[TR] Broken speedometer cable - again-

[Randall]

> My TR4A speedometer cable broke again yesterday!
> The needle on the second speedometer was kind of wobbly so I 
> suspect part of
> the problem is the speedometer itself but I think I would 
> like to know if
> anybody has any ideas.

That is exactly my idea; I've seen it several times.  Inside the speedo, there is a steel shaft that rides in a porous bronze
bushing.  When it gets old, the oil dries up and the bushing starts to bind just a little bit.  The speedometer still works, because
the shaft still turns, but it puts extra strain on the cable (and angle drive for cars that have one).  But the friction also
generates heat, which drives the oil out even more, and also tends to make the bushing grab the shaft harder.  Eventually, the cable
can't deal with the extra load, and breaks.

When I bought Stag #2, the previous owner had just "fixed" the speedometer by replacing the angle drive.  Speedo worked fine for
about 100 miles (on the way home) then the angle drive broke again.  When I got home, the speedo still worked, but I could feel the
drag in that bushing.  Added some lubrication to the bushing, and the replacement angle drive was still working fine at least 10,000
miles later (the car is off the road for other reasons).

The trip and odometer mechanisms also dry out and bind up (the ones on my TR3 were actually rusty inside), which adds load to the
cable and, at least in my case, caused fairly wide swings in the needle at speed.  The ratchet action that drives the dials means
the dials are only connected to the cable part of the time.  The needle drops down, because the speedo shaft is actually turning
slower as the inner cable twists under the extra load.  Then the ratchet lifts, disconnecting the dial, and the needle rises above
truth as the inner cable untwists.  If you've ever bent a wire back and forth until it breaks, you can see why the constant twisting
and untwisting shortens the life of the cable.

Randall