Speedo

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Ezmerelda1
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Joined: Mon Mar 29, 2021 2:47 pm
Location: Lancs UK

Speedo

Post by Ezmerelda1 »

My 1956 350 G3 LS Speedo is a bit notchy when speed is increasing does anyone know what problem may be please
Greybeard
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Re: Speedo

Post by Greybeard »

Its a chronometric speedo presumably - its how they work. A bit complicated to explain in a short post, but its like a clock mechanism rather than magnet driven. The needle needs a number of impulses from the drive before it jumps on a bit. You'll also find that if the drive suddenly fails the needle will stay at the last indicated speed until it gets another 'reminder' that its slowing down gradually.
Welcome to the world of engineering curiosities ;)

Steve
SPRIDDLER
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Re: Speedo

Post by SPRIDDLER »

Welcome to the club and Forums.
As Steve has said the complexity of the chronometric speedo is a black art to most of us.
The needle only moves in steps and not as a progressive movement.
Here's an extract from an article that gives you a flavour of its mechanicals:

The operation of the whole mechanism is thus governed entirely by the
escarpment, and as the teeth cut on the spindle H are drawn in and out of
engagement at regular intervals, the indicating hand connected to L is moved at
those periods only, at all other times being locked in position, and when it does
move it moves only the difference to correspond with any variation in speed
that has taken place during the last ¾ second.


If you're sitting comfortably have a look at the technical article from which the above quote was snipped:

http://archives.jampot.dk/technical/Spe ... ration.pdf
'There is a tide in the affairs of men
Which taken at the flood............'
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Group Leader
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Re: Speedo

Post by Group Leader »

I became fascinated by these marvellous instruments initially out of necessity as I attempted, successfully as it turned out, to coax my one back into life a while after getting my AJ.

Here's a a little video clip of my one showing how the "speed" is sampled by the rocking pinion engaging and then disengaging with the lowest wheel of the three on the main spindle for a fixed period of time. You can also see the pin on the lower wheel that pushes the next wheel up in the stack round if the speed has increased in the previous sample period. What you can't see is the cunning arrangements of cams and finger springs that stop the middle and top wheels immediately returning to 0 as each new sample is taken. There's another video clip that shows these working on my YouTube channel although you can't really see what they are actually doing.


What fantastic instruments Chronometrics are particularly when you realise the age of the design :beer:

Alan
1953 AJS 16MS, 1939 BSA 250 and a 1/3 scale Sopwith Triplane but that's another story ..... :lol:
Ezmerelda1
Posts: 18
Joined: Mon Mar 29, 2021 2:47 pm
Location: Lancs UK

Re: Speedo

Post by Ezmerelda1 »

Thank you for all your help guys? Very enlightening 👍
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clive
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Re: Speedo

Post by clive »

To be honest if they are working properly the movement should not be too notchy as a speedo, more so if braking heavily. As a rev counter they can tend to leap around more. I would be inclined to leave well alone until it goes wrong, and I used to repair them!
clive
if it ain't broke don't fix
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