[Rovernet] speedo is nervous

[Gordon Reddy]

Amazing! I will try this on my P5 this winter, thank you Bill!
Gord Reddy

On Tue, 19 Aug 2025 at 11:03, Jim Pile via Rovernet <rovernet at rovernet.org>
wrote:

> Amazingly detailed info, Bill. Thanks for explaining a ubiquitous issue
> with the earlier P6 models.
>
>
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> On Tuesday, August 19, 2025, 1:49 PM, magnet--- via Rovernet <
> rovernet at rovernet.org> wrote:
>
> Hi Larry,
>
> Speedo issues almost always come down to lubrication, or to be more
> precise the lack of it.
>
> As has been pointed out, the lube in the cable, and if present, in the
> right-angle drive behind the speedo head, dries out and gets gummy.
> Anything that resists the even turning of the cable, especially up near
> the head, can cause jitter, as the cable "winds up" and then "lets go"
> once it overcomes the resistance.  On the "let go" phase, the cable
> momentarily gives a little surge, resulting in a spike in the speedo
> reading.
>
> But a more insidious problem inside the speedo head itself often causes
> this surging, and lubricating the cable alone will not eliminate it.
> Smiths (aka British Jaeger) speedometers use a little pawl-and-ratchet
> arrangement to drive the mileage counter. A worm on the speedo input
> shaft turns a pinion gear, and this gear moves a pawl back and forth
> every time it goes around.  The pawl, in turn, pushes a toothed wheel
> around a few degrees on each stroke, and this wheel is coupled to the
> tenths-of-a-mile dial of the mileage counter.
>
> Now what happens when the lubricant of the pawl and the toothed wheel it
> drives dries out?
>
> The wheel gets harder for the pawl to turn.  The pawl thus has to work
> harder on the forward stroke as it turns the wheel. This causes a
> heavier load on the speedo cable, which winds up a bit. But the pawl's
> return stroke is easier, and the cable unwinds, giving a flip to the
> speedo needle.  Depending on your speed, this oscillation takes place a
> couple of times each second or faster, resulting in a small but
> noticeable jitter in the speedo needle. At some speed, this jitter
> coincides with the resonant frequency of the needle and hairspring
> assembly and you get a really big swing on the needle.
>
> But wait, it gets even more interesting!  Every time the tenths dial
> goes through a revolution, it moves the mile wheel forward by 1.  So the
> pawl has to work even harder to turn two dials, wnding up the cable even
> more, and causing a bigger flip when it relaxes.  So once every mile,
> you'll get a bigger swing of the needle.  And every ten miles, an even
> bigger swing, etc.
>
> Lubricating the pawl is easy, once you have opened up the speedo head,
> and greasing the pinion wheel shaft is pretty straightforward too.  In
> my case, binding of this shaft was the main culprit.  Just as well, as
> lubrication of the six-wheel dial assembly would have involved more
> disassembly than I was prepared to undertake.
>
> Cheers,
>
>  -- Bill Daddis
>
>
> On 19-Aug-2025 8:31 a.m., larry williamson via Rovernet wrote:
> > My speedo is steady sometimes and sometimes jumps around. What is the
> > reason and how to fix?  I can't seem to find out in the classic forum
> > these notes. Larry
> >
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