1927 Chummy #3 Speedometer over-reads

[Nick Salmon]

The Chummy's speedo is accurate to about 20mph (though the needle oscillates very fast, presumably because of the type of drive belt on the pulleys) and then dramatically over-reads so that by the time the car is doing 35mph the needle is off the scale at 60+mph. Pictures show speedo unit and the drive arrangement off the propshaft. Is this 'normal' or is there a job of work to be done in finding different sizes of pulley on order to get an approximation of true speed?

  speedo.jpg (Size: 74.84 KB / Downloads: 372)

  drive.jpg (Size: 98.2 KB / Downloads: 371)

[Alan]

My 27 Chummy has exactly the same problem. In my case it’s because the drive mechanism runs at 3360 turns per mile, whereas the speedo head is calibrated to 2240. I keep meaning to sort it out but as long as I know 30mph = 45 there’s no urgency. The easiest, if not cheapest solution is to work out what the two calibrations actually are and get a correction box made. Speedograph richfield or similar should sort you out.
Incidentally the drive belt should be a wire spring, available from various suppliers but that’s not the cause of this problem.

[Renaud]

This kind of belt I suppose?

[Mike Costigan]

Well, for starters that is not the correct speedo; Your car should have a silver-faced instrument with nickel-plated bezel - sorry, I don't know the correct type number.

[Parazine]

Well, for starters that is not the correct speedo; Your car should have a silver-faced instrument with nickel-plated bezel - sorry, I don't know the correct type number.

I just call these "A" types, they carry the letter A on the dial.



   





 All the ones I have come across are set up for 3360 TPM - the later PA type is 2240. I cannot find a definitive document that states what gearing was applied to which model number so an FFA, for example is so many TPM.

 I had to build a special, it looks like an "A" type but is geared for 2240 TPM by using the "guts" of a PN type from an early Austin 10. My Chummy is a changeover model, with the speedo still on the left but driven straight from the gearbox. I have been unable to find out what the correct speedo was for these models, built in the summer of 1927.

[Hugh Barnes]

Some research of the digitised Index cards on the Archive web site may help - for example page 16 of this resource http://archive.a7ca.org/wp-content/uploads/IC_A7_BG.pdf would seem to be relevant. That and some of the Parts Lists here http://archive.a7ca.org/collections/hand...rts-lists/ might also be of some help..

[Tony Griffiths]

The wire spring belts were used because the only other viable option at the time was leather - not an ideal material to use beneath a car. If originality doesn't matter, a better option is the genuine Swiss-made Polycord in its high-speed drive type. This can be ordered here However, if you phone instead of using the online system (mention Austin 7), there's a 25% discount.

[Parazine]

Some research of the digitised Index cards on the Archive web site may help - for example page 16 of this resource http://archive.a7ca.org/wp-content/uploads/IC_A7_BG.pdf would seem to be relevant. That and some of the Parts Lists here http://archive.a7ca.org/collections/hand...rts-lists/ might also be of some help..

Yep, I'd seen these. My mystery speedo is BG93, 2H352 sure enough (Chassis between 45016 and 46921) but what does that decode to on an autojumble stall?   

Hence building the special, which I have to say, is accurate from 20mph up to 50mph, checked with the SatNav!

[Mike Costigan]

BG93 should be a black-faced PA-style with the angled rear drive rather than the standard vertical drive.

[Parazine]

BG93 should be a black-faced PA-style with the angled rear drive rather than the standard vertical drive.

Thanks Mike! Sorry to hijack this thread!

Is BG93 labelled "PA" I wonder? If only there was a master decode sheet for all these mysterious Smiths Speedometers