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Thread: K100RS headlight question

  1. #1

    Question K100RS headlight question

    According to my owner's manual, my K100RS uses an H4 halogen 55/60 W bulb.

    Is it safe to use a brighter bulb with the stock electrical system? If so, which bulb?

  2. #2
    A bozo on the bus deilenberger's Avatar
    Join Date
    Mar 2003
    Spring Lake NJ, USA

    Re: K100RS headlight question

    Originally posted by Mudbug
    According to my owner's manual, my K100RS uses an H4 halogen 55/60 W bulb.

    Is it safe to use a brighter bulb with the stock electrical system? If so, which bulb?
    Assuming you mean a higher-wattage bulb the short answer - NO.

    The wiring and switch are marginal (IMHO) for the stock 55/60W bulb.

    On any older K bike that has seen any weather - you can typically measure 0.5-1.0V (or more) voltage drop between the battery and the rear of the bulb.

    The wiring for the bulb is much longer than you might think.. it goes from the battery up to the ignition switch, up to the right handlebar, back down to the left grip assembly and finally off to the bulb. I've looked at a harness diassembled and would estimate that it is over 1 meter in length. The Hi/Lo switch is also prone to failure (even with a stock bulb) since it is not well weather protected, and as it gets internally dirty - it becomes resistive (which means heating) and eventually the contacts melt into the plastic so they no longer work. This is made much worse when more than the designed current is passed through the switch.

    The other problem with higher wattage bulbs can be heating of the headlight assembly. This isn't a problem with 100/110W bulbs in the K100RS/RT housing, but people have experienced bulbs exploding or partial melting of the K75S housing when much higher than stock wattage bulbs are used.

    OK - what to do since I assume you want more light?

    1. Consider auxillary lights. There are lots of options out there - I've installed $30 auto-store clones of PIAA lights on my K75S, using brackets made for the PIAA's:

    Motolites also makes a nice kit (at a considerably higher price) for lights in the same position - mounted on the fork sliders.

    The $30 ones work just fine, and provide LOTS of light that is easy to adjust for where you want it to end up.

    2. Install relays to provide battery voltage without any drop to your existing headlight. Adding relays can typically result in 20-100% more light from your existing bulb (depending on how bad your drop was - light output falls as the square of the voltage drop.) There are instructions I wrote on the IBMWR K-tech pages, and there are premade kits which work quite nicely available from Jim Davis in Japan (I'm currently using this kit on the S bike):

    This is an easy way to solve the voltage drop problem, and to help preserve the hi/lo switch. The stock wiring now only has to carry the switching current for the relays (milliamps instead of amps), and this greatly extended the switch life.

    3. Consider higher-light output bulbs that retain the stock wattage. The higher output is accomplished at the expense of bulb-life. I'm currently using Philips Vision-Plus bubls that I ordered from the UK. Although PIAA makes a bulb they claim is higher output - tests I've seen make me doubt this claim. If you do a websearch on the Vision-Plus bulb - you'll turn up several places that can ship one from the UK to you.

    So - that's the longer answer :-)

    IMHO - the auxillary lights are the first step to take, since this gives you an alternative source of light (fail-safe) if your headlight bulb or wiring fails.

    Probably a longer answer than you asked for..
    Don Eilenberger
    Spring Lk Heights NJ NJ Shore BMW Riders
    '12 R1200R - I love this bike!

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