On 7 July Philippe Tusler wrote:
Speedo Calibration...
=
= >magnetic field strength. For my MG TF-1500 I set up a drill motor on
= the bench with a speed controller, optical shaft pickup and frequency
= counter to drive the speedo at known speeds. I put a large DC current
= thru a Radio Shack tape degausser to magnetize the speedo's bar to a
= high field strength, then carefully used AC on the degausser to decrease
= the field strength till the speedo read correctly. It worked!
=
= < -Dave Plumpe
=
=
= You've GOT to be kidding! High-tech meets Brit-tech! I should think
= that the the PROPER way to calibrate a Jaeger or Smiths speedometer
= would involve something like taking a small length of narrow diameter
= hose and feeding in through the back of the Speedometer and driving
= around while blowing into the hose with your ear to the instrument in
= order to listen for the proper change in pitch. If it's too high,
= dismantle the differential and replace the pinion gear with one
= of lower ratio. If too low, with one of higher ratio. Repeat
= until the pitch is right, or the car is totaled. "Reassembly is
= simply the reverse of the above."
=
= Seriously: is this how it's normally done, or do you now have something
= marketable?
Wellll - I don't know *fer shur* how it's normally done, but I strongly
suspect the magnetic field strength is the thing that's adjusted to
make it all come out right. It's the easiest I can think of and I know
it's worked at least once! I doubt it's marketable, but donations are
always welcomed by any LBC nut, including this one.
Maybe we should brainstorm a list of possible methods for calibrating
a LBC speedometer. I'll start it off with these first two, then add
a few -
1. Adjust magnetic field strength with tape degausser, using drill motor
and shaft rpm counter as standard.
2. Change differential pinion gear to make car speed agree with speedo
reading, using TRUE PITCH as the standard.
3. Change or shorten the pointer return spring to vary the spring constant
(small springs require many fewer thumbs than on my hands).
4. Spray paint the dial face, then draw new calibration marks and numbers
where the manufacturer SHOULD have put them in the first place (not
really so wierd - some precision analog meter faces really are individually
drawn!).
5. Burn rubber in the parking lot to reduce tire circumference, or wrap
electrical tape around treads to increase circumference 'till the speedo
reads right.
6. Move to a country which uses speed units compatible with your speedo's
markings (furlongs/fortnight, etc.), or get the definition of the
mile/kilometer/hour changed to agree with yours.
Any other methods out there in net.land? Please, nothing so plebian
as to simply change the speedo takeoff gear in the transmission - that's
unworthy of true LBC engineering.
Dave Plumpe
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