Cracked seatpost repair
 

Maybe a similar thread has been posted before, but just in case...
I changed frames on my main commuting bike late last year, and on the first ride, I noticed that the saddle would rotate axially on the seattube under extreme conditions (e.g. craning my neck backward to check traffic behind me).
I put this down to an error on my part of not tightening the seatpost clamp sufficiently. So, prior to the next ride, I cranked the bolt down a little more, and then noticed, since I was using the saddle for purchase against the wrench, that the top of the seatpost was rotating relative to the tubular part that slips into the seattube. OK, I thought, quality issue, I'll drill and tap for a setscrew or pop rivet, to prevent the cast knuckle from rotating relative to the tubular part.
Fast forward to today, when I had time to get down to business. I removed the seatpost and put it on my workbench, after I had located a suitable tap and drill to tap it out for a setscrew. That's when I noticed the real problem. There was a crack at the top of the tube in the area where the cast knuckle was inserted.

http://i.imgsafe.org/1073f01387.jpg

OK, so I quickly formulated and executed Plan B. I sawed off the tube below the end of the crack, clamped the cast knuckle into my vise (with aluminum soft jaws that I made on the spot for the purpose), and judicuously tapped the shortened tube back onto the knuckle with a largish ball peen hammer and a block of wood to soften the blow to the tube.

http://i.imgsafe.org/1073fcf4d5.jpg

http://i.imgsafe.org/1074108fd7.jpg

There was still enough length left on the seatpost to raise it to my customary height, fortunately.

http://i.imgsafe.org/fca13c48f7.jpg

Thinking about the history of the seatpost, it originally came with a 1987 Miyata 312 that had about a 53cm frame that I rode for a while as a stopgap during a nomadic period. Naturally the post was extended to nearly its limit.

http://i.imgsafe.org/5329eb0.jpg

Maybe that was a factor, maybe it wasn't. This is the first failure of this type I have experienced.

Handlebars, stems and seatposts are items I toss when compromised.

Thanks for the info. Dunno if the extension caused it, but I have the same seatpost on a bike and will have to keep an eye on it.

I've always been leery of these posts with a cast part jammed into and extuded one.

it should be fine you cut it off ahead of the crack...keep an eye on it.
JM2C's

+1.

Add to that anything aluminum and broken.

+1 Junk it. Severe injuries can result from a broken seatpost.

Nice save [MENTION=184345]old's'cool[/MENTION] . Why fix it when you can toss it and buy a new one ? Oh wait your fix was perfect .

I might have filed/sanded down the post clamp head insert portion so it was a just barely slip fit then LockTite it to reduce the stress on the post shaft. Is it an artifact of the fixturing or do I see stress lines on the shaft running down from the new top edge about 1"? I've seen this type of micro surface cracking with a tight press fit before. Andy

If you cut it off below the end of the crack you should be ok. That said, this is one reason why I prefer one-piece forged posts.

+1 on the stress risers along the seat post. Not worth the risk with the leverages involved. Think punji stick.

It might last forever, but in my opinion, it's compromised and not worth the risk.

Old's'cool, it looks like the cast head may have been machined just a bit too large. I wouldn't think you would need more than maybe .002" of interference. From the looks of the anodizing crazing on the new section, it may be much more than that. It'll probably be fine, but I wouldn't be terribly surprised if it cracked again at some point in the future.

I forgot to mention, that if it does crack, it will almost certainly be vertical like the first time. That is perpendicular to the hoop stress caused by the interference.

I dunno. Worst case scenario, it cracks again vertically, like it did before? It's not like to going to suddenly snap off horizontally and leave the rider somehow impaled on the seatpost stub.

Although, $10-20 gets you a lot seatpost on eBay. I'm pretty sure I'd have tossed the broken one into the recycling bin and just rode a different bike until a new seatpost arrived in the mail.

I tried the same repair once on the same post. For the life of me, I couldn't get the head fully seated again, either by hammering or squeezing with a pipe clamp, so I tossed it. Make of that what you will. At a minimum, two known instances of cracking isn't exactly a ringing endorsement.

DANGER, Will Robinson. Take heed.

Works for me..

Ok, dumb question time. Instead of cutting off the compromised area, could you remove the knuckle and flip the tube, installing the knuckle at the other end? Or is this the worst option, since the crack could continue to propagate? Presumably, the only reason to do that is if the shortened tube were too short. If that were the case, you'd be depending on the cracked area, so I guess I answered my own question.


Lol, I used to enjoy consulting. You'd ask the client clarifying questions, they'd figure out the answer on their own, and think you were smart and understood what was really going on. Oh, the good old days.

The OP did exactly what I would have done. Of course, he could have just wrapped some duct tape around it and gained honorary membership in Possum Lodge.

You could, provided the post had a uniform inside diameter. But this isn't always the case.

That is my plan. It certainly gave plenty of warning. Had the crack proceeded further, it would have reached a point where I could not help but notice how easily the saddle rotated.

Thanks for your concern. Like cracks in airplane parts, the crack has been dealt with, and the component is ready for a new tour of duty, subject to prescribed inspection intervals.

I considered that option briefly, with or without shortening the tube. I don't think a vertical crack at the bottom would actually propagate, since would be are no appreciable tensile stresses perpendicular to the crack. I abandoned the idea because the engraved markings in the tube would be UPSIDE DOWN. :twitchy:

Now why didn't I think of that? :foo:

Your endorsement is certainly appreciated, Sir! :beer:

Had I pursued that option, I would have miked both ends of the tube to check. :thumb:

We don't know if the crack was due to a metal flaw. And we don't know if the crack (or the flaw that allowed the crack) continues invisibly on down into the re-used part of the tube. Nice looking repair, but I think I'd look for a new post that was cast as one piece. Ironic that the brand name on the broken one is "Strong".


Reminds me of the outrageously funny thread on here somewhere where a few members shared their experiences with cheap carbon fiber seatposts failing. :)

Good spotting, I wondered if anyone would pick up the slight change in surface appearance of the tube where the knuckle was pressed in. Whatever caused the change, it wiped off easily, but I didn't take a photo at that stage.
Here is one I just took:

http://i.imgsafe.org/25a18607c1.jpg


If your hypothesis (regarding excessive interference) is correct, the outcome would depend a lot on the actual material properties of the tube (i.e. as manufactured, not as specified); for instance, UTS and elongation before fracture.
At any rate, It didn't seem to that the impact necessary to drive the knuckle into the tube was excessive.

:twitchy::lol:

Starting from ground zero, I'd be a little more concerned, but considering the miles I've put on this post before the issue, and the "gentle" failure mode, not to mention I have another post of the same design and manufacture, but 1mm smaller in diameter, with many more miles on it, still going "Strong" :innocent:

... Say, did any of those carbon seatposters attempt repairs when they failed? ;)

It didn't sound like there was enough left to attempt a repair. :lol: