[CR]Are stress risers really the culprit?

(Example: Framebuilders:Mario Confente)

To: classicrendezvous@bikelist.org
From: <marcus.e.helman@gm.com>
Date: Wed, 15 Sep 2004 09:16:25 -0400
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/bike/pic/fail-001/000.html

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