K-Bike Speedo Calibration
By: Frans Schrauwen
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.
- 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).
- Connect an electric soldering iron to the mains (230 V AC & 50 Hz in Europe).
- Switch on ignition.
- Hold the soldering iron close to the rear drive speedo pick up (don't touch!). 1 cm or 1/2 inch should be sufficient.
- 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.
- 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!
- 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!
- 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.
- 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.