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4 weeks ago I replaced the bottle dynamo on my bike (see this question).

Now, after 4 weeks of commuting for a total of something less than 600 km, the dynamo runner looks like this

bottle dynamo close up

It won't take longer to have the rest of the runner completely flattened, therefore I am considering moving to a better dynamo.

I can choose between:

  • low-end dynamo with steel runner (cost around 10 Euros)
  • high-end dynamo with plastic runner (cost around 40 Euros)

Aside from cost consideration, is it advisable to use a steel runner? Is there any risk that the steel runner can tear the side of my tyre?

P.S. Hub dynamo is out of question.

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    There is also a problem with the position of your dynamo, the dynamo axis looks like it is not perpendicular to the motion of the wheel at the point of contact. This means that as the wheel turns, the runner wants to roll away from its point of contact and needs to slip sideways against the wheel to stay in place, causing increased wear. This is compounded by the fact that only a small area of the runner is contacting the wheel. Fixing the alignment could reduce wear significanlty.
    – Toby
    Feb 27, 2017 at 10:15
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    It should run on that ribbed track under the treads. And the axis of rotation of the rotor needs to point very precisely at the axle of the wheel. (I've always checked that with a piece of string!)
    – Carel
    Feb 27, 2017 at 12:52
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    I'm astonished bottle dynamos still exist, given the improvements in battery tech and LEDs these days. 40 Euros would buy a reasonable middle of the range light, probably USB chargeable too.
    – Criggie ♦
    Feb 27, 2017 at 19:40
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    Because it's in the dirtiest possible place and rumbles against tread if the tread has any texture.
    – ojs
    Feb 27, 2017 at 22:24
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    @Criggie: Both dynamos and batteries have their pros and cons - see e.g. Why are battery-powered lights so popular? for details.
    – sleske
    Feb 28, 2017 at 19:46

2 Answers 2

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The roller of the dynamo should run on that ribbed track under the treads. And the axis of rotation of the rotor needs to point very precisely at the axle of the wheel. (I've always checked that with a piece of string!)

BTW: The roller doesn't look worn to me. It looks as if it had been machined that way, compare with a new one. And you may also find a rubber cap that fits over the steel roller although this will increase the diameter and make the dynamo turn at a lower RPM and dim the light in an unwanted way.

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    The "flat" part at the bottom of the roller wasn't flat when I installed it
    – L.Dutch
    Mar 1, 2017 at 8:40
  • Make sure that the top of the roller isn't sharp (it looks sharp) and cuts into your tyre!
    – Carel
    Mar 1, 2017 at 8:50
  • Can you suggest a video or an illustrated guide that shows how this is done?
    – ralien
    May 17, 2019 at 21:05
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Bottle dynamos wear down -- their other name are "friction dynamos". That being said, your one does seem excessive and may be a result of your friction wheel rubbing against the high end of the tire, near the treads. This area tends to be dirtier as well as has tire treads, so this might be causing premature wear.

Since you ruled out a hub dynamo, I'd suggest instead that you try another cheap dynamo but try to make sure the friction wheel hits the tire sidewall and not the tread. Notice how the sidewall has a friction track ||||||||||| that's intended for the bottle dynamo.

To the greatest extent possible, the dynamo body and friction wheel should be parallel to the bicycle wheel when engaged. The better friction dynamos use parallelogram linkages to accomplish this, the cheaper ones a simple hinge which will mean the two wheels are always at an angle to each other, causing a hot spot and bearing wear.

As far as steel vs plastic friction wheels. It's expensive to procure a plastic with the right friction coefficient that is hard wearing, won't melt, and won't damage the tire. Thus, cheaper dynamos go with a cheap metal casting. As @ojs notes, you may be able to get replacement rollers for higher end units.

If that fails, I might also suggest rechargeable battery lights. You can't go through many 10~40 eurodollar dynamos before a single 40 eurodollar USB rechargeable light makes more sense.

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  • I am fairly sure that the runner engages that thread (in the photo I posted it is not engaged). I'll double check for security.
    – L.Dutch
    Feb 27, 2017 at 7:03
  • Sure. Then again, 600km is a lot of distance for the little bottle dynamo.
    – RoboKaren
    Feb 27, 2017 at 7:24
  • @RoboKaren 600km is a lot? Wow, that's appalling. Running decent LED lights even on disposable batteries would surely be cheaper.
    – David Richerby
    Mar 1, 2017 at 9:24

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