Hi,
Can anyone present a lesson on speedometers? Have a '57 MGA and have
always
<snip>
My understanding is as follows:
A small gearwheel on the final drive on the transmission drives a cable
which is fitted inside a flexible sheath. The cable can turn freely inside
the sheath, and is routed to the rear of the speedo. At the rear of the
instrument the cable spins a magnet (the cable entry may be straight, or
via a crown or bevel gear at right angles due to space constraints behind
the dash).
In front of the speedo dial is the pointer, mounted on a pivot which goes
through the dial and through an aluminium disc behind the dial. The magnet
spins behind the disc on the same axis. The disc is prevented from
rotating in one direction by the pointer, which rests against a pin at 0
(or 5 or 10) on the speedo dial, and in the other direction by a small coil
spring.
The rotating magnet induces eddy currents in the disc, which set up
magnetic forces which cause the magnet to tend to drag the disc in the
direction of rotation (against the coil spring). The faster the magnet
spins the further the disc can rotate against the force of the spring, and
the amount of rotation is indicated by the pointer against the speedo dial.
So, the faster you are travelling, the higher the number that is indicated
on the dial.
Obviously the speedo calibration is dependent on many things including:
1) The tyre size (as the speedo is driven from the transmission, not the
road itself)
2) The gears at the takeoff point on the transmission
3) The construction of the speedo itself (disc size, spring tension, magnet
strength etc).
I think that explains it. By the way (BTW) one of the reasons the speedo
tends to jump is that there is something that catches the cable inside the
sheath, so it rotates, gets stuck, winds up and then breaks free, then
repeats.
Now the off-topic bit:
If anyone else on the list is an electronics enthusiast I can offer plans
and software for building an electronic speedo. It's not that hard, and
can be calibrated to any size wheel.
I originally designed it to use a digital display, but I recently thought
it should be quite easy to build a small stepper motor into a conventional
gauge so the pointer could be moved electronically.
This is probably a very off-topic [OT] thread, so to reduce list congestion
please reply directly. And if you are replying to the main reply please
snip judiciously.
Andy
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