[Rovernet] speedo is nervous

[larry williamson]

Thank for all that responded.  Now the work begins. Larry

On Tue, Aug 19, 2025 at 9:12 PM Gordon Reddy via Rovernet <
rovernet at rovernet.org> wrote:

> 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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