Technical FAQ: Checking crashed carbon, cleat position, derailleur storage
Recently had a collision with a van that slammed on the brakes which sent me headlong into the rear of the vehicle putting a crack in my fork. The bike is a Focus Izalco, full carbon. My question is, should I be wary of any internal damage to the frame, even though there is no visual evidence. My local bike mechanic said that without a frame x-ray, it is dangerous to assume the bike is safe to ride. I want to purchase a new fork but want to be sure that it would or wouldn’t be a good idea.
I would tend you agree with your local mechanic. My experience with metal frames is that, in the event of a head-on collision that damages the fork, there is almost always damage to the frame, and there is no reason to believe that a carbon frame would not be damaged, too.
Whether or not the frame was also damaged along with the fork depends on the ductility of the material the frame is made out of, which determines if the tubes were elastically deformed (sprang back without being damaged) in the collision or went beyond their elastic limit (aka yield stress) and were plastically deformed (resulting in permanent damage).
The best example I can point to is when people have brought bikes to me that were driven into the door frame of a garage while on the roof of a car. If the impact was big enough to damage the fork, how the frame fared depended on what metal it was built out of. Aluminum and magnesium frames tended to just break; the down tube and top tube crack cleanly through a couple of inches back from the welds at the head tube. Steel frames tended to be permanently deformed, even though their owners might not have noticed it; the top tube and down tube would be stretched on the top side right behind the weld and would have raised dimples on their underside, also right behind the weld (or end of a gusset, if present).
You can generally see stress lines or cracking in the paint as well as feel the stretching and dimpling. This permanent deformation results in a steeper head-tube angle, changing the bike’s handling and bringing the front wheel closer to the toe of the rider’s shoe when turning. It also eventually results in the tube failing (cracking) at the dimples if continued to be ridden long enough. When subject to similar impact, titanium frames often came through unscathed, despite massive damage to the fork, roof rack, and automobile gutters.
The difference between how frames of these different materials come through such a head-on event can be explained by the difference in ductility of the material, measured as the percentage length change the material can be stretched to and return to its original shape when the stress is removed. Elongation at yield stress of typical aluminum and magnesium tubes used in bike frames tends to be less than half that of chromoly steel tubes, which tends to be less than half that of 3Al/2.5V titanium tubes.
How does all of this relate to your carbon frame? Being a composite, one cannot make blanket statements about carbon construction, since it depends on the type and orientation of fibers used, as well as the resin used in the matrix. A ski is a good example of a composite designed to flex under stress, while a bike frame tends to be the opposite, built to be stiff. So, if your bike took a big enough impact to crack the fork, the frame is likely to have been damaged, even if there is no visible damage.
Carbon structures can yield through delamination of layers deep inside, while the top layer might look fine. Often, you can detect this sort of damage with a “coin test”: tap on the frame with the edge of a coin, especially the top and bottom of the top tube and down tube just behind the head tube. Compare the sound to tapping the top and bottom of the top tube and down tube with the coin near the seat lug. The clacking sound will be deadened if the structure has delaminated underneath. If your frame fails the coin test, you certainly ought to replace it. However, passing the coin test does not necessarily mean it is safe to ride, either.
Love the articles on cleat positioning for those of us with large flippers (size 14 in my case). I’ve always felt like too much strain is placed on my calves especially during climbing with a standard ball of foot over spindle positioning. It definitely feels like my quads are engaged more and I have less fatigue in the calves after moving the cleats back.
Here’s my question…is there a power loss using Speedplay pedals (Zero in my case) vs a Look pedal? The Look pedals seem to have much more surface area than the Speedplay lollipop (maybe the Speedplay cleat compensates for the small pedal??). Using Sidi road shoes with rigid bottom.
Speedplay claims that its cleat/shoe contact area is 2,849 square millimeters and calculates that of a Look Keo at 1,708 square millimeters. I’m not sure how those calculations are made. I think the fact that riders have won most of the world’s biggest races on Speedplay pedals, including the Tour de France, Giro d’Italia, Olympics, World Championships, and Paris-Roubaix is evidence enough that there is not a power loss to using them. Look pedals have won the same races, too, so power loss is likely not an issue with either one.
I had assumed your previous advice to take the tension out of a drivetrain applied to my Di2 as well, because of the B-spring. Why would that not be a factor? I have to admit I hope it’s not a factor, as I always feel slightly guilty that I almost never remember to move the gears at the end of the ride.
Well, I suppose one could be a stickler and worry about tension on the B-spring (the spring in the derailleur’s lower knuckle that rotates the jockey-wheel cage to tension the chain from the bottom of the chainring to the lower jockey wheel). However, that spring is a much bigger, heavier spring than the return spring that moves a cable-actuated derailleur to the smallest cog (which is the reason for storing the bike in the smallest cog), and I have never noticed a decrease in performance of that spring over time. So I doubt that there would ever be a noticeable difference to store a Di2 bike that way (and since there is no return spring in the front derailleur, either, then there is no point to storing it on the little-ring position).
Does it really matter what position the front and rear derailleurs are in during storage? I know about that conventional wisdom, but will it really affect the springiness of modern derailleur springs?
It’s a minor point. In theory, the return springs in the front and rear derailleur could weaken over time when under higher tension during storage. I have never bothered adhering to storing my bikes on the little chainring and smallest cog, and on derailleurs of the past 20 years, I’ve never noticed a decrease in return-spring tension.
Lennard Zinn, our longtime technical writer, joined VeloNews in 1987. He is also a custom frame builder and purveyor of non-custom bikes for big and tall riders. A former U.S. national team rider, co-author of The Haywire Heart, and author of numerous cycling books including Zinn and the Art of Mountain Bike Maintenance available also on DVD as well as Zinn and the Art of Road Bike Maintenance, Zinn and the Art of Triathlon Bikes and Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists. Zinn holds a bachelor’s degree in physics from Colorado College. Readers can send brief technical questions to email@example.com.