Dragonfly Smiths Chronometric Speedo WARNING

[cahallett]

Dear All,
Our lovely Smiths chronometric speedometer was sent away to London Instruments (London SE20 8RY) for a service/overhaul before being fitted back into the bike in time for its MOT.
Open fitting back onto the bike it was noticed that on tick over and in gear with the rear wheel raised off of the ground the hands on the speedo twitched but did not move.
On disassembling it was found that two screws securing the bridge that supports the MPH hand had been broken and fixed back into place with loctite and that the hands themselves had been fused together with loctite.
The internals where also found to be dirty.
So all I can say is do not pay £200.00 plus upfront to get your very rare Smiths Chronometric Speedo serviced at the above establishment!

One other note of warning, never get the dial wet. The numbers are like chalk and wipe away very very easily (probably why the modern replacements are screen printed)

Thanks for reading
Best Regards
Chris

[eddie]

Chris,
        Sorry to hear you have had a bad experience with your speedo. Most owners take one look inside the speedo head and are completely baffled by it's workings - but if you've got an hour to spare to check out it's operation, you will find that it is not that complicated. It consists mainly of an escapement (much like a watch) which trips the needle back - this works in opposition to the drive from the wheel - and the needle records the difference. A 'twitchy' needle is a characteristic of the chronometric unit - that is the reason for the 1mm driving pin fitting in a 3mm hole in the driving gear - the gear can then twitch backwards and forwards without transmitting that movement to the needle - instead, it gradually nudges the needle one way or the other. The actual speed recorded is governed by the number of extra weights added to the balance wheel - adding more weight slows the balance wheel down, thus giving a higher reading.
     Being purely mechanical, most problems with old units are down to dried up oil causing the delicate components to malfunction. Careful application of WD40 will usually get things working again - but be sure to relubricate before refitting.
     With regard to the smeared markings on the face - replacement faces are available from several reputable sellers at most autojumbles.

    Regards,
                Eddie.

[Red]

If you use the link below there is a very useful article on overhauling the Smiths Chronometric Speedo. It's in pdf format, but unfortunately the file is too large for me to upload it direct to this Web site.

Roy

http://www.hells-confetti.com/Technical%20data/Jaeger%20Instruments/Jaeger%20Chronometric%20Overhaul.pdf

[cahallett]

Thank you Roy,
That is an excellent article.
Best Regards
Chris