I have looked at a bunch of websites with no luck. But someone here is
sure to know. All the mechanical tachs I am familiar with work the
same, with an eddy current disc and a spinning magnet. I figured this
out when I fixed a speedometer in a VW Bug way back in 1980. But I
have seen several antique race cars with tachs that don't seem to work
this way because the needle moves in a jerky fashion. It will swing
fast to a position and then stop. The rpm will then audibly change and
there will be a time lag before the needle quickly swings to another
position and stops dead. It's as if the needle can only move in
discrete steps. I can't figure out why this is, if it's done on
purpose, or if it's desirable. I have seen this type of action on
several different makes of race cars. So, who here knows about this?
If you want to see this in action search Youtube for the TV show
"Victory By Design". If you like cars at all, and race cars in
particular, and have never seen any of these shows then by all means
look it up. A retired race car driver, Alain de Cadenet, does a
wonderful job driving and showing off the cars. His obvious joy
driving the cars and his obvious skill driving the cars makes for a
really enternaining show. He talks about brakes, engines, suspensions,
frames, superchargers, carbs, etc. But he never says anything about
the tachs.
Eric