james wrote:
>
> List, I just had Hans send me all of his list replies on his speedo problem,
> but they didn't seem to cover mine. So I'll start a similar thread to the one
> just finished.
>
> The car is a '71 roadster, the cable and housing are new, trans angle drive is
> rebuilt. The needle tends to run moderately steady about 30mph higher than
> the actual road speed. The odometer is dead on at all speeds when measured
> against mile markers of varying distances from 1 to 10 miles. The odometer
> always ticks correctly and it will roll over large (10k) digits. The needle
> bounces over about 5 to 10 mph range, but as stated reads very high.
> Typically the needle will read 50mph from just off the line until I start
> aproaching 40 mph where the needle then begins rising in time with speed
> changes. It would appear that on very hot days after prolonged drives the
> needle will start to fall back to the correct speed, but I can't call that a
> repeatable behavior since we haven't had a lot of hot days here lately.
>
> IMO it seems that the cable is working well since the odo is dead on. I infer
> from this that the odo is linked to the cable and that there is some form of
> spring restraining the needle and that there is a slipping or friction clutch
> between the cable and the needle so that the faster the cable turns the more
> friction and thus the needle is dragged, against the spring, farther into the
> range of the speedo. Since the speedo follows road speed over a certain speed
> it would appear that the spring is working correctly, but there is too much
> friction in the clutch and thus the needle travels too far into the range at a
> given speed. This also works with the needle dropping to better accuracy as
> heat is applied over long distances.
>
> So how far off am I? and what can I do about it?
Don't take this as the final answer, since the SMiths Speedo/Odos
I've worked on were not from MGBs. However, I'm guessing they are
similar to MGA and Mini, etc.
The "clutch" is just a magnetic linkage: one magnetic plate and one
ferrous plate with an airgap between. The needles on the units I
have can be turned on the wee shaft if the shaft is held stationary
with needle-nosed pliers or by holding the plate from turning. This
should allow you to adjust the speed shown to the actual speed.
I had the wrong speedo on a Mini I raced in the 60's, so it read
wrong at all speeds. Not knowing how to fix speedos yet, I doodled
around until I got the needle set where it would at least start out
at something like the right reading and then did a series of speed
runs, following a friend's car at a set speed and then marking the
needle position with a pencil. Then I repainted the face of the unit
to match the speeds penciled in. ;)
-Rock http://www.rocky-frisco.com
--
Red Dirt Rangers (Rocky on piano): http://www.reddirtrangers.com
JJ Cale Live (w/Rocky): http://www.rocky-frisco.com/calelive.htm
The Luggage Fan Club: http://groups.yahoo.com/group/luggage-fans
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