Vive memor quam sis aevi brevis.

Thursday, 17 March 2016

A Tale of Three Cranks

One fine spring evening several years ago, i was riding home from work. My morning commute had been ridden rather vigourously and aggressively -i'd been running late for work as usual- and i was dodging buses and taxis along the busy main street as if i were sprinting for time bonuses.

   The evening turn was a bit more sedate. The rush-hour traffic wasn't that much lighter, but i didn't feel the need to ride particularly hard or fast.  As i pedaled away from a stoplight, my right foot was suddenly dangling and i felt and heard the crankarm ping along the pavement...

I'm nowhere near being a strong rider, even 20 years ago, so i couldn't begin to guess why a relatively new Campag crankarm had disintegrated thus. I doubt it even had 1500km on it- probably much less. It'd never been raced nor crashed. When i looked into whether it was a warranty issue, the dealer i showed it to pointed out the rash on the end of the arm and told me it was clearly abuse -it wasn't. The rash came from the drop onto the pavement. I now owned a genuine Campagnolo paperweight.

Even today i shudder to think about what may have happened if it had let go when i was storming through the morning rush hour traffic.

It was years later that i learnt that Campagnolo Record cranks suffered from a basic engineering design flaw. The crank arm met the spider with a sharp angle that hadn't been properly radiused, leaving a stress riser that would often cause a fracture. Here are some pictures by way of example:

Dark arc of oxidised metal showing extent of crack before fracture.
If like me you have spent time and some little bit of cash searching for and collecting vintage parts, i will advise caveat emptor , especially when looking on that Really Big Auction Site. Here are two examples i encountered:

Side view:
The other side:
The one above was returned to the seller for refund. He didn't argue the point. Here's the second example:

This one got by me. If i recall correctly it came with a bike.

I'm given to understand that this problem can be corrected by the judicious use of a rifler file to round out the sharp edges of the joint. I also understand that Campagnolo knew about this flaw for about the whole production run of these cranks but kept mum and never corrected the problem. i don't know and cannot say for certain that this was the case. i'll also add that i've had and used several of these cranks with no sign of fracturing  and the two in current use here get a good inspection every time i ride them. The crank that broke under me gave no sign by way of creaks or pops before falling away, but any unusual sounds from that region of a bike should not be ignored. Also, i have heard- but not seen- of similar failures among older Shimano Dura Ace cranks and other Campag-inspired cranksets.

Just a cautionary tale. YMMV.

1 comment:

  1. Heavy alloy parts like that won't make a noise because the material does not resonate the way hollow frame tubes will. Crank cracks really are a silent killer. Weird, too, how removing material will improve strength, but once the crack starts in the non-structural web, it can proceed into the real load-bearing portion. Remove the web and the crack can't start.