800 as suggested elsewhere for motorcycle use.
  

Here's a table to get you started

Tires

WS - MPH

WS - Km/H

 

1,189

1,914

 

1,191

1,918

 

1,194

1,922

 

1,196

1,926

 

1,199

1,930

 

1,201

1,934

 

1,204

1,938

 

1,206

1,942

 

1,209

1,946

Metzeler ME 880 130/70 x18

1,210

1,948

 

1,211

1,950

 

1,214

1,954

 

1,216

1,958

 

1,219

1,962

 

1,221

1,966

 

1,224

1,970

 

1,226

1,974

 

1,229

1,978

 

1,231

1,982

 

1,233

1,986

Article By: David J. Morrow

Updated January 2005

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