Speedo drive

[Nick Salmon]

I have started to tackle the speedo drive on the Chummy. Found that the drive spindle which carries the pulley is flopping about in the housing. Could someone please tell me if the inner is a replaceable bush?

   

And second question - where would I go looking for a larger diameter pulley wheel? The speedo over-reads by a large amount and I want to experiment with other size pulleys. The one fitted by the P/O is 3inch.

   

[Austin in the Shed]

Nick, someone has mucked about with you speedo drive unit. The pulley has been replaced with a larger one already I think. The original one was from memory 60mm.I am surprised the original body has survived as they often snap off on the mazak ones I have seen.They were not bushed originally.
I wonder if you have the correct speedo fitted,the original being 3360 tpm on a belt drive.

[Nick Salmon]

Thanks for the reply. It isn't the correct speedo (the car is a 1927) so I am slightly shooting in the dark on how to get it somewhere near accurate.


  speedo.jpg (Size: 74.84 KB / Downloads: 413)

Any possible solutions gratefully received!

[Howard Wright]

Hi Nick

I can’t help with the take off pulley and drive but i have been through a PA speedometer rebuild recently and learned quite a lot.  According to Mike Flannery https://www.magnetic-speedometer-repair.com he can’t recalibrate these mechanical speedos so some form of gearbox on the drive may be necessary. Alternatively, in your case, the right combination of pulleys on the take off.

Firstly I would suggest checking the speedometer revolutions per mile.  I found that by using an electric drill I could determine the revs / minute to mph relationship on the speedo. I calibrated my drill using a strobe rev counter app on my phone.  Then checked what the output from the speedo drive was. This meant counting the turns of the drive to distance covered whilst pushing the car over a short distance. Then you can determine the gearbox ratio (or correct pulley size).

Carefully checking everything I found that the odometer was reading correctly (Using gps) but the speed under by about 5 to 10 mph.  This indicated that I had the right gear ratios but that the speedometer governor was out. Bearing in mind Mike’s advice I’ve decided to live with that and use a speedometer app on the phone to record speed. I am now pretty good at looking at the speedo and mentally correcting for true speed  .

Cheers

Howard

[Robert Foreman]

I would have thought a competent engineer would be make and fit a bush for you.

[Nick Salmon]

Oh Robert, if only I knew of one...

[Austin in the Shed]

Approximately how much does your speedo over read ? As you say fitting a larger pulley may get it reasonably accurate  .using a large O ring as a drive belt may be an option also.

[Nick Salmon]

The speedo overreads by probably about 25-30%%. By the time the car is moving at normal cruise (of about 45mph) the needle is off the end of the scale.

[Parazine]

The speedo is probably geared for 2240 TPM; apparently you can tell by reading a code on the back of the case....

You need an "A" type 3360 TPM speedo.

   


You also need the correct size pulley (mine is 3 1/4" OD), but the important thing is that it should provide 1:1 drive from the propshaft.


   

Then it should work OK.

My pulley bearing was quite sloppy, there's no bush and the shaft appears to be unhardened. I could have done something about it but decided it was OK if regularly greased.

[JonE]

could someone please confirm if this is the speedo cable end for a prop drive?
  2020-10-12 14.29.58.jpg (Size: 35.34 KB / Downloads: 213)