Revelation!

Of course, it's not necessary to understand how something works in order to enjoy it. I myself was a frequent and satisfied user of such popular products as gravity, photosynthesis and electromagnetic radiation for many years without the slightest idea how they worked. But if there is one thing I learned from my good old granddad, it's that knowledge opens the gateway to even more exciting discoveries ahead.

And so it is with the derailleur gears on my bike. I've used them every day for years and years, and although I always knew there were cables and cogs and clicky sounds, and that the cables had to be tight and the cogs free of gunk and the clicky sound just a "click" and not a "click-ick-ick-ick-ick-ick", I didn't really know how they did what they did.

All that changed when I watched this video:

  

There's little more explanation required, but I'm going to anyway. To my mind, it is actually much like gravity. The lowest gears, both front and back, are the ones where the chain is closest to the frame (the "ground"). A spring in the mechanism ("gravity") exerts a force to keep the chain at this level.

Then, when you change up a gear, the cable pulls on the mechanism, slightly overcoming its force,  guiding the chain out to the next highest gear, with the tension in the cable keeping it there. When you change down a gear, you're letting some of the tension out of the cable, and the mechanism's tendency to drag the chain inwards will take effect.

The key, therefore, to getting nice, smooth changes up and down is to ensure the tension in the cable increases and decreases enough to allow the mechanism to move just enough to go up or down a single gear. The people who make these gear mechanisms are (thankfully) very clever, and build them so that if you adjust the cable tension to get a smooth shift from the lowest gear to the second-lowest, it'll all just magically work for all of the other gears too.