donch@sri-unix (08/31/82)
Mandatory headlight laws for motorcycles pose serious problems for
older, expecially classic and vintage, bikes. Newer NHTSA-mandated
machines which don't allow a headlight option, i.e. no on-off switch,
can be rightly accused of adding to user expenses owing to shorter
lifetimes of bulbs and ensuing costs of replacements. I've tried a few
tricks and have some observations to offer on these two points.
Here in Oregon, it has been "lights on" for over 10 years. About that
long ago, I was rodding around on a Vincent Black Shadow which sported
a typical British lighting system: a 6-volt, 50-watt dynamo serving
the headlight, taillight and brakelight (a magneto provided sparks,
thank all the gods!) The headlight law put quite a strain on the dynamo,
particularly when the bike was fitted with a decent-wattage headlight.
As a result, daytime riding carried the risk of a citation because of
guaranteed dynamo failure, whereas night riding suffered from a much
worse potential fate. A Q-H light overloaded the system unless the
battery had just been externally fully charged and o;ne settled for a
short trip. This was assuming, of course, that the dynamo hadn't packed
it in yet.
The solution that got me by and later worked well on my l974 Moto Guzzi
850-T was quite simple. I installed a switch on the headlight shell that
hooked the low and high beam filaments in series. Since the voltage across
the serial filaments remains the same as a single filament, the summed
resistances yield a lower current drain. Ohm's law says so, if you don't
believe me. The result is less demand on the dynamo (or alternator for
newer machines) and a readily visible, but yellow, headlight to satisfy
Smokey. Now, if one hooks up a Q-H lamp, two further advantages show up.
One, the lower operating temperature of the series filaments gives longer
life. (Mini-tutorial: filament life is a direct function of temperature;
simply put, the tungsten evaporates from the filament, depositing on the
glass envelope, and eventually breaks because of weakness. The lifetime
vs. temperature curve is not linear but reveals dramatically shortened
life as temperature rises. Hence, a slight drop in temp. yields large
increases in life. That's the point of this modification.) Two, on
my Guzzi, which had adequate electrical power at normal revs, the difference
between low beam on the stock non-Q-H light and the serial filaments of
a Q-H was not very noticeable. Thus, for those of us who have experienced
positive benefits of visibility from having headlights on, going the serial
filament route is not a liability.
As far as my actual bulb-life is concerned, in the eight years I've
owned my Guzzi, only one filament has failed (the low beam on the original
QH bulb bought in l975). An autopsy of the filament in a scanning electron
microscope showed a great deal of faceting. I've since read that the only
demonstrated failure mode (other than mechanical or quality problems)
was due to crystal growth and fracturing , actually cleaving, along large
single-crystal planes. Since the serial filaments in the case of the Q-H
bulb do not get up to the temperature for the halogen cycle to work, there
probably was some evaporation occuring, albeit at a lower rate than a regular
bulb. But when the bulb was run at normal temp. in single-filament mode,
the halogen cycle takes over and maintains the health of the lamp.
This mod helped the Vincent, too. In recent years, however, a superior
fix was applied by enthusiasts in England. Some of their electronic
whizzes developed a voltage regulator that operated the stock dynamo
at 12 volts, which effectively doubled the wattage. Overheating hasn't
appeared as a problem, although the basic problem is still the old tired
dynamo.
Don Chitwood, Tek Labs
Tektronix, Inc.
Beaverton, OR