Jesus fuck, where to start. Well, let's go a long way back, to establish a baseline for bitchiness in the bike industry. I'm reading The Dancing Chain, by Frank Berto. Amazing book. Really well done, very complete look at the evolution of the bicycle drivetrain. A little on the geeky side, but I'm into that kind of thing.
It didn't take long to realize that there isn't a lot that's been truly new on a bike's drivetrain for a LONG time. The Frenchies were experimenting with derailleurs that look remarkably like today's in the early 1900s. Since then, it's been a steady progression of adding gears. Most of the changes we note (e.g. wider rear hubs/dropouts) were and are the result of accommodating more gears. So what. So my point is, even way back then, there were the retro grouches who bitched about the additional gears, and planned obsolescence, and they were debating whether or not cycling was a feat of physical fitness or a technological contest. Change some of the details, and you have today's stupid fucks forever bitching about the rise of disc brakes on road bikes.
Which brings us back to BBs, and importantly, to the process of evolution. Let's start with evolution. I'm going to simplify the shit out of this, because it's been a long time since I used my completely worthless biology degree (yes, I really have one), and because I trust if I get anything too wrong, some pedantic douchebag will be happy to correct me in the comments.
If we tear away all the details, evolution is change in the frequency of expressed genes, or stated another way, it is change in how often we "see" different traits expressed. With humans, those traits are things like hair color or height or cranial capacity, and what we "see" as humans evolve is an increase in cranial capacity and an increase in height. Why? Because those traits conferred a competitive advantage over humans who didn't have them.
Bottom brackets. Starting way back, we have the cottered crank. This was a round spindle with a keyway machined into it. The crankarm had a corresponding round hole and keyway. The crankarm slipped loosely over the spindle, and the cotter was hammered into place and bolted tight to keep it tight. For those lucky enough to have worked on cottered cranks, you know their weaknesses: the cotters need to be tightened regularly, are prone to rusting in place or otherwise getting stuck, and the whole system is flexy. When those old farts got sick of dealing with those limitations, they developed the square taper bottom bracket, in which the crankarm features a tapered, square hole, and the BB spindle a corresponding square taper. The crankarm is fitted over the spindle and a bolt pushes the crankarm tightly onto the spindle.
Thus, we are introduced to the first significant branch on the evolutionary tree of bottom bracket standards. To the "left," we have cottered cranks, based on a loose fit held in place with a key, and to the "right," we have square taper cranks, held in place by friction generated by pressing the tapered hole of the crankarm onto the tapered spindle.
Why? The evolutionary pressure most easily identified as resulting in this change is durability. Properly maintained, a square taper BB/crank requires much less maintenance than a cottered system. Secondarily, you might argue the system is stiffer, but I have no data to support that claim. In any case, the result was and is that the square taper system became virtually ubiquitous, and the cottered system all but became extinct.
Next post, we'll keep working our way up the tree to talk about why ISIS is just another version of the ol' square taper BB system.