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Speedometer Calibration

K-Bike Speedo Calibrator

By Jack Hawley
April 1998

Last summer I built a speedo calibrator which puts out very accurate rates of pulses per second to simulate the pulses normally supplied by the rear wheel pickup. I have been using it and have drawn up this finished schematic, which I will send to anyone who wants it.

Originally, I measured the distance the rear wheel travels, etc. to determine how many pulses per second causes the speedometer to register a mile an hour. I think I was within about 1 MPH at 75 MPH.

Not bad, but David Weiszbrod of Fuel Plus fame, supplied the fact that 75 pulses are input to the odometer to make it register 1 mile.

Bill Heckel supplied me with a data sheet for the chip inside the speedo and I determined from the schematic I drew of the circuit inside the speedo (below), that the chip divided the pulses from the rear wheel by 64. Working backwards then 64 X 75 pulses/mile = 4800 pulses/mile from the rear wheel. Then 4800 pulses/mile X 1 hour/3600 seconds = 1.333 pulses/sec for each MPH the speedo displays.

My box puts out 33.3, 50, 100, and 166.6 pulses per second as well as some other not useful rates (accurate to +/- 5 parts per million!). This corresponds to 25, 37.5, 75, and 125 MPH. I wired mine to put out 37.5 and 75 MPH selected by a momentary toggle switch which also applies the power from an internal 9 volt battery.

You can get the Epson 8651B osc/divider chip to build it from Digi-Key, 1-800-344-4539, for about $20 plus shipping. The rest of the parts can be gotten from Radio Shack.

 

 

 

 


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 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, above.

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.

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)

  • Remove four 4 mm Allen head cap screws with washers on the underside of the instrument housing. Be careful that these do not fall into the fairing when you pull off the instrument cluster.
  • Pull the instrument cluster up and away from the handlebar assembly. This will expose the connector on the rear of the housing. Remove this connector by undoing the small Allen head bolt holding the connector on. The instrument cluster will now come free of the bike.
  • Remove all Phillips head screws from around the periphery of the underside of the housing
  • Pull off the rear plate of the housing
  • Pull off the trip odometer reset knob. Be careful not to lose the small o-ring on the shaft.
  • Remove the Phillips head screws (6, I think) around the periphery of the now exposed interior of the instrument cluster.
  • Carefully remove the instruments from the housing. It may be necessary to gently pry the instruments out, using a small flat-bladed screw driver, working your way around the edge of the unit.
  • Remove the single, small, black, slot-head screw next to the silver metal bracket on the speedo side of the cluster. Photo here: http://skylands.ibmwr.org/tom/tech/speedo/speedo5.gif courtesy of Tom Coradeschi . This will allow the speedo subassembly to be lifted up enough to expose the adjustment screw.
  • Reconnect the instrument connector to the now disassembled instrument cluster, and let the unit rest on the handlebars.
  • Turn on the ignition
  • Energize the calibration transducer as described above
  • Lift up the speedometer to expose the adjustment trimming potentiometer on the inside (right side) edge of the printed circuit board on the speedo subassembly.
  • Using a small slot screw driver, adjust the trim pot so the speedometer reads the correct speed for the frequency you are feeding to the electromagnet
  • Reassemble everything

Happy cycling!

Jerry Skene – jerry@skene.org

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