BMW Speedometer Accuracy

By Joseph C. Dille
BMWMOA #24754

The above three words are a contradiction of terms. The legendary motorcycle of Germany has a small flaw -- the speedometer lies like a cheap rug, has for a long time. I can remember reading road tests in motorcycling magazines almost twenty years ago that said that the BMW speedometer was "quite optimistic". Mechanical speedometers have been in use since the turn of the century. I see no excuse for this inaccuracy. I did not want to put up with this fallaciousness, so I decided to take the speedometer apart and adjust it.

Before making any adjustment, determine how big the error is and if it is speedometer or odometer error. The odometer must be accurate within 5% before the speedometer can be adjusted. First determine the odometer error by driving down a road that has mile markers and noting how much the indicated mileage differs from the actual mileage. A big difference could be caused by incorrect tire size and/or pressure. It is also possible that the speedometer is not the proper one for your rear end gears. Different speedometer assemblies are made for each rear end ratio. If a higher numerical ratio gear set has been fitted for sidecar use or better acceleration, the speedometer will have to be changed to match. Your dealer should be able to tell if your rear end and speedometer are matched.

Next, check the calibration of the speedometer and record the error at one speed. To determine the speedometer error, find a fairly level section of highway with mile markers and get a stop watch. You will have to activate the watch while you are riding, so either wear the stop watch on your right hand or use a stop watch with a neck strap. Hold the bike at a constant speed and measure the amount of time that it takes to cover a mile. Repeat this several times and use an average time for the mile since accuracy is important for this step. Your actual speed is 3600 divided by the average number of seconds that it takes to cover a mile. In my case, my actual speed was 54.01 mph when my speedometer indicated 60 mph. This explained why everyone was passing me when I thought I was speeding!

Figure 1, Speedometer Cutaway
Before disassembling the speedometer and adjusting the works it is necessary to understand how a speedometer works. The mechanical speedometer operates on the eddy current principle. Whenever an electrical conductor experiences a change in magnetic field an electric current is generated. In an alternator the rotor provides a changing magnetic field that produces current in the stationary windings. Figure 1 shows the same arrangement as it appears in a speedometer. The rotating magnet on the drive shaft provides the changing magnetic field and the metal speed cup is the conductor. When current is generated in the windings of an alternator it can flow out to the rest of the bike's electrical system. The current that is generated in the speed cup is not so lucky; it has nowhere to go. This current is dissipated in the speed cup in the form of eddy currents, that spin around in the metal without doing any useful work.

The currents cause a twisting force on the cup. If the speed cup was free to rotate, it would spin faster and faster until it reached the same speed as the magnet. The speed cup is restricted from rotating by a hair spring. When the motorcycle is moving the speedometer cable rotates the magnet, which causes a twisting force on the speed cup. The speed cup will rotate until the force of the hair spring equals the force of the eddy currents. The relative position of the speed cup is indicated by the pointer. As the bike goes faster, the magnet generates more eddy currents in the speed cup. This creates a larger force and the speed cup/pointer assembly rotates to indicate a higher speed.

The speedometer is adjusted by changing the relationship between the angular velocity of the magnet and the position of the speed cup/pointer. The usual case is that the speedometer reads too high. This means that the magnetic force must be decreased or the restoring force of the spring must be increased. The hair spring can be adjusted, but it is quite small and in an inaccessible location. You are more likely to damage the spring than fix the speedometer. So a better way is to reduce the magnetic force.

There are two ways of reducing the magnetic force on the speed cup. The first is to subject the magnet to a strong magnetic field to demagnetize it slightly. This method is used at speedometer factories but I do not recommend it since it is a one way process. The second way is to increase the air gap by sliding the magnet down on the drive shaft. But how far should you move it?

Figure 2 Establishing the reference speed
with an electric drill.
(The ultimate form of bench racing).

The first step is to set up a reference speed. This is done by taking the speedometer off the bike and using a variable speed drill to simulate the rotation of the speedometer cable as shown in Figure 2. I squared off the end of an 8d nail to adapt the drill to the speedometer drive (Figure 3). The drill should be adjusted to the speed that was indicated during the error check. It is important that this speed can be repeated many times so, lock the trigger, place the drill in a vise (Figure 4) and start and stop the drill using the plug. It may also be a good idea to measure and record the electric line voltage, just in case it changes during the adjustments.

Figure 3, A squared-off nail is
used drive the speedometer

Figure 4, Hold the drill in a secure
place to prevent the speed from changing

After you have established and secured the reference speed, the disassemble the speedometer. These pictures and text depict a 1975 R90s speedometer. I believe all mid '70s and later boxer speedometers will be similar, but details may differ. In any case, the theory and adjustment method will be applicable. Remove the speedometer from the instrument housing and take off the angle drive assembly. Detach the lower half of the speedometer from the upper half. Be careful when removing the small clip under the drive gear. It can jump off the shaft and bounce off two walls before coming to rest under a tire -- trust me. Gently lift the steel ring out of the lower half using small needle-nose pliers. Remove the magnet and shaft assembly as shown in Figure 5.

Figure 5, Exploded view of the magnet
assembly. Do not loose the little clip!

Figure 6 Use a feeler gauge to
measure the shaft gap

Measure and record the shaft gap using a feeler gauge as shown in Figure 6. In my case the shaft gap was 0.011". The shaft gap, hence the air gap, is adjusted by gently sliding the magnet down on the shaft. Reducing the shaft gap will increase the air gap by the same amount. Gentle is the key word. The best way to do this would be with a press, but I used the Harley all-purpose adjustment tool shown in Figure 7. Take care not to damage the small hole in the magnet end of the shaft that acts as a bearing for the speed cup assembly. Tap, measure, tap, measure, until the magnet moves. The goal is to move the magnet on the shaft in 0.001" increments.

Figure 7, Gently adjust the shaft gap

Once the magnet moves on the shaft, measure and record the shaft gap as in Figure 6. Reassemble the speedometer and check the indication at the reference speed. In my case the indication was reduced 1 mph for every 0.001" reduction in shaft gap. Continue the disassemble, tap, measure, reassemble and check procedure until the indication is the actual speed calculated from the error check data. My final shaft gap was 0.005".

If the magnet is moved too far and the assembly binds, or if the indication is too low at the reference speed the magnet will have to be moved back. This procedure must be performed extra carefully to prevent damage to the bearing for the speed cup assembly. I supported the small washer on the back of the bearing flange with a small deep socket. Then I used a large nail set to drive the shaft back. The nail set has a concave point that will not damage the bearing hole if it is centered on the shaft.

When the indication at the reference speed matches the actual speed the adjustment is complete. While the assembly is apart, this is a good time to clean the inside the instrument lenses. Reassemble the instrument housing and mount it back on the bike. Return to that level section of road and check the accuracy again. In my case the result of six measured-mile speed checks at an indicated speed of 60 mph was 60.55 mph. This is less than a 1% error!

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