K-Bike Speedo Calibration

By: Frans Schrauwen
October 1999

Early this summer the speedo of my '88 K100RS developed intermittent malfunctioning. Moisture is often visible in my speedo, though it has the Gore-Tex® vents at the back. I disassembled the unit (as described by Tom Coradeschi) and cleaned the contacts where the speedo plugs into the printed circuit board. This worked.

I used the opportunity to calibrate the speedo/odometer. The trick I used is based on the initial idea of Jack Hawley.

Needed: tools to remove and open speedo unit, elec. soldering iron.

  1. Measure the distance of several rev of the rear wheel to find the rolling distance. I loaded the bike with my own weight and found 2.03 m/rev (6.66 ft/rev).
  2. Connect an electric soldering iron to the mains (230 V AC & 50 Hz in Europe).
  3. Switch on ignition.
  4. Hold the soldering iron close to the rear drive speedo pick up (don't touch!). 1 cm or 1/2 inch should be sufficient.
  5. See the miracle happen. My speedo indicated rocksteady 67 km/h (41.6 MPH). The rear drive sensor picks up the oscillating magnetic field of the soldering iron heating coil. It also works with a small household transformer or relay coil.
  6. The speedometer rotor in the rear drive has 6 teeth. With 50 Hz AC line frequency and 2.03 m rolling distance of the rear wheel this would give 60.9 km/h (37.8 MPH). My speedo did indicate almost 10 % too high!
  7. With the 50 Hz signal feed to the speedo pick-up, I measured the time to cover 1 km (0.621 mi) on the odometer. This was 59.4 sec, which corresponds to 60.6 km/h (37.7 MPH). The odometer is surprisingly accurate!
  8. I calibrated my speedo at approx. 120 km/h, the max speed on Dutch highways. Doubling of the effective frequency can be done by feeding the mains to the soldering iron or coil via a bridge rectifier. Actual calibration by turning the adjustment potentiometer as described by Brian Curry.
  9. The trick of wireless feeding the mains frequency into the speedo pick up is by far the simplest method to check all components in the line: pick up, wiring, connectors and speedo.