Technical Q&A with Lennard Zinn – Fixin’ carbon

Dear Lennard,In your Earth Day column there was a question about a cracked carbon frame. Since it is a save the earth day of posting I thought I would ask about the possibility of repairing a cracked carbon frame (small crack obviously). Since you can buy epoxy and carbon, would it be possible to repair a carbon frame? It would be ugly but would it be rideable? Scott Dear Scott,

By Lennard Zinn

If it's done right, a carbon frame can recover quite nicely from damage.

If it’s done right, a carbon frame can recover quite nicely from damage.

Photo: Calfee Designs

Dear Lennard,
In your Earth Day column there was a question about a cracked carbon frame. Since it is a save the earth day of posting I thought I would ask about the possibility of repairing a cracked carbon frame (small crack obviously). Since you can buy epoxy and carbon, would it be possible to repair a carbon frame? It would be ugly but would it be rideable?

Dear Scott,
Actually, Craig Calfee’s company repairs carbon frames. I should have mentioned that. It wouldn’t necessarily be ugly, either; I’ve seen repairs that Calfee has done, and they look very good.

I wouldn’t recommend embarking on repairing a carbon frame yourself without some prior experience working with the material.

Dear Lennard,
While I agree that “generally” carbon frames do not suffer from sweaty trainer sessions, there is one very big exception – carbon frames or other carbon parts that have aluminum inserts.

Sweat can speed the corrosion.

Sweat can speed the corrosion.


Though not as big an issue as it is in marine products (one of my areas of design/research), there can be issues with galvanic corrosion over time, especially with sweat or other electrolytic solutions. Please see the attached photos of the cable guide on my circa 2000 Trek 5500, and the bonded aluminum collar failure on a $200 carbon stem.

The first example is primarily cosmetic, but most people with an expensive carbon frame would not be happy with any cosmetic damage. The second example is much more serious, and much harder to discover. The aluminum collar failure was only seen when the stem was swapped out for one with different dimensions; otherwise it would still be on the bike. The bike had been ridden quite often on a trainer, and there was no visible damage while it was mounted.

Now, it’s possible that the stem would not have failed catastrophically as the part is in compression, but do you want to take a chance on that? Neither do I.

Aluminum and carbon don't work and play well with each other.

Aluminum and carbon don’t work and play well with each other.


We learned in the windsurfing industry that you never put aluminum and carbon together unless they are very carefully separated with a significant inert layer of fiberglass or some other impenetrable insulating material. Otherwise you will have failure, it’s only a matter of time. Again, bikes are not subject to the same degree of electrolytic/galvanic corrosion attack as marine products, but they are subject to it.

Dear Andrew,
Thanks. You bring up a very good point. That galvanic corrosion issue was a big one with early carbon frames with aluminum lugs that sometimes pulled completely apart in time or in a crash. Clearly, sweating on carbon frames or forks with aluminum parts bonded to them is not a great idea. Most carbon forks have aluminum dropouts, although the sweat would tend to drip off of that area. But as you say, corrosion around aluminum cable stops is an issue, and it is by no means limited to carbon frames with metal stops on them. This is a big issue where sweat collects around an area and surface tension keeps it there, like around a cable entry point, on any kind of metal frame other than titanium.

Dear Lennard,
I was curious about materials used for cycling. Steel has been used for virtually all applications of a bike build, the same goes and continues to be the case with aluminum and carbon. Ti also has its special places, but Magnesium is something I’m curious about.

SRAM uses magnesium on the Force brake levers and on the Force and Red rear derailleurs, if I’m not mistaken. Pinarello also makes a magnesium frame bike, and I’m sure that I read somewhere that magnesium has also been used for brake calipers (cross canti’s?). My question is has there been anyone who has made magnesium cranks? If so, who was it? If not, what are the pros and cons of such a material usage?

Dear Adam,
Here is a forged magnesium crank that’s not yet commercially available. It’s a joint project with Timminco, the local supplier of magnesium tubing for our frames.

As I own a company that builds magnesium frames, I can say that I am enthusiastic about the material’s potential. When drawn into tubes and welded, it is easy to work with, requires no post-weld heat treatment, can be made very light and strong, since it has 2/3 the density of aluminum with higher strength. It offers a great ride, thanks to superior vibration damping ability.

Cranks, however, cannot be built economically with welding, despite promising attempts in steel (Sweet Wings) and titanium (Morati and Sibex).

Most high-end cranks are cold-forged (the process of pounding solid metal into a form by striking it with great force). By aligning the grain of the material, cold-forging creates a stronger crank than does casting (pouring molten metal into a form), which creates brittleness due to the large crystals formed during cooling. Most forged cranks are machined afterward for the pedal threads, chainring tabs and holes, etc. Forging requires a large capital investment in forging presses and tooling for the specific part (i.e. forging molds and dies over which the parts are pressed), so machining is more economical than forging for making small quantities. Most low-production, high-end cranks (like ours) are completely machined, although a cold-forged crank of the same material and dimensions would be stronger.

Although magnesium can be forged, forged magnesium parts generally do not perform well, but the above-mentioned crank must represent a breakthrough in that.

Magnesium also can corrode rapidly, so it must be properly coated to last well in an application like a crank.

Machining magnesium has the potential of starting a very hot fire that cannot be extinguished with water or standard fire extinguishers; we and Paketa, another local builder of magnesium frames, keep buckets of sand around the machines to smother a fire that can develop in a pile of machined chips. But the fact that so many mag wheels exist on cars illustrates that it can be done safely (although there have been some big fires at mag wheel plants).

The bike industry has long shown that magnesium can be cast, then machined and coated, and then take heavy abuse including exposure to the elements, since most high-end suspension forks have lower legs made of cast magnesium. But this method would probably produce a brittle crank.

I am willing to bet that companies like Shimano and SRAM have investigated using magnesium as a crank material but ran into production roadblocks of the type I’ve described above that made it impractical relative to aluminum.

Feedback on hanging bikes.
Dear Lennard,
As a cycle mechanic for 23 years I am always prepared to learn. This article was timely as I have a customer with AVID brakes who stores his bike hanging from the front wheel. And guess what? It often has spongy brakes! I cure it by removing the lever from the bar, holding so that the port is uppermost and pump the lever. But in regular use the “metering” hole, in the piston’s chamber, is never the highest point and air is trapped there.

The solution is to bleed the system so that there is no air in the reservoir either, but I have difficulty doing this with the Avid. For fun, look at an old Hayes handpiece, they run “upside down” and will be “safer” stored hanging than any other position!
New Zealand (Down Under….. how does that affect the situation?)

Technical writer Lennard Zinn is a frame builder (, a former U.S. national team rider and author of numerous books on bikes and bike maintenance including the pair of successful maintenance guides “Zinn and the Art of Mountain Bike Maintenance” – now available also on DVD, and “Zinn and the Art of Road Bike Maintenance,” as well as “Zinn and the Art of Triathlon Bikes” and “Zinn’s Cycling Primer: Maintenance Tips and Skill Building for Cyclists.”

Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn. Zinn’s column appears here each Tuesday.

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