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Is using a metal seat post on a carbon fiber frame advisable or not? I want to use a seat post mounted rack on my road bike (no regular rack mount points are on this bike), but I have been told that is not a good idea for carbon seat posts. So I thought I get a whole new metal post & saddle and swap the posts as necessary for commuting versus recreation.

I ask primarily because the famous Sheldon Brown once wrote on his site "Using a carbon-fiber seatpost in a metal frame, or vice versa, invites problems." but didn't explain why.

Any thoughts?

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Not a direct part of my answer, but you may also consider a larger saddlebag like a Carradice. The issue with a seatpost rack on the carbon seatpost is concern for accidentally crushing or dimpling the tube with the (typically overbuilt, clumsy) clamps. A saddle-mounted bag neatly avoids these issues. –  lantius Jun 28 '11 at 2:34
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3 Answers

up vote 4 down vote accepted

It's likely that Sheldon's concern was for galvanic corrosion between carbon fiber and aluminum. This is an issue that's well-trod in both the bicycle and aeronautics fields. In essence, in the presence of an electrolyte (sea air, your sweat) and a cathode (carbon fiber-embedded resin) the anode (aluminum) will corrode. In the case of a seatpost/seattube interface, this corrosion will wedge the seatpost in place. This can, at best, make it extremely hard to remove the seatpost. At worst, it can render the frame unusable.

The first thing to do is to check to see if your frame was built with an aluminum insert for the seat tube. Given that you currently have a carbon seatpost this is unlikely, but it would alleviate the main concern.

If that's not the case, there are still some precautions you can take. As Calfee notes, anodization will provide some protection. As well, regular removal and cleaning and reinstallation with carbon paste can help prevent excessive buildup.

On the significantly more expensive side, you could also consider a titanium seatpost, which wouldn't corrode.

I'm not sure how the formatting in this article got so ruined, but you can read more in this VeloNews Technical Q&A with Lennard Zinn.

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Is lubing (light grease) the seat post a bad idea, prevents wedging? –  Moab Jun 28 '11 at 3:16
    
From what I've seen, manufacturers strongly contraindicate using grease, oil, or other lubricants on carbon components. Additionally, carbon seattubes are often supposed to be left ungreased due to slippage issues. As an alternative, there is carbon assembly paste that may help somewhat - it's actually designed to create friction and reduce slippage. How much it helps to prevent sticking from galvanic corrosion is dependent on environment and a myriad of other factors, as far as I can tell. –  lantius Jun 28 '11 at 3:21
    
Well thanks, lantius! That was exactly the type of answer I was looking for. I also like a suggestion a friend gave me - buy another bike with rack mount points. –  rsecca Jun 28 '11 at 3:52
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@rsecca I would tend to agree that a carbon bike is not likely to be the best fit for use that involves carting a lot of gear around. Buy a more solid 'tourer' style bike for when you want to fit your paniers. –  Mac Jun 28 '11 at 4:50
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One other thing to consider that lantius didn't mention is the possibility that using a seatpost mounted rack in even on a metal seatpost, may be contraindicated in some carbon frames. The one thing we know about carbon bikes is that they are generally manufactured with tight tolerances regarding weight and leverage points, and, even with a metal post, your frame might not support the added weight and leverage that add an unsupported rack might produce. I am not saying this IS a problem, only that it can be, and you should check with the frame manufacturer.

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I solved this problem with using a rack which mounts through the rear hub. It can't carry as much weight as a frame-mounted rack, but as others have pointed out - if I am hauling heavy stuff then I use a different bike.

This rack works great for me. Tubus makes some, and there are others.

Note that I haven't yet (knocking on wood) had to change a rear flat on this bike, and I expect it to be a pain (have to remove the quick release skewer to get the wheel off). That is the only downside, in my opinion.

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I have actually considered that option. The bike store near me says that style of rack has been their most popular this year. Thanks for the tip though. –  rsecca Jun 30 '11 at 2:14
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