Calibrating magnetic Smith speedometer

Now that your speedometer is cleaned with the aid of the previous articles, you may have realized that a calibration is needed.

This article will help you through this procedure. The tachometer is also based on the same principles and the same mechanical assembly. It could be adjusted the same way.

When I refer to RPM in this article this should be understood as input shaft spinning speed of the speedometer.

Remember this important diagram from the first article.

Theoretical speedometer curve

The offset parameter B should normally be nil since the needle should indicate 0 for a speed of 0 RPM.

As the instrument is mechanical, there are static forces that disturb the indication towards 0 RPM. So, the ideal function represented as a straight line in the diagram is actually distorted around the low speeds. We can adjust B so as to minimize the gap between the true function and the ideal straight line.

You may read many tricks for calibration just with tuning parameter B. Adjusting B is easily performed by rotating the needle on its axis so it rests in another position. It can be sufficient but adjusting the gain parameter (A) is rarely addressed.

On the dial of the TD instrument, we can read a small number print: “1600”. This magic figure is actually the A parameter. It means that when the input shaft rotates 1600 times, the odometer will show 1 mile. And if the input shaft is spun at 1600 turns per hour, the speedometer should read 1mph. Turns/hour is not a practical concept. Turns/minute is better to handle: RPM.

Hence the following formula:

The formula shows that increasing the RPM by a certain amount, the speed indication will also increase by a constant amount. And this is true even if B parameter is not zero. This is why A is the first parameter to adjust. As an example, for this calibration procedure, increasing the RPM by 800 RPM should show an increase of 30 mph on the dial.

Because of the non-linearity towards 0, it is preferred to calibrate the gain parameter at higher speed. I choose to calibrate at around 50 mph which is a speed when we need true indication from the instrument. The corresponding spinning speed of the input shaft is approximately 1300 RPM. Exact value is not important for the gain adjustment.

The set-up

You will need a drive motor with steady speed. Use a drill press, a lathe or make your own bench with a DC motor. It is important to have a faithful indication of the bench rotating speed. I use an optical tachometer that can be purchased on eBay for 20 euros. If you use a drill or a lathe, it should be set to reverse as shown in next picture.

Driving direction

The calibration bench

Sorry for the pictures that do not show a TD clock but another similar Smith instrument. I don’t want to disturb my calibration that was made previously. Connect the speedometer with a spare cable. Motorbike cables are shorter and thus easier to handle on the bench. Have a small magnet ready for adjustment. Rare earth small magnets (1 x 0.5 x 0.5 cm) can be purchased for a few euros.

The measurement procedure

1) Set the bench speed to 1300 RPM on the reference tachometer. Note the indication of the TD instrument.

Now set the bench speed to 1300+800 = 2100 RPM on the reference counter. The TD speedometer should read exactly 30 mph higher than the first measurement.

2) If this difference is greater, then you should decrease the magnet strength. If it is lower than 30 mph then you should increase the magnet strength. If it is equal, you can jump to B parameter adjustment.

Decreasing the magnet strength

Drive the input shaft at a speed between 500 and 800 RPM. No matter the exact speed. Watch the needle. Very slowly bring a small magnet close to the rotating plate. When the magnet is a few millimetres from the rotating plate you should see the indication of the needle decreasing. Get a bit closer if it does not decrease. But still very slowly and always watching the needle for slight decrease. Now perform the measurement procedure and loop to decreasing or increasing step if necessary.

Decreasing the magnet strength

Increasing the magnet strength

Stop the drive bench and slowly bring the magnet near the plate. Let the magnet land freely on the plate so that it is oriented along the existing field of the magnetic plate. Then slide it outward and take it away from the plate. Now do the measurement procedure and proceed to decreasing or increasing step if necessary.

If you cannot increase enough the strength then try a pair or a more powerful magnet.

Decreasing is proportional to the distance between the magnet and the rotating plate. Easy to dose.  Increasing is strong and may bring the indication too high at once. So accurate adjustment is generally obtained during weakening procedure.

Increasing the magnet strength.

B parameter adjustment

When you are satisfied with the A parameter check the absolute values indicated by the needle against the first graph. It may be necessary to adjust the offset parameter B. Adjusting B is simply made by shifting the needle on its axis. Drive the input shaft to the theoretical speed you want to set as a reference for the instrument. 45 mph is roughly at the middle of the scale and can be selected as a reference. Moreover, it corresponds to a round value of RPM: 1200. So, drive the shaft at 1200 RPM and note the indication of the needle if not correct.

Now stop the driving motor and rotate the aluminium plate with your fingers so that the needle indicates the previous noted speed. Extract the needle with the fork tool described in the first article and position the needle so that it shows 45 mph.

Still on the bench, you can then draw the actual curve of your speedometer from 0 to 2400 RPM and check it against the theoretical one.

When you are satisfied, fit the instrument on the dashboard and enjoy a drive in your car. I guess you will check the instrument against the GPS measurements. Both may disagree because of the diameter of the tyres, wear out or pressure. But now the instrument is calibrated as it was when new. This means that speedometer and odometer indications are consistent.

It is still possible to adjust A and B slightly differently to match the GPS but you should know that the odometer cannot be adjusted. It remains on the 1600 TPM ratio which is determined by the gears. That’s why this procedure cannot be used for a rear axle ratio upgrade.

I hope that you enjoy this series of articles. Delicate jewellery job in between greasy mechanical operations!

Laurent Castel