Except for the sake of rivet counting authenticity, it really does not matter weather you adopt a +ve or -ve earth system. The original reasoning behind the +ve earth system was that by changing the the direction of the electron flow, battery terminal corrosion would be inhibited, or so the theory went. In fact an unforeseen problem arose. As so often happens in life the solution to one problem unfortunately leads inadvertently directly to another problem. In this case it was chassis corrosion. Chassis corrosion had always been a problem, but matters were now made much was worse because the the chassis became a de facto sacrificial anode. Motorcycles were less affected by a +ve earth polarity configuration by virtue of the fact that motorcycle frames were generally made from heavier gauge metal than automobile bodies. It was for this reason that the +ve earth system was abandoned. In the case of our motorcycles if you are committed to using a 6v set up then the polarity of your circuit is irrelevant, you pay your money and take your choice. If you go down the 12v route then -ve earth has the advantage of there being a greater choice of LEDs available, and LEDs give the generator an easy life (relatively). LED luminaries for fitting into chronometric speedometers and rev counters for example are only only available as 12v -ve earth products. Now you might be thinking: "the speedometer lamp is only small that won't be drawing to much current" which in relative terms is true. But our generators are right on the edge and every little saving helps. Switch the lighting switch on to the parking light position with the conventional stop/tail lamp and speedo lamp fitted to your motorcycle and look at the ammeter discharge. Then fit an LED stop/tail lamp, and a LED lamp to your speedo and rev counter. You will see absolutely no ammeter pointer deflection at all, which means lots more power will be available for your headlight without flogging your generator to death. The dynamo is a willing provider and it will willing die in an attempt to meet all of your unreasonable demands. No one would consider it wise to run their engine flat out because it would soon go bang, and so it is with the dynamo. Do not expect it to run at maximum capacity for to long. It to will go bang. So to sum up: 6v -ve earth is fine with original 6v low power headlight bulb, but do not push your luck with higher power halogen bulbs. You will get away with it for a time, but eventually the dynamo will die. 12v +ve earth is also fine but again do not push your luck with high power headlight bulbs. 12v -ve earth is by far and away the best way to go if you want a decent headlight with dynamo reliability and the widest choice of lighting options. One last thought. Mechanical regulators. They are no match for electronic regulators, they were all that was available once upon a time, and they did a job in the absence of anything better, but the is no sensible reason to continue to use them today. You might be an old sentimentalist I suppose, but would you want to watch a Blue ray film on a 405 line monochrome television? Well, all right, I know there is going to be at least one of you that will say yes!
