Mountain Gear

Tech FAQ: Keeping Your Carbon Frame Clean and Secure

A riding reader asks Singletrack.com's tech expert Lennard Zinn about greasing the seatpost in his new carbon Gary Fisher Superfly Elite. And how do you keep carbon clean? LZ has the scoop…

Dear Lennard,
I have a Gary Fisher Superfly Elite 29er and I want to know how best to take care of the carbon fiber, since I want this bike to last me a long time. I was told by one friend not to grease the seatpost because the grease would damage the carbon fiber, and another friend told me that the aluminum seatpost could corrode to the carbon fiber if I didn’t put grease on it. What should I do?

Also, the same guy who told me not to grease the post said that I can’t let chain lube drip onto the chainstay or it will cause it to crack. Should I wrap up the right chainstay with something and seal it up? With what? What about the oil that gets on other parts of the frame?
-George

Dear George,
Galvanic corrosion can be an issue that can indeed adhere an aluminum seatpost pretty solidly to a carbon seat tube. You should definitely put some kind of lube on the post and inside the seat tube, and, no, it won’t damage the carbon. You should also pull out your seatpost periodically and after every ride in wet conditions and leave the frame to sit upside down to drain water out of the seat tube. Then re-lube the post before insertion.

If the seatpost slips down with grease, then use carbon assembly paste. Otherwise, grease is fine.

And don’t worry about getting chain lube or other oils or greases on your chainstay or any other part of the frame. It won’t damage it.

Here are the results of Cervelo’s testing of lubes, grease, oil, solvents and cleaners on carbon fiber structures.
-Lennard

Background

Picture 3The epoxy used as the binding matrix absorbs moisture and presumably other small liquid molecules by what the scientists call Fickian diffusion. For epoxies, it takes quite a long time or highly elevated temperatures for this diffusion to occur to any significant degree. The end result, if you could force it into such a condition, is a minor degradation of mechanical properties and a small amount of swelling. The test we designed was to demonstrate that any effect of compounds typically used around a bike is negligible.

Micro-cracking

Micro-cracking is present in essentially all epoxy/carbon parts made from prepregs or cycled at elevated (or very, very low) temperatures.  It is a result of the differential thermal expansion of the resin compared to that of the fiber that occurs when cooling down from the cure temperature. The cracks may increase upon initial loading or initial temperature excursions then basically go along for the ride after that with no increase in micro-cracking. Any effect caused by micro-cracking is already accounted for in the material properties and is present in any test and any bicycle component. Except under very high loads that case that is near static failure these micro-cracks do not contribute to a failure. In particular, the carbon fibers make excellent crack stoppers. That is why when we do extended fatigue tests on frames, we have a good chance of failing the test equipment before we fail the composite parts. The exceptions to this would include high stress concentrations due to poor design or manufacturing problems.

Short Beam Shear Tests

Picture 1From the ppt slides, you can see that small samples are conditioned by immersion in various liquids at an elevated temperature. The edges of the samples expose the maximum amount of contaminant to the epoxy matrix and the heating increases the rate of absorption to a greater degree than you will find in any reasonable ambient conditions. This conditioning is more aggressive than exposing a carbon structure such as a bicycle frame with some type of chain lubrication. This particular test method places high shear loads in the epoxy matrix and would show any degradation of the matrix or the interface of the matrix to the fibers in reduced strength values. (A tensile test would not show similar behavior because it relies heavily on the fiber strength, not the matrix properties). These samples already have their naturally-occurring micro-cracks plus the additional cracks and damage along the machined edges.

Don Guichard, Cervelo Cycles

And finally, a little feedback from a previous column on XTR:

Dear Lennard,
Not trying to be a stickler here, but in 1991, Shimano had 3 mountain bike groups when XTR was introduced, Deore XT, Deore DX (previously Deore II) and Deore LX (previously Mountain LX).  The greatest thing that the original XTR group introduced, besides the Campy like finish of the components, was Rapidfire.  Remember the original under bar “push-push” shifters introduced by Shimano?  They were awful, which is why everyone was clamoring to get their top mounts back and which gave SRAM a toe-hold in the shifter world (I had those atrocious, giant original Grip Shifters).

Thanks for letting me ramble for a minute.
-Chris

Readers can send brief technical questions directly to Lennard Zinn.

Technical writer Lennard Zinn is a frame builder (www.zinncycles.com), 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.

Follow Lennard on Twitter.