I have had some NR crank arms snap back in the '70's. Here is my opinion on what is going on. I sold these cranks to just about all my bike customers when I built bikes. Most people never had a crank arm break, but some did including me. I have had at least 4 right side arms crack, and when it comes apart it is quite a supprise. During this time period Campi USA would just give me a new arm if a customer gave me a broken one, no questions asked. The arms that I broke were all new, no crashes or hitting a curb during there life. But I think two things greatly increase the odds of failure on the right arm. First is the riders weight. I never saw any arm break with lighter riders. When I was fit and strong I weighed around 175 lbs. The 2nd thing that accelerates the failure is riding in the city with lots of stop and go. I used to track stand at traffic lights, then really jump when the light changed. After breaking arms in less than one year I looked at what I was doing to cause this a noticed that I all ways jump on the right side when I take off. I never broke an arm on my touring rides, and some of these rides were many months long.
These arms need to be carefully inspected often. The most common crack is at the spider on the back in the area where the machining leaves a sharp edge. Look for a crack on this sharp edge. The other area is where the pedal threads into the arm. These arms always had a tight fit with the pedal, not quite sure why the crack started here. The other area is at the top of the flute. Because the finish is usally pretty good the cracks are easy to see, no aircraft crack inspection methods are needed in my experiance.
After I changed to Dura-Ace cranks I had no more problems. I also know Jobst Brant broke many of these arms. I think the alloy is not correct for the application.
Jim Merz Bainbridge Is. WA
-----Original Message----- From: classicrendezvous-admin@bikelist.org [mailto:classicrendezvous-admin@bikelist.org] On Behalf Of PBridge130@aol.com Sent: Wednesday, August 28, 2002 7:36 AM To: classicrendezvous@bikelist.org Subject: [CR]Predicting/preventing NR/SR crank breakage
Has this subject been done to death prior to my arrival?
I went down very hard once, when my N/SR crankarm snapped, a couple inches up from the pedal. I was going fairly fast at the time. In addition to the normal rash, I had a helmet that was crushed on one side, and only the slightest of headaches (there's gotta be a lesson in there somewhere).
I think it's understandable that my enthusiasm for riding hard on similar cranks has been dampened, perhaps explaining my enthusiasm for Stronglight-equipped bikes (I've never heard of one of those breaking, and if they do, I don't think I want to know about it).
I just measured the broken end. Break occurred two inches from center of pedal, straight across the arm, 172.5 arm, "11" in a square.
So. What do I watch for? How do I anticipate catastrophic failure, and
avoid it? Hanging the bike on the wall and admiring it while quaffing a
brewski and smoking a cigar might be an acceptable strategy, but probably isn't the ideal one.
Thanks y'all,
Peter Bridge
DenCO