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Shaggy Dog Bike Repair Story (with SCIENCE)

March 6, 2010

Jack wanted me to raise his seatpost, how hard could it be?But the seatpost was stuck, tight. I’ve unstuck seatposts before, so I tried what had worked in the past (get the bike good and cold, pour boiling water on frame, twist seat). No luck.

Tried that three times, no luck.

Tried putting the seatpost in a vice, then twisting the bike. No luck.

Took a little time at this moment, to check that all the other seatposts on all the other bikes, were well greased and loose enough. They were.

Tried cut-the-top-and-slot-the-seatpost, but this was a long seatpost, and it was most of the way down, and hacksaw blades don’t come longer than 12 inches. Chances were, I might cut the seat tube instead.

What now?

Turns out, you can use electrolysis to dissolve metal (you can use lye to disssolve aluminum, if you like chemical burns, but I don’t).

So, how do I fill a seat tube with water?

Take out the bottom bracket, (this next bit was Stephanie’s idea, and a good one) pull an inner tube through, and inflate it to seal it from below.

Except, taking out the bottom bracket required a trip to the hardware store, because it was really tight. Really, really tight, so I had to get a bolt to hold the splined tool on. (Get a bolt that matches the threading of the bolt that holds on the cotterless crank, only longer.)

The tube trick would have worked better the first time, if I had not used the tube original with Jack’s (used) bicycle, which was stuffed full of green slime. When you inflate a tube full of green slime, outside of a tire, it stretches and all the accumulated holes expand, and leak green slime and air. So I used a different tube, and it worked better.

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The negative electrode is a copper pipe wrapped in cloth and tape.

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The positive electrode is the old seatpost.

The power is a bunch of AA batteries. What could possible go wrong with a pack of itty-bitty batteries?

Electrolyte #1 was sodium bicarbonate (I had read that alkali was good for attacking aluminum). But it was slow.

Electrolyte #2 was plain old salt. Oh, my. I smelled that I had a problem when I stepped out the door. I nearly melted the battery case.

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I put a fan in series (as many people on the net recommend), and started again.

The inside of the seat tube looks like this:

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It looks like this will be a relatively slow process, but it doesn’t really need my help at this point. It’s slow enough that I am not going to disappear the aluminum completely and put a hole in the seat tube.

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March 8

Turns out, it’s really slow. Over time I tried various electrolytes to see which produces the best effect, and none were really spectacular. Bases are supposed to be better for attacking aluminum, so I tried sodium bicarbonate, sodium carbonate, and ammonia. All of these clearly produced weird aluminum hydroxide gel, but not enough of it. I also tried salt and epsom salts; salt led to impressive currents (thus the cooked battery holder). I think, if I had had the nerve, that lye would have worked best, but I am no chemist and paranoid about chemical burns.

I went back to mechanical methods:

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from left to right, the negative terminal from the electrolytic etcher, a dowel with a carbide-grit cutter wrapped over its end, and a small file mounted in a drill bit extender.

resulting, finally, in this slotted and and extracted seatpost:

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Visual monitoring was not sufficient; I had imagined that I would see a difference when I hit steel, but did not. I also regularly checked the filings that came out of the seat post to see if they were magnetic; when they were, I got more careful in my filing, and attempted to squeeze and twist the seat post. There’s some visible filing deep in the seat tube, but a metal probe cannot distinguish it from the other corrosion in the seat tube.

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