Sunday, November 3, 2013

Fabricating a Bicycle Seatpost

I had intended to make a new seatpost for my bike because the original one wouldn't go high enough.  My bike's frame is almost 10 sizes too small for me, but I've been able to manage by getting a stem extender and raising the saddle as high as it would go.  So high, that the minimum insertion line was exposed, and it still felt too low.

I was planning on finding some tube to turn one down from until I found some conspicuously shaped tubing for free on the streets.  They were about 4 feet long 1" tubes that were reduced to 7/8" on the ends.  7/8" just happens to be a standard dimension for saddle attachment hardware, and 1" happens to be a standard seatpost dimension.  The problem is, my seatpost is a 26.4mm (actual dimension 1.037" or 26.3mm) which meant I would need to make a shim.

The shim would have to be .018" thick, but I went for .020" because I accidentally measured the seatpost to be 1.040".  I tried boring out a section of the 1" tube to make it to a .020" wall thickness.
The problem with using my lathe was the chuck didn't hold it on center that well, and with the thin wall thickness, it gave a nasty "3-jaw effect" in addition to inconsistent thickness.
So I used one of the chucker lathes at school to make the shim.  This time I drilled and bored it from a scrap piece of 1" aluminum round stock.  This is a textbook example of the advantages of using a collet instead of a 3-jaw chuck, it grips the whole circumference with less pressure required, and has a much better concentricity.
After slitting the side with a hacksaw, I checked the measurement over a 1" shaft.  It read 1.040".
This is where tolerance once again comes to bite me in the ass.  Had I actually carefully measured the seatpost and made the shim .018, it would've slid in the seat tube nicely.  It did go in, slightly, but took a lot of force even after sanding it (should've sanded it more).
For now that will suffice. Were the old seatpost at this height, there would be less than 2" holding it in the seat tube.  This new one extends well into the seat tube.
I had previously made a seatpost like this for a beach cruiser seat, but it wasn't nearly as long.  Also it was heavy as heck because it's made from pipe and the 7/8" part, which was welded on, had even thicker a wall.
Updation:
Looks like my no-budget seatpost was too good to be true. After riding it for a while it started to lean noticeably to the back.  I figured it added setback which my bike needed anyways.
Well after 2 weeks, it completely bent over. Looks like thin wall tubing isn't as good as they say it is. It was not comfortable riding like this either.
What's even more embarrassing is when I went to make a temporary replacement from the other piece, I had to smash it in half with a crowbar because I didn't have a saw at my disposal.
Looks like the scrap won this time, but I will get my revenge.  As usual you can't say "success" without first saying "sucks"  Also this seems like my second 2-week-failure after the pedal straps.

Update:
See part 2

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