Tech FAQ: Zinn on Seatposts, Rust and Pros Riding 26ers

Zinn answers your questions on not converting to a 29er, rusty frames, and carbon seatposts in Ti frames

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Dear Lennard,

I’ve benefited from your straight-to-the-point tech advice for years. Don’t believe you’ve answered this one yet (or if so I couldn’t find it.)

Q: I have a 4-year-old Gary Fisher Ferrous 29 frame that has developed surface rust in a few places (i.e., braze-ons) that don’t bother me much. But there’s also a thin line of rust peaking out under the edges of the gussets at the top tube-head tube and down tube-head tube junctions. Is this likely to lead to structural problems?

— Chris

Dear Chris,

That’s not a good place for rust, as it is a high-stress area and is constantly being worked as you ride. I’d sand/wire brush it and spot prime and paint it. Then watch it carefully for signs of cracking.

— Lennard

Dear Lennard,

I very much like your tech answers on VeloNews and have learned a lot from them. I have a question for you regarding Ti frames and carbon seatposts.

We all know that when mounting an aluminum seatpost into an alu or steel frame, a thin coat of grease should be used to prevent corrosion and fusion of the two materials. When mounting a carbon seatpost into an alu or carbon frame, a thin layer of friction paste should be used to prevent seatpost slippage. Here alu and carbon corroding together is not an issue.

What about carbon seatposts in Ti frames? In theory, corrosion here should not be an issue, because both carbon and Ti are non-corrosive. Are carbon posts known to slip in Ti frames?

I have done a couple of short test rides since building up a new bike and thus far no slippage. I also noted that the inside of the seat tube of this new bike is rougher or more textured compared to an aluminum seat tube, perhaps from a milling process.

Thanks for any advice you can offer.

— Sebastian

Dear Sebastian,

If there is no aluminum sleeve inside that frame (i.e., if the post is contacting only titanium), then there should be no corrosion and no need for grease.

Carbon seatposts can get compressed down to a smaller diameter and can slip because of that, in which case carbon assembly paste can fix it.

— Lennard

Dear Lennard,

From my perspective it appears that virtually all the top world cup women mountain bike racers have migrated over to 29ers, and from what I can tell, there are no jolly blonde giants among them. Most in fact are quite short. In this case, size doesn’t seem to matter, except when it comes to wheels. But while it seems overall that the 29er hardtail has also become the weapon of choice with the men, it’s also my impression that a lot of the top world cup men are still on 26″ bikes – Absalon comes to mind. Is that true, and if so, why would you think that is?

— Steve

Dear Steve,

So far, that is true, although the new breed of men is on 29ers. The thing that makes it hard for a top World Cup racer to switch to a 29er is the extra weight, because by the time they’ve reached the top, they have all become weight weenies and never want to add any. The only way Nino Schurter and Florian Vogel race them is when they received carbon tubular rims and tubular Dugast 29er tires. And top World Cup racers have gotten used to a certain setup, and it has worked well for them. So in the case of a guy like Absalon, he hasn’t seen that he’s getting left behind on the descents, and he probably believes that he would climb better and be more nimble with the 26er.

— Lennard

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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” – 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.

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