Joe Foster's cracked stem This stem cracked suddenly with a "Tink!" sound on the top (seen at roughly 10 o'clock to 3 o'clock) while Joe was descending a mountain road at 50mph. At that point the stem was still attached by the remaining metal on the bottom, although the crack eventually worked its way through the entire stem, causing separation by the time he came to a stop.
Note that the top of the stem is under tension and the bottom is under compression. The crack propagates quickly through material under tension usually with an explosive expansion. The crack continues to lengthen through material under compression if the material is being worked in torsion, as a partially-attached stem would be while its rider is desperately trying to stop in a controlled fashion. In this case, the Joe was able to stop before completely losing control of his bike.
Photo courtesy of Joe Foster
