Thread: Series 2A/Stage1 speedo question

OK here is what I found...I did also find the link to the article about Smiths/Jaeger speedos... was very usefull.

To summarise what I have learned...
S2A Speedo conveys TWO things, 1)Odometer, 2) Speed in either MPH or Kms

1) Odometer is determined by gears and ratios only.
Diff ratio[Originally 4.7:1 for S2A] & propshaft to speedo cable drive from gearbox drive ratio [2.2] & Turns per mile/kilometer gearing in the speedo [in the case of 88" SWB originally fitted with 600 or 650 tyres @ 1504 turns per mile (TPM) which is determined by A) the worm drive to ODO wheel gearing expressed as teeth on the gear that is driven by the worm, the choices for landrover [mostly 32] triumph and MG appear to be 20,25 and 32. Re-note that I have not seen anything but 32 tooth worm gears in either LR or Triumph speedos and although you may be able to acquire 20, 25 or 32 tooth worm driven gears, you probably cant get the worm drive with the right gearing. Since this confused me for a while, I will explain...the thing that spins concentrically with the cable drive, is the worm drive, documents dont seem to reference this when talking about worm drive ratios instead they reference the gear that is driven and that has X teeth however, the worm drive can only drive ONE of the worm gear teeth ratios either 20 or 25 or 32.
To get accurate ODO you simply need to have the correct gearing in the speedo to allow for i) tire/tyre size [turns per KM or mile], ii), diff ratio, gearbox propshaft to speedo ratio [2.2 for S2A], and turns per mile/kilomere gear settings [worm drive gear teeth and ODO wheel teeth in the speedo.

2) Speedo reading is completely separate from ODO reading and relates to turns of the speedo cable which directly spins a magnet in the close presence of an aluminium disk which is connected to a hair-spring and has the speedo needle attached to it. The magnet spinning near the aluminium disk creates eddy currents in the aluminium that resist the induced magnetisim causing the aliminium disk to move in opposition to the movement of the magnet which causes the speedo needle (which is attached to the aluminium disk) to move against the hair spring which attempts to keep the needle at Zero. To calibrate a speedo (not ODO), depends on MAGNETISM of the magnet that spins, pre-spring of the hair spring and the speedo may be calibrated at a certain speed and be less accurate at higher and/or lower speeds.
Important is the fact that for any given speedo needle setting, you can put behind it a 70MPH back plate or a 90 or a 120...so the calibration of the backplate is also relevant and important.

For my LR which has 7.50 tyres, and 3.54:diffs [ex RR] I calculate that I need 1052 TPM or 653 TPK at the speedo for my set up.
Remember for a speedo (NOT ODO) it doesn't matter if its TPM or TPK as long as your display [the print on the speedo face] matches. To put this another way, if you have a requirement (as I do) for a speedo with 1052 TPM, if I found a speedo with 1052 TPK on it...I would just imagine that the K's were Miles.

My calculations for ODO wheels are:
With 20 tooth worm gear (you probably won't have this but may get one from a triumph or MG?) you need 53 tooth ODO wheel.
With 25 tooth worm gear (etc) you need a 42 tooth ODO wheel
With 32 toothe worm gear (as all the Series LR's that I have seen have) you need 33 tooth ODO wheel.

For magnetisim level on the spining magnet...stay tuned!!

Side note: I could go to a specialist and pay money to get this stuff done...I'm not against paying the money (I may eventually end up doing this) but for me...part of the S2A restoration journey is figuring this stuff out with research and help from others...I love it and hope the above although incomplete may be useful.

Originally Posted by Aaron IIA

This same device could be used as a tachometer on a diesel vehicle. Do they come in a 1.5" or 2" guage, what ever the smaller standard size is?

Aaron.

Here's a link to the catalogue that Wolfman_TWP provided in my rebuild thread:

http://www.adelaideinstruments.com.a...automotive.pdf

http://www.aulro.com/afvb/series-3/7...-new-post.html