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RE: speedo

To: "" <mgs@Autox.Team.Net>
Subject: RE: speedo
From: Andrew Errington <>
Date: Wed, 22 Apr 1998 16:13:28 -0700

Can anyone present a lesson on speedometers?  Have a '57 MGA and have 


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 

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 

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.


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