[DeTomaso] Speed error and how the mechanical speedometer works

[Dagny Kristine Bakken]

In a mechanical speedometer, a rotating cable is attached to a set of gears
in the automobile's transmission or one wheel. This cable is directly
attached to a permanent magnet in the speedometer assembly, which spins at a
rate proportional to the speed of the vehicle. As the magnet rotates, it
manipulates an aluminum ring, pulling it in the same direction as the
revolving magnetic field; the ring's movement, however, is counteracted by a
spiral spring. Attached to the aluminum ring is the pointer, which indicates
the speed of the vehicle by marking the balance between these two forces. As
the vehicle slows, the magnetic force on the aluminum ring lessens, and the
spring pulls the speedometer's pointer back to zero.A correction can be done
by repositioning the needle to the pin its often mounted to.A thing often
performed by shops specialiced in instrument restoration/calibration.Hope
this helps :)

Øyvind Bakken 

-----Opprinnelig melding-----
Fra: detomaso-bounces at realbig.com [mailto:detomaso-bounces at realbig.com] På
vegne av JDeRyke at aol.com
Sendt: 9. mars 2009 06:47
Til: michaelsavga at gmail.com; detomaso at realbig.com
Emne: Re: [DeTomaso] Speed error

The speedo head drives a separate geartrain inside the speedometer case to
turn the odometer dials. Thus there's a chance one function will be accurate
and 
the other off at some, or all, speeds.   In running T & D rallies years ago,

I found the stock odometer, with 26-1/2" OD rear tires, to be very accurate-
at least at the speeds rallies were run (always a legal speed). During this
time the speedo was always optimistic. You're probably better off using a
calibrate-able electronic unit, but even then, one function may be more
accurate than the other- dunno how the mechanism inside works. Good luck- J
Deryke



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