Bike speedometer

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I have Bell branded bike computer that measures speed, distance, and time. It flashed the warning that the battery was low, so I peeked at the watch battery inside so I would know what replacement to get.

It is a CR1142 battery.

And now the computer has lost its settings. I tried to find the user manual on the web, but alas, Bell doesn't seem to make or support anything but helmets anymore. So I'm stuck with trying to find the user manual in somewhere in meat space. Which is NOT Google indexed.

Target values

But Google can help with calibrating this device even without the user manual. Google Earth says that my route is 5.97 miles long. So that is the figure I'll shoot for as I reverse engineer the calibration user interface.

Also, an on-line calorie calculator says that I burn about 285 calories by biking 6 miles. Before the system reset, the computer was saying that I burn an average of 300 calories for the 6.2 mile trip.

Calibration user interface

When the computer is reset, it follows this sequence for calibration:

  1. it flashes all LED components
  2. a 4-digit number. with a default of 2124 (set to 2126, assuming 26 inch tires)
  3. it prompts for Km/hour or mph.
  4. it prompts for pounds or kilograms.
  5. it prompts for a 3-digit number of pounds. (is this body weight?) with a default of 80, setting to 170

The top button accepts and advances to the next selection. The bottom button changes the current selection.

That 4-digit number is still a puzzle. Ignoring the first two digits doesn't sound like a good solution.

Given a 26 inch diameter tire, that would be 66.04 centimeters, and the circumference would be 207.46. That still isn't a 4-digit number.

It is entirely possible that the 4-digit number is a code for a table lookup and so I am lost without the user manual.

Bike geometry

The specs for my bike are available on the Trek web site. But what the computer is probably asking for is wheel size. Here is what Trek says about my model's wheels:


Bontrager Select: Durable welded rims, machined for smooth braking; OSB rear rim provides good stiffness for any size rider; Hi-Lo ATB rear hub design w/excellent spoke bracing angles for improved lateral wheel rigidity; Cro-Moly skewers; 780 g, 24h front/1045 g, 28h rear


Bontrager Jones AC, folding, 26x49/54

What does that mean? Sheldon Brown explains that the first number is the outside diameter of the tire and the second number is the width of the tire. Given the numbers in this example, it looks like the units are inches x millimeters.

All new computer

Elliott lost the old bike computer for me, so I don't have to worry about trying to figure it out anymore. The new computer uses an LR54 1.5 volt battery. This time, I've scanned the instruction page knowing that I'll lose the paper someday.

The important detail is that the code 208 indicates that my wheel circumference is 26 inches. (I never would have been able to figure out the old computer.) My tires may actually be 26 1/8 inches. That would be 209. That number is simply the circumference of the tire in centimeters.