Seatpost of a folding bike is always dirty, filed down by frame?

Question

I fold and unfold my Dahon Speed D7 at least four times a day, and each time its seatpost is being pulled up or pushed down the frame tube.

Problem is that the seatpost becomes dirty really quickly even on a dry weather. I suspect the aluminium it's made of is softer than the frame alloy and every time I move it up or down the tube it's being filed down a bit, and so the dirt is actually very fine chips of the seatpost itself. However, it hasn't been changing for a couple of years (shouldn't it already be polished enough by this moment?)

It's hard to tell for sure from the dirt look itself though. Is it possible for the road dust to get that quickly through the opening of the frame below? Would oiling the seatpost help or it would only make it worse (dry dirt is much more preferable to deal with than when mixed with oil)?

Maybe it makes sense to replace the seatpost with a steel one (not sure if they exist as a specific diameter and length are required).

Must be a common problem for folding bike owners, how do you cope with that?

Answer 1

If you have an old inner tube available cut a section slightly longer than your seatpost. Slip it over the seat post and zip tie it in place. It will act as a somewhat water/dirt resistant boot, similar to what mountain bikes have on the front shocks. You may have to look for a fairly wide tube to fit over the post. Check with your local shop to see if they have any unrepairable ones left laying around.

Answer 2

Another option is to remove the seat post from the saddle and the bike, then polish it.

You could polish it by hand, but a far easier solution is to spin it in a lathe and apply a light touch with sandpaper and WD40 as a lubricant.

Use a light touch, this will take some material off the outside diameter of the seat post. A solution to that is to apply a wide-spaced knurl, also using the lathe.

To provide a nice finish, you could apply buffing compound and bring it to a shine, and then top-coat with multiple layers of clearcoat. This will wear off over time but will keep the seatpost looking shiny.

If you have access to the gear, anodising the aluminium is also a valid solution. This lets you choose a colour too, and the duration spent in electrolysis will govern the thickness.

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Separately, you want to clean the bore in your frame too. Use a suitably-sized bottle-brush with a long handle of twisted wire. Fit the handle to the chuck of an electric drill, and "hone" the inside of the frame tube. You need to use degreaser and flowing water, so this is an outdoor task. If you feel or see any daggs or obstructions that could scratch your seatpost, try and file or sand them smooth. Bright torchlight helps here.

Hopefully your seat tube does not exit into the BB housing - most folders avoid this but I can't see in your photo. If it does, all this wash-water will drop into the BB area contaminating the bearings there. Avoid by removing the cranks and bearings.

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On reassembly, make sure everything is completely dry. Using grease will make a mess of the seatpost when extended, so I'd avoid grease.

If your seatpost does not exit the bottom of the frame when bike is folded, then I'd suggest making some kind of cap for the bottom of that tube to keep dirt and debris out.

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If none of this seems feasible, then you might choose to buy a new seatpost.

TERN bikes offer a two-stage telescoping post like this:

And there are doubtless seatposts with a hinge to allow folding rather than collapsing. I'd personally be leery of them but they're an option too.

Answer 3

I might be a bit late on this one. It looks to me that you have a few burs on the inside of your frames seat tube. There is a tool called a flex hone that you can buy, chuck into a drill and stuff down there with cutting oil to smooth evrything out. I do this on all my new bikes to prevent the seat tube getting scratches. Clean it out after with a rag and iso. Then lube your seatpost. Maybe keep a rag tied to the bike someplace to clean the post off when you need to fold the bike. 4 times a day is a lot of folding, I'm impressed.