Saturday, March 07, 2009

7 Steel Fork Blade Failure

The following graphic shows an old steel fork of a bicycle which, during some repair work at a bike shop, lost its right blade due to a failure at the crown of the fork. The pictures are from Rivendell Bicycle's 2009 web reader, in an article titled "Time & Misplaced Crown Point Broke An Old Fork, Too".

The failure, at the crown on the rear side of the fork, is attributed to the "catching" of the rearward flex of the fork from braking. The flex was directed to a point of the crown which became a stress-riser. Over 30 years, a crack formed, crept around the blade under increasing stress of continued riding. The last picture labels the zones of failure and explains how this fatigue worked its way and some tell tale signs of the failure that can be observed by anyone.

In the rest of the article not shown here, Rivendell writes :

"And just for the record, we consider this fork to be worn out, not defective. It was a well-ridden 30 year old bike, and just wore out. Things wear out, even good things. Its hard to call this a defect. Still, there was a reason, as you'll see-and keep in mind that this fork lasted five to ten times as long as many carbon forks made today can expect to last."


  1. I don't quite understand this Ron. Wouldn't upon braking compress the fork in the rear and stress or try to sheer it on the front of the fork.

    During natural riding with the weight of the rider and the rake of the fork make it want to do the opposite of braking?

    Set me straight please?


  2. A drawing of the actions during braking and riding would be nice...


  3. Anonymous11:01 AM

    Interesting. Why did the right blade fail before the left?

  4. Anonymous4:35 PM

    Go STEEL!

  5. Blue : I'll think about doing that. Meanwhile, to visualize things like this, what I find helpful doing is imagining that your rigid bicycle is actually made of soft, cooked sphagetti. Now envision with your intuition what would happen as you break, pedal hard etc. You can even build a small model for yourself to test your thoughts. :)

  6. Anonymous8:15 PM

    Hmmm... a bicycle made of spaghetti? The Flying Spaghetti Monster is pleased by your creative explanation. May you be touched by his noodley appendage and prosper.


Thank you. I read every single comment.