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MG TD TF 1500 - speedometer calibration

Hello everybody,

My speedometer is really out of calibration (stock rear axle).40 to 50% optimistic.
I though the reason was the return spring has become weak.

So I tried to bend it backward
- With the needle, several turns around the clock (about two)
- At its fixed attachment

No effect. I thougt the first method was easier and the entire spring would be bent slightly. Actually just the end part of the spring was bent and it eventually foul itself.I managed to solve it.

Then I tried to shorten the spring at its fixed attachment by removing the tiny taper pin. No effect.
I realized that the 3 methods actually changed the free position of the spring, hence the zero reading of the counter but not the gain factor which is related to the strenght constant of the spring.
The strength constant is a function of the material and the cross section of the spring. So there would be no solution but changing the spring.
But I'm wondering about the original manufacturing of these speedo. I'm sure that each assembly would need a final tuning on a calibrating bench.
So what was the tuning procedure at smith's factory ?

Tuning the magnet needs to disasemble the counter.I don't thing this solution is industry efficient.

They might have choosen the magnet depending on each spring ?

Is someone aware of the original tuning method used by Smith or Jeager ?


Laurent.





LC Laurent31

Actually the length of the spring will make all the difference. Think of the adjustment on a clock with an excapement movement. The clock is adjusted to give accurate time by effectively shortening or lengthening the hair spring by moving a "clamp" along its length. Or think of a long coil spring with a weight attached to one end and the other end fixed, set it in vertical motion and note the period of oscillation. Now cut the spring in half, attach the same weight and set it in vertical motion as before. The period of oscillation will be considerably slower. As well as that, the "stretch" of the spring when the weight is added will be much less with the short spring than with the long one.
In other words changing the length of the spring changes the "spring constant".

Hugh Pite
H.D. Pite

The major adjustment for speedo calibration is as you stated the strength of the rotating magnet, the secondary adjustment if the depth the rotating magnet protrudes into the aluminiuim cup.

Another factor is the drive ratio that is used to rotate the magnet. Declan can assist in this area with his custom built speedo drive reduction boxes.

I would be carefully inspecting to see if the aluminium cup is not dragging on the rotating magnet, a major cause of speedos becoming "optimistic".
G Evans

Laurent,

Usually it is not necessary to disturb the hairspring for a good calibration. As stated by G Evans, the most practical way of calibrating the speedo is by changing the force of the magnets in the input wheel (very easily done with another more powerful magnet).

There is a detailed description of the whole process (for Triumph gages, but identical to ours in operation principle) in the thread:

http://www.britishcarforum.com/bcf/showthread.php?94885-Instrument-Faces/page6&highlight=speedo

(you have to scroll to page 6 and following, but the whole thread is very useful)

I have used this method successfully on several gages, in particular in the TD speedo and revmeter.

There is also a very useful document “Repairing Jaeger & Smiths Speedometers”, out in the web:

http://obswww.unige.ch/~wildif/cars/docs/Smith-jaeger_speedo_repair.pdf

If you think that I can be of some help, do not hesitate in asking.

Jesus
J Benajes

Thank you Jesus,
I knew the Anthony Rhodes document but you made me discover this outstanding thread about restoration and calibration. What a clever guy !
I'll try the magnet. I'm sure this was the procedure Smiths used for calibrating their counters.

It remains that taking out the machanism off the casing is a hard moment ! (the cotter pin of the reset shaft through the bulb holder)


Laurent.

LC Laurent31

It works !
My speedometer is calibrated using the magnet method.

However I would suggest a more straight forward approach than the iterative process from the thread.

First : calibrate the gain (the magnet strength).
Second : adjust the offset (the needle position versus drag cup)

1) The ratio of our speedometer is written on the face : 1600 Turns Per Mile.

So an increase of 400 RPM at the input shaft should show an increase of 15 mph.
Just set a given speed rotation at the input shaft,Let's say 800RPM for exemple. Write down the reading of the speedometer.
Now set the input speed 400 RPM faster. The speedometer should read 15 mph faster.

Tune with the magnet procedure described in the thread for higher or lower reading.

2)Set the test bench to 1200 RPM and adjust the needle position to read 50 mph (center of the scale).

Laurent.
LC Laurent31

This thread was discussed between 07/02/2016 and 12/02/2016

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