[Rovernet] speedo is nervous

[magnet at ican.net]

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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