[TR] Speedo indicator calibration

[Anthony Rhodes]

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