Sigma 400 Bike Computer: Setting It Up for a 28-Inch Wheel
I found my notes where I calculated the setting for a Sigma 400 bike computer for a 28″ wheel.
The sheet had been lying around for a year. I had to do the math myself, because the table in the manual didn’t have a value for my tire.
What value to enter
A bike computer calculates speed and distance from wheel revolutions, so you need to give it the wheel circumference in millimeters. For a 28 × 1.35 tire I calculated 2180, while the table gives 2185 — a small difference.
Now there’s a more complete table:
How to calculate it yourself
The picture shows two ways to get the number:
- Measure the wheel diameter including the tire and multiply by 3.14.
- Mark a point on the tire and measure how far the wheel rolls in one full revolution.
For accuracy, measure in real conditions: inflate the tire the way you usually ride, sit on the bike, and only then take the measurement.
P.S. The Sigma 400 is a decent bike computer; mine worked for a little over two years. Then the battery died and the sensor wire seems to have frayed somewhere. I replaced the battery but not the wire, so for now I’ve just taken it off. I need to use a smartphone for this instead — then I could build a track right after each ride.