Mark (MileHigh), thanks for creating the diagram. I made some measurements while changing my left-hand bulb last week and I was going to create a similar diagram. It would not have looked anywhere near as good as yours.
Being on the forum for a number of years now I always took it with a grain of salt whenever I would run across a comment from someone saying that their left hand light burned out. I naturally assumed that both bulbs would have been basically wired the same and that it was sheer coincidence that more left hand bulbs were being replaced than right side ones.
But after losing an Osram Silverstar bulb on the left hand side after only installing it seven months prior, I began to wonder if there really was some truth to the grumblings.
After getting my hands on a readable copy of the wiring diagram I could indeed see that there was a distinct difference in the manner in which both bulbs were powered. Which is what you can see in Mark's diagram. Essentially the left hand bulb gets its power directly from the battery via the contacts of two relays (one for low beam, one for high). The right hand bulb receives its power from a different battery circuit that doesn't utilize any relays but passes through various switches (ignition, starter and headlight). In a perfect world it wouldn't make any difference, after all a relay is simply another form of a switch but in the real world there are IR losses (voltage drops) across all switches that need to be taken into account. In the ST's case this results in more losses in the right hand circuit versus the left.
I took the time to make some voltage measurements at the headlight connector of each bulb. The first set of measurements used the ground terminal of the respective headlight bulb as the negative input for the multimeter and the second set used the negative terminal of the battery as the negative input of the multimeter.
- Left - low beam > 14.33V (gnd @ bulb)
- Left - high beam > 14.31V (gnd @ bulb)
- Left - low beam > 14.29V (gnd @ battery)
- Left - high beam > 14.30V (gnd @ battery)
- Right - low beam > 13.85V (gnd @ bulb)
- Right - high beam > 13.87V (gnd @ bulb)
- Right - low beam > 13.90V (gnd @ battery)
- Right - high beam > 13.94V (gnd @ battery)
If you average out the difference in voltage between the bulbs for each condition listed above you get 0.4175V (0.48, 0.44, 0.39, & 0.36).
Most people might think that 0.4V doesn't sound like much BUT for an incandescent light bulb (which is what these are even though their called halogen lamps) that difference has a HUGE IMPACT on the life of a bulb!!!
The relationship between the life factor of a light bulb to its design voltage varies by a factor of 12! For an illustration of this refer to the two charts located at the bottom of this
link.
As an example using the low beam operating at 14.31V (ignoring other loads on the electrical system that might bring this voltage down) and the bulb is rated at 13.2V, the bulb is being operated at 1.08 factor of its design which corresponds to an operating life 0.4 times its rated value. If the bulb had a low filament rating of 800 hours, operating it at the higher level reduces its theoretical life to 320 hours.
You might be surprised to discover that many of the available aftermarket bulbs that you can put in your ST (9003, H4 types), the rated life of the high beam element is half of the low beam. So if you generally travel during the day with your high beams on you will find that you are replacing these more often. [Note: I could not located any technical specs for the actual bulb that Honda uses in the ST... both elements might be rated for the same length]
I'd be willing to bet that most owners who place auxiliary lighting on their bike wire the lights similar to how the left hand bulb is wired, that is, running the power from the battery through a set of relay contacts. If the right hand side bulb was wired identically I guess that there never would have been anything to talk about because the bulbs would blow on average at the same rate. So the next time you lose your left hand headlight... thank Honda for increasing the reliability of the right hand side bulbs!
