Letters From Readers - Tachometry For The Impecunious, January 1948
It was with some trepidation, to use a well-worn phrase, that I decided to release the following, as one never knows to what lengths powerful business interests might go if their markets were threatened. On reflection I decided that the kind of people who can afford to fit laboratory-tested rev.-counters will probably not be affected and my remarks must, therefore, be taken as addressed to that large but not vocal section of the enthusiast community who would like to know just what they do scream up to in second, but can see little likelihood of optimistic guesses ever being confirmed.
Of course speedometers can be, and indeed often are, calibrated for each gear, but the bottom-end errors become rather impressive in the lower gears and no indication is available until the vehicle is moving, which is not always convenient.
As our rather elderly Type 40 Bugatti was bereft even of a speedometer and some qualms were felt about the inertia-forces arising (on carefree occasions in second) it was decided that some form of rev.-counter must be developed. An inquiry to the usual quarter soon confirmed that the exchequer would not stand it.
Hence attention was turned to a couple of rather ancient Smith’s speedometers which were lying around. Obviously these were in effect tachometer’s; the only trouble was what did the little symbols on the front mean in terms of revs per minute of the little square shaft in rear ?
To determine this a stroboscopic method was used. As most people know, the powers that be provide alternating current in our houses which maintains its cycles per second with sufficient regularity to keep those irritating clocks with the large second-hands within easy reach of G.M.T.
If one connects a neon-lamp (such as is used for showing the energisation or otherwise of electric cookers) to said alternating-current mains, it will flash 100 times per second with some accuracy.
The trick is to have such a lamp in a darkened room in close proximity to a lash-up consisting of a small electric motor (such as a fan-motor) connected to the speedometer drive (we used a piece of rubber tubing as a universal) and on the same shaft a small cardboard disc having six radial spokes marked on it and alternate sectors blacked-out. (Three white – three black!) The motor is started up, and restraining its tendency to scream its fool head off with a pencil used as a brake, note is taken of the speedometer reading at which the pattern on the disc appears to stand still. This will be the 2,000 rev. mark, and assuming that you are going to drive the thing at camshaft speed, will represent 4,000 r.p.m. of your engine.
Both the speedometers we calibrated clocked precisely 36 at this point which probably has some significance. We were quite content to take it as read and mark in 3,000, 2,000 and 1,000 at 27, 18 and 9 on the clock, respectively.
For owners of really high-revving machinery it may be of interest to note that if the calibration rig is given its head further fixes can be obtained at no extra charge at 8,000 r.p.m. (engine), 12,000 and so on, but even the first of these is rather easy to miss unless you have a very powerful neon and vibration is controlled. A better plan is to make a quartered disc which will give a fix at 3,000 camshaft revs. – 6,000 engine.
As to obtaining a drive, an ordinary speedometer-drive can usually be adapted if access to an engine shaft turning at camshaft speed can be obtained. In our case the camshaft had a slot in the end, and in view of the low power transmitted, pieces of Meccano (tut! tut!) were pressed into service for a temporary arrangement which has now done 4,000 miles and appears to have settled in, both psychologically and mechanically.
It has been well worth the trouble to know that the appalling din caused by making “full use of the gears” corresponds to a mere 2,500 r.p.m. or so and that flat-out cruising in top means 3,000, hence all the remorse, hallucinations of a persistent knock, etcetera, which we used to suffer, were groundless! To disgusted Bugattisti I would remark that the carburetter in use has a ridiculously small bore and something suitable has been on order for months. No doubt when the missing thousand or so revs do appear they will bring in their train a load of typically Bugatti grief and frantic letters will be despatched to Mr. Lemon Burton and others! Well, at least we shall know what the safe revs, for a 1929 con.-rod were!
I am, Yours, etc.,
” Jongleur.”