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Old 04-11-06, 06:53 AM   #1
kyselad
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Expander seatpost mod?

I've got an old Kabuki (Bridgestone) with an expander seatpost. There's no seatpost clamp, thanks to aluminum lugs (Sheldon rails on these bikes http://www.sheldonbrown.com/japan.html#kabuki).

Can anyone think of a workaround for using a standard seatpost? As noted in the above link, a clamp won't work on the aluminum lug. I realize I can probably find a seatpost-friendly frame of comparable quality for next to nothing, but what fun is that?

This question has come up occasionally in other threads, without a straightforward solution. So I figure I'd throw it out here. Thanks.

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Old 04-11-06, 09:08 AM   #2
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This shouldn't be hard, all you need is a correct diameter seat post, the wedge assembly out of an old, long stem, and a cap with a hole in it. Cut a 45* angle in the bottom of the post, assemble, and call it a day. If you want to get really fancy you could weld the cap on. If you can't find an old stem that has a long enough bolt, I'm sure you could either buy a bolt or do it with all thread.

The only complication I could think of is if the seat post inside diameter would allow the wedge to slip inside, in which case you just need somebody to weld a piece of metal onto the cut end. This might be necessary anyway if the wedge wants to bind against the edge of the seat post tubing, but if you get it really smooth this shouln't be a problem.
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Old 04-11-06, 11:55 AM   #3
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other mods?

Thanks for the post.

I think I see how this could work. I'm a noob, so I don't have a lot of experience with stems (or seatposts), but it sounds like this fix is basically re-creating an expander seatpost of sorts (yes?). Would this work with a standard seatpost that has a seat mount built into the end? If I'm understanding correctly, the mod using the stem expander would require an open-ended post or something that could be drilled through the top. Then I'd need an extra clamp from the post to the seat. Sadly, that's the one part on my setup that's busted. I could hunt down a replacement, but if there's some way I can rig in a regular seatpost, all the better.

Many new seatposts do have a pretty tight bend near the top. Maybe drilling into the bend in line with the bottom opening would permit insertion of the wedge assembly? I'm unsure whether this would give a reasonable contact surface with the bolt head for torquing it down. And would this damage the integrity of the post?
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Old 04-11-06, 12:20 PM   #4
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Yes, if you look at an old style stem from a threaded headset, you will see that it's basically two opposing wedges. The stem is a solid piece of (usually) Al with a hole down the middle for the bolt, and then the bolt screws into the wedge piece (usually steel; go go gadget galvanic corrosion!). You crank down on the bolt at the top of the stem, the wedge ramps against the stem, and things lock up nice and firm.

I wouldn't try the mess I'm suggesting with anything but a straight seat post -- the bend is the weakest part, and you don't want to be drilling there. My package once had an abrupt meeting with my back wheel when a layback seat post on a BMX failed. Actually, if your current seat post isn't hosed, it sounds like it could be reused. What parts exactly are broken?
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Old 04-11-06, 12:27 PM   #5
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There are two main types of seat posts. Shaped either like an | or like a upside down L.

The | type is generally the cheap ones, there is another bracket that attaches to the top of it. That bracket also attaches to the saddle.

The L type has the saddle mount on the horizontal section of it. This is the type you described.

If you got a | type you could probably convert it to quill style. Check out this thread:
Bye Bye Seat Tube!
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Old 04-11-06, 01:00 PM   #6
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Also, I just looked and you can find this part on ebay, so no need for DIY. And yeah, you have to run a cheapie style saddle with a separate clamp.
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Old 04-11-06, 02:13 PM   #7
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Quote:
Originally Posted by Landgolier
Also, I just looked and you can find this part on ebay, so no need for DIY. And yeah, you have to run a cheapie style saddle with a separate clamp.
You would need a separate clamp, but you could use that clamp on whatever saddle was desired.

I use a clamp on my Brooks B-17... It may look strange on the new ultra-light racing saddles, but they would work on them as well. They will work on any saddle with rails.

To get a seat clamp, if you have no other source, buy a cheap seat at *Mart and remove the clamp from the seat rails... I am not sure about all of the *Mart seats, but I have seen the clamp on the rails on some of them. About $8.00 for the seat to use the clamp, but probably cheaper than ordering and paying postage.
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Old 04-11-06, 04:11 PM   #8
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the clamp

Thanks for all the replies -- I think you've all hit on a good fix. The expander is serviceable; the clamp from the post to the seat is the broken part, and I just figured finding a new one would be hopeless. I didn't realize the el-cheapo bikes still had straight posts and the clamp I'd need. The pointer to that other thread is a great help -- I'd actually seen that thread but totally missed the saddle clamp! I'll see if the lbs has a clamp. If not, I'll hit the superstore.
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