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Old 03-31-12, 12:29 AM   #1
pocky
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Hairline crack in top of seatpost -- clamp around it from tandem stoker stem

Yes, I know, this is a terrible idea, and I am planning on replacing the seatpost eventually, but mechanically, I'm pretty sure I'm right about this.

I got this fine SR Sakae 26.6mm aluminum seatpost off a used hardtail mountain bike. I noticed it had a hairline crack at the top of it, and so I immediately replaced it with a 26.8mm seatpost -- the size that probably *should* have been in there to begin with. Here's what the crack looks like:



Fast-forward a few months, and I find myself in need of replacing a 26.6mm captain's seatpost on my tandem. (That's a whole other saga.) Well, so now I find myself in possession of this perfectly sized seatpost with a crack at the top.

Thing is, this is the captain's seat on a tandem, and the crack lives PRECISELY underneath where the stoker stem clamps on, like this:



Now, I intend to keep an eye on the crack and see if it grows, and plan to replace the seatpost for good measure anyway, but I think that the physics of this clamp should completely negate the weakness posed by the crack. Aren't I right, or am I smoking crack?
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Old 03-31-12, 07:28 AM   #2
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"I...plan to replace the seatpost for good measure anyway" I think that if it were my bike I would follow that plan before I rode the bike again. Why risk riding with a known, easily-correctable defect that could cause you injury? What other, unseen damage might there be? The clamp might hold it together but then again, it might not. Why take unnecessary chances?
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Old 03-31-12, 08:18 AM   #3
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Quote:
Originally Posted by dsbrantjr View Post
Why take unnecessary chances?
Because that's how I roll.

Point taken -- I agree, the risk to reward isn't necessarily worth it here. But still, the answer to the physics question is what I'm after.

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Old 03-31-12, 08:51 AM   #4
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I think the stroker stem clamp will act as a fine splint, for a temporary measure. However, it is a tandem, you are risking not just yourself if you have a stroker. Something to think about.
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Old 03-31-12, 09:16 AM   #5
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The split is in the Seat post tube, is because of the plug in the top,the saddle clamp portion,
and the rider's weight and the terrain ridden and hours of use combining to fatigue the aluminum.

past for a new seat post.

common to deal with cracks in aluminum is drilling a hole at the bottom of cracks ...

but this is replacement with zero hours time.
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Old 03-31-12, 02:47 PM   #6
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Geez, that's a disaster waiting to happen. The stoker stem is trying to crush the flawed seatpost and there is nothing "stenting" the inside to resist these crushing forces, is the way I see it. If you really can't find a 26.6 seatpost without a crack in it, I'll give you a spare SR Sakae I have, for free. There is nothing wrong with it. I had replaced it with a zero-setback post to give my stoker a little more room on our old tandem. Only thing is, I had sawn it off so the length from the centre of the saddle clamp cradles to the end is only 175 mm -- I'm a shortie. You won't be able to have as much sticking out above the seat lug as you show in your photo but really if you can use it, you're welcome to it.

PM me if you're interested.
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Old 03-31-12, 03:10 PM   #7
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The question you need to ask yourself is how deep deyond the crack would you want to insert the seatpost in a frame to feel safe. If you use the minimum insertion mark as a guide, then you'd want about 2" or so for the crack to not be a factor. Even allowing for pushing the limits you'd still want 1" of overlap at a bare minimum.

If the crack is perfectly centered in the stem, that's only 1/2" on either side. So would you ride a bike with a stem post or stem having only that much overlap?
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Old 04-01-12, 08:29 AM   #8
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Why would you even consider risking your life and the lives of others to save buying a $20 part?
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