Saturday, December 04, 2010


JB told me he wouldn't be coming... he's got some kind of throat bug. So I gladly slept an extra half hour and prep'ed for a solo ride. Forecast was for rain so I was expecting some Belgian Hardman points.

CLICK ON THE PHOTOS! Blogger software is clipping them so you're only seeing half the shot... I'll get this figured out before my next post.

The weather was holding out though and things were going better than expected up until just past the Bump when my cleat appeared to come loose. I stopped to inspect and couldn't find anything wrong, then started up again. But something was definitely wonky so I stopped and pulled on my crank and tweaked on my cleat and still couldn't find anything wrong. Started up again and SNAP... my crank arm just broke off.

You have to click on these photos to see the whole thing... Blogger is cropping them in a weird way.

Limped my way to Encina where Tim told me he didn't have anything that would fit my Italian BB and suggested I buy something on-line and they'd install it for me. Managed to one-foot it back the rest of the way home.

A little looking around gave me some serious sticker shock on crank prices but I did find a good deal on a 105 version. Life's too short to ride crappy cranks but the crap line isn't always clear. In the end, I'm glad this didn't happen while I was sprinting up the Dips because broken cranks usually equal pain.


ben said...

I was just thinking today about how cranks should really follow the exo-skeleton design, hollow in the center with stronger I-beam-style broad flanges. I mean... why be thickest in the middle, a la Ultegra... they are the heaviest thing on a bike! And, as my thought continued, "I've never once seen a snapped crank, especially on a road bike! Well lo and behold, you have delivered that reply! It appears the failure is based not on rotational force around the BB, but around the longitudinal axis of the bike (direction of travel)
Would you agree? It also appears that it has been cracked for some time, letting dirt in. I guess you can assume the designer of the crank did not factor in your leg strength in his/her design. Very impressive!

Diablo Scott said...

This is classic fatigue failure and the dark portion of the cross-section isn't dirt but is where the early crack propagation happened; lots and lots of small increases in the crack size with every stress cycle. Then when the crack gets big enough, cracking the rest of the way happens with only a few more stress cycles.

So there was an initial tiny crack caused by who-knows-what, it started on the outboard side on that "hump" in the profile and in fact it looks like there's another crack a little closer to the pedal end of the arm too that would have eventually failed. You can sort of see it in the photo (must click to embiggen) that shows the broken piece still attached to my bike, paralleling the fracture between the break and the word GOSSAMER.

Most cranks break across the pedal eye... I've broken three this way and the problem is the threads (which are like initial scratches) and the way the shoulder of the pedal applies stress to the arm.

Lots of cool photos of broken cranks HERE

Keith said...

Broken cranks seem to happen on the left more often, at least in my unscientific survey. I broke a campy crank mid shaft like one of the photos in your link. I think the old "cove" detail weakens the structure. I have also broken a bunch of Look pedals, always on the left. Nice photos!

Rick said...

Make a mental note: A new landmark on the mountain, "Crankbuster Plateau"!
You should have continued up to the Junction for an extended one-leg drill!