Bikes & Tech
Photo: Brad Kaminski

Technical FAQ: When the carbon-fiber ‘coin test’ comes up short

After a crash, Lennard performs the coin test on his carbon fork, and the component sounds OK; it isn't until later that Lennard discovers the full extent of the damage.

I crashed hard on Wednesday, and my response to the crash, in retrospect, is concerning. I believe that I may not be alone in wanting to minimize the expense of replacing bike parts after a crash, and I’m taking this opportunity to tell a cautionary tale.

The short story is that I was riding on a sidewalk that had a section along a fenced roller hockey rink in which the light poles to illuminate the rink and the sidewalk come straight up out of the middle of the sidewalk. Despite the fact that I had ridden that stretch of sidewalk hundreds of times and had even had a few close calls with the light poles, on this occasion I was scrolling through options on my bike computer as I rolled toward them. Big mistake. I looked up just in time to see the first pole just as I ran straight into it.

Crashes in which your momentum goes immediately to zero tend to be very hard on the body, as well as on the bike, as Chris Froome can tell you after crashing into a wall a few months ago. When my front wheel stopped dead, I rotated forward with the rest of the bike, slamming my collarbone into the pole. Lying on the ground in immense pain with the wind knocked out of me, I didn’t want to touch my collarbone, because I was sure I would feel it in multiple pieces.

Finally, my lungs refilled, and I began moving and talking to the friends clustered around me (this had happened immediately following a weekly early-morning cyclocross group training ride). Turns out, my bone density must still be pretty good, because my collarbone is intact, albeit with a huge bruise on it. While the spasming of the muscles all around that area has not been fun, dealing with that is way better than healing a broken bone.

While slowly pedaling toward home afterward with friends from the cyclocross group, I explained to them that a carbon fork subjected to a head-on impact should always be replaced, and I was going to replace mine. Once I returned home, however, I began reconsidering this.

The fork looked fine, and it rode fine the rest of the way home. I thought I could perhaps reassure myself by doing the “coin test” all over the fork (clacking something hard, like a coin, against the carbon structure and listening for a deadening in the resultant clack, indicating delamination of underlying fibers).

I tend to always use any piece of equipment or clothing until it has breathed its very last gasp before giving up on it. Maybe that’s why I write maintenance books; I always would rather fix a piece of gear than replace it, and I tend to appreciate the character it and I have developed in going through so many shared experiences.

With that mindset, I shudder to think what I would have done if I didn’t own a bicycle manufacturing company and have many boxes lying around of the exact same kind of fork that I had on my bike. If I actually had had to go out and buy a new fork, rather than just grabbing one that was already in stock (even if I probably still have yet to pay the credit card bill that a whole slew of those forks were on), I don’t know that I would have done it.

Since I had so publicly announced that I was going to replace the fork, and since I had too much back, neck, and shoulder pain to sit down anyway, I at least removed the fork and inspected it thoroughly, including the coin test. The steering tube looked fine (I had concerns about the impact at the top of it from the stem hitting the light post), so I was more confident that there was no significant damage.

The shock came when I cleaned the mud from the training ride off of the fork legs, however. That’s when I saw the cracks that you can see in the photos. A coin test performed on the outside of the fork with the wheel still in the bike would not have revealed the damage except if I’d been careful enough to hit right on that one crack on the backside of the left fork leg directly under the carbon C-shaped clamp that holds the hydraulic brake hose in place.

Images of Lennard’s crashed fork show small cracks that somehow eluded the ‘coin test.’ Photo: Lennard Zinn

Clacking the outside of the fork legs sounds fine on both sides and does not give a hint of the shattered area on inner wall of the left leg.

There are some parts of the bike you never want to have fail, because the bike is uncontrollable after their failure. The fork is one. The stem and handlebars are others (I’ve inspected mine, BTW; in this case, they are aluminum, which gives more visible clues of damage). The front wheel is pretty high in that category as well (I’ve also thoroughly inspected mine). Most other parts of the bike don’t tend to pitch you over the front if they fail.

The point of all this is that there is no point economizing after a head-on crash. Replace the fork if it is carbon, and do the same with the handlebar if it is also carbon. They are costly and a hassle to replace—and much less costly and agonizing to rehabilitate than broken bones and brain injuries caused by their failure.
― Lennard


Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder (www.zinncycles.com) and purveyor of non-custom huge bikes (bikeclydesdale.com), a former U.S. national team rider, co-author of “The Haywire Heart,” and author of many bicycle books including “Zinn and the Art of Road Bike Maintenance,” “DVD, as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”
He holds a bachelor’s in physics from Colorado College.

Follow @lennardzinn