Quote:
Originally Posted by koan58
I stand to be corrected, but there seems to be one circuit which is neglected, which is the HT spark, which goes from coil to plug to engine, then back to coil. Your good grounding of your SPG to the frame doesn't guarantee the return from engine to coil (if the engine/gearbox/plates are all bolted to a painted frame, then the engine etc is electrically isolated from the frame). In practice I'd expect contact to break through the paint at some bolting point, but this is relying on luck rather than judgement.
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Dave -
Thanks for the input. Your point is well stated, but overlooks some important facts. And therein lies the answer, and more importantly
a learning opportunity.
You are very correct. The spark (HT) to the spark plugs is just like every other circuit on the bike. It needs a complete path back to the starting point. That is to say, it needs a complete path back to the ignition coil. Few people realize that.
However, the electrical path back to the coil is
NOT through the frame and back to the metallic body of the coil, as one might think at first glance. If it were true, then the rubber mounted coils on the OIF bikes wouldn't work at all. Believe it or not, the electrical path back to the coil is through the DC wiring harness.
That's all determined by how the HT coil is wired internally. You see the secondary return lead is not tied to the coil's body, but rather to one of the low voltage primary terminals. So the coil's "grounding" is actually done by the harness.
Therefore the proposed lead from the cylinder head back to the SPC does all the "grounding" the ignition system needs. So forget about rusty, greasy, or painted engine mounting plates.
They are not doing anything electrical.
Quote:
Originally Posted by koan58
To be sure, I would preferably put the SPG on an engine plate/head steady etc, or run a lead to such a place.
I don't see the point in scraping the paint off the frame for your SPG, as the frame is no longer doing any electrical function, your red leads are now doing what the frame was originally expected to do.
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For an education, please take a look at the mounting location for the rectifier on any 1963 to 1970 Triumph twin. The rectifier is the SPC on a stock Lucas system for those years. We can see that because all the red "ground" wires meet there. Now look closely at the rectifier mounting point.... the battery box. And the battery box
is rubber mounted. The battery box is actually electrically insulated from the frame !! So the whole point of SPC is
NOT to tie "ground" wires to the frame. That thinking went out in the 1940's. The object of SPC is to get all the "ground" wires gathered together in one place, whether it touches the frame or not. At that point you no longer have "ground" wires; the more appropriate term is "return" wires.
Quote:
Originally Posted by koan58
IMHO you don't need so many separate leads either, one unbroken lead running the length of the bike, with spurs where necessary, would do a neater, sturdier job.
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That's a matter of personal interpretation. Sure thing, one big wire will do it. But let me tell you why you might be better off to do it the way I advise. Using one big wire is "placing all your chickens in one basket". It's sort of like having your family all work for the same company. If there's a layoff, then you're all out of work. On the other hand, if you use multiple wires and one wire breaks, then you may loose your turn signals or your lights, but hopefully you can still ride home.
Another valid reason is this... In order to tap into that main wire you'll have to solder. (You wouldn't dare use connectors because connectors become unplugged.) Soldering hardens the copper making it brittle. So when you tap into this main "return" wire 4 to 8 times, that's 4 to 8 places for the vibration of a Brit bike to break the wire.
As always, you are very welcome to execute your wiring as you see best.
Every owner should. But I sincerely feel like multiple wires reduces the risk of walking home. My motto on Brit bikes has always been: "Ride bikes. Pick up chicks. Get laid." Personally, I have always felt that walking home was NOT conducive to the 3rd point.
All the best.
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