[TR] Speedo indicator calibration

[Geo Hahn]

Tony -

This is excellent information!  Your monograph on speedo repair and
calibration has been the gold standard for DIYers in the past - now this
addition may make it possible to overcome another hurdle in achieving
accuracy.

Apparently this technique is unknown to the big-name rebuilder who tried to
calibrate a speedo for me as they said (after failing badly):

Because of the wide variation between the original and current calibration
> of the Speedo, any further fine tuning of the calibration is limited to
> weighting the pointer and playing with spring rates both of which will
> produce results that are in all honesty, a compromise, I.e., weighing the
> pointer will drop the Speed down at the lower range but will also slow it
> down in the upper range, changing out the drum spring to one with a higher
> rate will help solve this problem, however there are not a broad range of
> different spring rates available.


Thanks for pointing the way.

Geo

On Sun, Feb 19, 2017 at 8:00 AM, Anthony Rhodes <spamiam at comcast.net> wrote:

> A friend has a TR4A with a 4.1 rear and 195 series tires and an OD with a
> lower than stock ratio in OD.  It is from a sedan of some sort.  This OD
> has a speedo drive with the wrong number of starts for the stock TR2-4
> speedo.
>
> The net result is it has about 940-950 turns per mile!  Nothing near the
> usual 1152-1184 TPM.
>
> He eventually found a nice speedo with odo gearing of 940 turns per mile.
> But that speedo had a dual face going to 140 mph instead of the stock 120
> on a TR speedo.
>
> I found that it was entirely possible to calibrate the speed indicator by
> overly re-magnetizing the bar magnet.  Then de-magnetizing it to get the
> pointer to read exactly what is required!
>
> I was inspired by finding a reference on how the factory calibrated the
> speedo originally.   They use a POWERFUL electromagnet to magnetize the bar
> magnet and then they mounted the speedo in a test machine fully assembled
> except for the outer case, but WITH the dual face and pointer properly
> installed.
>
> They would then spin the speedo at the required speed.  The pointer would
> read well over the desired reading.  They would then expose the speedo to a
> weak-ish electromagnetic field WHILE it was spinning and the pointer would
> gradually drop to the proper reading.  Then the calibration was finished.
> It would take maybe a minute to do
>
> I tried to make a big powerful electromagnet to exactly replicate the
> functions of the factory calibration bench that MAY have been possible but
> a pound of electromagnet wire later and my electromagnet was WAY WAY too
> weak.  My rare-earth coin magnets are much stronger than that so I bailed
> on the electromagnet plan.
>
> I decided to try to use the super magnet to magnetize the speedo magnet
> the same way I made magnets when I was a kid.  You can stroke steel with a
> magnet in one direction and the steel will pick up a magnetic field.
>
> I did this with a small stack of the super magnets to the speedo's bar
> magnet and then reassembled the speedo. I affixed a stock 120 mph face and
> tested the speedo at 940 RPM.  After a couple of false starts where I
> WEAKENED the speedo magnet, I got it reading about 75 mph at what should
> have been 60 mph.
>
> I then took a stack of the coin magnets (so it was easy to hold) and
> gradually and gently  brought the stack closer and closer the the speedo
> spinning speedo magnet.  Then the
> pointer was seen to slightly drop closer to 60.  I did this a few times
> waving the magnet near the speedo and the pointer ended up at precisely
> 60.  I then spun it faster to what should read 100 mph and it was dead-on.
>
> Job done.
>
> You just need to get a stack of the strongest rare earth magnets you can
> get.  I used 1/2" diameter, 1/8" thick magnets to re-magnetize.  And I
> could have used these to demagnetize, but I had a stack of 3/4" diameter
> similar magnets to use to de-magnetize.
>
> Since this is an analog reading, you can get ANY calibration on the
> pointer, not just the increments of 20 or32 turns per mile increments that
> the odometer gears allow
>
> -Tony
>
> Sent from my 1837 Babbage Analytical Engine
>
> ** triumphs at autox.team.net **
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