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When I ran my Instrument repair and supply business in Sydney, Australia until several years ago, we occasionally serviced and supplied the Smiths special "maximum hand" chronometric tachometers.... called this as they had an additional pointer, painted red and when at rest, sitting under the white working hand.
They were developed in the competition shop of Smiths Motor Accessories in Oxgate Lane, London for use with Formula 1 GP cars in the 1950s and 1960s.
Of course other racing applications also utilised them.
I acquired the remains of this competition shop from the last manager, the late Jack Owens in the 1980s.
When the engine speed increased, both hands moved upwards together until the engine ran steadily even it only for a fraction of a second say while a gear change was made...if the engine revs dropped, then the white, upper , working pointer fell down with the decreasing revs, but the red "maximum" hand remained at this highest engine speed, held by a spring loaded bellcrank against a small upper ratchet gear wheel.
If the engine revved higher than the red hand indicated, then the white working hand as it passed the red hand collected it and together they advanced up to the new highest revs position on the dial scale.
The only way to bring the red "maximum" hand back to the rest position, was by operating a reset button at the back of the tachometer case, always when the vehicle had stopped and the engine switched off.
The use of this was to let the pit crew/driver/rider know the maximum revs obtained, possibly for optimum gearing purposes. Racing boats used them to determine the propellor configuration. As well you could see if the engine was over-revved.
Rumour had it that Joe Craig had special "maximum hand" tachometers made for the works Nortons, without the reset button, so riders couldn't interfere and that the tachometers were taken to the Smiths garage in the IOM pits for resetting.... I can't confirm this...
But what has this to do with the special speedometer you can see pictured....?
All that I said above can be looked at in the mechanism of this speedometer by enlarging the photograph....
Some time in the past, Felix Tydeman, who worked for me, made this special chronometric speedometer for me as a birthday present.
There is no dial as such, the speedo scale is printed under the glass and of course you can observe the operation of the speedometer mechanism as you ride along... hence the title of this blog ..."crash gauge".... closely watching the facinating mechanism and not looking where you are riding sure makes for disaster....
I have it fitted on my 1954 MSS Velocette, hence the 140kph scale...adequate for the bike and of course Australia has been metric on the road since 1974.
On a final note, called as I said by Smiths a "maximum hand" chronometric tachometer, they were also known as "Tell Tale" tachometers... yep they told tales on the driver/rider to the mechanic in the pits...
Left click on the photo to enlarge it.