Velobrico Tinkering: Dynamo lights

Though I have absolutely no experience with dynamos, nor any real idea how they work, last Sunday I decided to tackle the non-functioning rear light from the Tigra ladies’ bike.

This bike has a dynamo installed, even though it is not such an old machine. I was recently told that in Germany it is illegal to ride a bike with battery powered lights. I’m not sure if that is true, (German cycling laws here…) as it does sound pretty nonsensical, though I’m pretty certain you are allowed to ride in Switzerland with whatever lights you want.

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Once I figured out how to engage the dynamo so it would work from tyre rotation (this seriously took me a few days of prodding and pulling to figure out), the front light functioned fine but the rear refused to shine.

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First, I removed the plastic housing and checked the bulb. Filament looked ok. Checked continuity with the multi-meter, all good. Screwed it back in. Crossed my fingers. Nothing.

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Then I had a look at the wiring. The dynamo has two wires coming out of it, one for the front light, another for the rear. Not sure how this can make a complete circuit, but I guess that’s not necessary for DC power. The bulb socket seems to be connected to the mudguard itself, so presumably being grounded is sufficient for a DC circuit. My lack of electrical knowledge is by now apparent.

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The wire hanging out of the bottom of the dynamo seemed loosely connected, but I noticed a spring-loaded connector in the bottom which holds it in place just fine. The wires head into the rear mudguard, then follow a crimp in the side until exiting through a hole in the rear, into the light fitting. This wire is then connected to another metal clip, basically wedged into a bent flap, held in place only by the “springy-ness” of the metal itself…

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The wire going into the rear light fitting looked a bit corroded, so I took it out, sprayed some white vinegar on it (helpful for oxidation), dried it, rubbed it with some steel wool. The copper colour shone nicely, so I “re-twizzled” the strands, rubbed the metal clip with the steel wool and reinserted the wire. Crossed my fingers. Tested it it again. Nothing.

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Then I removed the bulb and filed a tiny bit off the bottom to ensure good contact. No joy.

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I now removed the wires from the dynamo and did the vinegar, steel wool and “twizzling”. Crossed fingers. Tested again. Nothing.

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Not to be deterred, I removed the bulb again, and found bits of spider eggs or some other insect leftovers. Blew those out. Another round of vinegar, steel wool, scraped a bit with a screwdriver, dried it, sprayed a bit of WD40 on it.

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While I was at it, I sprayed a bit of WD40 on the metal clip and the wires at the dynamo. A bit of WD40 often works wonders.

Screwed the bulb back in. Crossed my fingers. And it worked!

A well earned cup of tea followed. If only every Sunday were so productive!

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