On Mon, 12 Sep 2005 15:37:36 -0700 (PDT) Christopher McCann
<[EMAIL PROTECTED]> wrote:
> SO, it's possible that that trip meter (and probably odometer) are
> correct and the speedometer wrong?
Yes, it's possible.
> I was being told that if the speedo was off 10%, then the trip meter/odo
> MUST be off 10% too.
That is wrong. The trip meter/odo MAY be off 10%, too, but they don't have
to be.
The speedometer and odometer work in the following way:
1. The car's wheels turn on the pavement due to friction. The number of
revolutions per mile is determined by the rolling radius of the tires
(relatively fixed).
2. The car's wheels turn the driveshaft through the differential, which
has gears, and thus a fixed ratio.
3. The car's driveshaft turns the speedometer (and odometer) pick off gear
through a solid mechanical shaft, and thus a fixed ratio.
4. The car's speedometer pick off gear turns the speedometer cable via
another gear, and thus a fixed ratio.
5. The car's speedometer cable turns the input shaft of the speedometer
through a (relatively) solid mechanical shaft, and thus a fixed ratio.
Now it gets a little tricky:
6. The input shaft of the speedometer does two things:
6a. It turns the odometer and trip odometer through a set of gears, and
thus a fixed ratio.
6b. It spins a permanent magnet near an aluminum disk. (There is a
solid mechanical connection between the speedometer's input shaft and
the permanent magnet.) The spinning magnet induces a current the
aluminum disk and causes a drag force proportional to the speed the
magnet is turning, and inversely proportional to the spacing between
the magnet and the disk. The disk is mounted on a rotating shaft along
with a rotary spring and the speedometer needle.
SO, the angular position of the speedometer needle is related to how fast
the magnet is spun by the speedometer's input shaft, and by extension how
fast the tires are rotating. BUT, it is ALSO related to the space between
the magnet and the aluminum disk AND where the speedometer needle happens
to be pointing relative to the aluminum disk.
The end result is that you can have a lot of variance of the indicated
speed for a specific rotation rate of the wheels. You have to juggle the
magnet/aluminum disk gap, the strength of the spring, and the pointing of
the speedometer needle to have the speedometer be accurate over its entire
range.
Craig
