Monday, January 19, 2015

Hike #7: Moving parts

Thursday, 2015-01-15
Out-and-back along trail 100, from the 40th St trailhead
½ hour
First after-work hike of the year

For all hikes this year other than hike #2 , I've used a bicycle to get to and from the trailhead. Hike #2 is exceptional because I hiked with my wife, who, other than her fear and loathing of bicycles, is a reasonable and thoughtful woman.

A bicycle is simple in both senses of the word. On one hand, a bicycle has fewer parts than automotive alternatives, and most of a bicycle's parts are some combination of visible, accessible, and modular, so a bicycle is simple as in not complex. On the other hand, a bicycle is easy to use—indeed, the very model thereof. We say of a skill not soon forgotten that it's like riding a bike. So a bicycle is simple as in easy to do.

Nevertheless, a bicycle has more moving parts than you would guess. Many of these moving parts are hidden. For example, there are dozens of ball bearings, such as those in the hubs of the two wheels or in the bottom bracket or the headset. But most riders never see the insides of these parts. Wheels come pre-built, modern bottom brackets have sealed cartridges, and a headset doesn't need servicing due to normal wear. And in any case, bearings make up only a small percentage of a bicycle's moving parts. Where are the others? Are they hidden? Not at all! They make up the chain.

Each chain link comprises four or five parts—two plates, a pin, a roller, and an optional bushing—so the 114-link chain of a typical upright bicycle contains 456 or 570 parts. Each of these parts rubs against one or more of the others, and the rubbing causes the chain's metal to wear away over time. Sometimes I think about all this rubbing and wonder at how my legs made only of flesh are able to grind down alloy steel.

Nevertheless, chain friction is nearly negligible, as a typical chain transfers about 98% of the power from chainring to sprocket. The other 2% is waste energy that escapes as heat and noise, with the noise sometimes crescendoing to the tell-tale squeak-squeak of a chain that's past due for maintenance—and that's less than 98% efficient. But mostly the waste is heat. On a bicycle with a rider pushing 200 watts to the chainring—a moderate effort for a fit rider—the chain disperses about as much waste heat as a 50-bulb strand of LED Christmas lights. That's cool to the touch.