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Why did old tachometers not read continuously?


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#1 MichaelJP

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Posted 10 October 2002 - 09:54

The thread about "sideways" tachos reminded me of something that I was wondering about recently:

- Why on many older motor racing films do the tachometers not read continuously? To elaborate, the needle sort of "ticks" up and down with a new reading every second.

This must have been difficult to read!

- MichaelJP

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#2 BRG

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Posted 10 October 2002 - 13:57

I think that these were mechanical tachometers, rather than electronic ones. They took their drive off the distributor in some way, similar to a speedo drive.

For some reason (?accuracy, or maybe reliability?) these were favoured for competition cars for a long while after electronic ones became available on road cars.

#3 bobdar

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Posted 10 October 2002 - 14:34

The motion you describe sounds like the Smith's Chronometric tach, a mechanical tach that usually is driven off the camshaft. They are the original tach for most historic race cars, but are not exactly "real time".

#4 VAR1016

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Posted 10 October 2002 - 14:46

Originally posted by bobdar
The motion you describe sounds like the Smith's Chronometric tach, a mechanical tach that usually is driven off the camshaft. They are the original tach for most historic race cars, but are not exactly "real time".


Yes, the Chronometric tachometer was always used on motorcycles.

Of course plenty of vehicles had conventional tachometers driven by cables from the camshaft which read just like speedometers. In one particularly pathetic instance ( the Mk1 Austin-Healey SPrite) the tachometer was driven from the back of the belt-driven dynamo! Very "engineering". :rolleyes:

I have always suspected the reason to be that they are resistant to vibration - certainly in this way the poor driver/rider would have a chance to read them.

I imagine that a chronometric tachometer would be essential on a Vanwall ;)

PdeRL

#5 MichaelJP

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Posted 10 October 2002 - 15:31

Originally posted by bobdar
The motion you describe sounds like the Smith's Chronometric tach, a mechanical tach that usually is driven off the camshaft. They are the original tach for most historic race cars, but are not exactly "real time".


That's the one - but why does it "tick"? After all, the camshaft is continuously rotating. Does anyone know how they work?

Also, you might overrev the engine between ticks, and not notice!

- MichaelJP

#6 bobdar

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Posted 11 October 2002 - 03:34

You'll have to take one apart to see what makes it "tick". :lol:

#7 Mark Beckman

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Posted 11 October 2002 - 04:16

The reason is simple, the Smiths Chronometric system makes a calculation every certain number of rotations and makes the adjustment then and moves the needle into that relevant position.

Modern instruments have a spinning magnet that drags the needle around and the faster you go the more the (sprung loaded) needle is dragged around with it.

I can get hold of a Smiths technical description of the workings if required, but make sure its required cause it will cost me an hours drive.

#8 dbw

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Posted 11 October 2002 - 08:00

ha! the old joke is that a chronometric tach will accurately tell you the engines rpm three seconds ago.... gp bugattis have a jaeger unit that i assume has a similar "clockwork" mechanism as the smith's....as opposed to the sprite's solution,consider this..
1. the bugatti's crank turns at crank speed..[duh]
2. the single overhead cam turns 1/2 crank speed...[still with me?]
3 as bugatti wanted to use the "best" bosch mag,[which ran at crank speed] a "step up" box was fitted to the cambox end to turn the mag at the proper speed.[now crank speed]
4.now as the jaeger tach[french,as one would expect] happened to turn at cam speed,so a 1:2 step-down flat leather belt and pulley unit,with spring tensioned idler no less, drove essentally a free spinning ball bearing alloy pulley on the engine facing firewall.
5. now the tach mounts facing rearward on the dash,with a 8 mm stump aiming directly at the back of the pulley[which coincidently has a similiar stump facing it.]
6. as there is about 4 inches of air between them a short bit of rubber hose connects the two and presto!at mag off the tach often reads 500rpm..and when the engine bursts into life,the twitching needle still lags the actual engine rpm by i misissippi; two misissippi .......

whew, it seems that at races and shows,with the hood open,more questions are asked about the kremlinsque tach drive than the truly baffling scroll advance/retard assy[sitting right there as well.....

even more hilarious were the little TT mg's with easily a 8+ inch dia. chrono tach sitting plunk in the middle of the facia....ablaze with colored pie wedges screaming at you where the needle was three seconds ago.

#9 MichaelJP

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Posted 11 October 2002 - 09:44

Originally posted by Mark Beckman
The reason is simple, the Smiths Chronometric system makes a calculation every certain number of rotations and makes the adjustment then and moves the needle into that relevant position.

Modern instruments have a spinning magnet that drags the needle around and the faster you go the more the (sprung loaded) needle is dragged around with it.

I can get hold of a Smiths technical description of the workings if required, but make sure its required cause it will cost me an hours drive.


Thanks for the info, Mark - it's not worth a special trip, just curiosity on my part. It doesn't sound the same as the Bugatti tach described by dbw, which is basically just geared down *a lot*.

But if anyone has the info easily to hand, I'd be interested to know how the Smiths system worked.

- MichaelJP