Date: Wed, 15 Sep 2004 09:16:25 -0400 From: marcus.e.helman@gm.com To: classicrendezvous@bikelist.org Subject: [CR]Are stress risers really the culprit?
I have read the bits about stress risers, which I take to mean the sharp edges on the back of the right crank arm as it joins the spider, but most of the failures seem to be at the pedal eye, or a short distance above it.
Robert Kaufman writes:
> Yesterday, I snapped my second NR Campy crank arm in three years. This
> time, the pedal eye "ripped" on a downstroke. (I fell over, but nothing
> more than road rash--the frame is unscathed). The first time, the arm
> cracked across the arm about a inch below the eye towards the axle. The
> cranks were installed at my LBS, and they are very good. So, I wouldn't
> fault them. I ride 3000-4000 miles per year. Once or twice a week, I ride
> hard (20-22mph) for 1-2 hour "time trials" but I am no monster. I weigh
> about 150 lbs. and am about 6 feet one inch tall. Clearly, I am no
> sprinter. I do climb well and like to get out of the saddle.
The pictures at this site also seem to be mostly pedal eyes and arm
failures, rather than arm/spider failures.
http://pardo.net/
So I wonder if the stress riser issue is as critical as some have suggested.
The comments on the site above also suggest that the flat part of the pedal spindle can wear away the crank arm right at the eye, contributing to failure. I have always wondered why people tighten their pedals so much. It certainly makes it harder to switch pedals on a crank. The usual answer has been "so that they won't spin off," but I have never heard of anyone spinning a pedal off a crank because it wasn't tight enough. I wonder if overtightening the pedals might not have contributed to some of these failures.
Not an engineer, Marcus Helman Huntington Woods, MI Marcus: You bring up a very good point. The majority of broken vintage Record crank arms *did not* fail at the arm-to-spider junction. Most that fail do so somewhere in between there and the pedal eye. Those small cracks at the sharp edge of the arm-to-spider area can be present for literally decades without the crank failing. (N.B.: I'm not saying don't worry about those cracks. Everyone should regularly & carefully inspect many vintage cranks, and bars and stems, etc. that can fail catastrophically, and if you aren't happy, don't use 'em. Period). Indeed, after Campy added the "bumps" at the arm-to-spider joint (that were present years earlier on some clones) in about 1985, the arms still broke sometimes. However, they did it only at all the other failure points, such as right through the stamped shield logo (!!), at the narrowest cross-section about 1" or so above the pedal eye (that certainly seems logical), at voids in the forging (rare, but it can happen), in the pedal eye itself, etc. What we have to keep in perspective is that these cranks are now in their fifth decade of use in some cases, and some sets may have hundreds of thousands of mile on them. It's amazing they've lasted this long! Most other similar vintage cranks (which were nearly all made in larger numbers than Record ones), are much less common now. Campy Record cranks have been cycled through 2, 3, 4, or 17 users in some cases (no pun intended). They are an icon of vintage Campagnolo-ness. It's no coincidence that they are on the cover of the book "The High Tech Bicycle." Cheers, Greg "cranked" Parker Chief nerd Engineer Dexter, MIchigan http://www.bicycleclassics.com