I am curious if anyone knows how a calibration is done to Smith
speedometers. Is the amount of magnetism on the spinning disk changed or
the spring tension of the needle altered?
I was fiddling with a speedometer last night (to a GT6). There was
excessive end play inside the unit and often the needle would jump
*alot* when the wrong pieces came in contact. I got that problem fixed
-- the needle now runs steady -- but the calibration is off quite a bit.
Please, a response like: "To calibrate a speedometer, send unit and
dollars to....." I'm just curious of the process, not wealthy enough to
send it out to end the irriatation.
For those who haven't had the pleasure, when you pull the guts out of an
LBC speedometer, there are three assemblies to deal with:
The first deals with the cable coming into the unit. The cable
terminates at a bushing. Aft of the bushing is a spiral-cut gear that
engages a right angle drive. That back of the drive is a cam that moves
a lever and the lever ratchets the odometer.
The odometer is the second assembly and it is pretty straightforward.
Someone figures out the ratios and it counts how many times the cable
rotates. The little numbers to the right of the model munber on the face
of the speedo is the rating for your instrument: The number of cable
rotations that will equate to one mile for the odometer.
Getting back to the first assembly, after the spiral-cut gear, the cable
end terminates with, effectively, a magnetized plate. The plate rotates
just as fast as the cable.
The third assembly is the speedometer which begins with what appears to
be a small cake pan. The magentized plate spins inside this little pan.
The pan is pinned at its center to a rod that extends out to the face of
the instrument. A needle is attached to indicate speed.
The rod connecting the small pan and the needle is also attached to a
weak, clock spring. So what happens is that the spinning, magnetized
disk will try to rotate the metal pan surrounding it. The spring keeps
the pan from spinning. The force of the spring fighting against the
magnetizism of the spinning plate is what is being measured on the face
of the speedo -- and, supposedly, once calibrated for MPH.
Incidently, the operation of the speedometer and odometer are completely
independent and that is why you can, and probably do, have varying
accuracy between the two.
For those still following this, I cannot find any adjustment for either
the spring or the magnetized disk. I am just curious which is changed to
effect calibration: the tension in the spring or the degree of magnetism
in the spinning disk. The phrase "no user servicable parts inside" comes
I've noticed both the C/GT and the TR6 have wildly optomistic
speedometers. As if, over time, the spring tension has weakened thus
showing too high an indicated speed. The odometer on the C/GT is off,
too, but that is likely due to a rear-end gear change (3.7 to 3.9). The
odometer in the TR6 is within 3%.
Bob Allen, Kansas City, '69CGT, '75TR6