Vitage Motorcycling Matters

Since few people seem interested in what I have posted so far, here is something completely different: a piece I wrote for the Deal and District Motorcycle Club:

Twelve-Volt Conversion

On my first encounter with the illustrious and knowledgeable Ladies and Gentlemen of the DDMCC at the Magnet, I heard some mutterings about the conversion of 6-volt electrics to 12-volt; I think I even remember (I’d had a couple of pints, and was dazzled by the array of fine machinery) someone mention ‘getting a 12-volt dynamo’. Conversion to 12 volts is not actually a big deal: all you need to change are the bulbs, the battery, and the regulator. If you have the usual sort of regulator, with two little bobbins in the box and terminals marked F,A,D, and E, (’Fade’ being what  British bike lights generally do), then you can simply find room to squeeze in one of the similar but larger boxes old cars used.

But what about the dynamo? Won’t it have to work harder to make 12 volts, and if it’s already ancient, might that not prove the last straw, causing burning smells followed by total black-out? Paradoxically, it’s the other way round. Power, used by the lights and put out by the dynamo, is measured in watts. Watts are the product of volts and amps. Amps are what make things like wires get hot. So if your total lighting load is, say, 60 watts, then on a 6-volt system the current is 10 amps, but if you change to twelve volts and replace the bulbs with 12 volt ones of the same wattage, there’ll only be 5 amps running through the wires, and that includes the windings in the dynamo. Your aged ‘6-volt’ dynamo will be cooler and happier and longer-lived.

But won’t the dynamo have to spin faster to make twice the voltage? Well, yes, but not twice as fast; the need to get the lights going will not count as an excuse for breakneck speed on the way home from the Magnet. The relation between dynamo revs and output volts is not linear; nothing much happens at a few hundred r.p.m., and then, just below normal idling revs, it jumps up enormously. In fact, if you disconnected the dynamo output wires from the system and put a voltmeter across instead, you’d probably get a good fifty or sixty volts. That’s what the regulator’s for.

What about the coil, in the case of those bikes having the curious combination of dynamo with coil ignition? Should it be changed for a 12-volt one? Yes, that probably would be as well. As with bulbs, the increased voltage will push more amps through it and it might burn out.

If your bike has an alternator rather than a dynamo, most of the above still applies, but it gets just a touch more complicated in the regulator department.

Still doubtful? Who is this guy who’s only just joined and has the nerve to tell us how to do things? Feel free to get in touch, preferably by e-mail.