Wednesday, December 5, 2012

We Are Golden in the Night: Pedal Reflectors

As I ride more and more into the darkness...

As I ride more and more into the darkness in these near-to-Solstice evenings, I become more and more aware of my visibility, as well as that of other cyclists (or lack of). Every reflective bit catches my attention, every dark lurking ninja rider my silent well-wishing for brighter equipment. In that theme, I have gone ahead and deployed the pedal reflectors, thus joining those who ride golden in the night.

These came off some forgotten set of pedals and out of the reflective / light-up parts box. Yes, there is one of those. I tried to use the "Drooping Marlo" Park MT-1 to tighten the nuts on the back. Alas! The Drooping Marlo has too many protruding bits to fit into the tight space inside the pedal cage to tighten the nuts, which meant I was forced to use a plain old wrench.

I like pedal reflectors because their opposing dancing motion suggests feet pushing pedals so clearly that they read "bike" to almost everyone. Viewed directly from the front or rear and assuming no pedal movement about the pedal axle, and circular chain rings, this appears as a simple harmonic motion on each side, 180 degrees out of phase, A cos omega T where A=length of the crank arm, ignoring that I have Biopace chainrings. Since my crank arms themselves don't change length, the result may be close enough to A cos omega T for blog work. Let's just say constant velocity um-kay? The motion is sinusoidal in time and demonstrates a single resonant frequency. If it reminds you of two weights hanging on springs moving in coordinated but exactly opposite phase, you're exactly right. And you could view the graph of that sine wave in space if you followed the motion of the pedal as a point from the side. Hopefully your body is not making a visible side-to-side sine wave as you ride, though.

But enough math
Or, I'll pause in considering the simple harmonic motion of the dancing golden pedals and look at the sunset for a moment. I would think I made that up, if I wasn't there and took a photo. Perhaps I did imagine it, some sort of golden harmonic motion hallucination.

Anyway, back to business. Who else is with me on the pedal reflectors? Who else appreciates the reflective golden harmonic motion dancing in the night? For greater visible harmony!


Equation for the period of a harmonic oscillator mass-spring system. Noticing that the period of a spring and mass does not depend on the displacement could lead us down a discussion of trying to match the stiffness of the springs of a front fork to the effective mass such that it has the same period as the observed motion of the pedal reflectors, but that's just crazy talk. Although I would enjoy a nice bike ride in the countryside with Robert Hooke to discuss it. Making sure we deployed pedal reflectors in case we were out after dark.


  1. I think that it is the relative motion between the motorist (relatively stable) and the "moving and grooving" cyclist that is important and real. I don't have a formula for that. My worry about pedal reflectors is that they can be obscured by panniers.

    Judicious application of yellow reflective tape way down on the seat stays combined with a decent flashing red taillight?

  2. Interestingly, it is the law in the UK that when riding a bike at night pedal reflectors are to be fitted....mind you it seems to be a law that is often overlooked by cyclists and rarely enforced by the police...


  3. Reflectors are cool again. Especially moving ones.


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