Speedometer Calibration Tools and Techniques

By: Jerry Skene
November 2000

The speedometer pickup on BMW's K&R bikes senses a magnetic signal generated by a rotating vane in the rear wheel assembly. It is possible to generate your own, calibrated magnetic signal, and so determine and adjust the accuracy of your speedometer.

The method is quite simple, as described below.

Have your computer play a sine wave sound file of a specific frequency from the table below, send this signal through a power amplifier, and in place of a loudspeaker, connect an electromagnet a coil of wire wrapped around a steel bolt. I used about 200 turns of enameled magnet wire (Radio shack sells this stuff part #278-1345) wrapped around a steel bolt. A variable speed electric drill is helpful in making this magnet. You can use an old horn or headlight relay here also, but it is only good up to about 45 mph.

Note: be sure to use at least 200 turns of wire, unless your amplifier can tolerate a very low speaker impedance. The author takes no responsibility for damage that may occur to your amplifier. Use the lowest volume control setting that gives a reliable activation of the speedometer.

If you have a laptop and portable amplifier, of course you can cart all this stuff out to your bike. If you have a desktop PC and a non-portable power amp, then you can connect your power amp to the electromagnet through a long extension cord, running out to the garage.

Place the electromagnet (I call it the transducer component of the precision computerized velocity measurement calibration system) next to the magnetic speedometer pickup on your rear wheel hub. When you turn on your ignition, your speedometer will now read a speed close to the speed from the table below. If the speed is off by much (mine was about 10% too high), you can adjust the speedo as described below.

An extremely simple alternative is to place a tape demagnetizer next to the speed sensor. This should read out 45 mph if you are using 60 Hz current, 37.5 mph if you are using 50 Hz current

Table of speed vs. frequency, for K1100RS (other models may require different frequencies)

Speed, MPH Frequency, Hz
10 13.33
20 26.67
30 40.00
40 53.33
45 60.00
50 66.67
55 73.33
60 80.00
70 93.33
80 106.67
90 120.00
100 133.33
110 146.67
120 160.00
140 186.67
150 200.00

Sound files for 55 and 100 mph are available at: http://skene.org/kspeedo/ These are .wav files, 20 seconds long, and can be played through most audio playback programs, including Microsoft's Windows Media Player that comes with Windows. You may want to select the option "Repeat Forever" to have this file played continuously.

You can generate your own sound files using any one of a number of audio programs available on the Web.

To adjust the speedo, proceed as follows:
(also described, with some excellent photos, at: http://skylands.ibmwr.org/tom/tech/speedo/speedo.html)

Happy cycling!

Jerry Skene - jerry@skene.org

 

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