How to Check a Speedometer Magnet

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Model T Ford Forum: Forum 2015: How to Check a Speedometer Magnet
Top of pagePrevious messageNext messageBottom of page Link to this message  By Alan Woolf on Thursday, January 29, 2015 - 08:35 pm:

We are working on a Stewart Speedometer and suspect the magnet is too weak. Is there a seat of the pants type test to determine if the magnet needs to be charged?

A good example of such a test would be a magnet from a magneto. If the magnet is well charged it will pick up a certain amount of weight. I need to determine a similar test for a speedometer magnet.

Alan


Top of pagePrevious messageNext messageBottom of page Link to this message  By Russ Furstnow on Friday, January 30, 2015 - 08:24 am:

Alan, I've worked on a few speedometers in the past, and there is no "test" to determine the strength of the magnet. Stewart had a variety of hair springs that had different tension to make up for the strength of the magnet. I hope this helps.
Russ Furstnow


Top of pagePrevious messageNext messageBottom of page Link to this message  By Alan Woolf on Friday, January 30, 2015 - 08:57 pm:

Russ,
Is there source for speedometer hair springs that would be appropriate for a Model 26 speedometer?

Any thoughts on charging magnets?

Thanks

Alan


Top of pagePrevious messageNext messageBottom of page Link to this message  By Russ Furstnow on Saturday, January 31, 2015 - 09:47 am:

Alan,
As you know, there are a variety of Model 26 speedometers, and each has a main shaft that has a different diameter. That being said, I know of no one who supplies these hair springs. When I work on a speedometer, I use my calibration machine to determine the speed (either too slow or too fast) and install the appropriate spring. The earliest Model 26 speedometers have no speed adjustment, making the hairspring the only way to adjust the speed.

If you have a later Model 26 (1912 or later), there is a small adjustment screw on the back side of the head and you can adjust the distance between the aluminum speed cup to the magnetic drum. Turn the screw clockwise to increase the speed and counter clockwise to slow the unit down. The closer the aluminum cup gets to the magnetic cylinder, the greater the speed.

Finally, if you want total accuracy in your speedometer, get a GPS. With the variety of tire sizes (oversized 30X3 1/2" tires, etc.) it is sometimes difficult to get your speedometer to indicate an accurate speed.
I hope this helps,
Russ Furstnow


Top of pagePrevious messageNext messageBottom of page Link to this message  By Chester Leighton on Saturday, January 31, 2015 - 10:55 am:

Russ I have a Stewart Model 102 speedometer. Is there a speed adjustment on the head as you describe or will the hair spring need to be changed to adjust indicated speed?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Russ Furstnow on Saturday, January 31, 2015 - 12:13 pm:

Chester,
If you look underneath the magnetic drum, you will see a movable steel device that fine tunes the speed. Generally this flat piece of steel is found close to where the magnet is split. If you move this steel piece counter clockwise, it will increase the speed and moving it clockwise to reduce the speed. This fine tuning adjustment is found on all Stewart speedometers after 1912.

If the speed is dramatically low or high, this fine tuning adjustment will not be enough to compensate for the speed, and the hair spring will need to be replaced.
I hope this helps,
Russ Furstnow


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