Re: [CR] Crankarm Crack Developing


Example: Component Manufacturers:Avocet

In-Reply-To: <4D22749F.3020807@aol.com>
References: <4D22749F.3020807@aol.com>
Date: Mon, 3 Jan 2011 20:28:26 -0500
From: Eric Keller <eekeller@psu.edu>
To: Classicrendezvous@bikelist.org
Subject: Re: [CR] Crankarm Crack Developing


I've seen pictures of pedal end cracks on modern cranks. One source of cracks is those scratches from toe clip straps that are prominently evident on your Stronglights. Modern crank manufacturers also seem to not be aware that their name embossed into the metal is enough to start cracking. I also have seen pictures of a TA crank that apparently came from a milling mark that would normally be hidden under the TA sticker.

I can't really argue with your analysis, but I think it may just be a case of a surface wear mark being in the wrong place. In my work I have created hundreds of fatigue cracks. During the initiation phase there are many cracks that can get fairly large until one crack face wins out. Depends on the alloy as to how pronounced this might be. It can be amazing and disheartening how small of a flaw will cause a crack to start even when there is a much larger flaw nearby. Eric Keller Boalsburg, Pennsylvania USA

On Mon, Jan 3, 2011 at 8:15 PM, verktyg <verktyg@aol.com> wrote:
> This weekend I was assembling another French bike from the frame on up. I
> had a set of Stronglight 105 cranks that I wanted to use on it. While
> cleaning the left arm I noticed a small spot of dirt that wouldn't clean up.
>
> On closer examination it looked like a crack was starting so I examined the
> arm under my 10x-60x-200x digital microscope. The crack was easy to see at
> 10x and 60x it looked like the Nile River Delta!
>
> The crack is at the pedal end of the forged in flute.
>
> http://tinyurl.com/284wtqa
>
> There's one picture that I drew in a red line to show that the crack is not
> only across the arm at 90° but there is some laminar failure under the
> surface to.
>
> What I suspect is that the billet may not have been hot enough when the
> flute was hammered in and/or there was a lap seam flaw in the aluminum
> billet.
>
> (LAP \u2014 A surface irregularity appearing as a fissure or opening\u201a caused by
> the folding over of hot metal\u201a fins or sharp corners and by subsequent
> rolling or forging (but not welding) of these into the surface.)
>
> This makes 2 orphan Stronglight 105 RH crankarms... ;-(
>
> Stronglight produced the 105 crank arms with the forged in logo from around
> 1875 until about 1980, After that they switched to a shallower flute (see
> last picture).
>
> That's around the same area I've seen a number of Campy cranks crack. In the
> early 80s most crank makers switched to flat top or slightly curved top
> crank arms. I've not seen a pedal end crack like this one on any of the
> latter style cranks.
>
> So, another suggestion for those long winter nights, closely examine your
> crankarms for any signs of cracks developing.
>
> Good riding...
>
> Chas. Colerich
> Oakland, CA USA