> On Feb 12, 2015, at 2:31 AM, Benedikt <neutralbuoya...@comcast.net> wrote: > > I seem to remember seeing a similar post to this a few weeks/months ago but > here it is ... > My VO crank that had 13,000 miles on it busted this morning on my way into > work. I was at a stop. Pushed down with my right foot, locked my left clip > in, pushed down with the right and "clunk" crank arm came right off. > Fortunately I wasn't going that fast, hammering down the road. What causes > this? Do ALL cranks have a life span? These are an aluminum alloy. > Here's a pretty good picture of the break - > https://www.flickr.com/photos/neutralbuoyancy/16320815710/
That looks like it may have failed along a machining line, with the crack initiating where there was a step-up from the flat of the back of the arm to the curve up to the spider. You can see another line between the arm and the spider. It may have been enough to cause a stress riser, especially if the alloy was not well-chosen. The VO site indicates it is 7075 but does not indicate whether it is one of the heat-treated varieties; this seems to be a pretty standard alloy for bike cranks. These sorts of failure are very, very serious and can in fact be fatal, if the crank breaks and dumps you in front of a moving vehicle- they almost always break when you are standing on them. I would examine the replacement crank very closely before installing it to check for machining ridged, lines, etc. Looking at the photos on the VO site, the machining looks quite clean and well-polished. You want to see no abrupt transitions. All cranks do have a lifespan, in part because that is the nature of aluminum- it cracks after enough repeated load cycles. Once a crack starts on the face of the material, it propagates through the metal. A collection of crank failure photos: http://pardo.net/bike/pic/fail-001/000.html#Crank -- You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group. To unsubscribe from this group and stop receiving emails from it, send an email to rbw-owners-bunch+unsubscr...@googlegroups.com. To post to this group, send email to rbw-owners-bunch@googlegroups.com. Visit this group at http://groups.google.com/group/rbw-owners-bunch. For more options, visit https://groups.google.com/d/optout.
