Confederacy of a Dunce - single speed cycle irritation

Tuesday, February 21, 2006

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As I've mentioned before, my commuting bike is a nice single speed mountain bike. One of the main issues with single speed bikes is maintaining chain tension, as a loose chain is liable to hop right off the chainring whenever you hit a bump. On a typical multispeed bike, tension is provided by a derailleur (or "derailer") which uses a pulley system to keep the chain tight. With single speed bikes, though, such a complicated system is not needed as the derailleur's main function is to provide tension across a wide variety of gears. On some single-speed bikes, chain tension is adjusted by moving the back wheel forward or backward; this is only possible if the bike frame has horizontal dropouts. Another possibility (most often used on bikes that are converted from multi-speed use) is to use a very simplified version of a derailleur: a single spring-loaded pulley that holds a loose-ish chain tight; the Singleator is probably the best known example. My bike, instead, has yet another system for keeping the chain tight. Instead of moving the back wheel, or taking up the slack on the chain itself, the only remaining option is to move the drive chainring (ie, the one attached to the crank & pedal) forward or back. This is done using an odd gadget called the "eccentric bottom bracket":

A normal bottom bracket has a circular surface, and the drive mechanism (axle, bottom bracket, whatever you want to call it) goes right through its center. As you can see from the picture above, however, the eccentric bottom bracket has an elliptical surface and the drive mechanism is offset from the center. This allows adjustment of the chain tension by rotating the bottom bracket, causing it (along with the chainring, pedals etc) to move forward or backward (there is also likely to be an issue of vertical displacement: as there are two positions that provide appropriate tension [unless it's at the maximum or minimum distance]. Rider's choice whether to go high or low). It's crucial that this mechanism be tightly fastened in place, otherwise it'll lose tension. And that's what's happened to me: somehow it loosened just a little bit during my normal commuting activities. And all of a sudden I had to take extra care to make sure the chain didn't hop off whenever I hit a bump. Not such an easy challenge as there are many, many bumps (speed humps, flawed road surfaces, construction zones, various obstacles). It's possible to achieve suitable tension by never coasting (always keeping forward pressure on the pedals, using the front chainring to keep the chain on) but this is quite a hassle. No problem, I thought. I'd just adjust the tension, quite easily done with this setup. As pictured above, you first release the mechanism using an Allen wrench, then use a specialized tool (a pin spanner, which I have) to rotate the unit. Get the correct tension and tighten it back down, easy as pie.

Easy as pie, that is, as long as your 4mm Allen bolt is in pristine condition. Somehow, it appears that my 4mm Allen bolt has become rather decrepit, or to put it more specifically, stripped. OK maybe it didn't find itself magically stripped by the bolt-stripping pixies, but through my brutish handling of the Allen wrench which quickly illustrated the geometric relationship between a hexagon and a circle both with radius n. I'm still mulling my choices, whether I admit defeat and take it to my LBS (local bike shop), or take the macho approach: try and drill the remains of the bolt out myself without destroying the rest of the bottom bracket and/or the frame. I can see the latter option degenerating into a situation in which the bike is entirely reduced to scrap, there's a sizable hole in the wall and/or floor, I'm bleeding profusely from hands and arms, and my hair is on fire. Anyway, I've spent too long writing about this, now I'm off to find the drill.

bike

Tuesday, February 21, 2006 2:30:10 PM (GMT Standard Time, UTC+00:00)