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  1. #1
    Senior Member Giant Doofus's Avatar
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    Ground wire for tail light?

    I've just installed a new B&M Cyo Premium head light on my bike to replace the Lumotec Classic that came with the bike. Everything is working well, except I can't figure out what to do with the ground wire for the tail light. The old head light just had one wire that ran from the head light to the tail light. The Cyo came with two wires: one that obviously connects the head and tail light and another with a white stipe on it (the ground). Any idea where the ground wire should go?

    Two details that may or may not matter:
    - My tail light is the B&M seculite (the kind that goes on the fender).
    - The wiring for the tail light runs through the frame and is pretty new, so I didn't replace it. I just connected the new (short) wire that was sticking out of the Cyo to the existing wiring where it exits the frame near the fork crown.

    Thanks for any advice you can offer.

  2. #2
    Senior Member
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    Classic bike lighting systems were similar to automotive in that they used a single hot wire, and the frame or chassis as the ground. No special ground is needed since the bracket was attached to bare metal, and what wasn't usually ha a small "piercing" screw to poke through the paint and make contact.

    However the advent of carbon has eliminated the option of using the frame as a ground, so many modern systems use 2 wires (hot and ground). How to wire depends on your frame and fork. If it's possible to use the frame as a ground do so by attaching the ground wires to any bare metal part. Otherwise wire the entire system as a closed 2 wire circuit without using the frame.
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  3. #3
    Senior Member Giant Doofus's Avatar
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    Thanks. It's a steel frame, so i'll just attach it that way.

  4. #4
    coprolite fietsbob's Avatar
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    AC dynamos want send(+) and return (-) twin lead or CoAx wires.. My B&M taillight uses the mounting bolt and the (-) wire both as a Ground/cold-return wire..
    Last edited by fietsbob; 11-02-14 at 09:44 AM.

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