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Some days ago I noticed the light cast from my bike was not steady, despite I was travelling at constant speed. Checking the dynamo when engaged I noticed it was touching on the side of the tire, but below the strip where it is supposed to.

I fixed this, and after just one day of proper working the dinamo head now simply slips on the tire. I ruled out the possibility of too low contact pressure beacuse if I try to roll the wheel backward the dynamo rotates properly.

Should I check something else, or should I better replace the dynamo?

  • It works okay rotated the other way? A quick fix may be to move it to the other side of the bike. These dynamos make 6V of AC electricity so they don't care which way they spin. However the engagement lever might be hard to reach. – Criggie Jan 26 '17 at 8:54
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    Depends on the mounting method. If it came with the bike, it might be on a brazed on mount so it can't be flipped to the other side. Of course, jubilee clips and an L bracket from the hardware store can fix everything. :-) – RoboKaren Jan 26 '17 at 8:58
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    But I think what he's saying is that when the wheel rotates slowly backwards, there's enough friction to rotate the dynamo but at speed forward, it loses friction and/or moves off the wheel. – RoboKaren Jan 26 '17 at 9:00
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    @Criggie, switching side is not an option. When engaged it bends to its right, so on the right side it would push outwards – L.Dutch - Reinstate Monica Jan 26 '17 at 9:53
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It sounds like you have a bottle dynamo. Many are constructed poorly and have matching short lifespans.

Bottle dynamo

Typical failure modes are:

  • Friction wheel is made of cheap plastic, rubber, or pot metal -- and wears down, breaks, or flies off
  • The wiring was done by drunken lemurs and breaks off inside
  • No sealing against water/dust/salt and it corrodes to pieces
  • The hinge mechanism is poorly constructed and breaks

In your case, it does sound like maybe the spring that forces the dynamo onto onto the tire sidewall isn't apply enough pressure. I'd put some oil on the spring and hinges and work the hinge back and forth a few times. The dynamo may have also rotated (fallen) off its mounting bracket slightly, you may need to rotate it back into position and tighten its screw more tightly.

The good news is that bottle dynamos are cheap enough to be easily replaceable when they do break.

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  • Note that I may be more skeptical of bottle dynamos than some other b.se folks. This may be because they are ubiquitous in Japan -- and so even the BSOs have them. And the BSOs had BSO-ish dynamos which quickly broke. In some other places (Europe?), they are more rare and perhaps higher quality. YMMV. – RoboKaren Jan 26 '17 at 8:57
  • Those lemurs though! – Ross Jan 26 '17 at 19:09
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    I gave a better look at the friction wheel, and indeed at closest inspection it is evidentent it is worn out. That explains the lack of grip. – L.Dutch - Reinstate Monica Jan 27 '17 at 6:23
  • There are (were?) rubber caps that can be slipped on the metal friction wheel. Maybe you can find one of those at an older LBS. – Carel Feb 27 '17 at 12:56

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