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MG Midget and Sprite Technical - Frogeye dynamo to engine speed ratio

Just realised that the chappie who recalibrated my mechanical tacho didn't take into account the step-up from engine to dynamo speed. I seem to recall that it's 1.4:1 but can't find the official figures. Anyone know for sure? That sounds about right looking at the diameters.
L B Rose

Ive not seen that question before

If it helps I think ?? The dyno kicks in around 2400 rpm

I know my GM..alt kicks in at 800rpm
Prop and the Blackhole Midget

Pretty easy to confirm if it's 1.4:1. Just whip out the plugs, mark each pulley 12 o'clock with a dab of Tippex and turn the engine over by hand until both marks are back at 12 o'clock at the same time.
If it takes 5 turns of the crank and 7 of the dynamo the ratio is 1.4:1

If it takes more or fewer turns a little arithmetic will find the ratio, but it might take a LOT of turns, depending on what the ratio is.

Much easier still if you have a degree disc. They are very cheap. (Mine came free with a copy of Classic Bike years ago). Stick it to the dynamo pulley with duck tape or whatever and set up a pointer on 0 degrees. Turn the engine over 360 degrees and read off the disc. I'd be tempted to repeat it a couple of times for confirmation.
Then work out the ratio between 360 and what you got from the disc.

(If you'd like a free degree disc I can scan mine and post the image on the bbs. Just save the image and print it, then glue it onto something suitable and Voila).

Unfortunately it's not as simple as measuring the pulleys because of the vee profiles the belt runs in.

Hope this helps.
Greybeard

Tacho drive gearbox on my frog has 1.4:1 embossed on it.
Simon
Simon Wood

Thanks guys. Yes I have a degree disc. My gearbox says nothing but I have an old broken one that says 4-1. Counting the gear teeth, that's correct - 10 teeth on one and 40 on the other. The engineer was I think confused because the tacho dial says 3-1. Is that because it compensates for the engine to dynamo ratio? Maybe I have that the wrong way round!
L B Rose

Don't you need to combine the effect of both the drive pulley and the gearbox on the back end? You would need to disconnect the tacho drive cable and count the number of revs on that compared with one revolution of the engine.
Graeme Williams

Not really Graeme, as I know the gearbox is 4:1. I have now calculated that indeed the gearbox is designed to compensate for the pulley ratio, so when the tacho dial says 3-1 it means tacho vs engine. So the engineer was by accident correct!
L B Rose

This thread was discussed between 13/01/2016 and 15/01/2016

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