A fascination with chainrings and front shifting

It’s been a while. It hasn’t been easy juggling work commitments and an ongoing course in Web browser test automation, while reserving mental capacity to write new posts. But here I am.

Look closely. Note the angled teeth and little notches on some of the cogs on this cassette.

Ever since I was first introduced to road bikes with my dad’s 1981 Peugeot P8, I’ve always been fascinated with the mechanics of how a chain literally jumps over to shift to another gear. On the rear of the bike, this is obvious for all the world to see. This funny-looking thing with the little pulleys on it, dangling on the underside of the frame, called a “rear derailleur,” somehow magically shoves the chain up and down the cogs. On newer cassettes, the cogs are shaped with little divots and other features which are meant to smoothen this process.

Up front, however, this is a lot less obvious, because chainrings are essentially huge cogs put in backward. All the little features and mechanisms that facilitate a front shift are hidden as they are sandwiched between the drive-side crank arm and the bottom bracket shell.

The front derailleur doesn’t help, either. Next to the rear derailleur, the front derailleur is a much more simple-looking mechanism, not doing as much to reveal the magic behind performing a front shift – arguably surrounding it with more mystique.

Curiosity taking over me, I decided to take a closer look.

Performing a downshift from the big chainring to the small chainring is a pretty straightforward affair, with the front derailleur cage doing the bulk of the work, along with any chain catchers or chain spotters. The chain is just pushed off, dropping onto the smaller chainring.

The real magic is how the chain gets to climb from the small ring to the big ring. How does it do that?

The front derailleur is just there to initiate the shift, again, by pushing the chain outboard sideways with brute force. That’s only half the job; the remainder of the shift is completed by the many features on the inside of the big chainring.

Looking on the inner edge reveals a few interesting things. There are little oval divots which seem to match the links of a chain, and they feed into raised shift pins. The divots and pins look like they follow a line tangential to the small chainring. This is effectively the other, hidden half of the front shift mechanism. On this Shimano 105 chainring, these ramps and pins are still rather simplistic compared to later iterations or more expensive chainrings, but for illustrative purposes they do the job.

Performing a front shift and breaking it down step by step reveals how the chain climbs a 16-tooth gap between chainrings. With the chain already traveling outboard from the initial push of the front derailleur, the shifting pin grabs it onto the big chainring, while the tangential divots serve as ramps to help up the rest of the chain off the small chainring.

As my chainrings have already been worn by many, many front shifts over many kilometers of riding, you can see on the inside of the front chainring just how the individual links have slid on and off.

That said, these are obviously worn chainrings, and the wear has made itself felt while riding. I used to be able to smoothly upshift in my third-largest 24T cog; now, the increasing failure rate of front upshifts is pretty bad when I’m in a cog larger than my sixth-largest 17T. Next time I’ll walk through chainring replacement.


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