Aluminum is a wonderful, awesome material. Heat treat it properly, and you can get a strength on par with structural steel. Okay, maybe only ordinary structural steel, but you still get a two thirds savings in weight. It’s why design engineers love aluminum. Plus it won’t rust. But screw up the heat treating, and well, you get this thing called grain boundary embrittlement that’s really, really bad. Cracks can form, which can lead to a complete failure of the part.
And that’s not at all good when that part is the bottom bracket spindle on a bicycle’s crankset –the stuff of my nightmares, actually. Cannondale Bicycle Corp. is recalling some 2007 and 2008 road and mountain bikes for precisely this reason. It claims in the recall notice that the problem is a manufacturing error and that its vendor (FSA) incorrectly heat treated the spindle (the end is visible at the center of the photo at left) installed on some of these bikes. “The part was probably made out of aluminum tubing,” says Ken Russell, a forensic engineer and professor of materials science at MIT. “Most likely, it cracked during the drawing process and nobody noticed. Or, it was heat treated incorrectly, though in that case cracks would form over time.” A report in Bikemag.com offerred up this explanation: “A small percentage of spindles were essentially overcooked during heat treatment, causing very small cracks in the wall of the spindle,” said Chris Peck, Cannondale’s vice president of R&D.” What’s surprising, says Russell, is that people have known how to heat treat aluminum for a couple hundred years and it’s not that easy to screw it up. “Steel is the one that’s really prone to cracking under heat treatment — aluminum parts are more the devil when it comes to casting.” Hydrogen porosity is the big problem in casting, as illustrated in this example of a crappily cast motorcycle brake.